Calorimetric Investigation of the Antiferromagnetic Transition in NiCl2· 6H2O

Abstract

Measurements of the heat capacity of Ni${\mathrm{Cl}}_2$ · 6${\mathrm{H}}_2$O were made in the vicinity of the antiferromagnetic transition using calorimetric techniques of sufficient resolution so that the observed behavior was intrinsic to the samples studied. Two crystals of different origins were investigated in zero field, and one crystal was investigated with external fields applied parallel to the easy axis. The temperature of the heat-capacity maximum $T_{max}$ differed by approximately 3 mK between the two crystals, but the heat-capacity anomalies of each were best described with the same value of $T_N$, 5.348 K. This value of $T_N$ was 9 and 12 mK above the temperatures at which the maximum heat capacity occurred. The data implied critical exponents of 0.20 ± 0.03 above the transition and 0.00 ± 0.01 below. An analysis of the data in terms of the imaginary temperature described the rounding of the calorimetric anomaly which occurred below $T_{max}$. The analysis yielded best values for the imaginary part of the temperature comparable to the shift between $T_N$ and $T_{max}$. The application of an external field caused a slow monotonic decrease in the amplitude and an increase in the width of the calorimetric anomaly. The width could be described by ${\Delta }^2={{\Delta }_0}^2+aH$, suggesting that the broadening of the transition could be described by two independent mechanisms, one intrinsic to the sample and the second field dependent. The shift of the $T_{max}$ with field was proportional to $H^2$.