Hirofumi Wada

Bio: Hirofumi Wada is an academic researcher from Kyushu University. The author has contributed to research in topics: Magnetization & Valence (chemistry). The author has an hindex of 29, co-authored 193 publications receiving 3935 citations. Previous affiliations of Hirofumi Wada include Sanden Corporation & University of Tokyo.

Giant magnetocaloric effect of MnAs1−xSbx

Abstract: A giant magnetocaloric effect was found in MnAs, which undergoes a first-order ferromagnetic to paramagnetic transition at 318 K. The magnetic entropy change caused by a magnetic field of 5 T is as large as 30 J/K kg at the maximum value, which exceeds that of conventional magnetic refrigerant materials by a factor of 2–4. The adiabatic temperature change reaches 13 K in a field change of 5 T. The substitution of 10% Sb for As reduces the thermal hysteresis and lowers the Curie temperature to 280 K, while the giant magnetocaloric properties are retained.

Negative magnetocaloric effect at the antiferromagnetic to ferromagnetic transition of Mn3GaC

Abstract: The magnetocaloric effect of Mn3GaC, which shows an antiferromagnetic to ferromagnetic transition at 165 K has been investigated. In this compound, magnetocaloric effect obtained at the transition is opposite to that of ordinary ferromagnetic systems, namely, negative magnetocaloric effect. It was found that a large magnetic entropy change, ΔSmag, of 15 J/kg K is obtained under an applied field of 2 T. The adiabatic temperature change, ΔTad, reaches 5.4 K in a field change of 2 T. At higher magnetic fields, both ΔSmag and ΔTad retain a large value over wide temperature range, exhibiting characteristic temperature dependence of a trapezoidal shape. These features are attributed to a sharp first-order transition retained in high magnetic fields as well as small magnetocrystalline anisotropy.

Giant magnetocaloric effect of MnAs1-xSbx in the vicinity of first-order magnetic transition

Abstract: The magnetocaloric effect (MCE) of Mn AS 1− x Sb x has been examined for 0⩽ x ⩽0.3. The magnetic entropy change caused by a magnetic field of Mn AS 1− x Sb x reaches 25–30 J/K kg, which is about twice as large as those of conventional materials near room temperature. The origin of the large MCE of Mn AS 1− x Sb x is unusual magnetic behavior in the vicinity of the first-order magnetic transition.

Extremely Large Magnetic Entropy Change of MnAs1-xSbx near Room Temperature.

Abstract: Magnetization of MnAs 1-x Sb x was measured as functions of temperature and magnetic field for 0 < x ≤ 0.4. The entropy change caused by a magnetic field, ΔS mag , was estimated on the basis of the Maxwell relation. The ΔS mag for 0 ≤ x ≤ 0.3 in a field change of 5 T reaches 25-30 J/K kg, which exceeds that of other materials by a factor of 2-4. The substitution of Sb for As can tune the Curie temperature between 230 K and 315 K without any significant reduction of ΔS mag , The large ΔS mag originates in a paramagnetic to ferromagnetic transition induced by a magnetic field. These results indicate that MnAs 1-x Sb x is a promising material for a working substance in magnetic refrigeration near room temperature.

Recent developments in magnetocaloric materials

Abstract: The recent literature concerning the magnetocaloric effect (MCE) has been reviewed. The MCE properties have been compiled and correlations have been made comparing the behaviours of the different families of magnetic materials which exhibit large or unusual MCE values. These families include: the lanthanide (R) Laves phases (RM2, where M = Al, Co and Ni), Gd5(Si1−xGex)4 ,M n(As1−xSbx), MnFe(P1−xAsx), La(Fe13−xSix) and their hydrides and the manganites (R1−xMxMnO3, where R = lanthanide and M = Ca, Sr and Ba). The potential for use of these materials in magnetic refrigeration is discussed, including a comparison with Gd as a near room temperature active magnetic regenerator material. (Some figures in this article are in colour only in the electronic version)

Ferromagnetic materials

Abstract: In this chapter, we will restrict our attention to the ferrites and a few other closely related materials. The great interest in ferrites stems from their unique combination of a spontaneous magnetization and a high electrical resistivity. The observed magnetization results from the difference in the magnetizations of two non-equivalent sub-lattices of the magnetic ions in the crystal structure. Materials of this type should strictly be designated as “ferrimagnetic” and in some respects are more closely related to anti-ferromagnetic substances than they are to ferromagnetics in which the magnetization results from the parallel alignment of all the magnetic moments present. We shall not adhere to this special nomenclature except to emphasize effects, which are due to the existence of the sub-lattices.

Magnetic materials and devices for the 21st century: Stronger, lighter, and more energy efficient

Abstract: A new energy paradigm, consisting of greater reliance on renewable energy sources and increased concern for energy effi ciency in the total energy lifecycle, has accelerated research into energy-related technologies. Due to their ubiquity, magnetic materials play an important role in improving the effi ciency and performance of devices in electric power generation, conditioning, conversion, transportation, and other energy-use sectors of the economy. This review focuses on the state-of-the-art hard and soft magnets and magnetocaloric materials, with an emphasis on their optimization for energy applications. Specifi cally, the impact of hard magnets on electric motor and transportation technologies, of soft magnetic materials on electricity generation and conversion technologies, and of magnetocaloric materials for refrigeration technologies, are discussed. The synthesis, characterization, and property evaluation of the materials, with an emphasis on structure‐property relationships, are discussed in the context of their respective markets, as well as their potential impact on energy effi ciency. Finally, considering future bottlenecks in raw materials, options for the recycling of rare-earth intermetallics for hard magnets will be discussed.

Magnetic-field-induced shape recovery by reverse phase transformation.

Abstract: Large magnetic-field-induced strains1 have been observed in Heusler alloys with a body-centred cubic ordered structure and have been explained by the rearrangement of martensite structural variants due to an external magnetic field1,2,3. These materials have attracted considerable attention as potential magnetic actuator materials. Here we report the magnetic-field-induced shape recovery of a compressively deformed NiCoMnIn alloy. Stresses of over 100 MPa are generated in the material on the application of a magnetic field of 70 kOe; such stress levels are approximately 50 times larger than that generated in a previous ferromagnetic shape-memory alloy4. We observed 3 per cent deformation and almost full recovery of the original shape of the alloy. We attribute this deformation behaviour to a reverse transformation from the antiferromagnetic (or paramagnetic) martensitic to the ferromagnetic parent phase at 298 K in the Ni45Co5Mn36.7In13.3 single crystal.