Nature of the Antiferromagnetic-Paramagnetic Transition in MnCl2·4H2O

The heat capacity and magnetization at constant field versus temperature, as well as the adiabatic variation of temperature with magnetic field, have been determined in fields directed along the $c$ axis of a large spherical single crystal of Mn${\mathrm{Cl}}_2$·4${\mathrm{H}}_2$O to virtually the limits of resolution of the dc methods used. In this study of the behavior of these quantities in the neighborhood of the antiferromagnetic-paramagnetic transition, one specific goal was to observe the (near) singularity in ${\left(\frac{\partial M}{\partial T}\right)}_H$ at $T_N\mathrm{}\left(H\right)\mathrm{}$. In addition, we sought to test the predictions that isentropes cross the phase boundary (defined as the locus of maxima in $C_H$) tangentially, and that this crossing point should prove to be the point of inflection of the isentropes provided $C_H$ does not diverge too strongly. A test for determining the existence of a divergence without the necessity of measuring infinitely high values is outlined. The fact that the maximum in the zero-field adiabatic susceptibility occurs at a temperature $T_{max}>T_N\mathrm{}\left(0\right)\mathrm{}$ has been found to be reflected in the persistence of a minimum in plots of the isentropic variation of $T$ versus $H$ up to $T=T_{max}$. This curious behavior has led us to speculate on a larger coexistence region of somewhat different character than has heretofore seemed reasonable.