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Recent progress in magnetic refrigeration has been reviewed. Adiabatic demagnetization refrigerators (ADR) are under development at NASA for large deployable reflectors and X-ray spectrometers. The refrigerators provide temperatures of 2K at 0.027W for 165s and 0.1K at 0.01W for 13h using gadolinium gallium garnet and paramagnetic salt crystals. The Gas-Gap heat switch is used for the isothermal portion of a Carnot cycle. Active magnetic regenerator (AMR) cycles have the advantage of the Carnot cycle for larger temperature span and refrigeration capacity, in particular, above 20K. A test apparatus of the AMR demonstrated that the refrigeration capacity of 1.5-3.5W was obtainable at the temperature spans of 13-25K with a Carnot efficiency of 42%. Practical applications of magnetic refrigeration are also described.

Thermal conductivity of Dy3Ga5O12 garnet single crystal has been measured between 2 and 20K with various magnetic fields between 0 and 5T. We have found a large magnetic field dependence of the thermal conductivity (λ); λ decreases with the magnetic field from 0 to 1T and then increases slightly above 1.5T. The maximum reduction in λ is found at 3.5K and 1.5T. The value is -70 times smaller than that at 3.5K and 0T. The experimental result shows that Dy3Ga5O12 single crystal has the thermal switch function controlled by the magnetic field between 0 and 1.5T in the temperature between 2 and 10K. The large magnetic field dependence of λ can be explained by the resonant type phonon scattering corresponding to the phonon-induced transition between the energy levels of rate earth ions which are modified by the magnetic field.

The Carnot type magnetic refrigeration in the temperature range from 4.2K to 15K has been studied. This refrigeration system mainly consists of the following components; the magnetic working material, the thermal switches at 4.2K and 15K, and the superconducting pulsed magnet. Gd3Ga5O12 (GGG) single crystal was used as the working material. The entropy diagram and thermal conductivity were obtained. It was concluded that GGG provides large refrigeration capacity (1-3W/mol) in the Carnot cycle. A heat pipe was used as the thermal switch at 4.2K. Thermal conductivity of the heat pipe at 4.2K was as large as that of OFHC copper and switch ratio between 4.2K and 15K was over 103. Some testpieces consist of GGG and the heat pipe were experimented. The difficulty concerning with the thermal resistance between non-metallic material (GGG) and metal (heat pipe) wat solved out by using the specially deviced heat pipe where GGG liquefies directly gaseous helium. The maximum liquefaction efficiency was -70%.Based on these preriminary studies, the Carnot type magnetic refrigerator was constructed. The average liquefaction efficiency for the one-shot demagnetion process was -40%. For continuous operation at 3.1 Tesla between 4.2K and 10K, the refrigeration capacity was 0.34W and the Carnot efficiency was 17%. This efficiency was not taken account of the loss from the superconducting pulsed magnet. This trial magnetic refrigeration system gave the possibility for realizing the small refrigerator with high efficiency at 4.2K.