Bose-Einstein Condensation of Magnons Pumped by the Bulk Spin Seebeck Effect

TL;DR

This paper demonstrates that a heat flow can induce Bose-Einstein condensation of magnons in a magnetic insulator, with a detailed analysis of the physical conditions and thresholds involved.

Contribution

It introduces a novel method to achieve magnon BEC via thermal gradients and provides quantitative estimates for the required conditions in yttrium iron garnet.

Findings

Magnon BEC can be induced by a boundary heat flux in magnetic insulators.

The threshold bias for condensation depends on overcoming damping and radiative losses.

Different regimes of condensation are identified and contrasted with bulk instabilities.

Abstract

We propose inducing Bose-Einstein condensation of magnons in a magnetic insulator by a heat flow oriented toward its boundary. At a critical heat flux, the oversaturated thermal gas of magnons accumulated at the boundary precipitates the condensate, which then grows gradually as the thermal bias is dialed up further. The thermal magnons thus pumped by the magnonic bulk (spin) Seebeck effect must generally overcome both the local Gilbert damping associated with the coherent magnetic dynamics as well as the radiative spin-wave losses toward the magnetic bulk, in order to achieve the threshold of condensation. We quantitatively estimate the requisite bias in the case of the ferrimagnetic yttrium iron garnet, discuss different physical regimes of condensation, and contrast it with the competing (so-called Doppler-shift) bulk instability.