Nobuyuki Hiratsuka

Fine structure of acicular BaCoXTiXFe12−2XO19 particles and their magnetic properties

Abstract Acicular barium ferrite fine particles doped with Co 2+ ions and Ti 4+ ions were prepared by a conventional sintering method. We investigated particle fine structure and magnetization mechanism of the acicular BaCo X Ti X Fe 12− 2X O 19 ( X = 0–1.0) fine particles. By TEM observation for the acicular BaCo X Ti X Fe 12− 2X O 19 fine particles, it was clarified that the particles consisted of some grains and their c -axis was aligned along the short axis of the acicular particle. Moreover, magnetic tapes coated with these particles and binders under a magnetic field of 7 kOe were prepared to investigate their magnetization mechanism. It was confirmed that the magnetization mechanism of the acicular BaCo X Ti X Fe 12− 2X O 19 fine particles was rotation magnetization by the measurement of the angular dependence of their coercivity.

Hereditary properties and stability of ferromagnetic amorphous ferrites

Abstract Several ferromagnetic amorphous oxides were found to inherit certain characteristics from the mother ferrites, which were used as the starting materials. The stable range of the amorphous oxides in the ternary system CoO-P 2 O 5 -Fe 2 O 3 was also investigated.

Crystallographic and magnetic properties of Cu2X, Co2X, and Ni2X hexaferrites

We have investigated the synthesis conditions and the magnetic properties of X-type hexagonal ferrites. It is found that Ba2Cu2Fe28O46 (Cu2X), Ba2Co2Fe28O46 (Co2X), and Ba2Ni2Fe28O46 (Ni2X) ferrites can be synthesized at a sintering temperature of 1250 °C. The x-ray diffraction patterns for Cu2X, Co2X, and Ni2X samples are in good agreement with the pattern calculated from the atomic coordinates for Ba2Fe30O46 (Fe2X), where two diffraction peaks at Q=2.24 and 2.30 A−1 are clearly observed in contrast with other M-, W-, Y-, and Z-types hexagonal ferrites. It is also found that the low-temperature spontaneous magnetizations of Cu2X, Co2X, and Ni2X ferrites are 47.5 μB/f.u., 43.4 μB/f.u., and 43.2 μB/f.u., respectively. The cation distributions for Cu2X, Co2X, and Ni2X are discussed within the model of a Neel-type collinear ferrimagnetic structure.

Annealing Temperature Dependences of Ferroelectric and Magnetic Properties in Polycrystalline Co-Substituted BiFeO3 Films

Multiferroic Co-substituted BiFeO3 films were fabricated by chemical solution deposition method followed by post deposition annealing at various temperatures. The substitution of cobalt of B-sites for iron in BiFeO3 was promoted at relatively high temperatures. The B-site substitution by cobalt promoted increases in saturation magnetization and spontaneous magnetization. By substitution, leakage current density was suppressed in a high-electric-field region, and ferroelectric hysteresis (P–E) loops became measurable even at room temperature. The optimal annealing temperature for the coexistence of a high remanent polarization and a high remanent magnetization was 923 K having a high B-site substitution ratio of cobalt.

Magnetic properties of acicular BaFe/sub 12/O/sub 19/ particles prepared by microwave plasma sintering

Acicular alpha -FeOOH particles coated with barium ions were irradiated with microwaves under an oxygen pressure of 20 torr to prepare fine BaFe/sub 12/O/sub 19/ particles by a plasma reaction. The fine acicular particles has an aspect ratio of about 13 and a magnetization of 43.5 emu/g and contained small amounts of alpha -Fe/sub 2/O/sub 3/ and BaFe/sub 2/O/sub 4/. The particles were mixed with a binder, and the resulting mixture was formed into a coating under a magnetic field of 7 kOe applied vertically to the surface of the film. The squareness ratio in the direction perpendicular to the surface of the coating film was 0.83 without any correction for the demagnetizing field. Transmission electron microscopy (TEM) showed that the long axes of the BaFe/sub 12/O/sub 19/ particles all were aligned parallel to the film base. The cross-sections of the particles were rectangular when observed by TEM. It follows that the easy axes of the particles are perpendicular to their long axes. >

Preparation of amorphous cobalt ferrite films with perpendicular anisotropy and their magnetooptical properties

Amorphous CoFe 2 O 4 films were made from CoFe 2 alloy and P 2 O 5 as the starting materials by the double source evaporation method. As the films were made in various levels of pressure of evaporated P 2 O 5 (P p2 o 5 ) in the bell jar, different crystal phases and perpendicular anisotropy constants (Ku) of the films were obtained. More precisely, when Pp 2 o 5 was higher than 5×10-3torr, the amorphous films were produced and under Pp 2 o 5 of higher than 10-2torr the maximum Ku of the films with the perpendicular anisotropy was 1.8×105erg/cm3. The perpendicular anisotropy seemed to be induced by both shape anisotropy with the columnar structure and uniaxial magnetic anisotropy of Co2+and Fe2+. For amorphous CoFe 2 O 4 films, the dependence of the polar Kerr rotation angle on the wave length became complicated with the increase in Pp 2 o 5 . A curve showing the relation between polar Kerr rotation angle and the wave length had a broad, positive peak in the region from 300 nm to 500 nm which was characteristic of amorphous CoFe 2 O 4 films.

Simple Process Synthesis of BaTiO3–(Ni,Zn,Cu)Fe2O4 Ceramic Composite

Ceramic composites (2Ni 0.41 Zn 0.41 Cu 0.18 Fe 2 O 4 –BaTiO 3 ) were successfully prepared by a direct solid-state reaction of raw materials (BaCO 3 , CuO, α-Fe 2 O 3 , NiO, TiO 2 , and ZnO). X-ray diffraction (XRD) and electron probe micro analysis (EPMA) measurements were performed for these samples and it is confirmed that the composites consist of spinel ferrite and BaTiO 3 phases. The composites are so homogeneous that the ferrite and BaTiO 3 grains do not react with each other and have radii in the range of 1–5 µm. Hexagonal BaTiO 3 ( h -BaTiO 3 ) can be made in this composite form with a sintering temperature of 1200 °C, although h -BaTiO 3 can be usually synthesized above 1460 °C. The freezing-point depression of BaTiO 3 takes place due to the mixing with the spinel ferrite, which may result in the formation of h -BaTiO 3 at a low temperature of 1200 °C.

Epitaxial growth of [111] oriented manganese zinc ferrite thin films and their magnetic properties

Manganese zinc ferrite (MnZn ferrite) thin films were prepared on quartz substrates buffered by a ZnO underlayer by rf magnetron sputtering. The ZnO underlayers exhibited preferentially [001] orientation for any thickness. When Mn-Zn-Fe oxide layers as-deposited on the ZnO underlayers were annealed in reduced atmosphere at 800/spl deg/C for 5 hours, single-phase spinel type ferrites were formed. The films were preferentially oriented to the [111] direction normal to the film plane. Thin ZnO underlayers successfully induced [111] orientation in plane because of good lattice matching between the ZnO and the spinel phase. Films with a composition of Mn:Zn:Fe=23:9:68 had a saturation magnetization value of 410 emu/cm/sup 3/. The relationship between the composition and the magnetization for the films was similar to that for the bulk material.

Magnetic and Electronic Properties of BaTiO3―(Ni,Cu,Zn)Fe2O4 Ceramic Composite: Reflection of Kepler Conjecture

Ceramic composites [ x Ni 0.15 Cu 0.30 Zn 0.55 Fe 2 O 4 –(1- x )BaTiO 3 ] were successfully prepared by a direct solid-state reaction of raw materials (BaCO 3 , CuO, α-Fe 2 O 3 , NiO, TiO 2 , and ZnO). The composites are so homogeneous that the ferrite and BaTiO 3 grains do not react with each other. The x -dependent permeability and permittivity are found to obey Maxwell–Garnett effective medium theory, which suggests that the composites consist of ferrite particles with barium titanate medium. This model, however, starts to deviate from the experimental data at x = 0.75, and the roles of medium and inclusions seem to be exchanged. It can be qualitatively explained by the fact that geometrical close-packing of spheres is limited up to about 74 vol % (Kepler conjecture).

Ferroelectric and magnetic properties of multiferroic BiFeO 3 -based composite films

BiFeO₃-based composite films were fabricated onto the Pt/Ti/SiO₂/Si(100) substrates by a chemical solution deposition (CSD) method using the precursor solutions with various excess iron composition followed by annealing at 923 K for 30 minutes under oxygen gas flow. Coexistence of spontaneous magnetization and remanent polarization could be obtained in the BiFeO₂-based composite films with high excess iron composition. The remanent magnetization of almost 20 emu/cm³ and the magnetic coercive field of 1.5 kOe were obtained at the iron composition ratio of Fe/Bi = 1.25. In this specimen, the remanent polarization at 90 K was approximately 10 muC/cm² at the electric field of 1500 kV/cm. Structural analysis suggested that the remanent polarization has a possibility to increase by suppressing the formation of the secondary phases of Bi₂Fe₄O₉ and alpha-Fe₂O₃, these are the nonferroelectric material as well as antiferromagnetic phase.