Magnetism and exchange in the layered antiferromagnet NiPS3

The anisotropic susceptibility of the layered antiferromagnet NiPS 3 (T N =155 K) has been measured between 45 K and 650 K. The system may be described by the effective spin Hamiltonian H=DS iz 2 - Sigma ij J ij S i .S j , with the quadratic single-ion anisotropy terms introducing anisotropy in an otherwise isotropic situation. The exchange J and single-ion anisotropy parameter D were determined from an analysis of the anisotropic susceptibility data for two different models: (i) the Oguchi model, in which a pair of spins chosen at random is treated exactly while its interactions with the rest of the crystal are approximated by a mean field and (ii) the correlated effective field (CEF) approximation developed by Lines, which reduces the many-body problem to a single-particle, non-interacting ensemble form, by the introduction of static temperature-dependent correlation parameters, which are evaluated by forcing consistency with the fluctuation-dissipation theorem. It is found that the CEF approximation is superior to the Oguchi model in describing the susceptibility of NiPS 3 . The exchange and crystal held parameters for the CEF approximation are J/k=-58.0 K; D/k=16.1 K; g/sub ///=2.05 and g perpendicular to =2.13.