Current-induced noise and damping in non-uniform ferromagnets

TL;DR

This paper investigates how electric current noise influences magnetization fluctuations and damping in non-uniform ferromagnets, revealing nonlocal effects and the dominance of shot noise at low temperatures.

Contribution

It introduces a detailed analysis of current-induced noise and damping in non-uniform ferromagnets, highlighting nonlocal tensor structures and the role of shot noise.

Findings

Current noise causes fluctuating spin-transfer torque.

Magnetization damping acquires a nonlocal tensor form.

Shot noise dominates at low temperatures in biased ferromagnets.

Abstract

In the presence of spatial variation of the magnetization direction, electric current noise causes a fluctuating spin-transfer torque that increases the fluctuations of the ferromagnetic order parameter. By the fluctuation-dissipation theorem, the equilibrium fluctuations are related to the magnetization damping, which in non-uniform ferromagnets acquires a nonlocal tensor structure. In biased ferromagnets, shot noise can become the dominant contribution to the magnetization noise at low temperatures. Considering spin spirals as a simple example, we show that the current-induced noise and damping is significant.