PHOTOACOUSTIC INVESTIGATION OF THE TEMPERATURE AND MAGNETIC-FIELD DEPENDENCE OF THE SPECIFIC-HEAT CAPACITY AND THERMAL-CONDUCTIVITY NEAR THE CURIE-POINT OF GADOLINIUM

The photoacoustic technique was used as a simple but accurate tool to simultaneously determine the specific-heat capacity C and thermal conductivity kappa of gadolinium in the neighborhood of the Curie point. One polycrystalline sample and two single crystals of different quality have been investigated as a function of temperature and external magnetic field B. One of the single-crystal samples was the same as the one for which Bednarz et al. [Phys. Rev. B 47, 14 247 (1993)] obtained high-resolution C(T,B=0) data by means of ac calorimetry. Our photoacoustic results are in full agreement with the Bednarz et al. data, thus confirming the complex crossover behavior found by these authors. The photoacoustic results in the presence of low external fields clearly were affected by demagnetization effects resulting in the occurrence of kink temperatures in the C-B(T) curves below the Curie temperature. Information on the temperature dependence of the spontaneous magnetization could be deduced from the experimentally obtained kink temperatures. Scaling properties for the B dependence of the specific-heat capacity have also been investigated. The thermal conductivity kappa(T) for B=0 shows a clear minimum at the Curie point for the best sample. The minimum is diminished with poorer sample quality and in the presence of an external magnetic field.