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)

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.