Thermally-induced mimicry of quantum cluster excitations and implications for the magnetic transition in FePX₃

Magnetic order in two dimensions typically relies on strong magnetic anisotropy, which protects magnetic order from both quantum and thermal fluctuations. In the quasi-two-dimensional family of antiferromagnets FePX₃, an orbital degeneracy gives rise to strong easy-axis anisotropy and magnetic order is preserved up to ~100 K [1].

Close to the magnetic ordering temperature, broad cluster-like excitations have been observed in neutron scattering measurements, raising interesting questions about the nature of the magnetic phase transition and the possible presence of quantum cluster excitations in this family of compounds [2].

In this talk, we offer a new perspective on the nature of the magnetic excitations in FePX₃. Using stochastic Landau-Lifshitz calculations to capture the finite-temperature dynamics, we reproduce the observed neutron scattering spectrum. Through these calculations, we argue that the cluster-like patterns have a classical origin but nonetheless offer important insights into the nature of the phase transition in FePX₃ [3].

[1] G. Le Flem et al, Magnetic interactions in the layer compounds MPX₃ (M = Mn, Fe, Ni; X = S, Se), J. Phys. Chem. Solids 43, 455 (1982).

[2] L. Chen et al, Thermal evolution of spin excitations in honeycomb Ising antiferromagnetic FePSe₃, Npj Quantum Mater. 9, 1 (2024).

[3] H. Lane and M. Mourigal, arXiv:2405.17308 (2024).