Many-body theory of spin-current driven instabilities in magnetic insulators

TL;DR

This paper develops a self-consistent theoretical framework for understanding spin-current induced instabilities in magnetic insulators, accounting for magnon interactions and interface effects, with implications for low-temperature and thin-film systems.

Contribution

It introduces a novel self-consistent Keldysh effective action approach that includes magnon-magnon interactions and interface effects in magnetic insulators.

Findings

Magnon gas stability is affected by magnon-magnon interactions and interface coupling.

The phase diagram of instabilities is modified by interference effects.

These effects are significant at low temperatures and in thin magnetic insulators.

Abstract

We consider a magnetic insulator in contact with a normal metal. We derive a self-consistent Keldysh effective action for the magnon gas that contains the effects of magnon-magnon interactions and contact with the metal to lowest order. Self-consistent expressions for the dispersion relation, temperature and chemical potential for magnons are derived. Based on this effective action, we study instabilities of the magnon gas that arise due to spin-current flowing across the interface between the normal metal and the magnetic insulator. We find that the stability phase diagram is modified by an interference between magnon-magnon interactions and interfacial magnon-electron coupling. These effects persist at low temperatures and for thin magnetic insulators.