Recently, investigation on magnetic refrigerators, especially in the temperature range below-15K has been considerably progressed. By the noticeable works on the magnetic refrigerators for producing superfluid He and for liquefing He gas, it is verified that those refrigerators have very high efficiency. In this review, the present stage of the investigation and development on the magnetic refrigerators will be briefly introduced and the prospect of those efforts will be discussed. First, the principle of the magnetic refrigerators and important problems necessary for development of those are shown. And then, the several examples of the magnetic refrigerators are shown and given brief consideration in connection with the problems discussed first. Finally, the current of the present investigations on the magnetic refrigeration is introduced briefly.
An important problem in developing magnetic refrigerator is selection of the magnetic refrigerants. The main purposes of this paper are to discuss the physical properties necessary for the magnetic refrigerant, such as the magnetic, thermal and magnetocaloric characters, and to show a brief review on our recent investigation. Magnetic refrigerators can be roughly divided into two groups; one for the Carnot type magnetic refrigerator below 20K and the other for the Ericsson type refrigerator. Therefore, in the present paper, the physical properties requisite for those two kinds of refrigerants are separately discussed. Then, the recent several results are shown. Finally a brief prospect of the magnetic refrigerant is given.
To study magnetic refrigerants useful at low temperatures below 20K, new pseudo-binary garnet single crystals of Gd3(Ga1-xAlx)5O12 with x=0.1, 0.2, 0.3 and 0.4, having about 25mm in diameter and 50mm in length, have been grown using the Czochralski technique. The effective magnetic moments p of the crystals are about 7.9μB, independent of x, and the paramagnetic Curie temperatures θp are-0.60 to -1.11K, showing the minimum at x=0.2. Based on magnetic measurements and adiabatic demagnetization experiments in magnetic fields up to 75 and 60kOe, respectively, the magnetic entropy change ΔSM and the entropy S are estimated to be almost the same for all the crystals.