Out-of-plane magnetization oscillation in spin Hall device assisted by field-like torque

Abstract

An excitation of a large-amplitude out-of-plane magnetization oscillation in a ferromagnet by the spin Hall effect is of great interest for practical applications such as microwave generators and neuromorphic computing. However, both experimental and theoretical works have revealed that only small-amplitude oscillation around an in-plane easy axis can be excited via the spin Hall effect. Here, we propose that an out-of-plane oscillation can be excited due to an assistance of field-like torque. We focus on an in-plane magnetized ferromagnet with an easy axis parallel to the current direction. We notice that the field-like torque with an appropriate sign provides an additional field, modifying the dynamic trajectory of the magnetization, and drives the auto-oscillation. The condition on the sign of the field-like torque is satisfied for a typical nonmagnet used in spin Hall devices such as tungsten.