Spin reorientation inNi2+compounds

It is shown that spin reorientation in ${\mathrm{Ni}}^{2+}$ ($S=1\mathrm{}$) compounds can be understood on the basis of a simple molecular-field approximation assuming a zero-field splitting $D$ and an anisotropic exchange tensor. The conditions set upon $D$ and the exchange anisotropy which limit the possible occurrence of spin reorientation are derived. The results are applied to Ni${\mathrm{Cl}}_2$ · 2${\mathrm{H}}_2$O and compare favorably with the experimental data.