Observation of self-induced spin-orbit torques in Ni-Fe layers with a vertical gradient of magnetization

Single ferromagnetic material generates transverse spin currents via in-plane charge current application and can exert spin-orbit torques (SOTs) on its magnetization, namely, self-induced SOTs, when the inversion symmetry in the bulk and/or interface is broken. In this Letter, we experimentally demonstrate that introducing a vertical gradient of magnetization within a ferromagnet is an effective way to generate self-induced SOTs. Self-induced SOTs in Ni-Fe alloys with a magnetization gradient are studied using spin-torque ferromagnetic resonance. It is found that the gradient direction of magnetization in the Ni-Fe layer can control not only the magnitude of the SOTs but even the sign of the torques. We propose a model to analyze self-induced SOTs and reveal that the asymmetric SOT profiles within a ferromagnet due to spatial variation of the magnetic moment play a dominant role in generating efficient self-induced SOTs. Our findings provide an alternative perspective on understanding the mechanism of generating self-induced SOTs and how to enhance their torque efficiency without the use of heavy metal elements.