Coexistence of spin-orbit torque and unidirectional magnetoresistance effect induced by spin polarization with spin rotation symmetry in Co/Cu/Co structures

Abstract

The conventional spin-orbit torque (SOT) and magnetoresistance effect observed in normal-metal (NM)/ferromagnet (FM) bilayers originate from the interaction between magnetic moments and spin with in-plane transverse polarization (${\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{\ensuremath{\sigma}}}_{y}$). In FM/NM/FM trilayer structures, the presence of an extra FM layer breaks the symmetry, resulting in spin polarization other than ${\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{\ensuremath{\sigma}}}_{y}$ and the corresponding SOT. However, the study on the unidirectional magnetoresistance (UMR) effect induced by the spin polarization with spin rotation (SR) symmetry is still missing. In this work, we investigate the SOT and UMR effect in a Co/Cu/Co structure with crossed anisotropy by utilizing the harmonic longitudinal voltage measurement. We demonstrate that, in addition to the harmonic terms originating from the fieldlike SOT of the spin polarization with spin Hall and SR symmetry, two different unidirectional harmonic signals are observed in the angular and field scan measurements. Further, combining the SOT effective fields results from the harmonic Hall voltage measurement, we successfully separate the contributions of the observed unidirectional harmonic signals, which include the dampinglike SOT contribution through the giant magnetoresistance effect, and the UMR induced by the spin current with different polarization directions through spin-dependent scattering mechanism.