Spin dynamics of antiferromagnets in the presence of a homogeneous magnetization

TL;DR

This paper investigates the spin excitations in isotropic antiferromagnets with homogeneous magnetization, revealing diverse modes and spectra depending on the magnetization's conservation, with implications for quantum phase transitions and magnetic metal behaviors.

Contribution

It provides a hydrodynamic framework to analyze spin dynamics in antiferromagnets with homogeneous magnetization, including cases with induced or spontaneous magnetization, and explores their excitation spectra.

Findings

Propagating, diffusive, and relaxational spin modes identified.

Spectral differences depending on magnetization conservation.

Foundation for understanding quantum phase transitions in magnetic systems.

Abstract

We use general hydrodynamic equations to determine the long-wavelength spin excitations in isotropic antiferromagnets in the presence of a homogeneous magnetization. The latter may be induced, such as in antiferromagnets in an external magnetic field, or spontaneous, such as in ferrimagnetic or canted phases that are characterized by the coexistence of antiferromagnetic and ferromagnetic order. Depending on the physical situation, we find propagating spin waves that are gapped in some cases and gapless in others, diffusive modes, or relaxational modes. The excitation spectra turn out to be qualitatively different depending on whether or not the homogeneous magnetization is a conserved quantity. The results lay the foundation for a description of a variety of quantum phase transitions, including the transition from a ferromagnetic metal to an antiferromagnetic one, and the spin-flop transitions that are observed in some antiferromagnets. They also are crucial for incorporating weak-localization and Altshuler-Aronov effects into the descriptions of quantum phases in both clean and disordered magnetic metals.