**Best suited protocol for applying magnetic field **

**in**** magnetic refrigeration**

Ryo Tamura

ICYS-SENGEN

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 *H*_{max}(*T*)
rather than zero. From the existence of *H*_{max}(*T*),
we proposed the suited protocol for the antiferromagnet where the magnetic
field is changed from finite to *H*_{max}(*T*) [1]. 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).

Reference:

[1] R. Tamura, T.
Ohno, and H. Kitazawa, Appl. Phys. Lett. **104**, 052415 (2014).