Low-Temperature Magnetic Susceptibilities of the Hydrated Nickel Nitrates

The zero-field magnetic susceptibilities of Ni(NO₃)₂·{}2H₂O, Ni(NO₃)₂·{}4H₂O, and Ni(NO₃)₂·{}6H₂O 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 FeCl₂, or FeCO₃, 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 $S=1$ 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 $g=2.25$, D/k=-6.50^∘K, n₁=+0.32 mole/cgs, n₂=-2.12 mole/cgs, where n₁ and n₂ 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 E/k=-2.66^∘K, D/k=-8.67^∘K, and $g=2.25$.