Best suited protocol for applying magnetic field
in magnetic refrigeration
Development of more environmentally-friendly and highly versatile cooling technology is important issue in our life. The magnetic refrigeration, which is expected to solve the environmental problems in the gas refrigeration and recent energy problems, is receiving attention as a next generation cooling technology. The key of the magnetic refrigeration is the temperature change induced by the magnetic entropy change in magnetic materials when the applying magnetic field is varied, which is the magnetocaloric effect (MCE). Normally, ferromagnets are used in magnetic refrigeration, because ferromagnets exhibit a large magnetic entropy change around the Curie temperature. On the other hand, recently, to obtain larger magnetic entropy change, MCEs in various types of magnets have been investigated. However, microscopic features of MCE in non-ferromagnets have not been well understood.
To present microscopic features of the MCE depending on the magnetic ordered structure, we study the MCEs in the Ising model on a cubic lattice by Monte Carlo simulations. We investigated the temperature dependences of magnetic entropy of the ferromagnets and the antiferromagnets. As a result, we found that in the ferromagnet, the magnetic entropy always decreases when the magnetic field increases. Thus, the suited protocol to obtain the maximum magnetic entropy change is the protocol in which the magnetic field is changed from finite to zero. On the other hand, in the antiferromagnet, there is the case that the magnetic entropy increases as increasing the magnetic field below the Néel temperature. Furthermore, the magnetic entropy reaches the maximum value at a finite magnetic field Hmax(T) rather than zero. From the existence of Hmax(T), we proposed the suited protocol for the antiferromagnet where the magnetic field is changed from finite to Hmax(T) . In addition, our proposed protocol to achieve a large magnetic entropy change can be widely applied to not only antiferromagnets but also non-ferromagnetic materials.
The collaborators in this work are Dr. Hideaki Kitazawa (NIMS) and Dr. Takahisa Ohno (NIMS).
 R. Tamura, T. Ohno, and H. Kitazawa, Appl. Phys. Lett. 104, 052415 (2014).