A complex magnetic material for use in the Ericsson magnetic refrigerator was investigated. In order to refrigerate effectively in the high temperature range above 10K, it is necessary to utilize the Ericsson cycle that includes two kinds of iso-magnetic field process, because lattice entropy for a magnetic refrigerant amounts to the same order of magnitude as that for magnetic entropy. Therefore in the Ericsson magnetic refrigerant, magnetic entropy change ΔSM is required to be constant in the refrigeration range. This property, however, cannot be satisfied by usual ferromagnetic material. In this paper, the complex RAl2 magnetic mixture (where R denotes heavy rare earth atoms) in grain size scale is prepared by shock compression method. Magnetic entropy changes are determined from the specific heat measurement. This magnetic mixture has high density, which is composed of several RAl2 compounds with different Curie temperatures, where each RAl2 is surrounded by gold. From these experimental results, it is concluded that the complex magnetic mixture with gold, such as (ErAl2)0.3 (HoAl2)0.2(Dy0.5Ho0.5Al2)0.5, shows an almost temperature independent magnetic entropy change ΔSM in some temperature range, which is suitable for the Ericsson cycle, satisfying the Carnot principle.
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