Spin Currents and Magnon Dynamics in Insulating Magnets

TL;DR

This paper reviews magnon transport in insulating magnets, discussing theoretical insights, analogies with electron transport, and the effects of quantum fluctuations, aiming to advance understanding and measurement of spin currents.

Contribution

It provides a comprehensive review of magnon transport phenomena, introduces new measurement proposals, and discusses the impact of quantum fluctuations on spin transport in low-dimensional systems.

Findings

Magnon transport exhibits analogies with electron transport laws.

Quantum fluctuations significantly alter spin transport in one-dimensional antiferromagnetic chains.

Proposed methods for direct measurement of magnon currents.

Abstract

Nambu-Goldstone theorem provides gapless modes to both relativistic and nonrelativistic systems. The Nambu-Goldstone bosons in insulating magnets are called magnons or spin-waves and play a key role in magnetization transport. We review here our past works on magnetization transport in insulating magnets and also add new insights, with a particular focus on magnon transport. We summarize in detail the magnon counterparts of electron transport, such as the Wiedemann-Franz law, the Onsager reciprocal relation between the Seebeck and Peltier coefficients, the Hall effects, the superconducting state, the Josephson effects, and the persistent quantized current in a ring to list a few. Focusing on the electromagnetism of moving magnons, i.e., magnetic dipoles, we theoretically propose a way to directly measure magnon currents. As a consequence of the Mermin-Wagner-Hohenberg theorem, spin transport is drastically altered in one-dimensional antiferromagnetic (AF) spin-1/2 chains; where the N\'eel order is destroyed by quantum fluctuations and a quasiparticle magnon-like picture breaks down. Instead, the low-energy collective excitations of the AF spin chain are described by a Tomonaga-Luttinger liquid (TLL) which provides the spin transport properties in such antiferromagnets some universal features at low enough temperature. Finally, we enumerate open issues and provide a platform to discuss the future directions of magnonics.