Distinct Characteristics between Magnon and Spin Accumulation Contributions to the Unidirectional Spin Hall Magnetoresistance*

Unidirectional spin Hall magnetoresistance (USMR) is a novel magnetoresistance family that arises from spin current generation in ferromagnet (FM)/nonmagnetic heavy metal (HM) bilayers. It has received significant attention in recent years because, unlike other magnetoresistances, USMR is a nonlinear effect that violates the Onsager reciprocity, being odd under either magnetization or current reversal. Because USMR exhibits this asymmetric behavior owing to the current or external magnetic field directions, it is easy to quantify the amount of charge-to-spin conversion in a system and the sign of the spin current. USMR has been explained using two major mechanisms: spin accumulation at the FM/HM interface and electron–magnon scattering in an FM layer.

In this study, we investigated the thickness and temperature dependence of the USMR of Ta/Co and Pt/Co bilayer structures and numerically analyzed the contribution of the electron–magnon scattering using a self-developed quantitative analysis model. The magnon USMR is dominant in the Pt/Co samples, and the spin accumulation USMR and magnon USMR trends are separated in the Ta/Co samples by thickness. The magnitude of the USMR strongly depends on the choice of the HM. Our findings provide a method to quantitatively separate the contributions of each mechanism.