Magnetism in MnPSe₃: a layered 3d⁵antiferromagnet with unusually largeXYanisotropy

The anisotropic magnetic susceptibilities of single crystals of the isostructural layered antiferromagnets, MnPS 3 ( T N = 78 K) and MnPSe 3 ( T N = 74 K), have been measured as functions of temperature. In both compounds, divalent manganese is present in the high-spin S = 5/2 state. The anisotropies in the susceptibilities of the two are, however, very different; while the susceptibility of MnPS 3 is isotropic, that of MnPSe 3 shows a large XY anisotropy, unusual for a manganese compound. The anisotropic susceptibilities are described by the zero-field spin Hamiltonian: = DS iz 2 − ∑ J ij S i · S j with the quadratic single-ion anisotropy term introducing anisotropy in an otherwise isotropic situation. The exchange J and the single-ion zero-field-splitting (ZFS) parameter D were evaluated using the correlated effective-field theory of Lines. For MnPSe 3 , J / k = −5.29 K and D / k = 26.6 K, while for isotropic MnPS 3 , J / k = −8.1 K. It is suggested that the large value of the ZFS parameter for MnPSe 3 as compared to MnPS 3 could be due to the large ligand spin–orbit contribution of the heavier selenium.