Local thermomagnonic torques in two-fluid spin dynamics

TL;DR

This paper develops a phenomenological framework for understanding how thermal magnons influence ferromagnetic insulators' dynamics, enabling control of topological solitons without electrical conduction.

Contribution

It introduces a general formalism for local thermomagnonic torques and reciprocal pumping, applicable to various magnetic materials and long-wavelength dynamics.

Findings

Thermal magnons can exert local torques on magnetic order parameters.

The formalism applies to ferromagnetic insulators with U(1) symmetry.

Thermal magnon gases can control domain wall dynamics without electrical currents.

Abstract

We develop a general phenomenology describing the interplay between coherent and incoherent dynamics in ferromagnetic insulators. Using the Onsager reciprocity and Neumann's principle, we derive expressions for the local thermomagnonic torques exerted by thermal magnons on the order-parameter dynamics and the reciprocal pumping processes, which are in close analogy to the spin-transfer torque and spin pumping at metallic interfaces. Our formalism is applicable to general long-wavelength dynamics and, although here we explicitly focus on ferromagnetic insulators possessing U(1) symmetry, our approach can be easily extended to other classes of magnetic materials. As an illustrative example, we apply our theory to investigate a domain wall floating over a spin superfluid, whose dynamics is triggered thermally at the system's edge. Our results demonstrate that the local pumping of coherent spin dynamics by a thermal magnon gas offers an alternative route - with no need for conducting components and thus devoid of Ohmic losses - for the control and manipulation of topological solitons.