Anisotropic magnetic susceptibility of hexagonal Co-based diluted magnetic semiconductors

We present magnetic-susceptibility data for two new cobalt-based diluted magnetic semiconductor systems with the wurtzite lattice structure: ${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Co}}_{\mathit{x}}$Se and ${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Co}}_{\mathit{x}}$S. At low temperatures, the magnetic susceptibility of both systems is anisotropic. We analyze the low-temperature data within the framework of the crystal-field model, including pair interaction. The analysis yields the value of the zero-field splitting D/${\mathit{k}}_{\mathit{B}}$ of the ${\mathrm{Co}}^{2+}$-ion orbital ground state as 1.0\ifmmode\pm\else\textpm\fi{}0.1 K for the selenides, and as 0.85\ifmmode\pm\else\textpm\fi{}0.1 K for the sulfides. From quantitative analysis of the high-temperature data, we obtain the value of the nearest-neighbor ${\mathrm{Co}}^{2+}$-${\mathrm{Co}}^{2+}$ exchange integral J/${\mathit{k}}_{\mathit{B}}$ for ${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Co}}_{\mathit{x}}$Se to be -37\ifmmode\pm\else\textpm\fi{}5 K.