MAGNETISM AND EXCHANGE IN THE LAYERED ANTIFERROMAGNETIC NIPS3

The anisotropic susceptibility of the layered antiferromagnet NiPS3(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 NiPS3. The exchange and crystal field parameters for the CEF approximation are J/k = -58.0 K; D/k = 16.1 K; g(parallel-to = 2.05 and g(perpendicular-to) = 2.13.