Magnetic Properties of the Antiferromagnet CuCl2·2H2O

We use a Green-function technique to derive expressions for the Néel temperature and the susceptibility above the Néel temperature for the case of an antiferromagnet with an antiferromagnetic nearest-neighbor interaction and a ferromagnetic next-nearest-neighbor interaction. Comparing the results thus obtained with the experimental data on Cu${\mathrm{Cl}}_2$·2${\mathrm{H}}_2$O, we find for the nearest- and next-nearest-neighbor exchange interaction constants, respectively, $\frac{J_1}{k_{\mathrm{B}}}=6.78\mathrm{}°$K and $\frac{J_2}{k_{\mathrm{B}}}=0.89\mathrm{}°$K, corresponding to a ratio $\frac{J_2}{J_1}=0.131\mathrm{}$ which is close to an estimate by Marshall. The value of $J_1$ agrees also reasonably well with the value obtained from low-temperature perpendicular susceptibility measurements. We also give an expression for the critical field necessary for spin-flopping and estimate that this field changes by only about 10% between absolute zero and the Néel temperature.