Magnetic Susceptibilities of Transition Elements in Host Crystals. II.Ni2+in ZnO and CdS

Abstract

The magnetic susceptibilities of ${\mathrm{Ni}}^{2+}$-doped ZnO and CdS crystals have been measured along and perpendicular to the trigonal crystalline axes over the temperature range of 28-500°K. The magnetic susceptibilities of both crystals approach constant values at low temperature and decrease more rapidly with increasing temperature at $T>\mathrm{}60\mathrm{}°$K. An unusually large magnetic anisotropy is observed for ${\mathrm{Ni}}^{2+}$: ZnO. The non-Curie behavior can be explained on the basis that the ground state of ${\mathrm{Ni}}^{2+}$ is nonmagnetic ($A_1$) and the temperature-dependent susceptibility arises mainly from the ions in the first two excited states ($A_2$ and $E$). By fitting the theoretical susceptibilities to the experimental values, the spin-orbit coupling constant of ${\mathrm{Ni}}^{2+}$: ZnO is obtained as -175±25 ${\mathrm{cm}}^{-1\mathrm{}}$ and the trigonal field splitting of the $T_1\left[{}_{}{}^3{}_{}{}^{}T\mathrm{}\right(F\left)\right]$ state (the lowest $T_1$ state) as 100±10 ${\mathrm{cm}}^{-1\mathrm{}}$. The corresponding quantities for ${\mathrm{Ni}}^{2+}$: CdS are -170±10 ${\mathrm{cm}}^{-1\mathrm{}}$ and 10±4 ${\mathrm{cm}}^{-1\mathrm{}}$. In both crystals the $A_2$ component of the $T_1\left[{}_{}{}^3{}_{}{}^{}T\mathrm{}\right(F\left)\right]$ state lies below the $E$ level. The large reduction of the spin-orbit coupling constant from the free-ion value indicates a rather strong covalency between the ${\mathrm{Ni}}^{2+}$ ion and the ligands. Combination of the trigonal splittings of the $T_1\left[{}_{}{}^3{}_{}{}^{}T_1^{}{}_{}{}^{}\mathrm{}\right(F\left)\right]$ state with those of $T_2\left[{}_{}{}^3{}_{}{}^{}T_1^{}{}_{}{}^{}\mathrm{}\right(P\left)\right]$ observed in the optical spectra leads to a determination of the trigonal field parameters. The experimental values of the trigonal parameters are consistent with those calculated by using the point-charge model and assuming a local contraction of the lattice with a slightly larger contraction for the three anions off the trigonal axis than for the one on the axis.