Theory of Current-Induced Magnetization Precession

TL;DR

This paper demonstrates that unpolarized current flowing from a non-magnet into a ferromagnet can induce magnetization precession due to spin accumulation and angular momentum transfer, leading to a current-induced instability.

Contribution

It provides a theoretical analysis showing how unpolarized currents can cause magnetization precession via spin accumulation, expanding understanding of current-induced magnetic dynamics.

Findings

Current flow can induce magnetization precession.

Instability occurs at specific current densities and wave vectors.

Variation along the current direction significantly affects the behavior.

Abstract

We solve appropriate drift-diffusion and Landau-Lifshitz-Gilbert equations to demonstrate that unpolarized current flow from a non-magnet into a ferromagnet can produce a precession-type instability of the magnetization. The fundamental origin of the instability is the difference in conductivity between majority spins and minority spins in the ferromagnet. This leads to spin accumulation and spin currents that carry angular momentum across the interface. The component of this angular momentum perpendicular to the magnetization drives precessional motion that is opposed by Gilbert damping. Neglecting magnetic anisotropy and magnetostatics, our approximate analytic and exact numerical solutions using realistic values for the material parameters show (for both semi-infinite and thin film geometries) that a linear instability occurs when both the current density and the excitation wave vector parallel to the interface are neither too small nor too large. For many aspects of the problem, the variation of the magnetization in the direction of the current flows makes an important contribution.