Spin Hall Magnetoresistance in Metallic Bilayers

TLDR

Spin Hall magnetoresistance (SMR) is investigated in metallic bilayers composed of a heavy metal and a ferromagnetic metal. A model is developed to account for the absorption of longitudinal spin current by the ferromagnetic layer, a key feature of metallic ferromagnets. The study reports a nearly tenfold increase in SMR for W/CoFeB compared to HM/ferromagnetic insulator systems, a temperature‑dependent rise in SMR despite unchanged W resistivity, and a model that quantitatively captures HM thickness and temperature effects by incorporating FM spin polarization, highlighting the pivotal role of metallic ferromagnets in spin transmission across HM/FM interfaces.

Abstract

Spin Hall magnetoresistance (SMR) is studied in metallic bilayers that consist of a heavy metal (HM) layer and a ferromagnetic metal (FM) layer. We find a nearly tenfold increase of SMR in $\mathrm{W}/\mathrm{CoFeB}$ compared to previously studied HM/ferromagnetic insulator systems. The SMR increases with decreasing temperature despite the negligible change in the W layer resistivity. A model is developed to account for the absorption of the longitudinal spin current to the FM layer, one of the key characteristics of a metallic ferromagnet. We find that the model not only quantitatively describes the HM layer thickness dependence of SMR, allowing accurate estimation of the spin Hall angle and the spin diffusion length of the HM layer, but also can account for the temperature dependence of SMR by assuming a temperature dependent spin polarization of the FM layer. These results illustrate the unique role a metallic ferromagnetic layer plays in defining spin transmission across the $\mathrm{HM}/\mathrm{FM}$ interface.