FePS is a van der Waals compound with a honeycomb lattice that is a good example of a two-dimensional antiferromagnet with Ising-like anisotropy. Neutron spectroscopy data from FePS were previously analyzed using a straightforward Heisenberg Hamiltonian with a single-ion anisotropy. The analysis captured most of the elements of the data; however, some significant discrepancies remained. The discrepancies were most obvious at the Brillouin zone boundaries. The data are subsequently reanalyzed, allowing for unequal exchange between nominally equivalent nearest-neighbors, which resolves the discrepancies. The source of the unequal exchange is attributed to a biquadratic exchange term in the Hamiltonian, which most probably arises from a strong magnetolattice coupling. The new parameters show that there are features consistent with Dirac magnon nodal lines along certain Brillouin zone boundaries.
Evidence for biquadratic exchange in the quasi-two-dimensional antiferromagnet FePS3
A. R. Wildes, M. E. Zhitomirsky, T. Ziman, D. Lançon, H. C. Walker; Evidence for biquadratic exchange in the quasi-two-dimensional antiferromagnet FePS3. J. Appl. Phys. 14 June 2020; 127 (22): 223903. https://doi.org/10.1063/5.0009114
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