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. [ABSTRACT FROM AUTHOR]