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)

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] Ł. Gladczuk et al., Phys. Rev. B 103, 064416 (2021)