Enhancement of room-temperature unidirectional spin Hall magnetoresistance by using a ferromagnetic metal with a low Curie temperature

Abstract

The unidirectional spin Hall magnetoresistance (USMR) effect is useful to detect the direction of magnetization in a ferromagnetic metal/nonmagnetic metal bilayer, which is the typical geometry used in a spin-orbit torque switching device. In this Letter, we demonstrate the enhancement of the USMR effect by reducing the Curie temperature ${T}_{\mathrm{C}}$ of the ferromagnetic (FM) layer in a FM/platinum bilayer. The USMR ratio was maximized when the thickness of each FM layer was consistent with the spin diffusion length. We found that the maximum USMR ratio can be doubled by replacing ${\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}$ (${T}_{\mathrm{C}}=854$ K) with ${\mathrm{Ni}}_{85}{\mathrm{Cu}}_{15}$ (${T}_{\mathrm{C}}=527$ K) as the FM. This enhancement of the USMR effect is attributed to an increase of electron-magnon scattering in accordance with Bloch law. We also found that the use of ferromagnets with face-centered-cubic structures, such as Ni, ${\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}$, and ${\mathrm{Ni}}_{85}{\mathrm{Cu}}_{15}$, increased the USMR effect relative to that of body-centered-cubic Fe.