Formation of new phase inclusions in the system of quasi-equilibrium magnons of high density

TL;DR

This paper investigates how high-density magnons in a magnetic sample can lead to the formation of new magnetic phases, including domains and lattices, competing with Bose-Einstein condensation.

Contribution

It introduces a theoretical model describing phase formation in magnon systems using the Landau-Lifshitz equation with relaxation, diffusion, and pumping effects.

Findings

New phase regions form when pumping exceeds a critical threshold.

Regions of opposite magnetization orientation can appear as droplets or lattices.

The process competes with magnon Bose-Einstein condensation.

Abstract

The paper studies the spatial variation of the magnetization in a nonconducting magnetic sample with an excess number of magnons in comparison to the equilibrium. The phenomenon is considered using the Landau-Lifshits equation with additional terms describing the longitudinal relaxation of the magnetization, the magnon diffusion and the magnon creation by external pumping. The free energy of the system is presented in the mean field approximation. It is shown that, if the pumping exceeds some critical value, regions of the new phase arise where the magnetic moments are oriented opposite to the magnetization of the magnetic sample. The phenomenon is similar to the appearance of droplets of the condensed phase in the supersaturated vapor. The appearance of the new phase either in the form of a single domain or a periodical lattice is demonstrated. The studied process is a competitor to the process of the Bose-Einstein condensation of magnons.