Anisotropic exchange and non-collinear antiferromagnets on a noncentrosymmetric fcc structure as in the half-Heuslers

TL;DR

This paper investigates how anisotropic exchange interactions in noncentrosymmetric face-centered cubic antiferromagnets, specifically half-Heusler compounds, influence ground state degeneracy and promote complex non-collinear magnetic orders.

Contribution

It derives a comprehensive exchange model including Kitaev, Gamma, and Dzyaloshinskii-Moriya interactions for half-Heusler compounds and analyzes their effects on magnetic ground states.

Findings

Anisotropic interactions lift the degeneracy of isotropic fcc antiferromagnets.

Certain regimes favor multi-q non-collinear/non-coplanar states.

Quantum fluctuations and magnetic fields can stabilize non-coplanar triple-q states.

Abstract

One of the signatures of the face-centered cubic (fcc) antiferromagnet as a typical example of a geometrically frustrated system is the large ground state degeneracy of the classical nearest neighbor and next-nearest neighbor Heisenberg (isotropic) model on this lattice. In particular, collinear states are degenerate with non-collinear and non-coplanar ones: this degeneracy is accidental and is expected to be lifted by anisotropic exchange interactions. In this work, we derive the most general nearest and next-nearest neighbor exchange model allowed by the space-group symmetry of the noncentrosymmetric half-Heusler compounds, which includes three anisotropic terms: the so-called Kitaev, Gamma and Dzyaloshinskii-Moriya interactions -- most notably, the latter is allowed by the breaking of inversion symmetry in these materials and has not been previously been studied in the context of the fcc lattice. We compute the resulting phase diagram and show how the different terms lift the ground state degeneracy of the isotropic model, and lay emphasis on finding regimes where multi-q (non-collinear/non-coplanar) states are selected by anisotropy. We then discuss the role of quantum fluctuations and the coupling to a magnetic field in the ground state selection, and show that these effects can stabilize non-coplanar (triple-q) states. These results suggest that some half-Heusler antiferromagnets might host rare non-collinear/non-coplanar orders, which may in turn explain the unusual transport properties detected in these semimetals.