Antiferromagnetic $H-T$ Boundaries and Apparent Molecular Fields for Mn${\mathrm{Cl}}_2$·4${\mathrm{H}}_2$O and Mn${\mathrm{Br}}_2$·4${\mathrm{H}}_2$O

Experimental study and analysis have been made of non-Brillouin magnetic moments of two hydrated manganous salts of similar structure. The moment ($M$) was measured as a function of field at constant temperature ($T$) and as a function of temperature at constant field ($H$). From the former, analysis of the extent of the nonsuperposition of $M\left(\frac{H}{T}\right)$ for different temperatures gave apparent molecular fields of 14 000 gauss and 18 000 gauss, respectively, for manganous chloride tetrahydrate and manganous bromide tetrahydrate. The Van Vleck antiferromagnetic exchange coefficient $\gamma$ is experimentally determined, corresponding to antiferromagnetic exchange energy densities of the order of ${10}^6$ ergs/${\mathrm{cm}}^3$. The constant field measurements gave points on antiferromagnetic transition curves such that $\frac{H_m}{T_N}$ was about 8000 gauss per degree for each salt. If the constant temperature moment is plotted against the Van Vleck parameter $\frac{(H-\gamma M)}{T}$ instead of the usual Brillouin parameter $\frac{H}{T}$ there is superposition of isotherms for points in the $H-T$ plane outside the antiferromagnetic region, thus giving experimental support to the Van Vleck model for simple antiferromagnetics. The method of measurement is the same as for our previous measurements. The temperature range was 1.3°-4.2°K and magnetic fields up to 58 000 gauss were applied, the greater part of the range being such as to prevent antiferromagnetic ordering.