Non-collinear ground state from a four-spin chiral exchange in a tetrahedral magnet

TL;DR

This paper introduces a new four-spin chiral interaction in cubic ferromagnets that induces a transition from a collinear to a non-collinear ground state, affecting magnon spectra and spin wave behavior.

Contribution

It proposes a novel quartic chiral term in the energy density and demonstrates its role in stabilizing non-collinear magnetic states in tetrahedral magnets.

Findings

Identification of a quartic chiral interaction term.

Transition from collinear to non-collinear magnetic ground state.

Modification of magnon spectra and spin wave propagation.

Abstract

We propose a quartic chiral term $m_x m_y m_z \nabla \cdot \mathbf{m}$ for the energy density of a cubic ferromagnet with broken parity symmetry (point group $T_d$). We demonstrate that this interaction causes a phase transition from a collinear ferromagnetic state to a non-collinear magnetic cone ground state provided its strength exceeds the geometric mean of magnetic exchange and cubic anisotropy. The corresponding non-collinear ground state may also be additionally stabilized by an external magnetic field pointing along certain crystallographic directions. The four-spin chiral exchange does also manifest itself in peculiar magnon spectra and favors spin waves with the wave vector that is perpendicular to the average magnetization direction.