Giant response to spin-orbit torques in heavy-metal/ferromagnetic bilayers associated with magnetic reversal

Abstract

Using bilayer films of $\ensuremath{\beta}$-Ta (5 nm)/${\mathrm{Ni}}_{0.8}{\mathrm{Fe}}_{0.2}$ (2 nm), we fabricate elliptical structures which exhibit uniaxial magnetic anisotropy, resulting in single magnetic domain behavior. We study induced spin-orbit torques (SOTs) in these devices with first- and second-order harmonic Hall measurements for current flowing along the long axis of the ellipses and external magnetic field applied in the film plane. We observe a giant response to the SOTs associated with magnetization reversal of the NiFe layer, and we correlate it quantitatively with a $d{\ensuremath{\varphi}}_{m}/d{\ensuremath{\varphi}}_{H}$ term, where ${\ensuremath{\varphi}}_{m}$ and ${\ensuremath{\varphi}}_{H}$ are the in-plane angles of the magnetization and the magnetic field, respectively. We discuss theoretical and applicative implications of this intriguing behavior.