A. Fujita

Large magnetocaloric effect in La(Fexsi1-x)13 itinerant-electron metamagnetic compounds

The magnetocaloric effect (MCE) originated from the itinerant-electron metamagnetic transition for La(FexSi1−x)13 compounds has been investigated. With increasing Fe concentration, the MCE is enhanced and both the isothermal magnetic entropy change ΔSm and the adiabatic temperature change ΔTad for the compound with x=0.90 are −28 J/kg K and 8.1 K, respectively, by changing the magnetic field from 0 to 2 T. Similar large MCE values are achieved around room temperature by controlling the Curie temperature by means of hydrogen absorption. Consequently, La(FexSi1−x)13 compounds are promising as magnetic refrigerant materials working in relatively low magnetic fields.

Magnetic and martensitic phase transitions in ferromagnetic Ni–Ga–Fe shape memory alloys

Ferromagnetic shape memory alloys with a body-centered-cubic ordered structure in a Ni–Ga–Fe system have been developed. The alloys with the composition range of Ni 27 at. % Ga (20–22 at. %)Fe exhibit a thermoelastic martensitic transformation from a B2 and/or an L21 parent to a martensite phase, with a seven-layer modulated (14M) and a five-layer modulated (10M) structure, in the ferromagnetic state. The parent phase transforms from the B2 to the L21 structure at about 970 K during cooling, and the degree of the L21 order in the parent phase is increased by annealing at 773 K, resulting in the increase of both the martensite starting and the Curie temperatures. The ductility of these alloys is improved by introducing of a small amount of a γ-phase solid solution. Consequently, we can conclude that the present alloys are promising for ferromagnetic shape memory alloys.

Magnetocrystalline anisotropy in single-crystal Co–Ni–Al ferromagnetic shape-memory alloy

The magnetocrystalline anisotropy in a single-crystal Co37Ni34Al29 ferromagnetic shape-memory alloy has been investigated. The prestrain was applied to the parent phase in order to nucleate the specific variant in the sample cooled down through the martensitic transformation temperature. The applied magnetic field facilitates the growth of variants parallel to the applied magnetic field in analogy with the prestrain. From these results of selective nucleation of variants, the magnetocrystalline anisotropy energy in the single crystal Co37Ni34Al29 β′ martensite phase is estimated to be 3.9×106 erg/cm3. In the single crystal, the observed magnitude of the reversible magnetic-field-induced strains is 0.06%.

Phase Separation and Magnetic Properties of Half-metal-type Co2Cr1-xFexAl Alloys

The phase stability and magnetic properties of the half-metal-type Co2Cr1−xFexAl alloy system were investigated. It was found that the occurrence of two-phase separation is unavoidable in a concentration range of less than x=0.4, leading to deviation of the saturation magnetic moments from the generalized Slater–Pauling line. The L21-type phase becomes stable in a concentration range of more than x=0.7, where no half-metallic behaviors are present. Consequently, it is concluded that the most favorable concentration for applications to spintronic devices is located around x=0.4 in Co2Cr1−xFexAl alloys having the B2-type phase.

Thermal transport properties of magnetic refrigerants La(FexSi1−x)13 and their hydrides, and Gd5Si2Ge2 and MnAs

La(FexSi1−x)13 and their hydrides exhibit large magnetocaloric effects due to the itinerant-electron metamagnetic transitions in a wide temperature range covering room temperature. Thermal conductivity and diffusivity of La(Fe0.88Si0.12)13 and La(Fe0.88Si0.12)13H1.0 have been investigated, together with those of other candidates for magnetic refrigerants working in the vicinity of room temperature such as Gd, Gd5Si2Ge2 and MnAs. The thermal conductivity in the vicinity of room temperature for La(Fe0.88Si0.12)13H1.0 is larger than that for Gd5Si2Ge2 and MnAs, and almost identical to that for Gd. Furthermore, the thermal diffusivity in the vicinity of room temperature for La(Fe0.88Si0.12)13H1.0 is as large as that for Gd and Gd5Si2Ge2, and larger than that for MnAs. Consequently, La(FexSi1−x)13 and their hydrides are promising as the magnetic refrigerants from the standpoint of thermal transport properties.

Magnetic properties and band structures of half-metal-type Co2CrGa Heusler alloy

The saturation magnetic moment Ms and the Curie temperature TC of the half-metal-type Co2CrGa Heusler alloy have been investigated. The value of Ms at 4.2K for the L21-type Co2CrGa alloy is 3.01μB∕f.u., in agreement with the generalized Slater-Pauling line and the theoretical calculation. The Curie temperature TC and the phase-transformation temperature Tt from the L21 to B2-type structure are 495 and 1050K, respectively. The band calculations disclose that the spin polarization is also relatively high in the B2-type structure, although its estimated TC is about 100K lower than that of the L21-type structure.

Large magnetocaloric effects in NaZn13-type La(FexSi1−x)13 compounds and their hydrides composed of icosahedral clusters

Abstract The magnetocaloric effects (MCEs) in cubic NaZn13-type La(FexSi1−x)13 compounds and their hydrides composed of icosahedral clusters have been investigated because these compounds exhibit the itinerant-electron metamagnetic (IEM) transition just above the Curie temperature Tc: The value of Tc can be increased continuously up to about 336 K by hydrogen absorption into the La(FexSi1−x)13 compounds. Therefore, the values of the isothermal magnetic entropy change ∆Sm and the indirectly estimated adiabatic temperature change ∆Tad due to the IEM transition become -31 J/kg K and 15.4 K, respectively, in the magnetic field change from 0 to 5 T at Tc = 287 K for the La(Fe0.90Si0.10)13H1.1 compound. The MCEs in the compounds having the IEM transition are much larger than thosein the compounds exhibiting the second-order magnetic transition at Tc: The direct measurement of the adiabatic temperature change has confirmed such a large value. Consequently, the La(FexSi1−x)13 compounds and their hydrides having the IEM transition are promising as high performance magnetic refrigerants working in a wide temperature range covering room temperature in relatively low magnetic fields.

Giant volume magnetostriction due to the itinerant electron metamagnetic transition in La(Fe-Si)/sub 13/ compounds

Ferromagnetic La(Fe/sub x/Si/sub 1-x/)/sub 13/ compounds with x/spl ges/0.87 show an itinerant metamagnetic transition above the Curie temperature T/sub c/. Since a large volume change of about 1.5% is caused by the transition, the present compounds have a possibility for practical applications as giant magnetostrictive materials. To adjust T/sub c/ around room temperature, the effect of the substitution by other magnetic elements on the Curie temperature was examined. The Curie temperature of La(Fe/sub 0.88/Co/sub y/Si/sub 0.12-y/)/sub 13/ compounds increases with increasing y and the itinerant electron metamagnetic transition appears above T/sub c/. For y=0.03 and 0.04, a huge volume change of 0.6/spl sim/0.9%, or a linear change of 0.2/spl sim/0.3%, due to the itinerant metamagnetic transition is observed around 250 K.

Half-metallic properties of Co 2 ( Cr 1 − x Fe x ) Ga Heusler alloys

Magnetic and half-metallic properties of

Temperature dependence of magnetocrystalline anisotropy constants in the single variant state of L10-type FePt bulk single crystal

{\mathrm{Co}}_{2}({\mathrm{Cr}}_{1\ensuremath{-}x}{\mathrm{Fe}}_{x})\mathrm{Ga}