Fate of Uniform Ferromagnetism under Spin Current: Domain Nucleation

A key mechanism of the current-induced magnetization dynamics is the spin torque from a spin polarized current (spin current), which is exerted on spatial gradient of magnetization. Recently, it has been pointed out that a large spin current applied to a uniform ferromagnet leads to a spin- wave instability. In this work, we show that such instability is absent in a state containing a domain wall by evaluating the spin-wave dispersion around it. This may indicate that nucleation of magnetic domains occurs above a certain critical spin current. This scenario is supported by an explicit energy comparison between the uniformly magnetized state and the domain-wall state under spin current.