SPECIFIC-HEAT OF CD1-XCOXS AND CD1-XCOXSE AT LOW-TEMPERATURES

The specific heat of two diluted magnetic semiconductors, Cd(1-x)Co(x)S and Cd1(-x)Co(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 congruent-to 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 Co2+ ion. From quantitative analysis we have obtained D/k(B) = 0.97 +/- 0.03 K for Cd(1-x)Co(x)S, and D/k(B) = 0.72 +/- 0.03 K for Cd(1-x)Co(x)Se. For larger concentrations x of Co2+ 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 J(i)(R(i))approximately(R(i))-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.