Magnetic entropy change of magnetic refrigerants with first order phase transition suitable for hydrogen refrigeration

Magnetic material with first order phase transition has been studied from the view point of magnetic refrigerant. Gd₅(Si_xGe_1-x)₄ has field induced metamagnetic transition, whose temperature can be varied by changing Si concentration. It was shown that suitable transition temperature for hydrogen magnetic refrigeration can be obtained. Magnetization was measured as functions of temperature (M-T) and magnetic field (M-H). Magnetic entropy change has been calculated from the magnetization of M-T and M-H using Maxwell relations. This compound showed the hysteresis in M-H so that we obtained magnetic entropy changes in both the magnetization and demagnetization process. The magnitude of entropy change agreed with each other but the temperature range of large entropy change had a small difference. Entropy changes of this compound obtained from M-T and M-H were in reasonable agreement. The entropy change of the compound with x <0.25 is qualitatively different from that with x >0.25.