Kazuaki Fukamichi

Bio: Kazuaki Fukamichi is an academic researcher from Tohoku University. The author has contributed to research in topics: Curie temperature & Amorphous metal. The author has an hindex of 42, co-authored 379 publications receiving 8558 citations. Previous affiliations of Kazuaki Fukamichi include Hokkaido University & Ehime University.

Itinerant-electron metamagnetic transition and large magnetocaloric effects in La(FexSi1-x)13 compounds and their hydrides

Abstract: The itinerant-electron metamagnetic (IEM) transition and magnetocaloric effects (MCE's) in the $\mathrm{La}({\mathrm{Fe}}_{x}{\mathrm{Si}}_{1\ensuremath{-}x}{)}_{13}$ and $\mathrm{La}({\mathrm{Fe}}_{x}{\mathrm{Si}}_{1\ensuremath{-}x}{)}_{13}{\mathrm{H}}_{y}$ compounds have been investigated. The $\mathrm{La}({\mathrm{Fe}}_{x}{\mathrm{Si}}_{1\ensuremath{-}x}{)}_{13}$ compounds exhibit large values of both the isothermal entropy change $\ensuremath{\Delta}{S}_{\mathrm{m}}$ and the adiabatic temperature change $\ensuremath{\Delta}{T}_{\mathrm{ad}}$ around the Curie temperature ${T}_{\mathrm{C}}$ in relatively low magnetic fields. Such large MCE's are explained by a large magnetization change at ${T}_{\mathrm{C}}$ and a strong temperature dependence of the critical field ${B}_{\mathrm{C}}$ for the IEM transition. By hydrogen absorption into the compounds, ${T}_{\mathrm{C}}$ is increased up to about 330 K, keeping the metamagnetic transition properties. Accordingly, the extension of the working temperature range having the large MCE's in relatively low magnetic fields is demonstrated by controlling y in the $\mathrm{La}({\mathrm{Fe}}_{x}{\mathrm{Si}}_{1\ensuremath{-}x}{)}_{13}{\mathrm{H}}_{y}$ compounds. The correlation between the increase of ${T}_{\mathrm{C}}$ and the large MCE's in the $\mathrm{La}({\mathrm{Fe}}_{x}{\mathrm{Si}}_{1\ensuremath{-}x}{)}_{13}{\mathrm{H}}_{y}$ compounds is discussed by taking the magnetovolume effects into consideration.

Chemical-order-dependent magnetic anisotropy and exchange stiffness constant of FePt (001) epitaxial films

Abstract: Anomalous Hall voltage was measured for FePt ${L1}_{0}$ films having very high magnetic anisotropy. The magnetic anisotropy ${K}_{1}$ and ${K}_{2}$ were determined with high accuracy by analyzing the magnetization curves obtained from the Hall voltage measurement. The saturation magnetization ${M}_{s}$ of the samples with different chemical-order parameter (S) exhibits a different temperature dependence, implying that the Curie temperature weakly depends on S. The first-order anisotropy ${K}_{1}$ gradually increases with S, while the second-order anisotropy ${K}_{2}$ remains almost constant of about $5\ifmmode\times\else\texttimes\fi{}{10}^{6}\mathrm{e}\mathrm{r}\mathrm{g}/\mathrm{c}\mathrm{c}.$ The temperature dependence of ${K}_{1}$ is correlated with S, that is, ${K}_{1}$ with a small S is more temperature dependent than that with a large S. These behaviors are quite similar to the temperature dependence of ${M}_{s}$ with different S, and can be explained by the conventional model based on thermal spin fluctuations. The domain wall energy ${\ensuremath{\sigma}}_{w}$ evaluated by the theoretical analysis of the stripe-domain structure tends to increase linearly with S, in a similar manner as that of ${K}_{1},$ whereas the exchange stiffness constant A of about $1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}\mathrm{e}\mathrm{r}\mathrm{g}/\mathrm{c}\mathrm{m}$ deduced from ${\ensuremath{\sigma}}_{w}$ and ${K}_{u}{(=K}_{1}{+K}_{2})$ hardly depends on S.

Promising ferromagnetic Ni–Co–Al shape memory alloy system

Abstract: A system of ferromagnetic β phase Ni–Co–Al alloys with an ordered B2 structure that exhibits the shape memory effect has been developed. The alloys of this system within the composition range Ni (30–45 at. %) Co–(27–32 at. %) Al, undergo a paramagnetic/ferromagnetic transition as well as a thermoelastic martensitic transformation from the β to the β′(L10) phase. The Curie and the martensitic start temperatures in the β phase can be controlled independently to fall within the range of 120–420 K. The specimens from some of the alloys undergoing martensitic transformation from ferromagnetic β phase to ferromagnetic β′ phase are accompanied by the shape memory effect. These ferromagnetic shape memory alloys hold great promise as new smart materials.

Magnetization reversal due to vortex nucleation, displacement, and annihilation in submicron ferromagnetic dot arrays

Abstract: Magnetization processes are analytically described for the arrays of soft ferromagnetic polycrystalline circular dots with submicron dimensions, wherein the magnetization reversal accompanied by nucleation, displacement, and annihilation of magnetic vortices. Magnetostatic, exchange, and Zeeman energies are taken into account for the analysis. The magnetic state of each dot in an applied magnetic field is treated as an off-centered rigid vortex structure; i.e., the vortex keeps its spin distribution while being displaced. This rigid vortex model yields analytical expressions for the size-dependent initial susceptibility, the vortex nucleation, and the annihilation fields. The interdot magnetostatic interaction plays an important role in the magnetization reversal for the arrays when the interdot distance is smaller than the disk radius, where the initial susceptibility increases and both the nucleation and annihilation fields decrease. The analytical predictions are compared to the micromagnetic calculations, and limitations of the model are discussed.

Itinerant-electron metamagnetic transition and large magnetovolume effects in La(Fe x Si 1-x ) 13 compounds

Abstract: Magnetic properties and magnetovolume effects have been investigated for itinerant-electron metamagnetic $\mathrm{La}({\mathrm{Fe}}_{x}{\mathrm{Si}}_{1\ensuremath{-}x}{)}_{13}$ compounds. At the Curie temperature ${T}_{C1},$ a first-order magnetic phase transition takes place in the concentration range $0.86l~xl~0.88.$ With increasing Fe concentration, the Curie temperature decreases and the critical temperature of the itinerant-electron metamagnetic (IEM) transition increases, accompanied by a more sharp IEM transition. For the compound with $x=0.88,$ a large volume change of about 1% follows the thermal induced transition at ${T}_{C1}.$ The value of ${T}_{C1}$ is significantly decreased by applying hydrostatic pressure, whereas the pressure dependence of the spontaneous magnetization is relatively small. These results are explained by the negative mode-mode coupling among spin fluctuations.

Mechanical Alloying And Milling

Abstract: Mechanical alloying (MA) is a solid-state powder processng technique involving repeated welding, fracturing, and rewelding of powder particles in a high-energy ball mill. Originally developed to produce oxide-dispersion strengthened (ODS) nickel- and iron-base superalloys for applications in the aerospace industry, MA has now been shown to be capable of synthesizing a variety of equilibrium and non-equilibrium alloy phases starting from blended elemental or prealloyed powders. The non-equilibrium phases synthesized include supersaturated solid solutions, metastable crystalline and quasicrystalline phases, nanostructures, and amorphous alloys. Recent advances in these areas and also on disordering of ordered intermetallics and mechanochemical synthesis of materials have been critically reviewed after discussing the process and process variables involved in MA. The often vexing problem of powder contamination has been analyzed and methods have been suggested to avoid/minimize it. The present understanding of the modeling of the MA process has also been discussed. The present and potential applications of MA are described. Wherever possible, comparisons have been made on the product phases obtained by MA with those of rapid solidification processing, another non-equilibrium processing technique.

Bulk metallic glasses

Abstract: Amorphous alloys were first developed over 40 years ago and found applications as magnetic core or reinforcement added to other materials. The scope of applications is limited due to the small thickness in the region of only tens of microns. The research effort in the past two decades, mainly pioneered by a Japanese- and a US-group of scientists, has substantially relaxed this size constrain. Some bulk metallic glasses can have tensile strength up to 3000 MPa with good corrosion resistance, reasonable toughness, low internal friction and good processability. Bulk metallic glasses are now being used in consumer electronic industries, sporting goods industries, etc. In this paper, the authors reviewed the recent development of new alloy systems of bulk metallic glasses. The properties and processing technologies relevant to the industrial applications of these alloys are also discussed here. The behaviors of bulk metallic glasses under extreme conditions such as high pressure and low temperature are especially addressed in this review. In order that the scope of applications can be broadened, the understanding of the glass-forming criteria is important for the design of new alloy systems and also the processing techniques.

Recent developments in magnetocaloric materials

Abstract: The recent literature concerning the magnetocaloric effect (MCE) has been reviewed. The MCE properties have been compiled and correlations have been made comparing the behaviours of the different families of magnetic materials which exhibit large or unusual MCE values. These families include: the lanthanide (R) Laves phases (RM2, where M = Al, Co and Ni), Gd5(Si1−xGex)4 ,M n(As1−xSbx), MnFe(P1−xAsx), La(Fe13−xSix) and their hydrides and the manganites (R1−xMxMnO3, where R = lanthanide and M = Ca, Sr and Ba). The potential for use of these materials in magnetic refrigeration is discussed, including a comparison with Gd as a near room temperature active magnetic regenerator material. (Some figures in this article are in colour only in the electronic version)

Ferromagnetic materials

Abstract: In this chapter, we will restrict our attention to the ferrites and a few other closely related materials. The great interest in ferrites stems from their unique combination of a spontaneous magnetization and a high electrical resistivity. The observed magnetization results from the difference in the magnetizations of two non-equivalent sub-lattices of the magnetic ions in the crystal structure. Materials of this type should strictly be designated as “ferrimagnetic” and in some respects are more closely related to anti-ferromagnetic substances than they are to ferromagnetics in which the magnetization results from the parallel alignment of all the magnetic moments present. We shall not adhere to this special nomenclature except to emphasize effects, which are due to the existence of the sub-lattices.