Magnetothermal properties of sintered Gd3Ga5O12

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Abstract
The feasibility of sintered gadolinium gallium garnet (GGG) as refrigerant for magnetic refrigerators has been investigated by measurement of its specific heat and magnetisation. The heat capacities of sintered samples with different densities were determined between 2 K and 300 K and below 40 K in applied magnetic fields up to B=6 T. The specific heat was found to be within 5% identical to that of monocrystalline GGG; the density of the sintered specimen is only of minor influence upon the heat capacity. The magnetic moments were measured at magnetic fields of 0.1, 1.0, 2.5, and 4.0 T in the temperature range from 4 to 30 K. The authors present a phenomenological model for the splitting of the energy levels in GGG. This model enables them to describe the specific heat of GGG accurately for T>4 K in the various applied magnetic fields, to calculate the temperature and magnetic field dependence of the magnetisation and to explain the magnetic susceptibility. They separated the lattice contribution and the magnetic contribution of the specific heat, and derived the Debye temperature as a function of temperature. Thus, they provide a numerical method to calculate the magnetic entropy. They found that the same entropy-diagram S(T,B) can be used for both sintered and monocrystalline GGG. The thermodynamic standard functions (in zero magnetic field) have been calculated.