Magnetism and exchange in the layered antiferromagnet $NiPS_3$

Abstract

The anisotropic susceptibility of the layered antifemomagnet $NiPS_3(T_N = 155 K)$ has been measured between 45 K and 650 K. The system may be described by the effective spin Hamiltonian $H = DS^2_{iz}-\sum_{ij} J_{ij}S_i . S_j$, with the quadratic single-ion anisotropy terms introducing anisotropy in an otherwise isotropic situation. The exchange I and single-ion anisotropy parameter D were determined from an analysis of the anistropic 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 parameter, which are evduated by forcing consistency with the fluctuation-dissipation theorem. It is found that the CEF approximation is superior to the Ognchi model in describing the susceptibility of $NIPS_3$. The exchange and crystal field parameters for the CEF approximation are J / k = -58.0 K; D / k = 16.1 K; g_l_l = 2.05 and $g_\perp =2.13$