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.

我们使用弹性和非弹性中子散射（INS）来研究二维（2D）之字形反铁磁体 FePSe ₃中的反铁磁（AF）相变和自旋激发。通过确定 AF 相变的磁序参数，我们得出结论，FePSe ₃中的 AF 相变本质上是一阶的。此外，我们的 INS 测量表明，AF 有序态的自旋波具有较大的易轴磁各向异性间隙，与伊辛哈密顿量一致，并且可能存在双二次磁交换相互作用。当T _N变暖时，我们发现与三倍旋转对称 AF 波动相关的色散自旋激发转变为高于T _{N 的}FM 自旋波动。这些结果表明，FePSe ₃中的一阶AF相变可能是由C ₃对称AF和C ₁对称FM自旋波动围绕T _N之间的竞争产生的，而不是传统的二阶AF相变。