In the temperature range above ∼20 K, the Ericsson magnetic refrigeration cycle has to be adopted instead of the Carnot cycle, because the lattice entropy becomes no longer negligible as compared with the magnetic entropy. Moreover, in this range since the thermal agitation energy ∼kT of spin also increases, the ferromagnetic materials have to be used for the refrigerant to obtain large entropy change enough to refrigerate. However, temperature variation of the magnetic entropy change of ferromagnetic material is not suitable for the Ericsson cycle. In the present paper we will give the answers to the following questions; (1) what kinds of ferromagnets must be used for the refrigerant? (2) what kind of method to compose the refrigerant using those ferromagnets is promising?.
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