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26 results on '"Koike, Kunihiro"'

2. Large difference between the magnetic properties of Ba and Ti co-doped BiFeO$_3$ bulk materials and their corresponding nanoparticles prepared by ultrasonication

Author

Ahmmad, Bashir, Kanomata, Kensaku, Koike, Kunihiro, Kubota, Shigeru, Kato, Hiroaki, Hirose, Fumihiko, Billah, Areef, Jalil, M. A., and Basith, M. A.

Subjects
Condensed Matter - Mesoscale and Nanoscale Physics, Condensed Matter - Materials Science
Abstract

The ceramic pellets of the nominal compositions Bi$_{0.7}$Ba$_{0.3}$Fe$_{1-x}$Ti$_x$O$_3$ (x = 0.00-0.20) were prepared initially by standard solid state reaction technique. The pellets were then ground into micrometer-sized powders and mixed with isopropanol in an ultrasonic bath to prepare nanoparticles. The X-ray diffraction patterns demonstrate the presence of a significant number of impurity phases in bulk powder materials. Interestingly, these secondary phases were completely removed due to the sonication of these bulk powder materials for 60 minutes. The field and temperature dependent magnetization measurements exhibited significant difference between the magnetic properties of the bulk materials and their corresponding nanoparticles. We anticipate that the large difference in the magnetic behavior may be associated with the presence and absence of secondary impurity phases in the bulk materials and their corresponding nanoparticles, respectively. The leakage current density of the bulk materials was also found to suppress in the ultrasonically prepared nanoparticles compared to that of bulk counterparts., Comment: 9 pages in Journal of Physics D: Applied Physics, 2016

Published
2016
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10. Maximum energy product of exchange-coupled Sm(FeCo)12/α-Fe nanocomposite particle.

Author

Uda, Ryusei, Koike, Kunihiro, Inaba, Nobuyuki, Kato, Hiroaki, Itakura, Masaru, Okubo, Susumu, Ohta, Hitoshi, and Tsuchiura, Hiroki

Subjects
*NANOCOMPOSITE materials, *MICROMAGNETICS, *DEGREES of freedom, *SURFACE coatings, *MAGNETS
Abstract

The effects of the coating surface orientation of the α-Fe soft magnetic layer on the Sm(Fe0.8Co0.2)12 hard magnetic phase and the volume fraction of α-Fe, VFe, on the maximum energy product, (BH)max of exchange-coupled Sm(Fe0.8Co0.2)12/α-Fe nanocomposite magnet particles were micromagnetics OOMMF package was systematically investigated. The (BH)max of the reference model, Sm(Fe0.8Co0.2)12 particles without Fe layer, was 630 kJ/m3. In contrast, in the nanocomposite magnet particle model with soft magnetic layers on both sides of the hard magnetic phase, (BH)max reached a maximum value of 657 kJ/m3 at VFe = 12% (Fe layer thickness, tFe = 2 nm). In the model with α-Fe coating on the top and bottom surfaces of the hard magnetic phase, (BH)max = 636 kJ/m3 at VFe = 4% (tFe = 2 nm). Furthermore, the coating of the soft magnetic phase on both sides of the hard phase particles reduces the magnitude of the demagnetizing field, Hd of the nanocomposite magnet particles, indicating that the side coating of the soft magnetic phase is effective in increasing (BH)max. These findings allow for a greater degree of freedom in the design of nanocomposite magnets by adjusting not only the VFe volume fraction of the hard/soft phases but also their arrangement. [ABSTRACT FROM AUTHOR]

Published
2023
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15. Damping constants of Ni-Fe and Ni-Co alloy thin films

Author

Inaba, Nobuyuki, Asanuma, Hironori, Igarashi, Satoshi, Mori, Shunnsuke, Kirino, Fumiyoshi, Koike, Kunihiro, and Morita, Hiroaki

Subjects
Nickel alloys -- Magnetic properties, Dielectric films -- Magnetic properties, Thin films -- Magnetic properties, Damping (Mechanics) -- Analysis, Business, Electronics, Electronics and electrical industries
Abstract

Gilbert's damping constants, [alpha], of Ni-Fe and Ni-Co thin films are determined by Q-band ferromagnetic resonance (FMR) analysis. [alpha] is calculated from the resonance width of the FMR spectrum, [alpha] of [Ni.sub.85] [Fe.sub.15] film is found to be 0.0034, which is one-fourth of those of Ni film ([alpha] = 0.013) and Co ([alpha] = 0.013). [alpha] of the [Ni.sub.100-x] [Co.sub.x] specimens increases with decreasing the Co concentration from 100 to 68 at %, and the [alpha] value of [Ni.sub.32] [Co.sub.68] film has the maximum value of 0.0021. The [alpha] values of the Ni-Co specimens are nearly the same as those of the Co-Cr-Ta and the Co-Cr-Pt. Index Terms--Damping constant, ferromagnetic resonance, Ni-Co alloy thin film, Ni-Fe alloy thin film.

Published
2006

16. Reduction of coercivity in FePt--FeRh bilayer films by heating

Author

Koyama, Shigenobu, Ogata, Hidehiro, Konno, Masaki, Goto, Takao, and Koike, Kunihiro

Subjects
Ferromagnetic materials -- Research, Iron -- Magnetic properties, Business, Electronics, Electronics and electrical industries
Abstract

In FePt(10 nm)--FeRh(25 nm), a large reduction of coercive field in the FePt layer occurs at a temperature just above the antiferro--ferromagnetic transition temperature of FeRh, which is attributable to the existence of an exchange spring mechanism. FePt(10 nm)--FeRh(7.5 nm) film does not show coercivity reduction due to the magnetic transition because of a high nucleation field for magnetization twisting of FeRh layer. The coercive field of the FePt layer is largely enhanced by field annealing. Index Terms--Coercive force, FePt-FeRh, interlayer exchange, magnetic films.

Published
2005

25. Synthesis of Metal Doped BiFeO3 Nanoparticles and their Dielectric and Magnetic Properties

Author

Ahmmad, Bashir, Basith, M.A., Koike, Kunihiro, Kubota, Shigeru, and Hirose, Fumihiko

Abstract

Dielectric and magnetic properties of BiFeO3 samples, co-doped with Gd or Ba and Ti in place of Bi and Fe, respectively, were investigated. The bulk powder samples of nominal compositions of Bi0.9Gd0.1Fe1–xTixO3 and Bi0.7Ba0.3Fe1-xTixO3 (x=0.00-0.25) ceramics were synthesized by conventional solid state reaction technique. The as prepared ceramics showed improved dielectric and magnetic properties. Also, ultrafine nanoparticles of the samples were prepared by milling assisted ultrasonic fragmentation of the bulk powder. The particles size was studied as a function of sonication time with TEM imaging and electron diffraction. These confirmed the formation of a large fraction of single-crystalline Bi0.9Gd0.1Fe1–xTixO3 nanoparticles with a mean size of 11-13 nm. The synthesized nanoparticles exhibit an improved magnetic behavior compared to their bulk counterparts at room temperature. The preparation and characterization of bulk ceramics as well as their nanoparticles were reported.

Published
2015
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