Specific heat of Ni(NO3)26NH3between 0.4 and 20 K

The specific heat of Ni${\left(\mathrm{N}{\mathrm{O}}_3\right)}_2$6N${\mathrm{H}}_3$ has been measured between 0.4 and 20 K in a mechanical heat switch calorimeter. A sharp $\lambda$-like anomaly occurs at the Néel temperature $T_N=1.345\mathrm{}$ K. The experimental entropy of the transition was determined to be $\Delta S=9.125\mathrm{}$ J/K mol, in quite good agreement with the theoretical value $R\ln (2\mathrm{}S+1)=9.137\mathrm{}$ J/K mol for $S=1\mathrm{}$. Since EPR measurements indicate that the anisotropy in these crystals is not large, a Heisenberg three-dimensional Hamiltonian has been used for modeling the magnetic behavior. The mean-field approximation indicates that the antiferromagnetic ordering is of the second kind in an fcc lattice. A best fit of the experimental data to a high-temperature expansion yields values for the exchange parameters $J_1=-0.125\mathrm{}$ K and $J_2=-0.38\mathrm{}$K which are consistent with this kind of order.