Stabilizing Mechanism for Bose-Einstein Condensation of Interacting Magnons in Ferrimagnets and Ferromagnets

TL;DR

This paper introduces a stabilizing mechanism for magnon Bose-Einstein condensation in ferrimagnets and ferromagnets, emphasizing the role of sublattice structure in maintaining stability despite magnon-magnon interactions.

Contribution

It demonstrates that sublattice structure is crucial for stabilizing magnon BEC in interacting systems, resolving previous experimental-theoretical discrepancies.

Findings

Magnon BEC remains stable in ferrimagnets with sublattice structure.

Magnon BEC becomes unstable in ferromagnets without sublattice structure.

The sublattice structure is key to stabilization, regardless of spin alignment.

Abstract

We propose a stabilizing mechanism for the Bose-Einstein condensation (BEC) of interacting magnons in ferrimagnets and ferromagnets. By studying the effects of the magnon-magnon interaction on the stability of the magnon BEC in a ferrimagnet and two ferromagnets, we show that the magnon BEC remains stable even in the presence of the magnon-magnon interaction in the ferrimagnet and ferromagnet with a sublattice structure, whereas it becomes unstable in the ferromagnet without a sublattice structure. This indicates that the existence of a sublattice structure is the key to stabilizing the BEC of interacting magnons, and the difference between the spin alignments of a ferrimagnet and a ferromagnet is irrelevant. Our result can resolve a contradiction between experiment and theory in the magnon BEC of yttrium iron garnet. Our theoretical framework may provide a starting point for understanding the physics of the magnon BEC including the interaction effects.