Thermal Behavior of the Antiferromagnet MnBr2·4H2O in Applied Magnetic Fields

The heat capacity $C_p$ of a single-crystal specimen of Mn${\mathrm{Br}}_2$·4${\mathrm{H}}_2$O has been measured between 1.4 and 8°K with magnetic fields from 0 to 15 kOe applied along the $c^{\prime }$ axis. This axis, which is orthogonal to the $a$ and $b$ axes of this monoclinic substance, is thought to be close to the direction of preferred spin orientation in the antiferromagnetic state [$T_N\mathrm{}(H=0\mathrm{})=2.13\mathrm{}°$K]. Temperature changes associated with adiabatic magnetization (magnetocaloric effect) have also been observed. For $H>~0$, $C_p$ exhibits at $T_N\mathrm{}\left(H\right)\mathrm{}$ a $\lambda$ anomaly whose maximum shifts to lower temperature with increasing $H$, tracing out in the $H-T$ plane a portion of the boundary between antiferromagnetic and paramagnetic phases. Isentropic lines originating within the antiferromagnetic region of this diagram show cooling with initial increase of $H$, appear to intersect the phase boundary at their inflection points with common tangent, and exhibit temperature minima in the paramagnetic region. The phase boundary is nearly parabolic, with the same curvature as is deduced thermodynamically from zero-field susceptibility and heat-capacity data.