Low-Temperature Magnetic Susceptibilities of the Hydrated Nickel Nitrates

The zero-field magnetic susceptibilities of Ni${\left(\mathrm{N}{\mathrm{O}}_3\right)}_2$·2${\mathrm{H}}_2$O, Ni${\left(\mathrm{N}{\mathrm{O}}_3\right)}_2$·4${\mathrm{H}}_2$O, and Ni${\left(\mathrm{N}{\mathrm{O}}_3\right)}_2$·6${\mathrm{H}}_2$O have been measured in the liquid-hydrogen and liquid-helium ranges. The dihydrate is obtained by evaporation of a solution at 105°C. Its powder susceptibility has a large, sharp, peak at 4.20°K, where it reaches 0.74 cgs/mole, then drops down to 0.2 cgs/mole below 2°K. When measured along the $a$ axis, the susceptibility of monoclinic single crystals of the dihydrate shows a similar peak. It reaches 1.5 cgs/mole, but drops to vanishing values at lower temperatures. The susceptibility in the bc plane reaches only 0.3 cgs/mole, and is nearly isotropic. It drops little below 4.20°K. This behavior is similar to that of Fe${\mathrm{Cl}}_2$, or FeC${\mathrm{O}}_3$, and suggests the existence of two magnetic sublattices, with strong ferromagnetic interactions within each sublattice, and weaker antiferromagnetic interactions between one sublattice and the other (metamagnetism). A spin Hamiltonian with $S=1\mathrm{}$ and uniaxial one-ion anisotropy gives results in fair agreement with the experimental data if the exchange interactions are described in the molecular-field approximation. The best fit corresponds to $g=2.25\mathrm{}$, $\frac{D}{k}=-6.50\mathrm{}°$K, $n_1=+0.32\mathrm{}$ mole/cgs, $n_2=-2.12\mathrm{}$ mole/cgs, where $n_1$ and $n_2$ are, respectively, the antiferromagnetic and the ferromagnetic molecular-field constants. In the case of the tetrahydrate and of the hexahydrate, the powder susceptibility approaches a constant value of 0.35 cgs/mole below 2°K; the data can be fitted to the spin Hamiltonian for a nickel ion in a rhombic field, without exchange, with $\frac{E}{k}=-2.66\mathrm{}°$K, $\frac{D}{k}=-8.67\mathrm{}°$K, and $g=2.25\mathrm{}$.