Magnetic Properties of CoCl2and NiCl2

Crystal field theory has been used to examine the origin of magnetic anisotropy in Co${\mathrm{Cl}}_2$ and Ni${\mathrm{Cl}}_2$. These salts are antiferromagnets in which the metal ions form ferromagnetic layers with alternate layers oriented in opposite directions. An exchange Hamiltonian is derived for ions in the ground state taking account of a ferromagnetic in-layer interaction $J_1$ and an antiferromagnetic between-layer interaction $J_2$, and it is found that the anisotropy may be approximately represented by including a single extra parameter. The cooperative problem is then treated by molecular field theory at extremes of high and low temperature, and by Green function techniques (using the results of the previous paper) for temperatures near the Néel point. Fitting low-temperature experimental results with the theory, the exchange interactions are calculated showing, in particular, that the ratio $\frac{J_1}{J_2}$ is large in both salts (11.6 for Co${\mathrm{Cl}}_2$, 13.1 for Ni${\mathrm{Cl}}_2$). Using these estimates the high-temperature susceptibility is derived and a good agreement between experiment and theory observed. For Co${\mathrm{Cl}}_2$ the anisotropy is considerable and $g$ values $g_{\mathrm{II}}=3.38\mathrm{}$ and $g_{\perp }=4.84\mathrm{}$ are estimated.