Anisotropic exchange and noncollinear antiferromagnets on a noncentrosymmetric fcc half-Heusler structure

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 noncollinear and noncoplanar 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, show how the different terms lift the ground state degeneracy of the isotropic model, and lay emphasis on finding regimes where multi-$q$ (noncollinear/noncoplanar) 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 noncoplanar (triple-$q$) states. These results suggest that some half-Heusler antiferromagnets might host rare noncollinear/noncoplanar orders, which may, in turn, explain the unusual transport properties detected in these semimetals.