Specific heat ofCd1−xCoxS andCd1−xCoxSe at low temperatures

The specific heat of two diluted magnetic semiconductors, ${\mathrm{Cd}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Co}}_{\mathit{x}}$S and ${\mathrm{Cd}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Co}}_{\mathit{x}}$Se, both having the wurtzite structure, has been measured in the temperature range 0.4–4.0 K, and in various magnetic fields up to 2.8 T. In the dilute limit (x≃0.001), both materials exhibit broad maxima in the specific heat resulting from the zero-field splitting, 2D, of the ground orbital state of the ${\mathrm{Co}}^{2+}$ ion. From quantitative analysis we have obtained D/${\mathit{k}}_{\mathit{B}}$=0.97±0.03 K for ${\mathrm{Cd}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Co}}_{\mathit{x}}$S, and D/${\mathit{k}}_{\mathit{B}}$=0.72±0.03 K for ${\mathrm{Cd}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Co}}_{\mathit{x}}$Se. For larger concentrations x of ${\mathrm{Co}}^{2+}$ ions (x=0.0077), exchange interactions have to be included in the theoretical model. We have used the nearest-neighbor pair-correlation approximation (NNPA), with the distance dependence of the exchange integral given by ${\mathit{J}}_{\mathit{i}}$(${\mathit{R}}_{\mathit{i}}$)∼(${\mathit{R}}_{\mathit{i}}$ ${)}^{\mathrm{-}\mathit{n}}$. This approximation describes the specific-heat data quite well in the present case. The best fit of the NNPA curve has been achieved for n=6.0±1.0, which is very close to that for Mn-based diluted magnetic semiconductors.