The understanding and ability to manipulate magnetization dynamics in magnetic multilayered structures offers great potential for the development of novel devices. Until recently, macroscopic understanding of the dynamic magnetization behavior in such materials was obtained through ferromagnetic resonance (FMR) measurements where micromagnetic simulations and theoretical modelling were used to predict the microscopic behavior. Here we present our recently developed synchrotron-based x-ray technique with the ability to uniquely resolve the dynamic magnetization behavior as a function of depth through magnetic multilayered systems [1-3]. Reflectometry ferromagnetic resonance (RFMR) brings together the power of two synchrotron x-ray based techniques: Firstly, x-ray detected ferromagnetic resonance (XFMR) provides a element-specific and time-resolved probe to explore the magnetization dynamics [4,5]. Secondly, soft x-ray reflectivity provides a measure of the depth dependence to both the chemical and magnetic structure through multilayered systems. In combination, RFMR reveals new and unique insights into the magnetization dynamics exhibited in layers beneath the surface in multilayered structures. We will present new understanding gained from such layer resolved measurements of the magnetization dynamics in a [CoFeB/MgO/Ta]4 multilayer structure. As expected, each of the magnetic layers exhibit a ferromagnetic resonance behavior. However, in contrast to a single magnetic thin film, additional coupling between the layers results in a phase lag relationship between the dynamics in adjacent layers [1].


[1] <a href="/publications/Burn_PRL_2020">D.M. Burn et al. Phys. Rev. Lett. 125, 137201 (2020).</a>
 [2] <a href="/publications/Burn_NanoLett_2020">D.M. Burn et al. Nano Lett. 20, 345 (2020).</a>
 [3] <a href="/publications/Burn_AIP_2021">D.M. Burn et al. AIP Advances 11, 015327 (2021).</a>
 [4] G. van der Laan, J. of Electron Spectrosc. Relat. Phenom. 220, 137 (2017).
 [5] <a href="/publications/Klewe_SRN_2020">C. Klewe et al. Sync. Rad. News 33, 12 (2020).</a>

Layer resolved dynamics of magnetic multilayers

Magnetic hard/soft bilayers are known to exhibit exchange spring states1, which are promising candidates for use in next generation magnetic data storage devices2. The dependence of the magnetic behaviour of Mn ions on their local magnetic environment has been well documented3. Therefore, a thin Mn layer inserted into the soft layer of a hard-soft bilayer can act as a probe of the soft layer magnetisation at a certain depth without disrupting the magnetic state4. Consequently, we have studied the influence of the Mn thickness on the static and dynamic magnetisation properties of L10 FePt/NiFe/Mn/NiFe thin films to find an ideal thickness for the Mn ‘spy layer’.



A series of samples with nominal structure MgO(110)/ L10 FePt(110) (100Å)/ NiFe(200Å)/ Mn(𝑥𝑥 Å)/ NiFe(200Å) were fabricated by sputtering with 𝑥𝑥 = 0, 5 and 10Å. X-ray magnetic circular dichroism (XMCD) was used to obtain element-specific hysteresis loops in order to explore the switching of the individual constituent layers (see Fig. 1). For 𝑥𝑥 = 0 or 5Å, the entire multilayer switches as a single rigid magnet. When 𝑥𝑥 = 10Å, the Mn layer appears to decouple the upper and lower NiFe layers. We explain the observed switching by attributing the Mn signal to two distinct Mn rich regions, each of which is strongly coupled to one of the NiFe layers. From VNA-FMR and XFMR measurements, we deduce that the upper NiFe layer resonance drives precession in one of the Mn rich regions as intended.


<img class="img-fluid" src="/images/presentations/abstract_figs/Newman_Magnetism_2021.png"/>


 Figure 1: XMCD hysteresis loops for Mn (blue), Ni (black) and Fe (red) obtained from samples with Mn layers of different thickness as indicated. The loops were acquired with the magnetic field in the plane of the sample.



 [1] <a href="/publications/Dabrowski_AMI_2020">M. Dąbrowski et al., Appl. Mater. Interfaces 46, 52116 (2020)</a> 
 [2] T. Seki et al., Nat. Commun., 4 1726 (2013).
 [3] C. Demangeat and J.C. Parlebas, Rep. Prog. Phys., 65 1679 (2002).
 [4] J. Li et al., Phys. Rev. Lett., 117 076602 (2016).

Influence of a Mn ‘spy layer’ on the static and dynamic properties of a FePt/NiFe bilayer

Heterostructures composed of ferromagnetic layers that are mutually interacting through a non-magnetic spacer are at the core of magnetic sensor and memory devices. Such devices can exhibit different types of coupling between magnetic layers, such as: exchange coupling and spin-current mediated coupling. The latter is often rather weak and difficult to observe. We used the technique of x-ray detected ferromagnetic resonance (XFMR) [1] to study the dynamics of a Co/MgO/Permalloy magnetic tunnel junction (MTJ). The experimental results were quantitatively compared to the Landau-Lifshitz-Gilbert-Slonczewski model, and an indepth statistical analysis based on a likelihood ratio test was employed to determine the presence of spincurrent mediated coupling between the two magnetic layers [2]. A strong proof of the existence of spincurrent-like coupling was obtained and the contribution of exchange and spin-current coupling to the overall interaction between magnetic layers was quantified (Fig. 1)


<img class="img-fluid" src="/images/presentations/abstract_figs/Gladzuck_Magnetism_2021.png"/>


 Figure 1: (a) XFMR of the MTJ’s Co layer at 80 K measured for the indicated field values at an angle θH = 10º. The data points in panels (b) and (c) show the amplitude and phase, respectively, extracted from the XFMR signal as a function of magnetic field strength at θH = 20 º (Co - orange, Py - blue). The experimental results are compared with a model with only exchange coupling (dashed lines) and a model with both exchange and spin-current coupling (full lines). Panel (d) shows the combined information about the phase and amplitude on a single plot, where the green and red arrows for Co and Py, respectively, indicate the values of the X and Y parameters used in the likelihood ratio test at a magnetic field of 63.5 mT. 



 [1] G. van der Laan, J. Electron Spectrosc. Relat. Phenom. 220, 137 (2017).
 [2] <a href="/publications/Gladczuk_PRB_2021">Ł. Gladczuk et al., Phys. Rev. B 103, 064416 (2021)</a>

Determination of the existence and magnitude of spin-current and exchange coupling in MgO-based magnetic tunnel junctions

Synthetic ferrimagnets (SFM) two ferromagnetic layers with different magnetic moments, exchange-coupled via the RKKY interaction across a non-magnetic spacer layer, are of particular interest as a feasible media for all optical magnetic switching. Design of the magnetic properties of these SFM structures requires knowledge and control of the magnetisation, M(T), and anisotropy, K(T), of each ferromagnetic layer, and the strength of the exchange coupling between them, Jij(T). 



 We have shown it is possible to engineer the magnetic properties of series of compensating Ni3Pt/Ir/Co SFMs, fabricated by magnetron sputtering and exhibiting perpendicular magnetic anisotropy. The tuneability of this structure (through variation of the Ni3Pt deposition temperature and the thickness of each layer) provides an ideal system with which to investigate the effect of M(T), K(T) and Jij(T) on reversal mechanisms within SFMs, with the additional benefit of distinct magnetic species allowing for element and layer specific characterisation.



 Initial pump/probe experiments reveal that it is possible to switch these SFMs by application of optical laser pulses under only a small applied field. Full optical switching requires stronger antiferromagnetic exchange coupling than can currently be achieved in Ni3Pt/Ir/Co SFMs, as a result of the elevated deposition temperatures necessary for PMA in Ni3Pt. A possible solution is to replace the Ni3Pt layer with a Ni/Pt multilayer, in which PMA can be achieved by careful choice of layer thickness. This gives scope for improving the exchange coupling by removing the cooling step during deposition, while also maintaining element specificity and tuneability of the magnetic properties.



<img class="img-fluid" src="/images/presentations/abstract_figs/Scott_Magnetism_2021.jpg"/>


 Figure 1: Remanent magnetisation as a function of temperature of a Ni3Pt(7nm)/Ir(0.4nm)/Co(1nm) SFM with arrows depicting this sample’s layer alignment, Co (green) and Ni3Pt (red, solid), and noting the positions of the magnetisation compensation point, Tcomp and T-MR, the onset temperature of negative remanent magnetisation. The multilayer sample, 4x[Ni(0.6nm)/Pt(0.4nm)] /Ni(0.6nm)/Ir(0.4nm)/Co(1nm), shows the same temperature dependent behaviour.



 Figure 2: Element specific XMCD hysteresis loops of a Ni3Pt(8.5nm)/Ir(0.5nm)/Co(1nm) SFM recorded at temperatures T < T-MR (~200K), T ~ TMR (~220K) and T > T-MR (~250K), with Co (blue) loops shown on top and Ni (red) shown below. Arrows show the direction of the field sweep.






 [1] S. Mangin et al, Nature Materials 13, 286 (2014)
 [2] R. F. L. Evans et al, Appl. Phys. Lett. 104, 082410 (2014)

Tuneable, Perpendicular Ni, Pt, Ir and Co Synthetic Ferrimagnets

The understanding and ability to manipulate collective spin excitation modes in topologically ordered magnetic structures offers great potential for the development of novel devices. At present, the understanding of these dynamic magnetization modes is based on ferromagnetic resonance (FMR) studies, probing the bulk of the material system, in conjunction with micromagnetic and theoretical modelling. On the other hand, experimental tools to directly probe complex magnetization dynamics in such spin systems are lacking. 

Here, we will present a novel diffractive ferromagnetic resonance (DFMR) technique with the ability to uniquely resolve the dynamic behavior of individual resonance modes tied to topological spin structures [1]. This new technique brings together the power of two synchrotron x-ray based techniques: Firstly, x-ray detected ferromagnetic resonance (XFMR) provides a chemical- and time-resolved probe to explore the magnetization dynamics [2,3]. Secondly, resonant elastic x-ray scattering (REXS) selectively probes periodic spin structures within the magnetic material [4]. In combination, DFMR reveals new and unique insights into the dynamics of ordered spin systems. 

<img class="img-fluid" src="/images/presentations/abstract_figs/Burn_MMM_2020_a.jpg"></img>

Fig. 1 (a) Experimental DFMR schematic where x-rays are scattered from magnetic structures within the sample with magnetization dynamics driven by a RF magnetic field. (b) Example data showing the variation of the dynamic signal with linear polarization angle and pump-probe delay for Y-type hexaferrite.

[1] D.M. Burn et.al. Nano Lett. 20, 345 (2020).
 [2] G. van der Laan, J. of Electron Spectrosc. Relat. Phenom. 220, 137 (2017).
 [3] C. Klewe, et.al. Sync. Rad. News 33, 12 (2020)
 [4] G. van der Laan. C. R. Phys. 9, 570 (2008)

Magnetization dynamics in topological spin structures revealed by diffractive ferromagnetic resonance (DFMR) INVITED

FeGe is a noncentrosymmetric helimagnet exhibiting non-collinear spin textures. Below the ordering temperature, Tc, bulk FeGe has a well-established and rich magnetic phase diagram showing helical, conical, and skyrmion states [1]. The skyrmion pocket has a wide, layer thickness dependent, temperature vs. field window extending up to room temperature [2-4]. These features make FeGe of great interest due to the potential for novel room-temperature devices based on the manipulation of topological magnetic states. 

 Here, we characterize the magnetic states in a thin membrane of FeGe with small angle soft x-ray scattering as a function of both temperature and out-of-plane magnetic field. We reveal a hook-shaped dependence of the modulation vector on the applied field whilst traversing the skyrmion phase pocket. This dependence arises due to competition between changes in the skyrmion size and packing density that occur as a function of field [5].

In addition to the helical, conical, and skyrmion phases, we observe an additional helical phase which coexists beneath the skyrmion phase in the phase diagram, towards the higher field boundary of the helical phase. In contrast to the other phases this additional helical phase shows strong uniaxial preferential alignment of the modulation vector [6]. 

Our findings build upon other work on FeGe, strengthening the case that the room-temperature topological magnetic properties are of great interest for the development of novel technological devices. Our work shows how multiple coexisting phases can be simultaneously and uniquely identified, which will provide further opportunities for potential information processing and storage methodologies.
 
 [1] A. Bauer et.al. Springer Series in Materials Science 228, 1 (2016).
 [2] X.Z. Yu et.al. Nat. Mater. 10, 106 (2011).
 [3] H. Wilhelm et.al. J. Phys.: Condens. Matter 24, 294204 (2012).
 [4] S. X. Huang et.al. Phys. Rev. Lett. 108, 267201 (2012).
 [5] D.M. Burn et.al. Phys. Rev. B 101, 014446 (2020).
 [6] D.M. Burn et.al. Phys. Rev. B 100, 184403 (2019).

Field and temperature dependence of the skyrmion lattice phase and the appearance of a second helical phase in FeGe

Synthetic ferrimagnets and antiferromagnets with perpendicular anisotropy have attracted great interest as they allow for stabilization of room-temperature skyrmions 1, fast domain wall motion 2, and all-optical switching of magnetization 3. Here we study Ni3Pt/Ir/Co structures in the vicinity of the compensation point temperature TM, where the magnetizations of the two layers have the same magnitude and are anti-parallel to each other. By measuring element-specific hysteresis loops using x-ray magnetic circular dichroism (XMCD) and wide-field magneto-optical Kerr effect (MOKE) microscopy, we explore the magnetization reversal and domain structure of the Ni3Pt and Co layers (Fig. 1). We show that the change of the magnetic state upon increasing the temperature through the compensation point is a continuous transition, associated with formation of a domain structure at remanence. Furthermore, we show that increasing the magnetic field sweep rate between saturation and remanence changes the mechanism of the magnetization reversal from propagation of domain walls to nucleation of reverse domains. Finally, we demonstrate how the layers can be switched by optical laser pulses, with the assistance of a small external magnetic field (down to 1 mT). The Ni3Pt/Ir/Co structures are promising candidates for all-optical control of magnetism in rare-earth-free synthetic ferrimagnets. 

[1] W. Legrand et al., Nature Materials 19, 34-42 (2020).
 [2] S.-H. Yang et al., Nature Nanotechnology 10, 221-226 (2015). 
 [3] J. W. Liao et al., Advanced Science 1901876 (2019).

Layer-selective magnetization reversal and optical switching in Ni3Pt/Ir/Co synthetic ferrimagnets

Magnetization dynamics in chiral spin structures revealed by diffractive ferromagnetic resonance (DFMR)

Coherent transfer of spin angular momentum within antiferromagnetic thin NiO films

Layer-selective magnetization reversal and optical switching in synthetic ferrimagnets

Magnetic skyrmions are nanoscale, particle-like entities made up of a swirling, topologically protected spin texture which exist in chiral magnetic systems. These skyrmions are promising candidates for novel memory technologies and spintronic devices due to the potential to encode information as the presence or absence of skyrmions, allowing for a racetrack memory system [1]. Techniques to manipulate the skyrmions in such a system are key to development of such devices. Here we present a novel method to manipulate skyrmion motion using magnetic field gradients (Fig. 1) [2].

Cu22OSeO3 is a well-established skyrmion system in which individual skyrmions assemble into a closed-packed hexagonally ordered magnetic lattice [3,4]. The magnetic lattice constant is relatively large ~(60 nm), allowing for the observation of skyrmions using resonant elastic x-ray scattering (REXS). This technique is ideally suited to studying slow dynamics in situ and uses polarized soft x-rays only available at synchrotrons. 

Our REXS experiments on beamline I10 at the Diamond Light Source show that the skyrmion lattice structures in single-crystal Cu22OSeO3 are highly mobile in a radial field gradient. The skyrmions rotate collectively in a shell-like structure following a given velocity-radius relationship. The direction of the field can be used to control the sense of rotation of the lattice. These results show that field gradients provide an effective mechanism for fine control of skyrmion motion. In this scheme, no local electric currents are needed, thus providing an alternative, lower energy approach to shift-register-type operations based on spin-transfer torque. Our findings open the door to a new way to manipulate and engineer the skyrmion state which will enable applications in the future.

<img class="img-fluid" src="/images/presentations/abstract_figs/Burn_MMM_19_b.jpg"></img>

Fig. 1. Manipulation of skyrmions along a linear racetrack and the application to a circular racetrack memory concept. (a) In a 1D linear racetrack scheme, skyrmions are moved past read/write heads collectively in a shift register fashion. (b) Morphing half of the 1D strip into a circular structure is the basis for the 2D racetrack memory element shown in (c). The required linear motion/rotation is controlled via an applied magnetic field gradient.

 A. Fert, V. Cros and J. Sampaio, Nature Nano. 8, 152 (2013).
 <a href="/publications/Zhang_NComms_2018">S.L. Zhang, W.W. Wang, D.M. Burn, et al., Nature Comms. 9, 2115 (2018).</a>
 S. Seki, X.Z. Yu, S. Ishiwata and Y. Tokura, Science 336, 198 (2012).
 <a href="/publications/Zhang_NanoLett_2016">S.L. Zhang, A. Bauer, D.M. Burn, et al., Nano. Lett. 16, 3285 (2016).</a>

Manipulation of skyrmion motion by magnetic field gradients

FeGe is a noncentrosymmetric helimagnet in which non-collinear spin textures result from a competition between the exchange interaction and the spin-orbit-induced Dzyaloshinskii-Moriya interaction. Below the ordering temperature, Tc, bulk FeGe has a well-established and rich magnetic phase diagram showing helical, conical, and skyrmion states [1]. The skyrmion pocket has a wide temperature-field window extending up to room temperature [2,3] which can be manipulated through changes to the sample thickness [2,4]. These features make FeGe of great interest due to the potential for novel room-temperature technological devices based on the manipulation of topological magnetic states. 

In this work, we characterize the magnetic states in a thin slab of FeGe with smallangle soft x-ray scattering on beamline I10 at the Diamond Light Source as a function of both temperature and magnetic field applied out-of-plane. In addition to the helical, conical, and skyrmion phases, we observe an additional helical phase which coexists beneath the skyrmion phase towards the higher field boundary of the helical phase. In contrast to the other phases this additional helical phase shows strong uniaxial preferential alignment of the modulation vector. 

We have also explored the variation in the magnitude of the modulation vector when traversing each of the magnetic phase pockets. For the helical and conical phases, this shows a field-dependent but temperature-independent variation, whereas for the skyrmion phase both a field and temperature variation is observed. Furthermore, the two helical phases show significantly different coefficients for the field-dependent modulation vector contraction. 

 Our findings build on other work for FeGe, strengthening the case that the room-temperature topological magnetic properties are of great interest for the development of novel technological devices. Our work shows how multiple coexistent phases can be simultaneously and uniquely identified which will provide further opportunities for potential information processing and storage methodologies.

<ol>
 <li>[1] A. Bauer and C. Pfleiderer, Springer Series in Materials Science 228, 1 (2016).</li>
 <li>[2] X. Z. Yu, N. Kanazawa, Y. Onose, K. Kimoto, W. Z. Zhang, S. Ishiwata, Y. Matsui, and Y. Tokura, Nat. Mater. 10, 106 (2011).</li>
 <li>[3] H. Wilhelm, M. Baenitz, M. Schmidt, C. Naylor, R. Lortz, U. K. Robler, A. A. Leonov, and A. N. Bogdanov, J. Phys.: Condens. Matter 24, 294204(2012).</li>
 <li>[4] S. X. Huang and C. L. Chien, Phys. Rev. Lett. 108, 267201 (2012).</li>
</ol>

Modulation vector dependence of a second helical phase pocket in FeGe

One of the most fascinating developments within spintronics in recent years has been the use of insulating antiferromagnets (AFM) as a medium to transport spin angular momentum. In particular, it has been observed that thin NiO films not only transmit but can also enhance a spin current [1, 2]. So far experiments have been limited to the detection of dc spin current and mainly performed on polycrystalline NiO layers. By employing element specific x-ray ferromagnetic resonance (XFMR) [3] we demonstrate transport of an ac spin-current across epitaxial NiO(001) layers, generated via spin pumping within adjacent NiFe layers. We show that in the case of a FeCo/NiO/NiFe multilayer, magnetization precession at the resonant frequency of the NiFe is also observed in the FeCo, with the phase of the precession being very similar in both layers. The amplitude of the precession of the sink FeCo layer (which is a measure of the amplitude of the spin current propagating through NiO) depends on the NiO thickness. The amplitude reaches a maximum at 6 nm and decays exponentially above this thickness (see Fig.1), in a similar manner to that in both previous experimental reports concerning the dc spin current [1, 2] and theoretical studies [4, 5]. 

For comparison, we present results for FeCo/NM(5nm)/NiO/NiFe multilayers, where the NiO is decoupled from the sink layer by insertion of a 5 nm thick non-magnetic (NM) layer of Ag or Pd. Although the magnetization precession of the FeCo sink layer at the NiFe resonance can be observed, its amplitude is nearly one order of magnitude smaller than for the case of directly coupled layers. Furthermore, we observe that the precession of the sink layer is characterized by a bipolar variation of the phase that is a signature of spin transfer torque due to propagation of ac spin current through the NiO layer [3]. Our results confirm the propagation of ac spin current within NiO layers mediated by coherent magnons, rather than by the diffusion of thermal magnons. 

<img class="img-fluid text-center" src="/images/presentations/abstract_figs/Dabrowski_MMM_19.jpg"></img>

 Figure 1. Ratio of the magnetization precession cone angles of Co (sink layer) and Ni (pump layer) for different thicknesses of NiO layers. The results were extracted from XFMR measurements on MgO(001)/MgO(5nm)/Fe75Co25(5nm)/ NiO(x)/Ni80Fe20(25nm)/MgO(3nm) structures at the excitation frequency f = 4 GHz.




<ol>
 <li> H. Wang, C. Du, P. C. Hammel, and F. Yang, PRL 113, 097202 (2014).</li>
 <li> C. Hahn, G. de Loubens, V. V. Naletov, J. B. Youssef, O. Klein, and M. Viret, EPL (Europhysics Letters) 108, 57005 (2014). </li>
 <li> J. Li, L. R. Shelford, P. Shafer, A. Tan, J. X. Deng, P. S. Keatley, C. Hwang, E. Arenholz, G. van der Laan, R. J. Hicken, and Z. Q. Qiu, Phys. Rev. Lett. 117, 076602 (2016).</li>
 <li> R. Khymyn, I. Lisenkov, V. S. Tiberkevich, A. N. Slavin, and B. A. Ivanov, PRB 93, 224421 (2016).</li>
 <li> S. M. Rezende, R. L. Rodríguez-Suárez, and A. Azevedo, PRB 93, 054412 (2016).</li>
</ol>

Experimental observation of AC spin current propagation through epitaxial NiO layers

Exchange Spring (ES) magnets possess a reservoir of potential energy that reduces the energy required from an external source, such as a laser pulse, to switch the magnetization direction [1]. ES structures are therefore attractive for future data storage device applications such as heat-assisted-magnetic recording (HAMR). In this work we explore the magnetization dynamics of ESs with different magnetic anisotropies and different soft/hard layer thicknesses using time-resolved magneto-optical Kerr effect (TRMOKE) measurements. We start with epitaxial Laves phase DyFe2/YFe2 structures with in-plane magnetic anisotropy, where winding and precession within the soft YFe2 layers in the [DyFe2(2nm)/YFe2(8nm)]x40 superlattice structure is demonstrated [1]. We show that the ES magnetization dynamics strongly depends upon the magnetic bias field orientation. In particular, for the magnetic field applied along the in-plane hard axis, the magnetization dynamics of the spring can be continuously excited in the field range from 0.5 T down to remanence, allowing the precession to be observed over a wide frequency range of 2.5–16 GHz. In contrast, for a DyFe2(30nm)/YFe2(7.5nm)/DyFe2(30nm) trilayer ES structure with substantially thicker hard layer and stronger in-plane magnetic anisotropy, the magnetization precession of the spring is heavily damped, with virtually no oscillations observed beyond an inflection point at around 30 ps after excitation by the fs laser pulse, regardless of the applied magnetic field direction and the pump laser fluence. Finally, we present a [Co(0.4nm)/Pt(0.9nm)]x10/Py(10nm) bilayer structure, where a 90° exchange spring between out-of-plane and in-plane magnetization orientation is formed within the Co/Pt layers. We show that for the field applied perpendicular to the sample plane the magnetization precession can be observed over a wide frequency range of 2.5–21 GHz. The precession is even present at the highest available magnetic field of 1.5 T, suggesting mutual imprinting of the spin configuration between the layers [2]. 

[1] L. R. Shelford, Y. Liu, U. Al-Jarah, P. A. J. de Groot, G. J. Bowden, R. C. C. Ward and R. J. Hicken, Phys. Rev. Lett. 113, 067601 (2014).
 [2] M. T. Bryan, G. Heldt, T. Thomson, L. J. Heyderman and G. Hrkac, Phys. Rev. B 94, 104415 (2016).

Ultrafast magnetization dynamics of in-plane and out-of-plane exchange spring magnets

Structures with canted magnetic moments are important for the delivery of reduced critical current densities and turn-on time within spin-torque oscillators [1-3]. Such a structure can be realised with an exchange spring. These are usually engineered with anisotropies such that the moments of both hard and soft layers are aligned in the same direction, either in-plane (IP) or out-of-plane (OoP). If instead the hard and soft layers have different anisotropy directions (IP and OoP) then the exchange coupling creates a canted magnetic state at remanence. Using x-ray magnetic circular dichroism (XMCD) and x-ray ferromagnetic resonance (XFMR) at the Co and Fe L2,3 edges [4], at both the Advanced Light Source in the US and the Diamond Light Source in the UK, we have studied the effect of inserting a Pt spacer layer on the exchange coupling within [Co/Pt]/(Pt)/Py (tri-)bi-layer exchange springs with canted magnetic moments (Py = Fe20Ni80).

 Samples of Pt(300)/[Co(2)/Pt(9)]10/Pt(x)/Py(y)/Pt(30) (thickness in Å) were grown on Al2O3 (11-20) substrates, using DC magnetron sputtering, with x = 0 and 15 Å and y = 550 and 100 Å. By making the Py layer (y = 550 Å) thick compared to the 110 Å [Co/Pt] layer, the Co moments are canted from the OoP direction. Element resolved hysteresis loops, measured by XMCD, show that the IP shape anisotropy of the Py dominates and pulls the Co moments from their preferred OoP direction towards the IP direction. Using the thinner Py layer, the Py at the interface is instead canted towards the OoP direction, while the Py magnetization relaxes towards the in-plane direction away from the interface. By inserting a Pt spacer layer, the exchange coupling was decreased, resulting in Co moments that were less strongly canted. For samples with the thickness in this study a static exchange coupling was always observed. The canting of the moments is in agreement with work by Hsu and Saravanan et al. [1, 5].

The dynamic exchange coupling between the layers was investigated using XFMR which gives the element resolved FMR. XFMR measurements performed with an IP bias field confirmed that the [Co/Pt] and Py layers are strongly coupled in the samples without the Pt spacer. For a 4GHz RF field, both Co and Fe exhibit a resonance field of 20mT, while in the sample with a Pt spacer only the Fe exhibits a resonance for an IP bias field. See Fig 1 for IP field scans of the resonance amplitude measured by XFMR at the Fe and Co L2,3 edges for a 4GHz RF-field. Vector network analyser FMR (VNA-FMR) was also performed with biasing field IP and OoP. VNA-FMR with IP biasing field was in agreement with the XFMR, while for OoP there was a complex behaviour.

<img class="img-fluid text-center" src="/images/presentations/abstract_figs/Frisk_JEMS_19.jpg"></img>

<ol>
 <li>J.-H. Hsu, et al., JAP, 117, 17A715, (2015) </li>
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 <li> G. van der Laan and A. I. Figueroa, Cord. Chem. Rev. 277-278, 95-129, (2014)</li>
 <li> P. Saravanan, et al., JAP, 115, 243905, (2014)</li>
</ol>

De-coupling of an exchange coupled CoPt/Py bilayer exchange spring with canted magnetic anisotropy using a Pt spacer

Synthetic ferrimagnets (SFM), two ferromagnetic layers exchange-coupled via the RKKY interaction across a nonmagnetic spacer layer, are of particular interest as a feasible media for all optical magnetic switching [1,2]. The atomistic simulation package, VAMPIRE [3], is used in conjunction with SQUID magnetometry and x-ray magnetic circular dichroism (XMCD), to characterise a series of perpendicular, compensating Ni3Pt/Ir/Co SFMs, fabricated by magnetron sputtering. Designing and creating these structures requires knowledge and control of several fundamental magnetic parameters, the magnetisation with temperature, M(T) and the anisotropy, K(T), of the individual ferromagnetic layers and the exchange interaction between them, Jij.

Unexpectedly, these Ni3Pt/Ir/Co SFM exhibit regions of negative remanent magnetisation, MR, observed in experiment and replicated in simulation, just below the compensation point, Tcomp.This can be described as an interplay between anisotropy, exchange and Zeeman energies of the system [4]. XMCD measurements confirm the distinct nature of SFM layers and the negative MR, Fig 1.

 VAMPIRE is used to successfully reproduce the main features of the magnetic behaviour and yield insights to the magnetisation dynamics on the atomic scale. The simulations can be presented as a map of the MR dependence on Jij and T so creating a ‘phase diagram’ (Fig 2), revealing the regions of distinct magnetic behaviour, for a particular SFM. This further demonstrates the ability of VAMPIRE to elucidate and enhance new synthetic material development. 

<img class="img-fluid text-center" src="/images/presentations/abstract_figs/Scott_Magnetism_19_1.jpg"></img>

Fig 1: Ni3Pt(8.5nm)/Ir(0.5nm)/Co(1nm) (a) SQUID measurement of MR vs T, (b) XMCD at T < T-Mr, Tcomp, (c) XMCD at T-Mr < T < Tcomp


<img class="img-fluid text-center" src="/images/presentations/abstract_figs/Scott_Magnetism_19_2.jpg"></img>

Fig 2: (a) VAMPIRE phase map of Ni3Pt(11nm)/Ir(0.5nm)/Co(1nm) of MR as a function of Jij and T. The red regions mark the point at which the Ni3Pt is the first layer to reverse. Below Tcomp, the higher moment of the Ni3Pt layer results in negative MR. The black line marks the experimental Jij vs T of a Ni3Pt(11nm)/Ir(0.5nm)/Co(1nm) SFM calculated from minor loops (b) MR vs T cross section of the data plotted on the VAMPIRE remanence map compared to that measured via SQUID


<ol>
 <li> S. Mangin et al, Nature Materials 13, 286 (2014)</li>
 <li> R. F. L. Evans et al, Appl. Phys. Lett. 104, 082410 (2014)</li>
 <li> R. F. L. Evans et al, J. Phys.: Condens. Matter 26, 103202 (2014)</li>
 <li> K. Takanashi et al, Appl. Phys. Lett. 63, 1585 (1993)</li>
</ol>

Mapping magnetisation in compensating synthetic ferrimagnets with negative remanence

Magnetic skyrmions are nanoscale, particle-like entities made up of a swirling, topologically protected spin texture which exist in chiral magnetic systems. These skyrmions are promising candidates for novel memory technologies and spintronic devices due to the potential to encode information as the presence or absence of skyrmions, allowing for a racetrack memory system [1]. Techniques to manipulate the skyrmions in such a system are key to development of such devices. Here we present a novel method to manipulate skyrmion motion using magnetic field gradients (Fig. 1) [2].
Cu2OSeO3 is a well-established skyrmion system in which individual skyrmions assemble into a closed-packed hexagonally ordered magnetic lattice [3,4]. The magnetic lattice constant is relatively large (60 nm), allowing for the observation of skyrmions using resonant elastic X-ray scattering (REXS). This technique is ideally suited to studying slow dynamics in situ and uses polarized soft x-rays only available at synchrotrons. 

Our REXS experiments on beamline I10 at the Diamond Light Source show that the skyrmion lattice structures in single-crystal Cu2OSeO3 are highly mobile in a radial field gradient. The skyrmions rotate collectively in a shell-like structure following a given velocity-radius relationship. The direction of the field can be used to control the sense of rotation of the lattice. These results show that field gradients provide an effective mechanism for fine control of skyrmion motion. In this scheme, no local electric currents are needed, thus providing an alternative, lower energy approach to shift-register-type operations based on spin transfer torque. Our findings open the door to a new way to manipulate and engineer the skyrmion state which will enable applications in the future.

<img class="img-fluid text-center" src="/images/presentations/abstract_figs/Burn_Magnetism_19_b.png"></img>

Fig. 1. Manipulation of skyrmions along a linear racetrack and the application to a circular racetrack memory concept where information is represented by the presence (1) or absence (0) of a skyrmion. (a) In a 1D linear racetrack scheme, skyrmions are moved past read/write heads collectively in a shift register fashion. (b) Morphing half of the 1D strip into a circular structure is the basis for the 2D racetrack memory element shown in (c). The required linear motion/rotation is controlled via an applied magnetic field gradient.


<ol>
<li> A. Fert, V. Cros and J. Sampaio, Nature Nano. 8, 152 (2013).</li>
<li> <a href="/publications/Zhang_NComms_2018">S.L. Zhang, W.W. Wang, D.M. Burn, et al., Nature Comms. 9, 2115 (2018).</a></li>
<li> S. Seki, X.Z. Yu, S. Ishiwata and Y. Tokura, Science 336, 198 (2012).</li>
<li> <a href="/publications/Zhang_NanoLett_2016">S.L. Zhang, A. Bauer, D.M. Burn, et al., Nano. Lett. 16, 3285 (2016).</a></li>
</ol>

Ferromagnetic resonance (FMR) is a standard technique for measuring magnetisation dynamics in thin film, multilayered and single crystal systems. The absorption of a RF magnetic signal supplied from a co-planar waveguide reveals the field and frequency dependence to the magnetic resonance in the sample. However, this technique comes with limitations, for example it is not possible to determine the origin of resonance modes in complex multilayered structures typically used in magnetism research today.
 
By combining FMR measurements with x-ray magnetic circular dichroism (XMCD) it is possible to overcome this limitation by exploiting the element selectivity available with x-ray absorption measurements. X-ray detected ferromagnetic resonance (XFMR) works by synchronising RF excitation of the sample with pulsed x-rays available at a synchrotron we stroboscopically probe the absorption as a function of pump-probe delay revealing the element specific magnetodynamics at picoseconds timescales [1,2].

We present a selection of recent XFMR results, including spin pumping across spin valves and spin transfer across a topological insulators [3-7], outlining the operating principles of the technique and the valuable insights it offers [8].

[1] A.I. Figueroa, et al., J. Magn. Magn. Mater. 400, 178 (2015).
 [2] M.K. Marcham et al., J. Appl. Phys. 109, 07D353 (2011).
 [3] G.B.G. Stenning et al., New. J. Phys. 17, 013019 (2015)
 [4] M.K. Marcham et al., Phys. Rev. B 87, 180403 (2013).
 [5] A.A. Baker et al., Sci. Rep. 5, 7907 (2015).
 [6] A.A. Baker et al., Sci. Rep. 6, 35582 (2016).
 [7] A.A. Baker et al., J. Magn. Magn. Mater. 473, 470 (2019).
 [8] G. van der Laan, J. Electron Spectrosc. Relat. Phenom. 220, 137 (2017).

X-Ray detected Ferromagnetic Resonance (XFMR)

Much of the intrigue and utility of nanomagnetic systems arises from the properties of specific system-wide magnetic configurations, or microstates. From practical applications such as data storage to microstates of fundamental physical and statistical interest such as ground states1,2 or metastable excitations3,4, the ability to prepare systems in any given microstate is of crucial importance. Historically, microstate preparation methods have been somewhat limited â€“ relying on globally-applied or stripline-generated fields and thermalisation protocols to access a limited range of states. Groups have recently demonstrated that the dynamic magnetic charge of a scanning MFM tip may be used to write all conceivable microstates5,6, but requires that the sample is situated within expensive and cumbersome SPM hardware.
 
We present developments and applications of the dynamic magnetic-charge writing method Topological Defect-Driven Magnetic Writing (TMW)6, outlining a fully solid-state and current-addressable SPM-free solution replacing the MFM tip with alternative magnetic charge sources. This improved technique is leveraged to realise novel reconfigurable magnonic circuit elements, implemented within strongly-interacting nanomagnetic networks.

<img class="img-fluid text-center" src="/images/presentations/abstract_figs/Carter_ICM_18.png"></img>

Figure 1:
 i) Schematic of dynamic magnetic charge writing process.
 ii) Spin-wave spectra for hexagonal NiFe nanoarrays prepared in two dynamic magnetic charge written states (a,b), exhibiting peaksuppression via microstate control.
 iii) MFM image of single pair of monopole excitations in hexagonal NiFe magnetic nanoarray, realised via dynamic magnetic charge writing.
 iv) MFM image of repeating columns of monopole excitations or monopole ladders, realised via dynamic magnetic charge writing.
 
 <ul>
 <li>[1] Morgan, J. P. et al. Nature Physics 7.1 (2011): 75.
 <li>[2] Perrin, Yann, et al. Nature 540.7633 (2016): 410-413. 
 <li>[3] Anghinolfi, L., et al. Nature Comms. 6 (2015). 231 3 Ladak, S. et al. Nature Physics 6.5 (2010): 359-363.
 <li>[4] Mengotti, E. et al. Nature Physics 7.1 (2011): 68. 
 <li>[5] Wang, Yong-Lei, et al. Science 352.6288 (2016): 962-966. 
 <li>[6] <a href="/publications/Gartside_NNano_2017">Gartside, J. C., et al. Nature Nanotechnology 13 (2017): 53-58.</a>
 </ul>

Nanomagnetic Writing via Dynamic Magnetic Charge: Magnonic Horizons & Developments Beyond Surface Probe Lithography

Antiferromagnetic NiO films have been studied as media for the effective propagation of spin currents, mostly by means of the inverse spin Hall effect (ISHE) [1]. However their study is still incomplete because ISHE has only been used to probe the dc spin current, while the ac spin current has not been measured. X-ray detected ferromagnetic resonance (XFMR) allows an ac spin current to be detected via the spin transfer torque exerted upon a ferromagnet [2]. To this end, time-resolved element-specific XFMR measurements were performed upon a CoFe/NiO/Fe/NiFe multilayer. Microwaves of 4 GHz frequency were supplied through a coplanar waveguide (CPW) with a variable phase relative to the x-ray pulses, so as to generate an in-plane rf magnetic field orthogonal to the axis of the CPW, while a static magnetic field of magnitude H was applied parallel to the axis. Figure 1(a) shows XFMR delay scans acquired at the Co and Ni edges of the CoFe and NiFe layers respectively. The amplitudes and phases were extracted by fitting sine curves to the data acquired at different values of H, as shown in Figure 1(b) and (c). The FMR field was close to 60 Oe for both the CoFe and NiFe layers, and the precession was found to have very similar phase for both layers. These results indicate that the CoFe and NiFe layers are strongly coupled and exhibit an acoustic precessional mode. Since the intrinsic FMR frequency of the CoFe layer should be much higher than 4 GHz, the XFMR measurements may be interpreted as a signature of spin current propagation mediated by magnons within the NiO layer.

<img class="img-fluid text-center" src="/images/presentations/abstract_figs/Nakano_EASTT_18.png"></img>

Figure 1. (a) XFMR delay scans measured at different H values, (b) fitted amplitudes, and (c) fitted phases for Co (blue squares) and Ni (red circles) L edges. Sine curves in (a) are fits to the experimental data

References
 [1] H. Wang et al., Phys. Rev. Lett. 113, 097202 (2014).
 [2] J. Li et al., Phys. Rev. Lett. 117, 076602 (2016).

Time-resolved X-ray detected ferromagnetic resonance measurements of a CoFe/NiO/Fe/NiFe multilayer structure

Antiferromagnetic NiO films have been studied as media for the effective propagation of spin currents, mostly by means of the inverse spin Hall effect (ISHE) [1]. However their study is still incomplete because ISHE has only been used to probe the dc spin current, while the ac spin current has not been measured. X-ray detected ferromagnetic resonance (XFMR) allows an ac spin current to be detected via the spin transfer torque exerted upon a ferromagnet [2]. To this end, time-resolved element-specific XFMR measurements were performed upon a CoFe/NiO/Fe/NiFe multilayer. Microwaves of 4 GHz frequency were supplied through a coplanar waveguide (CPW) with a variable phase relative to the x-ray pulses, so as to generate an in-plane rf magnetic field orthogonal to the axis of the CPW, while a static magnetic field of magnitude H was applied parallel to the axis. Figure 1(a) shows XFMR delay scans acquired at the Co and Ni edges of the CoFe and NiFe layers respectively. The amplitudes and phases were extracted by fitting sine curves to the data acquired at different values of H, as shown in Figure 1(b) and (c). The FMR field was close to 60 Oe for both the CoFe and NiFe layers, and the precession was found to have very similar phase for both layers. These results indicate that the CoFe and NiFe layers are strongly coupled and exhibit an acoustic precessional mode. Since the intrinsic FMR frequency of the CoFe layer should be much higher than 4 GHz, the XFMR measurements may be interpreted as a signature of spin current propagation mediated by magnons within the NiO layer.
 
 
<img class="img-fluid text-center" src="/images/presentations/abstract_figs/Nakano_ICMM_18.png"></img>

References
 [1] H. Wang et al., Phys. Rev. Lett. 113, 097202 (2014).
 [2] J. Li et al., Phys. Rev. Lett. 117, 076602 (2016).

Ferromagnetic resonance (FMR) is a standard technique for measuring magnetisation dynamics in thin film, multi-layered and single crystal systems. The absorption of a RF magnetic signal supplied from a co-planar waveguide reveals the field and frequency dependence to the magnetic resonance in the sample. However, this technique comes with limitations, for example it is not possible to determine the origin of resonance modes in complex multi-layered structures typically used in magnetism research today. 
 
By combining FMR measurements with x-ray magnetic circular dichroism (XMCD) it is possible to overcome this limitation by exploiting the element selectivity available with x-ray absorption measurements. X-ray detected ferromagnetic resonance (XFMR) works by synchronising RF excitation of the sample with pulsed xrays available at a synchrotron we stroboscopically probe the absorption as a function of pump-probe delay revealing the element specific magnetodynamics at picoseconds timescales [1,2].
 
We present a selection of recent XFMR results, including spin pumping across spin valves and spin transfer across a topological insulators [3-6], outlining the operating principles of the technique and the valuable insights it offers.
 
[1] A.I. Figueroa, et. al., J. Magn. Magn. Mater. 400, 178 (2015).
 [2] M.K. Marcham et.al., J. Appl. Phys. 109, 07D353 (2011).
 [3] G.B.G. Stenning et. al., New. J. Phys. 17, 013019 (2015)
 [4] M.K. Marcham et. al., Phys. Rev. B 87, 180403 (2013).
 [5] A.A. Baker, et. al., Sci. Rep. 5, 7907 (2015).
 [6] A.A. Baker et. al., Sci. Rep. 6, 35582 (2016)

 X-Ray detected ferromagnetic resonance (XFMR)

Antiferromagnetic NiO films have been studied as media for the effective propagation of spin currents [1]. In order to detect the ac spin current propagating through a NiO film, which has not been reported so far, we performed element-specific x-ray detected ferromagnetic resonance (XFMR) measurements [2] on a CoFe/NiO/Fe/NiFe multilayer.
 
Figure 1(a) shows the XFMR delay scans acquired at the Co and Ni L3 edges in the CoFe and NiFe layers respectively. The amplitudes and phases were extracted by fitting sine curves to the data acquired at different static magnetic fields, as shown in Figure 1(b) and (c). The FMR magnetic field was close to 60 Oe for both the CoFe and NiFe layers, and the precession was found to have the same phase in both layers. These unexpected results indicate that the CoFe and NiFe layers are strongly exchange coupled and rotate entirely together. An ac spin current could not be observed in this sample due to the exchange coupling. We suspect two possible origins for the coupling. Either the NiO layer contains oxygen deficiencies and metallic Ni components mediate the coupling, or else pinholes within the NiO layer allow the ferromagnetic layers to directly couple to each other. 

<img class="img-fluid text-center" src="/images/presentations/abstract_figs/Nakano_Magnetism_18.png"></img>
 
Figure 1: (a) XFMR delay scans with a static magnetic field H of 60 Oe, (b) fitted amplitudes, and (c) fitted phases for Co (blue squares) and Ni (red circles) L3 edges. The sine curves in (a) are fits to the experimental data. 4-GHz microwaves were supplied through the coplanar waveguide (CPW) with a variable delay relative to the x-ray pulses, and generated an in-plane rf magnetic field orthogonal to the CPW, while H was parallel to the CPW.
 
[1] H. Wang et al., Phys. Rev. Lett. 113, 097202 (2014).
 [2] J. Li et al., Phys. Rev. Lett. 117, 076602 (2016).

Time-resolved x-ray detected ferromagnetic resonance measurements in a CoFe/NiO/Fe/NiFe multilayer structure

Arrays of non-interacting nanomagnets are widespread in data storage and processing. As current technologies approach fundamental limits on size and thermal stability, enhancing functionality through embracing the strong interactions present at high array densities becomes attractive.
 
In this respect, artificial spin ices[1-4] are geometrically frustrated magnetic metamaterials that offer vast untapped potential due to their unique microstate landscapes, with intriguing prospects in applications from reconfigurable logic to magnonic devices6 or hardware neural networks[7].
 
However, progress in such systems is impeded by the inability to access more than a fraction of the total microstate space. To address this, we have developed a novel scanning probe technique, Topological DefectDriven Magnetic Writing (TMW)[5] , which provides full access to all possible microstates in artificial spin ices and related arrays of nanomagnets.
 
We employ TMW to create previously elusive magnetic configurations such as the spin- crystal ground state[8] of artificial kagome dipolar spin ices and high-energy, low-entropy monopole-chain states that exhibit negative effective temperatures.
 
<img src="/images/presentations/abstract_figs/Branford_Magnetism_18.png">
 
Figure 1: i)Micromagnetic simulation and accompanying schematic of writing process dynamics showing positions of magnetic charges (±Q). ii) MFM image of ladder and zigzag monopole-defect chain states written at RT on a NiFe ASI lattice. iii) MFM image of the spin-crystal chiral ground state of kagome ASI written at RT on a NiFe ASI rosette.
 
 [1] Wang, RF, et al. Nature 439.7074 (2006): 303-306.
 [2] Branford, W. R., et al. Science 335.6076 (2012): 1597-1600.
 [3] Zhang, Sheng, et al. Nature 500.7464 (2013): 553-557.
 [4] Perrin, Yann, et al. Nature 540.7633 (2016): 410-413.
 [5] <a href="/publications/Gartside_NNano_2017"> Gartside, J. C., et al. Nature Nanotechnology 13 (2017): 53-58.</a>
 [6] Heyderman, L. J., and R. L. Stamps. J. Phys.: Cond. Matt. 25.36 (2013): 363201.
 [7] Wang, Yong-Lei, et al. Science 352.6288 (2016): 962-966.
 [8] Anghinolfi, L., et al. Nat. Comms. 6 (2015).

Realization of ground state in artificial kagome spin ice via topological defect-driven magnetic writing

Networks of magnetic nanostructures are of broad interest from novel data storage and computation to magnonic crystals. One family of nanomagnetic arrays, characvterized by strong and frustrated magnetic interactions are the artificial spin ices (ASI). ASI structures have provided vast amounts of physical insight in recent years [1,2,3,4] in part due to their ability to model complex systems [5] and exhibit exotic phenomena such as â€˜magnetic monopoleâ€™-like states [6]. The power of these networks stems from the extraordinary number of unique microstates, even in systems comprising relatively few nanostructures. However, magnetic nanoarrays in general, and ASI structures in particular, have yet to realise their full potential as the majority of microstates remain inaccessible due to the rudimentary state-writing tools currently available. An experimental means to prepare all potential microstates has huge implications, including realising ASI as a tunable-bandgap magnonic crystal [7] or reconfigurable neural-network [8]. We present a novel MFM-tip based state writing technique, building on our previously demonstrated domain-wall injection process [9]. It requires no global fields and is applicable to all nanostructure architectures, providing control over the spin-configuration and access to every possible microstate. We demonstrate our method via realisation of several exotic and thus-far unobserved states, unachievable via global fieldprotocols: magnetic monopole-defect chains and the spin-crystal ground state [10] of kagome ASI. 
 
<img class="img-fluid text-center" src="/images/presentations/abstract_figs/Branford_Magnetism_18.png">
 
Figure 1: i) Micromagnetic simulation and accompanying schematic of writing process dynamics showing positions of magnetic charges (±Q). ii) MFM image of ladder and zigzag monopole- defect chain states written at RT on a NiFe ASI lattice. iii) MFM image of the spin-crystal chiral ground state of kagome ASI written at RT on a NiFe ASI rosette.
 
 [1] Wang, RF, et al. Nature 439.7074 (2006): 303-306.
 [2] Branford, W. R., et al. Science 335.6076 (2012): 1597-1600.
 [3] Zhang, Sheng, et al. Nature 500.7464 (2013): 553-557.
 [4] Perrin, Yann, Benjamin Canals, and Nicolas Rougemaille. Nature 540.7633 (2016): 410-413.
 [5] Qi, Yi, T. Brintlinger, and John Cumings. Phys. Rev. B 77.9 (2008): 094418.
 [6] Ladak, Sam, et al. Nature Physics 6.5 (2010): 359-363.
 [7] Heyderman, L. J., and R. L. Stamps. J. Phys.: Cond. Matt. 25.36 (2013): 363201.
 [8] Wang, Yong-Lei, et al. Science 352.6288 (2016): 962-966.
 [9] <a href="/publications/Gartside_SciRep_2016">Gartside, J. C., et al. Scientific Reports 6 (2016)</a>
 [10] Anghinolfi, L., et al. Nat. Comms. 6 (2015).

Topological magnetic writing: Defining specific magnetization states in nanostructures

Magnetic skyrmions in noncentrosymmetric chiral magnets appear as an ordered lattice state, with the individual skyrmions measuring 3-100 nm. Though rarely reported, this is a decent length scale for carrying out soft x-ray scattering experiments. Here we present our recent studies on the magnetic characterization of chiral magnets, using resonant elastic x-ray scattering (REXS). We demonstrate the unique advantages of REXS that allow for the study of the microscopic structural information of skyrmions, inaccessible to other characterization techniques. First, we show that REXS is extremely surface sensitive [1], which allows for the probing of the multidomain skyrmion lattice state - a surface state of the skyrmion lattice [2]. Second, the variable probing area allows us to map out the real-space skyrmion domain structure [3], which is important for future applications. Third, we show that using circularly polarised light, REXS is capable to accurately determine the topological winding number of a skyrmion, rigorously following its mathematical definition [4]. This 'topology determination principle' is a general experimental strategy, applicable to a wide range of topologically ordered magnetic materials. Finally, we show REXS results on grown thin films, which unambiguously reveals their magnetic structure [5].

[1]	S. L. Zhang et al., Phys. Rev. B 93, 214420 (2016).
 [2] <a href="/publications/Zhang_NanoLett_2016"> S. L. Zhang et al., Nano Lett. 16, 3285 (2016).</a>
 [3] S. L. Zhang et al., Appl. Phys. Lett. 109, 192406 (2016).
 [4] S. L. Zhang, G. van der Laan, and T. Hesjedal, Nat. Comms. 8, 14619 (2017).
 [5] S. L. Zhang et al., Sci. Rep. (2017). in press

Resonant Soft X-Ray Scattering Studies of Skyrmions in Chiral Magnets

Magnetic skyrmions are proposed as binary information carriers in racetrack-type memory device schemes. Therefore, the manipulation of skyrmion arrays in a shift-register-like fashion is one of the key technical requirements for practical device applications. Though moving skyrmions by spin transfer torque has been proven to be effective, the electrical current will eventually dissipate in the form of Joule heat. Here, we demonstrate local Joule heating-free method of manipulation that can translate skyrmions via a magnetic field gradient. A circular field gradient alone is sufficient to drive collective and continuous rotation of the skyrmion lattice. We were able to unambiguously confirm the motion in an in-situ resonant magnetic scattering experiment, as shown in Fig.1. The skyrmion lattice diffraction peaks hereby undergo a continuous rotational motion, generated purely by a set of permanent magnets. We further experimentally rule out other possible driving forces, such as thermal gradients, as well as x-ray-induced effects. We present a theoretical analysis of the skyrmion motion in support of our observations. The rotational motion is accompanied by a skyrmion lattice domain dynamics, which will be discussed in the context of complementary Lorentz transmission electron microscopy experiments.

<img class="img-fluid text-center" src="/images/presentations/abstract_figs/Hesjedal_Skymag_17.jpg"></img>

FIG 1: Real-time evolution of the skyrmion lattice peaks rotating in a circular field gradient.

Manipulation of Skyrmion Motion by Magnetic Field Gradients

The interactions between a magnetic material and an electrical current within it are governed by fundamental physical principles based on the spin-orbit interaction. As well as being a popular area for scientific study, the potential for the development of novel technological devices is also important. So far resistance changes as a function of magnetisation, known as magnetoresistance, has been most extensively explored. Now electrical control of the magnetisation is becoming a more popular avenue to explore [1]. In particular, systems with a magnetic layer and a layer with high spin-orbit coupling give rise to interesting interfacial effects. An electrical charge current in the high spin-orbit layer generates a transverse spin current. This spin current exerts a spin-orbit torque on magnetic moments in an adjacent magnetic layer and can manipulate the magnetisation [2,3].

 We have performed soft x-ray reflectivity measurements on CoFeTaB/Pt bilayers which probe the depth dependence to both the structural and magnetic properties within the system. Additionally, tuning energy to an absorption edge provides elemental selectivity and even allows the relative spin and orbital contributions to the moments to be investigated.

Both field- and current-driven magnetisation reversal hysteresis loops have been measured, which reveal a depth-dependence to the field-driven coercive field, and current-driven critical switching current density, for the system when changing the x-ray penetration depth into the sample. By applying additional static magnetic field or DC current, modifications to the hysteresis loop shape show that the current driven reversal is complex and cannot simply be described as a purely field-like effect. These modifications may be attributed to changes in the anisotropy with strong links to the spin-orbit interaction. Furthermore, circular dichroism in the x-ray absorption is only observed for the field-driven magnetisation reversal loops suggesting fundamental physical differences between the current- and field-driven magnetisation reversal processes.


<img class="img-fluid text-center" src="/images/presentations/abstract_figs/Burn_Magnetism_17.jpg"></img>

Fig 1: Current-driven magnetization reversal in CoFeTaB/Pt detected by X-ray reflectivity

[1] Slonczewski, J., Journal of Magnetism and Magnetic Materials 159, L1 (1996).
 [2] Miron, I. M., et al., Nature materials 9, 230 (2010).
 [3] Garello, K., et al., Nature Nanotechnology 8, 587 (2013).

X-ray detected current-driven magnetisation reversal in CoFeTaB/Pt

Networks of interacting magnetic nanostructures including artificial spin ice (ASI) have provided vast amounts of physical insight in recent years[1,2,3,4], in part due to their ability to model complex systems[5] and exhibit exotic phenomena such as magnetic monopole-like states[6]. The power of these networks stems from the extraordinary number of unique microstates, even in systems comprising relatively few nanostructures. 

However, ASI and related architectures are yet to realise their full potential as the majority of microstates remain inaccessible due to the rudimentary state-writing tools currently available. Researchers largely resort to global field protocols to stochastically select spin-configurations from a vastly reduced microstate space. An experimental means to prepare all potential microstates has huge implications, including realising ASI as a tunable-bandgap magnonic crystal[7] or reconfigurable neural-network[8]. 

We present a novel MFM-tip based state writing technique, building on our previously demonstrated domain-wall injection process[9]. It requires no global fields and is applicable to all nanostructure architectures, providing total control over the spin-configuration and access to every possible microstate. We demonstrate the efficacy of our method via the realisation of several exotic and thus-far unobserved states, unachievable via global field-protocols: magnetic monopole-defect chains and the spin-crystal ground state[10] of kagome ASI. 

<img class="img-fluid text-center" src="/images/presentations/abstract_figs/Carter_Magnetism_17.jpg"></img>

Figure 1: i) Micromagnetic simulation and accompanying schematic of writing process dynamics showing positions of magnetic charges (±Q).
ii) MFM image of ladder and zigzag monopole-defect chain states written at RT on a NiFe ASI lattice.
iii) MFM image of the spin-crystal chiral ground state of kagome ASI written at RT on a NiFe ASI rosette.

1 Wang, RF, et al. Nature 439.7074 (2006): 303-306.
 2 Branford, W. R., et al. Science 335.6076 (2012): 1597-1600.
 3 Zhang, Sheng, et al. Nature 500.7464 (2013): 553-557.
 4 Perrin, Yann, Benjamin Canals, and Nicolas Rougemaille. Nature 540.7633 (2016): 410-413.
 5 Qi, Yi, T. Brintlinger, and John Cumings. Phys. Rev. B 77.9 (2008): 094418.
 6 Ladak, Sam, et al. Nature Physics 6.5 (2010): 359-363.
 7 Heyderman, L. J., and R. L. Stamps. J. Phys.: Cond. Matt. 25.36 (2013): 363201.
 8 Wang, Yong-Lei, et al. Science 352.6288 (2016): 962-966.
 9 <a href="/publications/Gartside_SciRep_2016">Gartside, J. C., et al. Scientific Reports 6 (2016). </a>
 10 Anghinolfi, L., et al. Nat. Comms. 6 (2015).

Topological Lithography: Monopoles on demand and realisation of the kagome ASI spin-crystal ground state

Thin film magnetic materials continue to attract significant research attention due their potential for the development of technologically desirable magnetoelectronic, spintronic and spin-orbitronic devices. Such materials gain their magnetic properties from the electronic structure and fundamental physical interactions between neighbouring species in thin film systems. 

The role of interfaces in multi-layered materials is important and static properties have been well explored in recent years. Interfacial anisotropy and chirality effects from the Dzyaloshinskii-Moriya interaction are significant examples. Induced moments in non-ferromagnetic layers[1] and even a modification to the damping in NiFe/Cr bilayers[2] has been identified. However, in this latter case the elemental contribution to this dynamic effect is unknown.
 
With x-ray scattering techniques, the varying penetration depth of the x-ray probe with grazing incidence angle provides depth dependent interrogation of the structure. When performed with varying x-ray polarisation in an applied magnetic field, additional sensitivity to the magnetic moment arises. This makes x-ray reflectivity a valuable tool to interrogate element specific interfacial contributions to the magnetism in thin film multi-layered samples.
 
Key to the high speed operation of future devices is a solid understanding of the magnetisation dynamics and time-dependent magnetisation processes taking place in thin film multilayered materials. To investigate this temporal behaviour we employ a stroboscopic pump-probe measurement approach. Here, the time structure of x-rays produced by the synchrotron source is synchronised with a pulsed magnetic field with a variable delay from the arrival time of x-ray photons. This allows us to probe the element specific magnetisation dynamics within the layered structure with sub-nanosecond resolution. Results from a NiFe/Cr bilayer interface will be presented.
 

[1] <a href="/publications/Burn_JPCM_2014"> D.M. Burn, T.P.A. Hase and D. Atkinson, J. Phys.: Condens. Matter 26, 236002 (2014) Focused-ion-beam induced interfacial intermixing of magnetic bilayers for nanoscale control of magnetic properties </a>
 [2] <a href="/publications/King_APL_2014">J.A. King, A. Ganguly, D.M. Burn, et.al. Applied Physics Letters 104 242410 (2014). Local control of magnetic damping in ferromagnetic/non-magnetic bilayers by interfacial intermixing induced by focused ion-beam irradiation. </a>

Element specific magnetisation dynamics at NiFe/Cr interfaces

Domain walls (DWs) in magnetic nanostructures have long been a subject of much interest[1]. The majority of work so far has focused on 90° and 100° DWs. However, recently the field has seen promising developments in 360° DWs, stable topological defects consisting of two strongly-coupled 180° DWs of opposite chirality[2] 360° DWs have several interesting properties relating to behaviour in applied field[3], interactions with spin-waves[4] and high density DW systems such as racetrack memory[4]. Here we demonstrate their utility as a tool for controlling global magnetization states of nanostructures comprised of interconnected nanowires.

By using a novel 360° DW injection technique we are able to introduce DWs of desired chirality to any point in a
structure. Choosing these points carefully allows the injected DWs to mediate controlled switching events
throughout the nanostructure, resulting in new magnetization states without the application of a global field or current. This behaviour is confirmed via MFM data supported by micromagnetic simulations.

<img class="img-fluid text-center" src="/images/presentations/abstract_figs/Carter_JEMS_16.png"></img>

Figure 1: Micromagnetic simulation & corresponding MFM data showing 360° DW mediated switching. An initially left-magnetised structure i) has a 360° DW injected at its centre ii) resulting in a new magnetization state oriented down-left after the composite 180° DWs have spread through the structure iii).

[1] Klaui, M. JOP: Cond. Matt. 20(31), 2008
 [2] McMichael, R.D. and Donahue, M.J - IEEE Trans. Mag. 33(5), 1997
 [3] Mascaro, M. D. and Ross, C. A. PRB 82(21), 2010
 [4] Roy, P.E., Trypiniotis, T. and Barnes, C. H. W. PRB 82(13), 2010

Control over magnetization states of nanostructures via injection of 360 degree domain walls 

Artificial Spin Ice is a magnetic meta-material, where the nano-elements act as macrosized spins. The magnetic
 properties are governed by the collective behaviour of the macrospins in addition to the magnetic material. He we
 study honeycomb Artificial Spin Ice, where interconnected magnetic nanobars are arranged in a kagome network.
 The collective behaviour originates from the â€˜ice rulesâ€™ local ordering principle and also magnetic domain wall (DW)
 effects.[1,2]. The structures are rather interesting for data storage if a methodology can be developed to controllably
 select a single microstate of the array.[3]
 
 Here we explore spatially selective methods of magnetization reversing within the array, including selective domain wall propagation [1,2] off-axis global fields4 and injection of domain walls with locally applied fields from striplines and an MFM tip. The spatially resolved magnetisation reversal behaviour is imaged using MFM and focussed MOKE magnetometry.
 
 This research builds on our understanding of complex interconnected magnetic systems and opens pathways to
 further explore the manipulation of magnetic charges5 and potential technological applications.
 
 [1] <a href="#!/publications/Walton_NJP_2015">S.K. Walton et. al. New Journal of Physics 17, 013054, (2015)</a>
 [2] K. Zeissler et al. Sci Rep-Uk 3, 1252, (2013)
 [3] A. Pushp et al. Nature Physics 9, 505-511, (2013)
 [4] <a href="#!/publications/Burn_PRB_2015"> D.M. Burn et al. Phys. Rev. B 92, 214425, (2015) </a>
 [5] S. Ladak et. al. Nature Physics 6, 359-363, (2010)

Local manipulation of the frustrated magnetic state of artificial spin ice arrays 

The BLADE beamline at Diamond Light Source can is geared towards research into the magnetic properties of thin-film and nanostructured samples. With x-rays in the soft energy regime, element specific measurements can be performed on the L2,3 absorption edges of transition metals and the M4,5 edges in rare earth materials. Combined with control over the x-ray polarisation state, both arbitrary linear and circular, allows for the investigation of a wide range of magnetic dichroism phenomena.

Using a focussed x-ray beam and through x-ray scattering techniques it is possible to determine the spatial dependence to the magnetic behaviour within a sample. However with potential applications for magnetic materials in novel technological devices, the temporal behaviour of magnetic thin films and nanostructures is also of considerable interest. Our understanding of time-dependent dynamical processes is key to the high speed response of future magnetoelectronic, spintronic and spin-orbitronic technologies.

To investigate these time-resolved magnetic phenomena with element sensitivity we are developing pump-probe measurements using the intrinsic pulse structure of x-rays produced by a synchrotron source. By synchronising a magnetic field or electrical current pulse with a variable delay from the arrival time of x-ray photons we can probe the magnetisation dynamics with sub-nanosecond resolution.

Measuring element specific time-resolved magnetic phenomena

Domain walls (DWs) in magnetic nanostructures have long been a subject of much interest[1]. The majority of work so far has focused on 90o and 180o DWs. However, recently the field has seen promising developments in 360o DWs, stable topological defects consisting of two strongly-coupled 180o DWs of opposite chirality[1]. 360o DWs have several interesting properties relating to behaviour in applied field[3], interactions with spin-waves[2] and high density DW systems such as racetrack memory. [4].

Here we demonstrate a novel 360o DW injection technique using an MFM tip. This enables the introduction of DWs of desired chirality to any point in a structure. When the injection point is separated from other micromagnetic features by more than a 360o DWs width, the 360o DW is stable. When injected close to a magnetic junction, the 360o DW is unstable and collapses in a chirality-dependent manner. Choosing injection points carefully allows the injected DWs to mediate controlled switching events throughout the nanostructure, resulting in new magnetisation states.

Using MFM data supported by micromagnetic simulations we demonstrate the utility of this as a tool for controlling global magnetisation states of nanostructures comprised of interconnected nanowires without applied current or global field.

Control over magnetisation states of nanostructures via injection of 360 degree domain walls

Magnetic meta-materials exhibit interesting and complex magnetisation behaviour which originates from nanoscale patterning in addition to the magnetic material. Artificial spin ice is one example, where interconnected magnetic nanobars are arranged in a kagome network. Here, the collective behaviour originates from the â€˜ice rulesâ€™ local ordering principle and also magnetic domain wall (DW) effects[1,2]. The structures are rather interesting for data storage if a methodology can be developed to controllably select a single microstate of the array[3].

Here we explore spatially selective methods of magnetization reversing within the array, including selective domain wall propagation[1,2] off-axis global fields[4] and injection of domain walls with locally applied fields from striplines and an MFM tip. The spatially resolved magnetisation reversal behaviour is imaged using MFM and focussed MOKE magnetometry. This research builds on our understanding of complex interconnected magnetic systems and opens pathways to further explore the manipulation of magnetic charges[5] and potential technological applications.

[1] <a href="/publications/Walton_NJP_2015"> S. K. Walton et al. New Journal of Physics 17, 013054, (2015).</a>
 [2] K. Zeissler et al. Sci Rep-Uk 3, 1252, (2013).
 [3] A. Pushp et al. Nature Physics 9, 505-511, (2013).
 [4] <a href="/publications/Burn_PRB_2015"> D. M. Burn et al. Phys. Rev. B 92, 214425, (2015).</a>
 [5] S. Ladak et al. Nature Physics 6, 359-363, (2010). 

We acknowledge Leverhulme Trust grant RPG_2012-692 and UK EPSRC grant EP/G004765/1 for funding.

Local manipulation of the magnetic state of large area nanomagnetic arrays 

Cu2OSeO3 is a chiral ferrimagnetic insulator that carries a hexagonal array of spin whirls, known as a skyrmion lattice. We report the observation of a multidomain skyrmion state near the surface of a Cu2OSeO3 single crystal using soft resonant elastic x-ray scattering. This technique is an ideal tool to probe the magnetic order at the L3 edge of 3d metal compounds giving a depth sensitivity of ?50 nm. We show that for magnetic field directions deviating from the major cubic axes, the single-domain sixfold-symmetric skyrmion lattice breaks up into domains overcoming the propagation directions imposed by the cubic anisotropy. Our findings are discussed in the framework of competing anisotropies and magnetoelectric effects. In magnetoelectric crystals they open up a new way to manipulate the skyrmion lattice state.

Resonant elastic x-ray scattering on a multidomain skyrmion lattice in Cu2OSeO3

The BLADE beamline at Diamond Light Source can is geared towards research into the magnetic properties of thin-film and nanostructured samples. With x-rays in the soft energy regime, element specific measurements can be performed on the L2,3 absorption edges of transition metals and the M4,5 edges in rare earth materials. Combined with control over the x-ray polarisation state, both arbitrary linear and circular, allows for the investigation of a wide range of magnetic dichroism phenomena.

Using a focussed x-ray beam and through x-ray scattering techniques it is possible to determine the spatial dependence to the magnetic behaviour within a sample. However with potential applications for magnetic materials in novel technological devices, the temporal behaviour of magnetic thin films and nanostructures is also of considerable interest. Our understanding of time-dependent dynamical processes is key to the high speed response of future magnetoelectronic, spintronic and spin-orbitronic technologies.

To investigate these time-resolved magnetic phenomena with element sensitivity we are developing pump-probe measurements using the intrinsic pulse structure of x-rays produced by a synchrotron source. By synchronising a magnetic field or electrical current pulse with a variable delay from the arrival time of x-ray photons we can probe the magnetisation dynamics with sub-nanosecond resolution.

Cu2OSeO3 is a chiral ferrimagnetic insulator carrying a long-range order magnetic skyrmion phase. Here, we report a short-range ordered equilibrium skyrmion state on the surface of Cu2OSeO3 single crystal, studied by resonant soft x-ray scattering. Soft x-ray scattering at the L2,3 edge of 3d compounds is an ideal tool to probe the magnetic order, and is only sensitive to ~60-70 unit cells in depth of Cu2OSeO3. Our results show that under the arbitrary magnetic field directions that deviate from the cubic main axes, the six-fold-symmetric skyrmion order breaks into domains, and the initial, anisotropy-governed pinning of propagation directions is completely unwound. We show that uniform 360o Bloch domain walls form between the skyrmion domains. Our findings provide a new way to manipulate and engineer the skyrmion state locally, or even individually, on the surface which will enable applications in the future.

Skyrmion domains in Cu2OSeO3: Short-Range Order and Domain Wall Formation

Magnetic meta-materials exhibit interesting and complex magnetisation behaviour which originates from nanoscale patterning in addition to the intrinsic properties of the magnetic material. Artificial spin ice is one such example where interconnected magnetic nanowires are arranged in a kagome network. Here, the collective behaviour originates from multiple magnetic domain walls (DWs) which propagate along nanowires and interact at vertices.[1,2] 

In this work, spatially resolved magnetisation reversal behaviour in a kagome artificial spin ice structure is investigated through focussed MOKE magnetometry. With the application of a magnetic field, magnetisation reversal due to the nucleation and propagation of magnetic DWs is observed. Varying the applied field direction relative to symmetry axes in the patterning allows us to bias particular sub-lattice directions giving physical insight into the DW propagation and vertex interactions taking place.

Localised pulsed-field injection techniques[3] allow the DW propagation behaviour to be probed at fields below the intrinsic nucleation field of the system. This means that the relative influence of the DW nucleation and propagation processes upon the magnetisation reversal can be differentiated. Furthermore, the localisation of the DW injection shows behaviour associated with the bulk of the sample in comparison to that resulting from boundary effects at the edges of the structure. 

This research builds on our understanding of complex interconnected magnetic systems and opens pathways to further explore the manipulation of magnetic charges[4] and potential technological applications.

 [1] <a href="/publications/Burn_PRB_2014">D. Burn et.al. Phys. Rev. B 90, 144414 (2014)</a>
 [2] K. Zeissler et.al. Sci. Rep. 3, 1252 (2013)
 [3] <a href="/publications/Burn_PRB_2013">D. Burn et.al. Phys. Rev. B 88, 104422 (2013)</a>
 [4] E. Mengotti et.al. Nat. Phys. 7, 68 (2010)	

 We acknowledge Leverhulme Trust grant RPG_2012-692 and UK EPSRC grant EP/G004765/1 for funding.

Magnetic domain walls in interconnected nanowire structures

Advances in nanotechnology have enabled the creation of complex single domain ferromagnetic constructs, allowing the study of ferromagnetic interactions influenced by geometry. Artificial spin ice is such a playground which marries geometrical order with magnetic frustration. Recently published measurements have shown unusual features in the magnetotransport signature of cobalt and permalloy honeycomb artificial spin ice below 50 K and 20 K respectively [1,2]. Here we explore the changes in the magnetic reversal of a connected artificial spin ice array fabricated from permalloy below and above this transition temperature using magnetic force microscopy. The imaging shows a change in the magnetic reversal process at low temperatures, which results in a striking number of ice rule violations. These ice rule violations are rare occurrences at room temperature in permalloy arrays where switching occurs via propagation of transverse domain walls [3-5]. We present a link between the domain wall chirality with our low temperature observations.


 [1] Branford WR, et al. Emerging Chirality in Artificial Spin Ice. Science. 2012;335(6076):1597- 600 
 [2] Lee LB, et al. Effects of exchange bias on magnetotransport in permalloy kagome artificial spin ice. New J. Phys. 2015;17(023047) 
 [3] Ladak S, et al. Monopole defects and magnetic Coulomb blockade. New J Phys. 2011;13(023023) 
 [4] Zeissler K, et al. The non-random walk of chiral magnetic charge carriers in artificial spin ice. Sci Rep. 2013;3(1252) 
 [5] <a href="#!/publications/Walton_NJP_2015"> Walton SK, et al. New J. Phys. 2015;17(013054) </a>

Magnetic imaging of honeycomb artificial spin ice at low temperatures

Domain walls (DWs) in magnetic nanostructures have long been a subject of much interest . The majority of work so far has focused on 90 and 180 DWs. However, recently the field has seen promising developments in 360 DWs, stable topological defects consisting of two strongly-coupled 180 DWs of opposite chirality. 

360 DWs have several unique properties. They are stationary under applied magnetic fields , interact interestingly with spin-waves and have a small stray field, desirable for systems such as racetrack memory[4].

Current methods for generating 360 DWs rely on fabricating injection structures and applying rotating fields or current pulses . These techniques have several drawbacks including additional nanofabrication steps and the application of global magnetic fields.

We present a novel method for injecting 360 DWs into nanowires via MFM tip. Experimental evidence is presented from MFM images and supported by micromagnetic simulations. The method avoids fabricating injection structures and global field/current application. Additional benefits include localised injection via precise tip positioning and chirality selection of injected DWs.


 [1] Klaui, M. JOP: Cond. Matt. 20(31), 2008
 [2] McMichael, R.D. and Donahue, M.J - IEEE Trans. Mag. 33(5), 1997
 [3] Mascaro, M. D. and Ross, C. A. PRB 82(21), 2010
 [4] Roy, P.E., Trypiniotis, T. and Barnes, C. H. W. PRB 82(13), 2010
 [5] Diegel, M. Mattheis, R. and Halder, E. IEEE Trans. Mag. 40(4), 2004
 [6] Oyarce, A. L., Llandro, J. and Barnes, C.H.W. APL 103(22), 2013

360 degree domain wall injection via moving magnetic charge

Magnetic meta-materials exhibit interesting and complex magnetisation behaviour which originates from nanoscale patterning in addition to the intrinsic properties of the magnetic material. Artificial spin ice is one such example where magnetic nanowires are arranged in a kagome geometry connected by nanowire vertices. The nucleation and propagation of magnetic domain walls (DWs) is the predominant magnetisation reversal process for magnetic nanowires. However, additional complexities in interconnected systems arise due to DW interactions at the vertices[1,2] and the flow of magnetic charge throughout system[3]. These features are governed by the topological nature of the DWs and the artificial geometrical structuring in the material.
 In this work, spatially resolved magnetisation reversal behaviour in a kagome artificial spin ice structure is investigated through focussed MOKE magnetometry. The nucleation and propagation of magnetic DWs is observed when a field is applied to the system. Varying the applied field direction relative to symmetry axes in the patterning biases particular sub-lattice directions giving physical insight into the DW propagation and vertex interactions taking place. Additional localised pulsed-field injection techniques[4] allow the DW propagation behaviour to be explored at fields below the intrinsic nucleation field of the system. This allows further characterisation of the DW â€“ vertex interaction and gives control over the DW nucleation location.
 This research builds on our understanding of complex interconnected magnetic systems and opens pathways to further explore the manipulation of magnetic charges[5] and their use in potential technological applications.

 [1] <a href="/publications/Burn_PRB_2014"> D. Burn et.al. Phys. Rev. B 90, 144414 (2014)</a>
 [2] <a href="/publications/Walton_NJP_2015"> S.K. Walton et.al. New J. Phys. 17, 013054 (2015)</a>
 [3] K. Zeissler et.al. Sci. Rep. 3, 1252 (2013).
 [4] <a href="/publications/Burn_PRB_2013"> D. Burn et.al. Phys. Rev. B 88, 104422 (2013)</a>
 [5] E. Mengotti et.al. Nat. Phys. 7, 68 (2010)	

 We acknowledge Leverhulme Trust grant RPG_2012-692 and UK EPSRC grant EP/G004765/1 for funding.

Magnetisation reversal behaviour in interconnected artificial spin ice structures

We present SQUID and MOKE magnetometry and MFM imaging experiments on arrays consisting of ~106 single domain permalloy nano-disks with 60 nm diameter. The nano-disks were fabricated with different array geometries and varying disk separations. This was done to test whether nano-disk arrays can be well described as an artificial 2D-XY dipolar system [1-3]. The 2D-XY system with exchange interactions has been well understood by the Berenzinskii-Kosterlitz-Thouless (BKT) model [4, 5], and the 2D-XY system with purely dipolar interaction presents an interesting variation to study. A variety of differences are expected because dipolar interactions are long-range and anisotropic in nature, however, contradicting results about collective properties have been published in literature [1-3].

In this work, we study kagome and square lattices with lattice spacing varying from 160 to 450 nm. MFM imaging shows that the individual disks of this diameter are single domain and with approximately uniform magnetisation and without significant in-plane anisotropy. Zero field-cooled and field-cooled magnetometry measurements on the kagome and square lattices were performed. The kagome lattice shows a cooperative freezing transition while square lattice shows no such transition in the temperature range studied (2 - 300 K). This demonstrates that the magnetic properties of our arrays are governed by the lattice geometry, rather than the properties of a single disk. The observation that the square lattice is apparently less frustrated than kagome lattice is consistent with 2D-XY dipolar theory [3, 6]. Additionally, we explore the collective properties as a function of lattice spacing, to tune the dipolar interaction strength.

 [1] A. Carbognani, E. Rastelli, S. Regina, and A. Tassi, Phys. Rev. B 62, 1015 (2000).
 [2] S.K. Baek, P. Minnhagen, and B.J. Kim, Phys. Rev. B 83,184409 (2011).
 [3] L.A.S. MÃ³l and B.V. Costa, J. Phys.: Condens. Matter 22, 046005 (2010).
 [4] S.T. Bramwell and P.C.W. Holdsworth, J. Phys.: Condens. Matter 5, L53 (1993).
 [5] S.T. Bramwell, P. Day, M.T. Hutchings, J.R.G. Thorn

Collective properties of frustrated nano-disk arrays

Domain walls (DWs) in magnetic nanostructures have long been a subject of much interest[1]. The majority of work so far has focused on 90° and 180° DWs. However, recently the field has seen promising developments in 360° DWs, stable topological defects consisting of two strongly-coupled 180° DWs of opposite chirality[2].
360° DWs have several unique properties. They are stationary under applied magnetic fields[3], interact interestingly with spin-waves4 and have a small stray field, desirable for systems such as racetrack memory[4].
Current methods for generating 360° DWs rely on fabricating injection structures and applying rotating fields[5] or current pulses[6]. These techniques have several drawbacks including additional nanofabrication steps and the application of global magnetic fields.

 We present a novel method for injecting 360° DWs into nanowires via MFM tip. Experimental evidence is presented from MFM images and supported by micromagnetic simulations (example below). The method avoids fabricating injection structures and global field/current application. Additional benefits include localised injection via precise tip positioning and chirality selection of injected DWs.

<img class="img-fluid text-center" src="/images/presentations/abstract_figs/Carter_Magnetism_15.jpg"></img>

Figure 1: Micromagnetic simulation showing 360Â° DW injection process. An initially homogenous nanowire i) has an MFM tip scanned over it ii)-iii) resulting in a stable 360Â° DW after the tip has passed iv)

[1] Klaui, M. JOP: Cond. Matt. 20(31), 2008
 [2] McMichael, R.D. and Donahue, M.J - IEEE Trans. Mag. 33(5), 1997
 [3] Mascaro, M. D. and Ross, C. A. PRB 82(21), 2010
 [4] Roy, P.E., Trypiniotis, T. and Barnes, C. H. W. PRB 82(13), 2010
 [5] Diegel, M. Mattheis, R. and Halder, E. IEEE Trans. Mag. 40(4), 2004
 [6] Oyarce, A. L., Llandro, J. and Barnes, C.H.W. APL 103(22), 2013

Injection of 360 degree domain walls in magnetic nanostructures via MFM tip

Geometrically patterned magnetic nanostructures supporting individual magnetic domain walls (DWs) have been extensively studied due to the potential impact from the development of novel technological devices. Our current understanding has led to the development of magnetic meta-material systems such as artificial spin ice. Here, the collective behaviour originates from the DWs as they interact with the vertices between many interconnected nanowire structures.[1,2]

In this work, spatially resolved magnetisation reversal behaviour in a kagome structure is investigated through focussed MOKE magnetometry. With a combination of spatially uniform quasi-static fields and localised pulsed fields, the role of DWs in the reversal process has been investigated.[3] Localisation of the DW injection field allows the DW propagation behaviour to be probed at fields below the intrinsic nucleation field in the system. This behaviour can be differentiated from that associated with the boundary effects resulting from the edges of the structure. Furthermore, varying the applied field direction around particular symmetry can bias the DW propagation and vertex interactions giving further insight into the interactions taking place in these structures.

This research builds on our understanding of complex interconnected magnetic systems and opens pathways to further explore the manipulation of magnetic charges[4] and potential technological applications.

[1] <a href="/publications/Burn_PRB_2014">D. Burn et.al. Phys. Rev. B 90, 144414 (2014)</a>
 [2] K. Zeissler et.al. Sci. Rep. 3, 1252 (2013)
 [3] <a href="/publications/Burn_PRB_2013">D. Burn et.al. Phys. Rev. B 88, 104422 (2013)</a>
 [4] E. Mengotti et.al. Nat. Phys. 7, 68 (2010)

We acknowledge Leverhulme Trust grant RPG_2012-692 and UK EPSRC grant EP/G004765/1 for funding.

The propagation of magnetic domain walls in interconnected artificial spin ice structures

The control and understanding of magnetic damping is technologically desirable. For spin-transfer torque magnetic random access memory (STT-MRAM) and magnonic devices, lower damping allows a lower writing current and longer propagation of spin waves, while increased damping increases reversal rates and facilitates coherent reversal for fast switching in recording. In this work the influence of interfacial intermixing on the picosecond magnetization dynamics of ferromagnetic/nonmagnetic thin-film bilayers was studied. Low-dose focused-ion beam (Ga+) irradiation was used to induce intermixing across the interface between a 10 nm Ni81Fe19 layer and a 2-3 nm capping layer of either Au or Cr. Time-resolved magneto-optical Kerr Effect magnetometry was used to study magnetization dynamics as a function of ion-beam dose. With a Au cap the damping of the unirradiated bilayer was comparable with native Ni81Fe19 and increased with increasing ion dose. In contrast, for Ni81Fe19/Cr the damping was higher than that for native Ni81Fe19 but the damping subsequently decreased with increasing dose.
		

The high spatial resolution of the FIB technique combined with the capacity of very low irradiation doses to cause intermixing offers the ability to locally modify the precessional magnetisation behaviour of ferromagnetic materials on a micro- or nano-scale. (a) (b)


<img class="img-fluid text-center" src="/images/presentations/abstract_figs/King_UKIERI_15.png"></img>


Figure 1. The damping coefficient, alpha, obtained from the TR-MOKE data as a function of FIB dose at H = 1.5 kOe for (a) NiFe/Au bilayer (error bars are smaller than the data points), (b) NiFe/Cr bilayer.

Local control of damping in ferromagnetic / non-magnetic bilayers by interfacial intermixing induced by focussed ion beam irradiaiton

Magnetic meta-materials exhibit interesting and complex magnetisation behaviour which originates from nanoscale patterning in addition to the intrinsic properties of the magnetic material. Artificial spin ice is one such example where an array of magnetic nanowires are connected by vertices to form a kagome structure. The nucleation and propagation of magnetic domain walls (DWs) is the predominant magnetisation reversal process for magnetic nanowires. However, complexities in the behaviour of artificial spin ice systems arise due to DW interactions at the vertices[1,2] and the flow of magnetic charge throughout system[3].

In this work, spatially resolved magnetisation reversal behaviour in a kagome artificial spin ice structure is investigated through focussed MOKE magnetometry. With an applied field the nucleation and propagation of magnetic DWs is observed throughout the array. Varying the applied field direction relative to symmetry axes in the patterning biases particular sub-lattice directions giving physical insight into the DW propagation and vertex interactions taking place. Additional localised pulsed-field injection techniques[4] allow the DW propagation behaviour to be explored at fields below the intrinsic nucleation field of the system. This allows further characterisation of the DW - vertex interaction and gives control over the DW nucleation location.

This research builds on our understanding of complex interconnected magnetic systems and opens pathways to further explore the manipulation of magnetic charges[5] and their use in potential technological applications.

 [1] <a href="/publications/Burn_PRB_2014"> D. Burn et.al. Phys. Rev. B 90, 144414 (2014)</a>
 [2] <a href="/publications/Walton_NJP_2015"> S.K. Walton et.al. New J. Phys. 17, 013054 (2015)</a>
 [3] K. Zeissler et.al. Sci. Rep. 3, 1252 (2013)
 [4] <a href="/publications/Burn_PRB_2013"> D. Burn et.al. Phys. Rev. B 88, 104422 (2013)</a>
 [5] E. Mengotti et.al. Nat. Phys. 7, 68 (2010)

 We acknowledge Leverhulme Trust grant RPG_2012-692 and UK EPSRC grant EP/G004765/1 for funding.

Magnetisation dynamics in the picosecond regime are largely determined by damped precessional behaviour. Damping has, therefore, received significant research effort, both in terms of the fundamental physics involved and for technological applications. In general, the control of magnetic properties at the micro- and nano-scale is important for technological applications. Focused ion beam irradiation provides the opportunity for such highly-localised control and modification of magnetic properties. Here the influence of interfacial intermixing on the picosecond precessional magnetisation dynamics of ferromagnetic/non-magnetic thin-film bilayers was studied [1]. Low-dose focused-ion-beam irradiation was shown to affect various aspects of the intrinsic magnetisation behaviour in such bilayers [2]. The influence on damping of ion-beam induced intermixing across the interface between a 10 nm Ni81Fe19 layer and a 2-3 nm capping layer of either Au or Cr was studied here. Time-resolved MOKE was used to study magnetisation dynamics and determine the damping as a function ion-beam dose. Detailed structural analysis of NiFe/Au bilayers was used to show the nature of the intermixing at the bilayer interface [3].

With a Au cap the damping behaviour of the un-irradiated bilayer was comparable with native Ni81Fe19 and increased with increasing ion dose. In contrast, for the as-deposited Ni81Fe19/Cr bilayer the damping was higher than for native Ni81Fe19, but decreased with increasing ion beam dose. Furthermore, field-dependent damping measurements of the NiFe/Au films showed the damping decreased steadily with increasing frequency, indicating an extrinsic two-magnon type contribution to the damping, that may be attributed to increasing disorder at the interface. This contrasts with the behavior observed for MBE grown Au on Fe, where the damping was enhanced by spin-pumping effects. For the NiFe/Cr the damping was invariant with increasing precessional frequency. This indicates an enhancement of the intrinsic damping with Cr cap, which contrasts with the effect of Cr on Fe observed for MBE grown samples reported elsewhere.

 [1] <a href="#!/publications/King_APL_2014">J.A. King et al. Appl. Phys. Lett. 104, 242410 (2014) </a>
 [2] <a href="#!/publications/Burn_JPCM_2014"> D.M. Burn, T.P.A. Hase and D. Atkinson J. Physics: Cond Mat; 26(23): 236002 (2014)</a>
 [3] <a href="#!/publications/Arac_JAP_2012"> E. Arac et al. J. Appl. Phys. 111, 044324 (2012)</a>

Local Control of Damping in Ferromagnetic/Non-magnetic Bilayers by Interfacial Intermixing Induced by Focused Ion-Beam Irradiation

Geometrical patterning of a magnetic thin film allows for the manipulation of the shape anisotropy, modifying the magnetisation behaviour. Nanoscale magnetic bars in the single domain regime with uniform axial magnetisation have an associated magnetic charge at both ends. When arranged in a kagome lattice structure where three bars connect 120 degrees, a finite magnetic charge must reside at each vertex. These structures are known as artificial spin ice and allow for the investigation into fundamental magnetic frustration effects in a system where the interactions can be manipulated.

Magnetic charge defects in these structures move about due to charge carrying domain wall (DW) nucleation and propagation processes in the nanobars[1]. The charges follow a path determined by a combination of the spin configuration at the vertex, the micromagnetic structure of the DW and the frustration in the system[2].

In this work, experimental MOKE measurements have been used to probe the magnetisation behaviour in an ASI structure. Here, changes in the magnetisation indicate transitions in the magnetic configuration which take place at various switching fields. Variation of the sample alignment adjusts the bias on the relative sub-lattice bars whist manipulation of the applied field sequence allows different initial states to be investigated. Asymmetries in the angular dependence of the switching field reveal the influence of the spin structure at the vertex[3] upon the DW nucleation process. This allows direct DW nucleation switching processes to be compared with avalanche effects from transitions in neighbouring vertices. Additionally, micromagnetic simulations give further insight into the origins of these transitions based on the spin structure at the vertex and how this relates to the chirality of the nucleated DW.

This research builds on our understanding of fundamental magnetic frustrated systems and has potential for technological developments involving the manipulation of magnetic charge for on-chip applications.

 [1] E. Mengotti, et.al. Nat. Phys. 7, 68. (2010)
 [2] K. Zeissler, et.al. Sci. Rep., 3, 1252. (2013)
 [3] S.A. Daunheimer, et.al. Phys. Rev. Let. 107, 167201. (2011)

Magnetisation reversal processes at artificial spin ice vertices 

The behaviour of domains and domain walls (DWs) in magnetic nanostructures has led to many developments in our understanding of fundamental magnetisation reversal processes. Interactions between DWs and geometrical structural features are one area of interest where previous studies largely consider the quasi-static field driven behaviour of DWs based on a structure in an energetically minimised state[1]. In this regime the behaviour of DWs can be described by properties such as their type, chirality and topology[2,3].

More recently, an increasing interest in DW dynamics has developed due to the potential for novel spintronic devices and the desire for high operating speeds. In this dynamic regime, gyroscopic precession limited and time-dependent changes in micromagnetic structure become significant in addition to the minimised energy states which dominate in the quasi-static regime. The micromagnetic structure of propagating DWs become distorted, and with driving fields in excess of the Walker field periodic transformations in DW type and chirality take place[4].

In this work both experimental focussed MOKE measurements and micromagnetic simulations are used to explore the behaviour of a dynamically propagating DW arriving at a Y-shaped vertex between three nanowires with a 120 degree opening. The DW either propagates through one of the nanowire paths or becomes pinned at the vertex, the outcome being determined by the DW structure at the point of arrival at the vertex. Thus, the final magnetisation state strongly depends on Walker breakdown processes associated with the incoming DW dynamics.

This work builds on our understanding magnetisation reversal processes in artificial spin ice structures comprising of many nanowire vertices. In addition this work also has implications for the development of novel DW based spintronic devices.

 [1] <a href="/publications/Burn_PRB_2013"> D.M. Burn et.al. Phys. Rev. B. 88, 104422 (2013)</a>
 [2] K. Zeissler et.al. Sci. Rep., 3, 1252 (2013)
 [3] A. Pushp, et.al. (2013). Nat. Phys., 9, 505. (2013)
 [4] G.S.D. Beach, et.al. Nat. Mater., 4, 741. (2005)

Interactions between dynamic propagating domain walls and nanowire vertices

Ferromagnetic nano-dot arrays are interesting for data storage applications, but as the density of disks becomes high the dipolar interactions between disks become strong. In this work we study lithographically prepared arrays of densely packed single domain permalloy nano-disks where the dipolar correlations are significant. We study the collective magnetic array properties for different array geometries and varying disk separation and explore the effect of magnetic frustration in these systems.

Magnetic frustration in dipolar coupled nano-disk systems

Nanoscale patterning techniques can be used to fabricate magnetic structures with dimensions comparable with those of magnetic domains and domain walls (DWs). This can give control over the interactions taking place within the material allowing complex frustrated geometries to be designed such as artificial spin ice. In addition to advances in the fundamental physical understanding of magnetism from studying these nanoscale systems, research in this area may lead to the development of novel spintronic devices for technological applications. 

In artificial spin ice, magnetic nanobars can support magnetic DWs and the magnetisation reversal in an applied field is mediated through DW propagation. Interactions between the micromagnetic structure of a DW and the spin structure at the nanobar vertices determine the path of a DW and it's de-pinning field from the vertex. 

In this work the trajectory of a DW at a nanobar vertex is investigated as a function of the dynamic behavior of the propagating DW. This includes the time dependent periodic changes in the DW micromagnetic structure that results from Walker breakdown. These results have implications for future technological applications as well as suggesting processes that may govern magnetisation reversal in artificial spin ice structures.

Propagation of magnetic domain walls in artificial spin ice

With modern lithographic patterning techniques the fabrication of magnetic structures on a lengthscale comparable with that of magnetic domains and domain walls (DWs) has become prevalent. Studies on such structures have improved our understanding of fundamental magnetic and magnetisation reversal processes and also show potential for the development of novel spintronic devices.

Engineering of nanostructure geometry and applied field sequences have become a popular approach to investigate DWs and their interaction with geometrical structural features. Much of this previous work considers the quasi-static field driven behaviour of DWs with features described by their magnetisation, chirality and topology governed by energy minimisation effects.

More recently, an increasing interest in DW dynamics has developed due to the high speeds required for the development of novel spintronic devices. In this regime, time-dependent changes in micromagnetic structure become significant over the minimised energy states which dominate the quasi-static regime. Propagating DWs have distorted structures; and with driving fields in excess of the Walker field even periodic transformations in DW type and chirality take place.

In this work both experimental focussed MOKE techniques and micromagnetic simulations are used to explore these dynamic effects for a DW arriving at a nanowire vertex. The final trajectory shows how dynamic effects can dominate over a quasi-static understanding of this system. This work has implications for future technological applications as well as suggesting processes that may govern magnetisation reversal in artificial spin ice structures comprising of many nanowire vertices.

Dynamic structure of magnetic domain walls and their trajectory at nanowire vertices

I will discuss arrays of single domain nanomagnets. The shape of each nanomagnets controls the magnetic anisotropy and the elements are closely spaced so dipolar interactions are important. Lattices are chosen such that the geometry prevents all dipole interactions from being satisfied. The building block of such frustrated lattices is the equilateral triangle because it cannot support simple antiparallel ordering. A two dimensional array of corner sharing triangles is known as the kagome lattice and a three-dimensional array of corner sharing tetrahedral is known as pyrochlore. Magnetic pyrochlore chemical compounds (spin ices) have recently attracted much attention with the observation of emergent magnetic monopoles, but they have limitations as model frustrated systems: tuning the lattice parameter by chemical doping tends to break the symmetry, specific defects cannot be engineered and the spins cannot be directly imaged. The use of frustrated artificial nanostructures overcomes these problems through the tremendous versatility in array fabrication and compatibility with a suite of magnetic imaging techniques. Here I will show direct magnetic imaging studies of monopole defects [1-2] and magnetic charge flow. [3-4] The magnetic charge is carried by transverse domain walls and the chirality of the domain wall is found to control the direction of propagation. In addition to magnetic imaging studies of the magnetisation state, I will also present magnetoresistance and Hall effect measurements. These techniques probe the array as a whole and can be very sensitive to the details of the spin structure. A change in symmetry in the Hall response of connected honeycomb nanostructures is observed at low temperatures indicating a collective response of the array of nanomagnets. [5]

 [1] S. Ladak, D.E. Read, G.K. Perkins, L.F. Cohen, W.R. Branford. Nature Physics 6, 359, (2010).
 [2] S. Ladak, D. Read, T. Tyliszczak, W. R. Branford, L.F. Cohen. New Journal of Physics 13, 023023, (2011).
 [3] S. Ladak, S.K. Walton, K. Zeissler, T. Tyliszczak et al. New Journal of Physics 14, 045010, (2012).
 [4] K. Zeissler, S.K. Walton, S. Ladak, D.E. Read et al. Sci. Rep. 3, 01252, (2013).
 [5] W. R. Branford, S. Ladak, D.E. Read, K. Zeissler, L.F. Cohen. Science 335, 1597, (2012).

Control and influence of domain wall chirality in Artificial Spin Ice

Artificial Spin Ice, comprising ferromagnetic nanobars arranged in a honeycomb geometry, is a directly imageable frustrated system which has demonstrated rich Physics. Its magnetic reversal is mediated by domain wall propagation in the presence of external magnetic fields. These domain walls carry magnetic charge and have a distinct structure or "chirality", namely up or down in the transverse domain wall regime or clockwise or anticlockwise in the vortex domain wall regime. In this talk, both experimental Scanning Transmission X-ray Microscopy and micromagnetic simulations which suggest that the domain wall performs a non-random walk through Artificial Spin Ice due to its chirality are presented. In addition, the role of Walker Breakdown in both the transverse and vortex domain wall regimes is discussed. Furthermore, modes of controlling and measuring domain wall chirality are explored.

Collective Properties of Nanomagnet Arrays; Electric and Magnetic Currents in Artificial Spin Ice

Ferromagnetic nano-dot arrays are interesting for data storage applications, but as the density of disks becomes high the dipolar interactions between disks become strong. In this work we study lithographically prepared arrays of densely packed single domain perm-alloy nano-disks where the dipolar correlations are significant. We study the collective magnetic array properties for different array geometries and varying disk separation and explore the effect of magnetic frustration in these systems.

Study of dipolar interaction between nano-disks

Nanoscale patterning techniques can be used to fabricate magnetic nanowire structures where the behavior of individual magnetic domain walls (DWs) can be investigated. In addition to the fundamental physical understanding of magnetism, research in this area is also driven by the potential to realise novel spintronic devices for technological applications. 

Magnetic DWs can support a wide variety of micromagnetic structures with different magnetisation, chirality and topology based on their interaction with the nanostructure geometry. These interactions can govern the field dependent domain wall trajectory and subsequent magnetisation reversal that takes place within nanowire vertex structures. 

In this work the additional factors affecting the trajectory due to the dynamic behavior of propagating DWs are investigated. This includes the time dependent periodic changes in the DW micromagnetic structure from Walker breakdown. These results have implications for future technological applications as well as suggesting processes that may govern magnetisation reversal in artificial spin ice structures.

Trajectory of dynamically propagating magnetic domain walls at nanowire vertices

Local Control of Nanoscale Magnetic Properties by Interfactal Intermixing in Magnetic/Nonmagnetic Thin Film Bilayers

Time-Domain Magneto-Optical Kerr Effect Investigation of Precessional Dynamics in Ion Irradiated Transition Metal-Capped NiFe Thin Films

The application of the magneto-optic Kerr (MOKE) effect has become widespread for the investigation of magnetic thin-films, multilayers and nanoscale magnetic structures. With due care to the experimental arrangement MOKE magnetometry can be exquisitely sensitive to magnetisation changes and can be used to study magnetisation reversal in lithographically patterned structures such as small arrays of nanostructures and individual nanowires. Using the functionally simpler quasi-static focused MOKE arrangement, magnetisation reversal behaviour can be studied as a function of magnetic field. The magnetisation state can be interrogated during the reversal in the case of field driven rotational processes. However, in the case of rapid reversals, for example mediated by domain wall motion in nanowires, the magnetisation process can be followed by interrogating the magnetisation state before and after switching. In the latter case this also presents the opportunity to make multiple measurements and study the stochastic nature of the processes. For lithographically patterned nanostructures typical structures have thicknesses ranging from a few nanometres up to 20 nm. Since the MOKE sensitivity is limited by the skin-depth to the order of 20 nm the technique is highly suited to interrogating the entire thickness of such nanostructures. 

Examples of MOKE measurements on such lithographic nanostructures will be discussed. However, this penetration depth limit also makes this technique surface sensitive for materials and structures that are significantly thicker. New results and analysis of MOKE and anisotropic magnetoresistance measurements on the same 200 nm diameter cylindrical NiFe nanowires are compared and contrasted to show the surface magnetisation sensitivity and highlight the complexity of the magnetisation behaviour between the surface and the bulk of these nanowires. In contrast to the more widely used application of MOKE magnetometry for the study of magnetisation processes in thin-films and nanostructures, the magneto-optic Kerr effect can also be used to investigate fundamental aspects of magnetism. Here examples are included which show that under appropriate circumstances the MOKE amplitude obtained from quasistatic measurements can be used to inform on the changes in intrinsic magnetic moment. The method does not give absolute magnetic moment but can show relative changes in moment that can be correlated with direct moment measurements. Furthermore, dynamic MOKE magnetometry is a powerful technique that can be used to study intrinsic precessional magnetisation processes and inform on the magnetic damping. Here examples of the application of MOKE magnetometry to the study of precessional processes are presented that show modifications to the damping in ferromagnetic/non-magnetic bilayered samples by the choice of the non-magnetic layer and the effect of ion beam induced interfacial mixing.

Static and dynamic magneto-optic Kerr effect magnetometry for analysis of magnetisation processes and fundamental magnetic properties

Magnetisation reversal processes in ferromagnetic nanowires are of fundamental physical interest and also attract significant interest due to their potential exploitation in novel spintronic devices.[1,2] Techniques for the control of domain walls in nanowires are important and geometrical structuring is widely used for the controlled pinning of domain walls with individual geometrical features[3]. Nanowire structuring can even lead to improved domain wall dynamics due to the suppression of the Walker breakdown process[4].

Here the magnetisation reversal behaviour in lithographically defined edge modulated nanowire structures are studied in two regimes, first by nucleation controlled reversal and secondly, by domain wall propagation. In the latter case, domain walls were introduced both with 'nucleation pad' structures and through local pulsed field injection techniques.

The reversal behaviour shows a complex dependence on the modulation parameters involving competing contributions to the energetics of the magnetisation reversal. This gives rise to a minimum reversal field for specific modulation parameters showing that wire parameters can be optimised for both static and dynamic magnetisation processes.

The experimental results obtained here are supported by a detailed series of micromagnetic simulations and an analytic model, developed based on demagnetisation effects of the modulation upon the spin structure of the wire. This analysis highlights consistent trends in the reversal behaviour resulting from the modulation. Significantly, the reversal behaviour is found to be scalable in relation to the amplitude and the wavelength of the modulation[5].

 [1] D.A. Allwood et al., Science 309, 1688 (2005)
 [2] S.S.P. Parkin et al., Science 320, 190 (2008)
 [3] L.K. Bogart et al., Phys. Rev. B 79, 054414 (2009)
 [4] Y. Nakatani et al., Nat. Mat. 2, 521 (2003)
 [5] <a href="/publications/Burn_PRB_2013">D. Burn et al., Phys. Rev. B 88, 104422 (2013)</a>

Magnetisation reversal and domain wall propagation behaviour in edge modulated nanowire structures

The behaviour of domain walls in magnetic nanowires has received significant interest due to the potential for the development of novel spintronic devices. High operation speeds, reliability and packing density are important factors for the success of such devices. However, Walker breakdown [1] at high driving fields affects the wall chirality and limits domain wall mobility in both nanowires [2] and thin film [3] structures.


Nanowires with rough edges show improved domain wall mobility due to interactions between the domain wall and nanowire structuring.4 Here, artificial roughness is introduced in the form of periodic modulations to a nanowire's edge geometry. Experiments and micromagnetic simulations are used to investigate the influence of the modulation upon both the static and dynamic magnetisation processes.

Field driven reversal processes in these wires were investigated first via direct reversal from an initially uniform magnetisation state at a relatively high field and then through domain wall mediated reversal at lower fields. Experimentally determined reversal fields are shown in figures 1(a) and 1(b) for these two reversal modes respectively. These results have been interpreted using micromagnetic simulations and through the development of a model based on the demagnetisation effects of the modulation upon the spin structure of the wire.

Further micromagnetic simulations were used to investigate dynamic domain wall processes. Matching the periodicity of the wire structuring to the frequency of Walker breakdown structural transitions leads to the suppression of Walker breakdown by triggering energy dissipation through spin wave emission[5].


<img class="img-fluid text-center" src="/images/presentations/abstract_figs/Burn_CRIM_13_a.png"></img>
<img class="img-fluid text-center" src="/images/presentations/abstract_figs/Burn_CRIM_13_b.png"></img>

Figure 1: Magnetisation reversal field for a nanowire as a function of edge modulation wavelength. a) Direct nucleation limited reversal where the lines show the best model fit to the long wavelength data and b) domain wall mediated reversal using nucleation pad and stripline domain wall injection techniques.

 [1] N.L. Schryer and L.R. Walker J. Appl. Phys. 45, 5406 (1974).
 [2] G.S.D. Beach, C. Nistor et.al. Nat. Mater. 4, 741 (2005). 
 [3] <a href="/publications/Burn_JAP_2010">D.M. Burn and D. Atkinson J. Appl. Phys. 108, 073926 (2010)</a>. 
 [4] Y. Nakatani, A. Thiaville and J. Miltat Nat. Mater. 2, 521 (2003). 
 [5] <a href="/publications/Burn_APL_2013">D.M. Burn and D. Atkinson Appl. Phys. Lett. 102, 242414 (2013)</a>.

Magnetisation reversal and domain wall propagation in edge modulated nanowires

Interest in the dynamics of domain walls in magnetic nanowires has increased rapidly in recent years, largely due to their potential in novel spintronic applications. Domain wall dynamics are key to device operation speeds, which are ultimately limited by the domain wall velocity.

 Typically, domain wall velocity increases linearly with applied field up to the Walker field. This field marks the onset of Walker breakdown, after which a dramatic reduction in the time-averaged velocity occurs due to the nucleation of a vortex core originating from a structural instability in the wall. Further increase in the applied field leads to a gradual increase in velocity interrupted by further breakdown events. Walker breakdown can be suppressed with the application of transverse fields, which shifts breakdown to higher fields [1]. Edge roughness can also suppress Walker breakdown by dissipation of wall energy preventing vortex core formation, but by its nature this is hard to control [2]. Subsequent theoretical work has shown modulation of the edges modifies the energy landscape and thus affects wall propagation [3].

 Here, a detailed systematic micromagnetic analysis of small amplitude periodically modulated nanowires has been performed. The periodicity dependence shows how energy dissipation via spin wave emission from domain walls can reduce the structural instability of the wall. Edge modulated nanowires can be tuned to match the period of vortex nucleation giving rise to a regime where fast, constant domain wall motion can be achieved at fields above that usually resulting in Walker breakdown. 

 [1] M.T. Bryan et.al., J. Appl. Phys. 103, 073906 (2008).
 [2] Y. Nakatani et.al., Nature Materials 2, 521 (2003).
 [3] J. Ieda et.al., J. Magn. Magn. Mater. 322, 1363 (2010).

Suppression of Walker breakdown by spin waves in edge modulated planar NiFe nanowires

The control of domain walls in magnetic nanowires is a popular research theme as their use in spintronic devices for memory, sensing and logic processing applications grows. In such devices information is represented by the magnetisation direction of regions of the nanowire separated by domain walls. To achieve high density devices, controlling the pinning of these domain walls is essential.

Domain wall pinning has previously been achieved by modifying the structural geometry of the wire with notches or anti-notches [1]. Here we show that domain wall pinning behaviour can be achieved through local modifications to the underlying magnetic material of the nanowire through focussed Ga+ ion irradiation. With this technique the positioning and the potential energy of the pinning sites can be controlled without the need for lithographic structuring.

A detailed investigation has been performed on the magnetic changes (Fig 1(a)) taking place within NiFe / Au bilayers as a result of low dose irradiation induced interfacial intermixing [2]. This has revealed that fine control over the saturation magnetisation can be achieved for low dose irradiation. Localised control of domain wall pinning using irradiation induced intermixing has been demonstrated in nanowires here (Fig 1(b)). Furthermore, this approach could also potentially lead to a reduction in the critical current density for current induced domain wall motion when Ga+ irradiated pinning sites are used [3].


<img class="img-fluid text-center" src="/images/presentations/abstract_figs/Burn_JEMS_12_b.png"></img>
Magnetic properties of Ga+ irradiated NiFe / Au bilayer (a) structures and (b) nanowires.

[1] L.K. Bogart et.al., Phys. Rev. B 79, 054414 (2009).
 [2] <a href="/publications/Arac_JAP_2012">E. Arac et.al., J. Appl. Phys. 111, 044324 (2012)</a>.
 [3] K.J. Lee et. al., J. Magn. Magn. Mater. 304, 102 (2006).

Local modification of magnetic properties via Ga+ irradiation for the control of domain wall pinning in planar NiFe nanowires

The effect of low dose focussed ion beam (FIB) Ga+ irradiation on the structural and magnetic properties of NiFe/Au bilayer systems have been investigated. Irradiation induced modifications of the structural and chemical properties within the bilayer result in complex and unexpected changes to the magnetisation, magnetoresistance and fluorescence within the sample.

The structural changes resulting from low dose irradiation have been studied in detail using synchrotron radiation for grazing incidence x-ray reflectivity and angle dependent x-ray fluorescence. Analysis shows roughening at the NiFe/Au interface and the development of a compositionally graded NixFeyAu(1-x-y) alloy. At low dose some Au is lost to sputtering but a consistent Au layer remains on top of the NiFe. Magneto-optical Kerr effect (MOKE), VSM and SQUID measurements show a loss in ferromagnetic signal with increasing dose and also an increased coercivity from an increased density of pinning sites. More interesting is the complex dose dependent change in magnetisation that shows a reduction followed by a recovery of the magnetisation at low dose. This dip in magnetisation is also coincident with a maximum in the anisotropic magnetoresistance ratio (AMR) despite an increase in average resistivity. X-ray fluorescence ratio of Ni to Fe also shows unexpected changes with low dose irradiation, despite the presence of a gold capping layer.

These results indicate that low dose irradiation can have a significant effect on the magnetic moment and from the AMR, on the spin-orbit coupling. Supporting work using XMCD to differentiate the spin and orbital contributions to the magnetic moment are scheduled and will be included in this work.

Modification of magnetic moment and spin orbit coupling in NiFe/Au bilayers induced by Ga+ ion irradiation

The effect of low dose focussed ion beam (FIB) Ga+ irradiation on the structural and magnetic properties of NiFe/Au bilayer systems have been investigated. Irradiation induced modifications of the structure within the bilayer result in complex and unexpected changes to the magnetisation and magnetoresistance within the sample.

The structural changes resulting from low dose irradiation have been studied in detail using synchrotron radiation for grazing incidence x-ray reflectivity and angle dependent x-ray fluorescence measurements. Analysis shows roughening at the NiFe/Au interface and the development of a compositionally graded NixFeyAu(1-x-y) alloy. Magneto-optical Kerr effect (MOKE), VSM and SQUID measurements show a loss in ferromagnetic signal with increasing dose and also an increased coercivity from an increased density of pinning sites. More interesting is the complex dose dependent change in magnetisation that shows a reduction followed by a recovery of the magnetisation at low dose. This dip in magnetisation is also coincident with a maximum in the anisotropic magnetoresistance ratio (AMR) despite an increase in average resistivity.

These results indicate that low dose irradiation can have a significant effect on the magnetic moment and from the AMR, on the spin-orbit coupling.

Effect of Ga+ irradiation on the magnetic and structural properties in NiFe/Au bilayers

Control of electronic and magnetic behaviour in NiFe/Au nanostructures by Ga+ irradiation

Control over the magnetic properties such as anisotropy, coercivity and saturation magnetisation in magnetic structures is desirable. Modification of the local magnetisation for trapping and manipulating domain walls is highly sought-after for technological applications in information storage and processing.
	
Ion beam irradiation can induce interface mixing and strain relaxation between the different layers, and has become a very popular approach in out-of-plane magnetic systems for which it is well understood that interface anisotropy can be controlled by tuning the interface grading and roughness. More recently, there have been studies reporting ion beam induced changes in the magnetic properties of in-plane systems. However, a detailed study accounting for the origin of these changes is still missing for in-plane systems.

We have investigated the structural properties of Au/Ni81Fe19 bilayers as a function of Ga+ ion dose by means of X-ray reflectivity (XRR) and depth resolved x-ray fluorescence (XRF) using synchrotron radiation. We have also performed Monte-Carlo (SRIM and TRIDYN) simulations to provide further information on the compositional structure of the samples for varying Ga+ irradiation doses. We have discovered that for an irradiation dose of 10 pC/&mu;m2 and above the layered structures are destroyed and samples become amorphous-like. We observed that systematic changes in structural and compositional changes in response to Ga+ irradiation at low doses. XRR and TXRF measurements and the model used to fit the experimental data give evidence on the widening of the Au/Ni81Fe19 interface width due to an increase in topological roughness and chemical intermixing by increasing Ga+ irradiation dose. Moreover, experimentally observed results are well predicted by SRIM and TRIDYN simulations suggesting the validity of the presented model.

Structural studies in ion irradiated Au/Ni81Fe19 bilayers by XRR and XRF techniques

Control over the domain wall (DW) dynamics in small magnetic structures is highly sought-after for technological applications in information storage and processing. To address this issue, we use a novel approach for trapping and manipulating domain walls based on artificial domain structures created by low-dose FIB irradiation. This scheme allows us to tune sample parameters such as local anisotropy, domain geometry and domain wall location, which may be used to tailor domain wall dynamics in magnetic nanostructures.

Magnetic properties of Ni81Fe19 /Au bilayers were investigated as a function of Ga+ ion dose and Ni81Fe19 thickness by means of focused magneto-optical Kerr effect (MOKE) magnetometry. It was found that magnetic properties such as coercivity, remanence ratio and hard axis anisotropy field could be tuned. Possible reasons for the change in magnetic properties were investigated using Monte-Carlo simulations to provide information on layer intermixing and Ga+ implantation. Moreover, X-ray reflectivity measurements give experimental evidence on intermixing and roughening in the samples.

We will also present primary field and current driven DW measurements on nanowires containing artificial domain structures. Control over the depinning field is achieved by varying the irradiated dose which tunes the coercivity of the irradiated wire section. Furthermore, by local FIB irradiation, artificial domain structures with different geometry and internal structure will be used to optimise the critical currents for current induced domain wall motion and to gain an understanding over the dynamics of propagating domain walls.

Fabrication and modification of magnetic structures using dual-beam focused ion beam (FIB) system

Dual-beam FIB/SEM has become a very popular tool in nanomagnetism research. It can be used to manufacture nanostructures with resolution down to 10 nm by subtractive milling as well as modifying the local magnetic properties such as coercive field and remnant magnetisation. The latter can be achieved by ion beam induced interface mixing and strain relaxation between two different layers.
		
We present a complete process for the fabrication of nanowires and associated circuitry needed to perform pulsed field and current measurements on nanowires by a combination of ion beam milling and e-beam lithography by using FIB.

We also tried to tailor the switching field in NiFe nanowires by sinusoidally modulating the wire edges with periodicity and amplitude of modulation precisely varied by FIB. Ion induced local changes in magnetic properties and chemical composition were also investigated by MOKE and EDX techniques, respectively.

Magnetic nanowire and measurement circuitry fabrication by dual-beam FIB for studies of domain wall dynamics

The combination of an electron column and Ga+ ion source in a single instrument makes the Dual Beam FIB / SEM a versatile instrument for nanoscale device fabrication and structural analysis.

We have investigated the changes in magnetic properties within NiFe / Au bilayers induced by low dose Ga+ irradiation. A rise in coercivity with dose is found up to a critical dose where magnetic properties become quenched. With focused beams, local control over magnetic properties is used to create domain wall pinning sites in nanowires and will ultimately allow artificial domain structures to be created.

Following irradiation, electron microscopy and EDX compositional analysis give information from the sample surface whilst magneto-optical techniques are used to probe the magnetic properties. Together with models of ion interactions within these materials they give an insight into the processes such as Ga+ implantation and intermixing which bring about the changes in magnetic properties resulting from Ga+ irradiation.

Fabrication and structural analysis of magnetic nanostructures using dual ion and electron beam systems

The combination of an electron column and Ga+ ion source in a single instrument makes the Dual Beam FIB / SEM a versatile instrument for nanoscale device fabrication and structural analysis.

We have investigated the changes in magnetic properties within NiFe / Au bilayers induced by low dose Ga+ irradiation. A rise in coercivity with dose is found up to a critical dose where magnetic properties become quenched. With focused beams, local control over magnetic properties is used to create domain wall pinning sites in nanowires and will ultimately allow artificial domain structures to be created.

Following irradiation, electron microscopy and EDX compositional analysis give information from the sample surface whilst magneto-optical techniques are used to probe the magnetic properties. Together with models of ion interactions within these materials they give an insight into the processes such as Ga+ implantation and intermixing which bring about the changes in magnetic properties resulting from Ga+ irradiation.

Dual ion and electron beam system with multiple fabrication and structural analysis functionalities

Interest in thin-film magnetism has recently been stimulated by a wide range of potential applications ranging from sensors to devices such as hard disk read heads. The maximum operating speeds and reliability of such devices are ultimately limited by the switching behaviour taking place within thin-films inside the device. Thus, understanding the physical processes of magnetisation change in thin-films in response to very rapid magnetic field changes has significance for potential applications. 
		
Here we present experimental results for magnetisation switching in a NiFe thin-film measured using magneto-optic Kerr effect magnetometry. Pulsed magnetic fields from 10 ns to 1 us are created using a pulser circuit and microstrip line. These combine with a quasi-static field resulting in a total field which induces switching in the sample. When this total field exceeds the coercive field with a positive quasi-static contribution, successful switching is observed. As the quasi-static component is reduced but with total fields still in excess of the coercive field, complex switching behaviour arises which results in incomplete switching for large total fields. Figure 1a shows the experimental results where a change in behaviour occurs between positive and negative offset fields.
		
A qualitative model has been constructed to explain this complex behaviour where magnetisation switching in a thin-film takes place by the propagation of domain walls. By including a thermally activated domain wall process and Walker breakdown in domain wall velocity, this simplified model explains the main features of the complex behaviour found in the experimental results (see figure 1b).

<img class="img-fluid text-center" src="/images/presentations/abstract_figs/Burn_CMMP_09_a.png"></img>
<img class="img-fluid text-center" src="/images/presentations/abstract_figs/Burn_CMMP_09_b.png"></img>

 Figure 1: The change in magnetisation switching from simple step-like behaviour at positive offset fields to complex behaviour at negative offset fields. This has been observed in experimental results (a) where the key features are explained by a simple domain wall propagation model (b).