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Spherical GdN material with diameter of 0.85 ∼ 1.0 mm was synthesized by the hot isostatic pressing (HIP) method aiming at the application to the magnetic refrigeration for hydrogen liquefaction. Conditions of the HIP nitridation starting from metallic Gd spheres were investigated to obtain the spherical GdN materials free from unwanted deformation and/or cracks. We found that slower temperature rising rate, 10°C/h, around the melting point of Gd metal, 1300-1600°C, is favorable because it would allow the metallic Gd core to transform into nitride by reacting with nitrogen diffusing from the surface. We have successfully synthesized spherical GdN material of 300 g, which is enough to operate a test device for the magnetic refrigeration. Oxidization degree of this spherical GdN material was investigated by exposing it to air with some controlled humidity. It was found that only 2.5 % of the GdN was degraded after 56 hour exposition to air dried with silica gel.

Z-type hexagonal barium ferrite samples in which a part and/or all of Ba2+ was substituted for Sr2+ were prepared by the ceramic method, Ba3Co2Fe24O41, Ba1.5Sr1.5Co2Fe24O41 and Sr3Co2Fe24O41. The Sr2+ substitution was found favorable for lowering temperature and oxygen partial pressure in the sintering process. Ba1.5Sr1.5Co2Fe24O41 and Sr3Co2Fe24O41 were obtained at 1523K under Po2=101.3kPa and at 1483K under Po2=21.3kPa, respectively. We investigated crystal and magnetic anisotropies of the materials by measuring the magnetizations and XRD patterns of specimens which were shaped by pressing the sample powders in a magnetic field. Both the results indicated that the magnetic moment lay in the basal plane of the hexagonal structure in Ba3Co2Fe24O41 and Ba1.5Sr1.5Co2Fe24O41, whilst was significantly off from the basal plane in of Sr3Co2Fe24O41. We measured their powder neutron diffraction patterns and analyzed them by the Rietveld method to estimate site distributions of Sr2+-Ba2+ and Fe3+-Co2+, and orientation of the magnetic moment. The analyses revealed (1) Sr2+ preferentially occupied a site out of two sites available for Ba2+, (2) the magnetic cation distribution in Sr3Co2Fe24O41 was significantly different from those of the others, and (3) the magnetic moment angle with respect to the c-axis was 52.3° in Sr3Co2Fe24O41 whilst almost 90° in the others.

Conditions for synthesizing single phase materials of the hexagonal Z-type were investigated by preparing samples with various compositions (x=0, 0.2, 0.4, 0.6 in Ba3Co2-xFe24+xO41) and oxygen partial pressures, PO2=21.3, 31.3, 41.3, 61.3, 101.3 kPa, at T=1573 K. The phases formed in the samples were examined by X-ray diffraction technique and the dc magnetization was measured as a function of temperature by a vibrating sample magnetometer (VSM) at a magnetic field of 5.6 kA/m. The correlation between the phases occurring in the samples and the complex permeability μ, at 100MHz was investigated, and it was indicated that high μr is obtained when the hexagonal Z-type occurs without any other phase. The highest μr=20 was obtained with conditions of x =0.2, PO2=101.3 at 1573 K. The neutron diffraction patterns were measured between 294 and 773 for the sample of x=0.4. Two significant magnetic Bragg reflections were observed in the pattern measured at 294 K, but no peak at 773 K. Integrated intensities of the magnetic scattering peaks were plotted against temperatures, which was consistent with the magnetization vs. temperature curve measured with VSM. It was indicated that the hexagonal Z-type phase exhibits two stage magnetic transition at 500-540 and 680 K.

The iron oxide nanocrystalline were manufactured by a Inert Gas Condensation - Pulse Electric Current Sintering (IGC-PECS) process. The IGC method involves condensation of vapors of metallic or nonmetallic powders with a controlled residence time in a low pressure environment. The particle size was controlled by pressure of inert gas (He). The smallest average size of powders, 8 nm and the largest average size of powders, 28 nm, were used as a starting precursors for PECS. The smallest average grain size of sintered pellets is 33 nm under the condition at 1073 K with applied pressure of 100 MPa.