Theory of transverse magnetization in spin-orbit coupled antiferromagnets

TL;DR

This paper develops a comprehensive microscopic theory explaining the conditions under which transverse magnetization appears in antiferromagnets with spin-orbit coupling and cluster multipolar order, linking symmetry, ground state degeneracy, and transport phenomena.

Contribution

It provides the first general microscopic framework for transverse magnetization in antiferromagnets considering symmetry and ground state properties.

Findings

Transverse magnetization appears only when all symmetries except certain antiunitary ones are broken.

TM occurs when the ground state manifold of the spin Hamiltonian is discrete.

TM can induce an anomalous planar Hall effect, useful for probing magnetic structures.

Abstract

Some antiferromagnets under a magnetic field develop magnetization perpendicular to the field as well as more conventional ones parallel to the field. So far, the transverse magnetization (TM) has been attributed to either spin canting effect or the presence of cluster magnetic multipolar ordering. However, a general theory of TM based on microscopic understanding is still missing. Here, we construct a general microscopic theory of TM in antiferromagnets with cluster magnetic multipolar ordering by considering classical spin Hamiltonians with spin anisotropy that arises from the spin-orbit coupling. First, from general symmetry analysis, we show that TM can appear only when all crystalline symmetries are broken other than the antiunitary mirror, antiunitary two-fold rotation, and inversion symmetries. Moreover, by analyzing spin Hamiltonians, we show that TM always appears when the degenerate ground state manifold of the spin Hamiltonian is discrete. On the other hand, when the degenerate ground state manifold is continuous, TM generally does not appear except when the magnetic field direction and the spin configuration satisfy specific geometric conditions under single-ion anisotropy. Finally, we show that TM can induce anomalous planar Hall Effect, a unique transport phenomenon that can be used to probe multipolar antiferromagnetic structures. We believe that our theory provides a useful guideline for understanding the anomalous magnetic responses of the antiferromagnets with complex magnetic structures.