Anomalous Behavior of the Low-Temperature Magnetic Specific Heat of NiCl2· 2H2O

The specific heat of Ni${\mathrm{Cl}}_2$ · 2${\mathrm{H}}_2$O has been measured for 1.2 $K<T<\mathrm{}24.\mathrm{}5$ K. Two peaks are observed, at $T_{1c}=6.\mathrm{}309$ K and at $T_{2c}=7.\mathrm{}258$ K. The former can be described by a power law $C_{\mathrm{mag}}\propto {|T-T_{1c}|}^{-0.3}$. The $T_{2c}$ peak is asymmetric and displays power-law behavior with smaller exponents. Below ∼4K, $C_p\propto T$, but the line does not extrapolate to zero at $T=0\mathrm{}$. The conventional explanations for some previously observed double transitions in other substances do not seem to be applicable to Ni${\mathrm{Cl}}_2$ · 2${\mathrm{H}}_2$O, thus raising intriguing questions as to the origin of the double $C_p$ peaks. Two possible explanations are sketched.