Non-linear Hall effect due to asymmetric magnon scattering

The discovery of the unidirectional magnetoresistances (UMR) in magnetic multilayers has recently attracted experimental [1,2,3] and theoretical [4] interest. However, only the longitudinal contribution of these effects has been extensively studied, whereas little is known about its transverse contribution: the nonlinear Hall effect (NLHE). Its observation is for now limited to model systems: magnetic topological insulators [5] and ferromagnetic films with bulk inversion asymmetry [6]. The anomalous Nernst effect and the nonlinear contributions of the spin orbit torques to the Hall resistance make it difficult to evidence the NLHE without additional thermoelectric measurements and numerical simulations.

By using NiCu single layers with a large spin Hall angle [7] we observed UMR due to asymmetric magnon scattering in both the paramagnetic and ferromagnetic state. Close to the Curie temperature, where the magnon fluctuations are enhanced, we identify a NLHE signal with a field dependence similar to the UMR and different from other effects. This NLHE is attributed to a Rashba-like spin splitting at the NiCu interfaces in line with the observation of a large field like torque in the ferromagnetic state.