Evidence for biquadratic exchange in the quasi-two-dimensional antiferromagnet FePS$_3$

TL;DR

This study reanalyzed neutron spectroscopy data of FePS$_3$, revealing that including biquadratic exchange interactions in the Hamiltonian resolves previous discrepancies and indicates the presence of Dirac magnon nodal lines.

Contribution

The paper introduces the consideration of biquadratic exchange in modeling FePS$_3$, providing a more accurate description of its magnetic excitations and uncovering Dirac magnon nodal lines.

Findings

Biquadratic exchange explains discrepancies at Brillouin zone boundaries.

Unequal exchange between nearest neighbors is attributed to magnetolattice coupling.

Evidence of Dirac magnon nodal lines in FePS$_3$.

Abstract

FePS$_3$ 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 FePS3 were previously analysed using a straight-forward 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 reanalysed allowing for unequal exchange between nominally equivalent nearest-neighbours, 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.