Thermal evolution of spin excitations in honeycomb Ising antiferromagnetic FePSe₃

We use elastic and inelastic neutron scattering (INS) to study the antiferromagnetic (AF) phase transitions and spin excitations in the two-dimensional (2D) zig-zag antiferromagnet FePSe₃. By determining the magnetic order parameter across the AF phase transition, we conclude that the AF phase transition in FePSe₃ is first-order in nature. In addition, our INS measurements reveal that the spin waves in the AF ordered state have a large easy-axis magnetic anisotropy gap, consistent with an Ising Hamiltonian, and possible biquadratic magnetic exchange interactions. On warming across T_N, we find that dispersive spin excitations associated with three-fold rotational symmetric AF fluctuations change into FM spin fluctuations above T_N. These results suggest that the first-order AF phase transition in FePSe₃ may arise from the competition between C₃ symmetric AF and C₁ symmetric FM spin fluctuations around T_N, in place of a conventional second-order AF phase transition.