Magnetization isotherms have been measured for single-crystal Ni${\left(\mathrm{N}{\mathrm{O}}_3\right)}_2$.2${\mathrm{H}}_2$O between 1.4 and 4.2°K with fields up to 16 kOe applied along the $a$ axis. These isotherms exhibit the metamagnetic threshold phenomenon anticipated on the basis of earlier zero-field susceptibility measurements. The locus of the metamagnetic transition points in the ($H, T$) plane forms a smooth phase boundary separating antiferromagnetic and ferro- or paramagnetic regions. The extremities of this boundary occur at $T_N \mathrm{}(H=0\mathrm{})\mathrm{}\approx 4.2°$K and $H_c \mathrm{}(T=0\mathrm{})\mathrm{}\approx 3.4$ kOe. The thermodynamic character of the transition, as indicated by the shape of the isotherms, changes at $T_c\approx 3.85°$K. Below $T_c$, the transitions appear to be of first order, while between $T_c$ and $T_N \mathrm{}\left(0\right)\mathrm{}$ they are of higher order or perhaps $\lambda$-like. It has been possible to reproduce many features of the magnetization curves as well as the ($H, T$) and ($M, T$) phase diagrams of Ni${\left(\mathrm{N}{\mathrm{O}}_3\right)}_2$.2${\mathrm{H}}_2$O with $\mathrm{H}\parallel a$ by means of the same two-sublattice molecular-field model used to describe the zero-field susceptibilities. Spins on a given sublattice lie on alternate sheets of this layered structure. Intrasublattice (intrasheet) spin coupling is ferromagnetic and about 7 times larger in magnitude than the antiferromagnetic intersublattice (interlayer) interaction. Two variants of the model are considered, as are possible improvements to account for the consequences of the inequivalence of the two ${\mathrm{Ni}}^{++}$ ions per unit cell.

• Received 27 October 1969

DOI:https://doi.org/10.1103/PhysRevB.1.2250