Magnetic and thermal behavior of NiSnCl6·6H2O at low temperatures

NiSn${\mathrm{Cl}}_6$·6${\mathrm{H}}_2$O crystals have a trigonal structure containing one trigonally distorted ${\left[\mathrm{N}\mathrm{i}{\left({\mathrm{H}}_2\mathrm{O}\right)}_6\right]}^{2+}$ complex per unit cell. We have measured the magnetic susceptibilities ${\chi }_{\parallel }$ and ${\chi }_{\perp }$ and the heat capacity $C_p$ of single-crystal specimens of this salt down to ∼ 0.06 K in a ${}_{}{}^3{}_{}{}^{}\mathrm{He}$-${}_{}{}^4{}_{}{}^{}\mathrm{He}$ dilution refrigerator. The low- (zero-) field data are well described by a model in which the ${}_{}{}^3{}_{}{}^{}A_{2g}^{}{}_{}{}^{}$ ground state of ${\left[\mathrm{N}\mathrm{i}{\left({\mathrm{H}}_2\mathrm{O}\right)}_6\right]}^{2+}$ is split into a lower singlet separated from an excited doublet by $\frac{D}{k}=0.576\mathrm{}$ K, and significant antiferromagnetic interionic interactions exist which are too small relative to $D$ to produce long-range spin order. Using both simple mean-field and Oguchi approximations, we have determined the parameters $D$, $g (=2.24)$, and $\frac{\mathrm{zJ}}{k (=-0.044\mathrm{} \mathrm{K})}$. Some anisotropy of the subcritical coupling constant, probably of dipolar origin, is inferred from the susceptibility results. Magnetocaloric effect and heat capacity were measured for single crystals with $\overrightarrow{H}$ parallel to the trigonal axis. Cooling by adiabatic magnetization, followed by level crossing at $H_c^{\prime }=4100\mathrm{}$ Oe, was observed. However, attempts to observe field-induced long-range spin ordering were not successful.