Electronic vs magnonic contributions to unidirectional magnetoresistance

The effect of spin current on magnetoresistance is of central importance to modern spintronics. We have investigated unidirectional magnetoresistance (UMR) in Permalloy (Py)/Pt bilayers, a magnetoelectronic effect whose origin is currently debated. Two different mechanisms are believed to contribute to UMR. The first mechanism is due to magnon generation/suppression in Py by pure spin current produced by the spin Hall effect in Pt. The second one is due to spin dependent electronic scattering near the Py/Pt interface, where spin accumulation is modulated due to the spin current generated in Pt. By engineering the geometric and material properties of our samples, we enhanced magnon relaxation, thus suppressing the spin current-induced changes of magnon population. We observed a significantly reduced UMR, suggesting that this effect in the studied structures is dominated by the magnon contribution. We also discuss how the dependence of UMR on field temperature provides information about the spectral characteristics of the generated magnons, and their generation mechanisms. Our results suggest the UMR can be utilized as an efficient technique for the characterization of spin current-driven dynamical magnetization states.