Abstract

Inspired by the recent resurgence of interest in frustrated magnets with triangular motifs, we have derived a universal field theory for antiferromagnets with 3 magnetic sublattices. With sublattice magnetizations forming a rigid coplanar structure with 120º angles between them, the order parameter can be thought of as a rigid body resembling the Mercedes–Benz logo. Spin waves can be pictured as extended vibrations of these rigid objects in space. The field theory predicts a Pythagorean relation between the speeds of the three magnon branches. In a triangular-lattice antiferromagnet, where two of the three magnon branches are degenerate, circularly polarized magnons carry nonzero angular momentum and can be used to control the magnetic order. Topological terms allowed in this field theory give rise to helical edge modes with a nonzero spin current and endow domain walls with nonzero magnetization.