Thermally Activated Transitions Between Micromagnetic States

TL;DR

This paper reviews thermal activation processes in nanoscopic magnetic systems, focusing on transition states, energy barriers, and the dynamics of magnetic textures including skyrmions and droplets, highlighting both theoretical and computational challenges.

Contribution

It provides a comprehensive analysis of noise-induced magnetic transitions, including new insights into nonlocal interactions, non-Arrhenius rates, and topologically protected textures in nanoscale systems.

Findings

Activation barriers for magnetic texture creation and annihilation calculated.

Transition rates can be Arrhenius or non-Arrhenius depending on system parameters.

Droplet solitons can be metastable or transition states in magnetic systems.

Abstract

We review work by the authors on thermal activation in nanoscopic magnetic systems. These systems present unique difficulties in analyzing noise-induced escape over a barrier, including the presence of nonlocal interactions, nongradient terms in the energy functional, and dynamical textures as initial or saddle states. We begin with a discussion of magnetic reversal between single-domain configurations of the magnetization. Here the transition (saddle) state can be either a single-domain or a spatially varying (instanton-like) configuration, and depending on the system parameters can exhibit either Arrhenius or non-Arrhenius reversal rates. We then turn to a discussion of transitions between magnetic textures, which can be either static and topologically protected or dynamic and not topologically protected. An example of the latter case is the droplet soliton, a rotating nontopologically-protected configuration, which we find can occur either as a metastable or transition state in a nanoscopic magnetic system. After discussing various issues in calculating transition rates, we present results for the activation barriers for creation and annihilation of these magnetic textures. We conclude with a discussion of activated transitions between topologically protected skyrmion textures and other configurations, on which work is ongoing.