Magnon-phonon coupling unmasked: a direct measurement of magnon temperature

TL;DR

This study directly measures magnon temperature in a magnetic material under thermal gradient, revealing that magnons and phonons share the same temperature, challenging existing theories of spin caloritronics and the spin Seebeck effect.

Contribution

It provides the first direct measurement of magnon temperature distribution, showing magnons and phonons are thermally coupled, which questions current theoretical models of spin caloritronic phenomena.

Findings

Magnon and phonon temperatures are equal under thermal gradient.

Results challenge the current understanding of spin Seebeck effect.

Implications for theories of spin-lattice interactions in magnetic materials.

Abstract

Thermoelectric phenomena in magnetic materials present tantalizing possibilities for manipulating spin-information using heat in future 'spin caloritronic' devices. Key to unraveling their underlying physics is to understand spin-lattice interactions, i.e. the coupling between magnons (the quanta of magnetization excitations) and phonons (the quanta of lattice vibrations). Here, we present the first measurements of the spatial distribution of magnon temperature in a magnetic system subject to a lateral thermal (i.e. phonon temperature) gradient and demonstrate that, contrary to currently accepted theory, the magnon and phonon temperatures do not differ. This result has profound implications. In particular, it re-opens the question of how the spin Seebeck effect-which allows spin currents to be produced from thermal gradients, and is arguably the most intriguing and technologically relevant thermoelectric phenomenon of all-can exist, and which physics underpins it. Specifically, it reveals that if the general framework of the current theory of the effect holds, we must adopt a new concept of spectrally non-uniform magnon temperature.