Landau Theory of Altermagnetism

TL;DR

This paper develops a Landau theory for altermagnets, explaining their unique spin-split band features and linking theoretical order parameters to observable magnetic phenomena.

Contribution

It introduces a novel Landau framework incorporating spin-space symmetries for altermagnets, extending understanding beyond conventional models.

Findings

Relates order parameters to magnetization and Hall conductivity

Explains features of materials like RuO2, MnTe, CuF2

Provides a basis for understanding exotic magnetic systems

Abstract

We formulate a Landau theory for altermagnets, a class of colinear compensated magnets with spin-split bands. Starting from the non-relativistic limit, this Landau theory goes beyond a conventional analysis by including spin-space symmetries, providing a simple framework for understanding the key features of this family of materials. We find a set of multipolar secondary order parameters connecting existing ideas about the spin symmetries of these systems, their order parameters and the effect of non-zero spin-orbit coupling. We account for several features of canonical altermagnets such as RuO$_2$, MnTe and CuF$_2$ that go beyond symmetry alone, relating the order parameter to key observables such as magnetization, anomalous Hall conductivity and magneto-elastic and magneto-optical probes. Finally, we comment on generalizations of our framework to a wider family of exotic magnetic systems deriving from the zero spin-orbit coupled limit.