Skyrmions as quasiparticles: free energy and entropy

TL;DR

This paper investigates the thermodynamic properties of magnetic skyrmions in a specific bilayer system, revealing their quasiparticle nature and entropic stabilization effects through atomistic spin model simulations.

Contribution

It provides the first detailed calculation of free energy and entropy of skyrmions across a wide temperature range, demonstrating entropic stabilization beyond linear spin-wave theory.

Findings

Skyrmions are thermodynamically favored at low temperatures due to entropy.

A sign change in free energy and entropy occurs at higher temperatures.

Skyrmions exhibit quasiparticle behavior with temperature-dependent stability.

Abstract

The free energy and the entropy of magnetic skyrmions with respect to the collinear state are calculated for a (Pt$_{0.95}$Ir$_{0.05}$)/Fe bilayer on Pd(111) via atomistic spin model simulations. The simulations are carried out starting from very low temperatures where the skyrmion number is conserved up to the range where skyrmions are constantly created and destroyed by thermal fluctuations, highlighting their quasiparticle nature. The higher entropy of the skyrmions at low temperature leads to a reduced free energy, such that the skyrmions become energetically preferred over the collinear state due to entropic stabilization as predicted by linear spin-wave theory. Going beyond the linear spin-wave approximation, a sign change is shown to occur in the free energy as well as the entropy at elevated temperature.