MAGNETISM, EXCHANGE AND CRYSTAL-FIELD PARAMETERS IN THE ORBITALLY UNQUENCHED ISING ANTIFERROMAGNET FEPS(3)

FePS3 is a layered antiferromagnet (T(N) = 123 K) with a marked Ising anisotropy in magnetic properties. The anisotropy arises from the combined effect of the trigonal distortion from octahedral symmetry and spin-orbit coupling on the orbitally degenerate 5T2g ground state of the Fe2+ ion. The anisotropic paramagnetic susceptibilities are interpreted-in terms of the zero field Hamiltonian, K = SIGMA(i)[DELTA(L(iz)2 - 2) + \lambda\L(i) . S(i)] - SIGMA(ij)J(ij)S(i) . S(j). The crystal field trigonal distortion parameter DELTA, the spin-orbit coupling lambda and the isotropic Heisenberg exchange, J(ij), were evaluated from an analysis of the high temperature paramagnetic susceptibility dita using the Correlated Effective Field (CEF) theory for many-body magnetism developed by Lines. Good agreement with experiment were obtained for DELTA/k = 215.5 K; lambda/k = 166.5 K; J(nn)/k = 27-7 K, and J(nnn)/k = - 2.3 K. Using these values of the crystal field and exchange parameters the CEF predicts a T(N) = 122 K for FePS3, which is remarkably close to the observed value of the T(N). The accuracy of the CEF approximation was also ascertained by comparing the calculated susceptibilities in the CEF with the experimental susceptibility for the isotropic Heisenberg layered antiferromagnet MnPS3, for which the high temperature series expansion susceptibility is available.