Low-Temperature Heat Capacity of the Metamagnet Ni${\left(\mathrm{N}{\mathrm{O}}_3\right)}_2$·2${\mathrm{H}}_2$O

The heat capacity $C_p$ of Ni ${\left({\mathrm{No}}_3\right)}_2$·2${\mathrm{H}}_2$O has been measured in the range 1.5 $K\le T\le 10 \mathrm{K}$, revealing a sharp $\lambda$ anomaly characteristic of the transition to a magnetically ordered state at $T_N=4.105\mathrm{}\pm 0.005$ K. The magnetic contribution to $C_p$ has been separated from the lattice part and the temperature variation of the magnetic entropy has been calculated. The magnetic entropy at $T_N$ is found to be only 0.546 $R$, suggesting a two-dimensional character to the ordering which is compatible with the picture of this system as an antiferromagnetically coupled array of alternating sheets of ferromagnetically aligned ${\mathrm{Ni}}^{++}$ spins. This interpretation, however, is complicated by the effect of anisotropy on $C_p$.