Thermally-induced mimicry of quantum cluster excitations and implications for the magnetic transition in FePSe$_3$

TL;DR

This paper uses classical spin dynamics simulations to explain magnetic excitations in FePSe$_3$, suggesting they originate from thermal fluctuations overcoming anisotropy, which impacts understanding of magnetic transitions in 2D magnets.

Contribution

It provides a classical nonlinear spin dynamics interpretation of neutron scattering data, challenging the notion of entangled cluster excitations in FePSe$_3$.

Findings

Simulations reproduce neutron scattering measurements accurately.

Magnetic excitations are explained as classical thermal fluctuations.

The magnetic transition is driven by thermal energy overcoming anisotropy.

Abstract

In two dimensional magnets, the interplay of thermal fluctuations and spin anisotropy control the existence of long-range magnetic order. In the van der Waals antiferromagnets FePX$_3$, orbital degeneracy in the $t2g$ levels of the Fe$^{2+}$ ions in octahedral coordination yields strong uniaxial anisotropy, which stabilizes magnetic order up to T$\approx$100 K. Recent inelastic neutron scattering measurements around the magnetic ordering transition have shown the existence of a broad spectrum of magnetic fluctuations with nontrivial momentum dependence, which has been interpreted as evidence for localized entangled cluster excitations. In this paper, we offer an alternative interpretation using classical nonlinear spin dynamics simulations. We present stochastic Landau Lifshitz dynamics simulations that reproduce the neutron scattering measurements of Chen et al. [npj Quantum Mater. 9, 40 (2024)] on FePSe$_3$. These calculations faithfully explain the dynamical structure factor's momentum and energy dependence and point to a classical origin for the excitations observed in neutron spectroscopy and that the order-disorder transition can be understood in terms of thermal fluctuations overcoming the anisotropy energy.