Shun Fujieda

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.

Giant isotropic magnetostriction of itinerant-electron metamagnetic La(Fe0.88Si0.12)13Hy compounds

La(FexSi1−x)13 compounds exhibit an itinerant-electron metamagnetic (IEM) transition above Curie temperature TC. The IEM transition in the compound with x=0.88 is accompanied by a giant volume change. From a practical viewpoint, TC was controlled by hydrogen absorption in order to obtain such a giant volume magnetostriction at room temperature. For the La(Fe0.88Si0.12)13H1.0 compound, the IEM transition occurs above TC=278 K, and a significant isotropic linear magnetostriction of about 0.3% at 7 T is induced in the vicinity of room temperature. This large magnetostriction is attributed to the giant volume magnetostriction of about 1% by the IEM transition.

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.

Enhancement of magnetocaloric effects in La1- zPrz-(Fe0.88Si0.12)13 and their hydrides

The magnetocaloric effects (MCEs) in La1−zPrz(Fe0.88Si0.12)13 and their hydrides have been investigated. The values of the MCEs due to the itinerant-electron metamagnetic transition are enhanced significantly by a partial substitution of Pr for La. Furthermore, the Curie temperature TC of the La1−zPrz(Fe0.88Si0.12)13 is controlled by adjusting the absorption content of hydrogen in a similar manner as La(Fe0.88Si0.12)13. For the compounds having almost the same TC, the MCEs in the La1−zPrz(Fe0.88Si0.12)13Hy are larger than those in La(Fe0.88Si0.12)13Hy. To conclude, the La1−zPrz(Fe0.88Si0.12)13 and their hydrides are more useful for magnetic refrigerants working in a wide temperature range covering room temperature.

Active Magnetic Regeneration Behavior of Spherical Hydrogenated La(Fe0.86Si0.14)13 Fabricated by Rotating Electrode Process

To verify effectiveness for a magnetic refrigerant, we have fabricated spheres of La(Fe0.86Si0.14)13 by a rotating electrode process (REP). The Curie temperature of the spheres was increased up to around room temperature by hydrogen absorption. Putting these spheres into an active magnetic regenerator (AMR) test module, the temperature span of 16 K in the vicinity of room temperature was obtained in a steady state. Consequently, it has been confirmed for the first time that the La(Fe0.86Si0.14)13 spheres perform as an excellent material for the AMR.

Strong magnetocaloric effects in La1−zCez(Fex−yMnySi1−x)13 at low temperatures

The Curie temperature TC of La1−zCez(Fex−yMnySi1−x)13 is decreased by adjusting the composition, and hence the itinerant-electron metamagnetic transition is observed at 5K in La0.75Ce0.25(Fe0.850Mn0.035Si0.110)13 after zero-field cooling. As a result, the La1−zCez(FexMnySi1−x)13 compounds exhibit strong magnetocaloric effects in relatively low magnetic fields in a wide range of temperatures between about 19 and 180K. Consequently, the La1−zCez(Fex−yMnySi1−x)13 compounds are useful for the magnetic refrigerants working at low temperatures.

Magnetocaloric effect in spherical La(FexSi1−x)13 and their hydrides for active magnetic regenerator-type refrigerator

We have fabricated spherical La(FexSi1−x)13 compounds by a rotating electrode process (REP) to provide suitable refrigerants for active magnetic regenerator (AMR)-type systems. In the as-prepared state, the cross section exhibits a dendritic metallographic structure composed of two phases of α-Fe-based alloy with Si and LaFeSi equiatomic compound. After annealing at 1323 K for 5 days, the spheres become a single phase of La(FexSi1−x)13. From the temperature dependence of magnetic entropy change for the hydrogenated spheres, it has been confirmed that the hydrogen concentration in the spheres is homogeneous. In addition, it has also been demonstrated that the itinerant-electron metamagnetic transition appears in the spheres with the higher Fe concentration. Consequently, the spherical La(FexSi1−x)13 fabricated by the REP are suitable for AMR-type refrigerations.

Neutron Diffraction and Isotropic Volume Expansion Caused by Deuterium Absorption into La(Fe0:88Si0:12)13

The crystal structures and magnetic properties of La(Fe 0.88 Si 0.12 ) 13 D 1.5 and La(Fe 0.88 Si 0.12 ) 13 H 1.6 have been investigated. The powder neutron diffraction declared that the compound La(Fe 0.88 Si 0.12 ) 13 D 1.5 has a cubic NaZn 13 -type structure of the F m \bar3 c space group in analogy with La(Fe 0.88 Si 0.12 ) 13 . The deuterium atoms in the compound La(Fe 0.88 Si 0.12 ) 13 D 1.5 occupy the interstitial 24 d site surrounded by two La atoms and four icosahedral clusters. The x-ray diffraction data of La(Fe 0.88 Si 0.12 ) 13 H 1.6 are also consistently explained by the same site of hydrogen atom. The deuterium or hydrogen absorption brings out a notable increase of the Curie temperature T C . The neutron diffraction of the compound La(Fe 0.88 Si 0.12 ) 13 D 1.5 shows that the magnetic structure is ferromagnetic and the average magnetic moment per Fe is about 2.2 µ B , almost the same as that of La(Fe 0.88 Si 0.12 ) 13 . Such behaviors can be explained as characteristic magnetovolume effect...

Influence of partial substitution of Ce on the Curie temperature and magnetic entropy change in itinerant-electron metamagnetic La(FexSi1-x)13 compounds

In La(FexSi1−x)13, the partial substitution of Ce for La results in the decrease of the Curie temperature TC, and the isothermal entropy change ΔSm becomes larger with decreasing TC. The increase of ΔSm with decreasing TC is also confirmed by applying hydrostatic pressure, although the increment of ΔSm is not so large. Accordingly, another effect addition to the reduction of volume due to the small ionic radius of Ce should be considered for the enhancement of ΔSm. From the fitted isothermal magnetization curves, it is evident that the partial substitution of Ce enhances the thermal variation of the forth-order coefficient B(T) in the Landau expansion of the Gibbs free energy, being consistent with the increase of ΔSm. Therefore, it is concluded that the change in the band structure also contributes the enhancement of ΔSm.

Changes in electronic states and magnetic free energy in La1?zCez(FexSi1?x)13 magnetic refrigerants

The influence of partial substitution of Ce on the electronic structure and magnetic free energy has been investigated for La1−zCez(FexSi1−x)13. From the Mossbauer spectroscopy of La0.7Ce0.3(Fe0.88Si0.12)13, the distribution of the electric field gradient is found to be scarcely changed, therefore, the volume reduction by partial substitution is regarded as isotropic. The change of the isomer shift to positive sign after the partial substitution is closely correlated with the 5d and/or 4f electrons of Ce.The change in magnetic free energy has been examined for La0.7Ce0.3(Fe0.86Si0.14)13 having a large magnetic entropy change ΔSm and a small hysteretic behaviour. From the results analysed by the Landau expansion theory, the large ΔSm and the small hysteresis of this compound are attributed to the magnitude and thermal variation of the fourth-order Landau coefficient in magnetic free energy. Consequently, the combination of partial substitution and control of Fe concentration is useful for highly efficient magnetic refrigerants.