Abstract
The crystal structure and magnetic correlations in triangular antiferromagnet FeGa$_2$S$_4$ are studied by x-ray diffraction, magnetic susceptibility, neutron diffraction and neutron inelastic scattering. We report significant mixing at the cation sites and disentangle magnetic properties dominated by major and minor magnetic sites. The magnetic short-range correlations at 0.77 \AA$^{-1}$ correspond to the major sites and being static at base temperature they evolve into dynamic correlations around 30 - 50 K. The minor sites contribute to the magnetic peak at 0.6 \AA$^{-1}$, which vanishes at 5.5 K. Our analytical studies of triangular lattice models with bilinear and biquadratic terms provide the ratios between exchanges for the proposed ordering vectors. The modelling of the inelastic neutron spectrum within linear spin wave theory results in the set of exchange couplings $J_1=1.7$\,meV, $J_2=0.9$\,meV, $J_3=0.8$\,meV for the bilinear Heisenberg Hamiltonian. However, not all features of the excitation spectrum are explained with this model.
Highlights
The interest in the MT2X4 family of compounds (M = Ni, Fe; T = Ga, Al; X = S, Se) arises due to the quasi-twodimensional (2D) triangular geometry of the magnetic M sublattice
Our extensive experimental study of the triangular antiferromagnet FeGa2S4 implies that this material, and most probably NiGa2S4, has significant mixing of the M (M = Ni, Fe) and Ga cations between the octahedrally coordinated 1(b) and tetrahedrally coordinated 2(d ) positions
20% of the 1(b) sites forming magnetic triangular layers are occupied by nonmagnetic ions and nearly 10% of the 2(d ) sites forming nonmagnetic triangular layers host magnetic M ions
Summary
The interest in the MT2X4 family of compounds (M = Ni, Fe; T = Ga, Al; X = S, Se) arises due to the quasi-twodimensional (2D) triangular geometry of the magnetic M sublattice This family, especially NiGa2S4, are strong candidates as an experimental realization of the triangular lattice antiferromagnet (TLAFM), the extensively studied theoretical model in frustrated magnetism. Our x-ray diffraction study detected significant mixed occupancies (inversion) of the cation sites which we associate with highly nontrivial magnetic behavior of the MGa2S4 series. This feature is one of the important outcomes of our paper. We associate the unconventional magnetic properties with the nonideal structure of the MGa2S4 family
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