Koichi Kakizaki

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.

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.

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.

Synthesis and magnetic characterization of Sr-based Ni2X-type hexaferrite

We have investigated the synthesis conditions, and the magnetic properties of the Sr2Ni2X-type hexagonal ferrite, Sr2Ni2Fe28O46. The Sr2Ni2X-type hexaferrite was synthesized at 1240∘C. The spontaneous magnetization at 5 K was 44.2 μB/f.u., suggesting that most of the Ni2+ ions are at the up-spin octahedral sites in the spinel-structure blocks within the model of a Neel-type collinear ferrimagnetic structure. The Curie temperature of the Sr2Ni2X-type hexaferrite was estimated to be TC[Sr2Ni2X] = 472∘C. This is consistent with the difference of the block stacking structures of SrM-type, Sr2Ni2X-type, SrNi2W-type, and nickel spinel ferrites.

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.

Design of Spin Polarization Analyzer using Transverse-Longitudinal Correlation in Resistivities Induced by Spin–Orbit Interaction

We have theoretically studied a methodology for the measurement of the degree of spin polarization (P) in metals as well as semiconductors. Our principle is based on the correlation existing between transverse resistivity (ρyx) and longitudinal resistivity (ρxx), both influenced by transverse scattering due to a spin–orbit interaction (SOI) as well as longitudinal scattering due to usual mechanisms. Our spin polarization analyzer employs an unknown polarization conductor as a source electrode from which spin-polarized electrons are injected into a nonmagnetic (NM) channel region. The channel length is set to be much smaller than its spin diffusion length so that ρyx and ρxx in the NM region, both complementarily influenced by carrier spin polarization, would be measured to obtain the P value. Also, application to OR and XOR logic gates are discussed on the basis of our spin polarization analyzer.

Preparation of pyrolytic magnetic carbon under magnetic field

We have prepared pyrolytic carbon samples from triethylamine and investigated their magnetic properties. A ferromagnetic sample was obtained from the pyrolysis products even at room temperature, with spontaneous magnetization of 5 × 10−1 emu/g at 300 K. The magnetization was decreased with increasing temperature up to 500 K, but increased above 550 K. After this measurement, it was found that the magnetization at 300 K was changed to 6 × 10−1 emu/g. Another ferromagnetic sample has been also prepared by pyrolysis under high magnetic field of 2 Tesla. The spontaneous magnetization of this field-pyrolysis sample is 1.3 emu/g, which is twice as large as that of the above-mentioned sample prepared without magnetic field. Therefore, the magnetic field may help to form a ferromagnetic structure in pyrolytic carbon.