Tuning Unidirectional Magnetoresistance via Current-Induced Nonequilibrium Magnons

Unidirectional magnetoresistance (UMR) in magnetic heterostructures has garnered increasing interest due to its fundamental and practical implications. The effect generally arises from the interplay among magnetization, spin current/polarization, and inversion symmetry breaking. A critical challenge for it to be relevant to device applications is the enhancement of its magnitude. To address this, we theoretically explore the role of nonequilibrium magnons, identifying two key contributions: (1) Indirect role: Electron spin current is partially converted into magnon accumulations and magnon spin current, reducing UMR as electron-magnon coupling intensifies. (2) Direct role: Current-induced nonequilibrium magnons directly scatter electrons, increasing UMR. We present solutions of coupled kinetic equations for magnons and electrons in layered structures with proper boundary conditions, offering new insights into how tuning the competition between these roles can effectively enhance UMR.