Critical phenomena were studied by Raman scattering in the two-dimensional Ising-type antiferromagnet ${\mathrm{FePS}}_3$. Below ${\mathit{T}}_{\mathit{N}}$=118 K the Brillouin-zone-boundary phonon Raman peaks were observed because of the formation of a magnetic superstructure. Above and below ${\mathit{T}}_{\mathit{N}}$ a low-frequency broad band was observed, and it was interpreted in terms of Raman scattering from transverse acoustic (TA) phonons throughout the Brillouin zone, which was induced by the spin disorder. The spectrum reflects the two-dimensional one-phonon density of states of the TA phonons. The temperature dependence of the Raman intensity shows a maximum at ${\mathit{T}}_{\mathit{N}}$ and agrees with the Ornstein-Zernike form. A quasielastic component due to the magnetic critical scattering was first observed by light scattering. The intensity tends to diverge at ${\mathit{T}}_{\mathit{N}}$. As soon as the spin-disorder-induced Raman spectrum was quenched with decreasing temperature, a one-magnon Raman spectrum began to emerge. The Raman process was discussed using a phenomenological spin-dependent Raman theory.

• Received 19 March 1990

DOI:https://doi.org/10.1103/PhysRevB.42.8382