Chemical and magnetic ordering in Fe_0.5Ni_0.5 PS ₃

The MPS₃ family of layered magnetic materials (M = Fe²⁺, Ni²⁺, Mn²⁺, etc) shows many unusual properties. We have recently observed time-dependent magnetisation and two magnetic phase transitions in Fe_0.5Ni_0.5PS₃, and here we use neutron diffraction and Mössbauer spectroscopy to explore the magnetic and structural ordering. Neutron diffraction shows that the staggered magnetisation lies closest to the Brillouin curve for J = 1, which is the spin for quenched Ni²⁺. In agreement with neutron diffraction, Mössbauer spectroscopy shows a magnetic ordering temperature of ∼120K. It does not show any evidence of a second, low temperature (re)ordering, suggesting that the low temperature transition seen previously is a result of the time dependence of the magnetisation and is not apparent when the sample is given time to relax between measurements. The presence of three magnetic Fe-site environments when four chemical environments (Fe₃, Fe₂Ni, FeNi₂ and Ni₃) are possible may indicate that the mixture is not random, but shows some local ordering; the neutron results show evidence for a similar conclusion.