Kenji Hiraga

Plasma assisted molecular beam epitaxy of ZnO on c -plane sapphire: Growth and characterization

ZnO single crystal thin films were grown on c-plane sapphire using oxygen microwave plasma assisted molecular beam epitaxy. Atomically flat oxygen-terminated substrate surfaces were obtained by pre-growth cleaning procedures involving an oxygen plasma treatment. A two dimensional nucleation during the initial growth which is followed by a morphology transition to three dimensional nucleation was observed by in situ reflection high energy electron diffraction. X-ray diffraction (XRD) and photoluminescence investigations suggest that the ZnO epilayer consists of a high quality layer on top of a transition layer containing a high density of defects in the interfacial region. A full width at half maximum (FWHM) of 0.005° is obtained for the ZnO(0002) diffraction peak in an XRD rocking curve, while a broad tail extending from the peak can also be observed. The photoluminescence spectra exhibit dominant bound exciton emission with a FWHM of 3 meV at low temperatures and free exciton emission combined with a ver...

Permanent magnet materials based on the rare earth-iron-boron tetragonal compounds

Structural and metallographic studies were carried out on the Nd-Fe-B alloy system as well as the Nd-Fe-B tetragonal compound on which record high energy magnets have been developed using a powder metallurgical technique. The study on the new magnet has also been extended to other R-Fe-B componds containing various rare earths (R) and to R-Fe-Co-B alloys. The results are as follows; (1) The sintered Nd-Fe-B magnet is composed of mainly three phases, the Nd 2 Fe 14 B matrix phase plus Nd-rich phase and B-rich phase ∼ Nd 2 Fe 7 B 6 ) as minor phases. (2) Nd 2 Fe 14 B has the space group of P4 2 /mnm. The crystal structure of this phase can be described as a layer structure with alternate stacking sequence of a Nd-rich layer and a sheet formed only by Fe atoms. The sheet of Fe atoms has a structure similar to the σ-phase found in Fe-Cr and Fe-Mo systems. (3) The Nd-rich phase containing more than 95 at.% Nd, 3∼5 at.% Fe and a trace of B has fcc structure with a=0.52 nm. This phase is formed around grain boundaries of the matrix phase. Nd 2 Fe 7 B 6 phase has an one-dimentional incommensurate structure with a=a o and c\simeq8 C o , based on a tetragonal structure with a o =0.716 nm and c o =0.391 nm. (4) In the as sintered Nd 15 Fe 77 B 8 alloy periodic strain contrasts are observed along grain boundaries, which disappear after annealing at 870K. This may be related to the enhancement of the intrinsic coercivity of the sintered magnet by post sintering heat treatment. (5) Stable R 2 Fe 14 B phases are formed by various rare earths except La. Of all the R 2 Fe 14 B compounds, Nd 2 Fe 14 B has the maximum saturation magnetization as high as 1.57 T. Dy and Tb form R 2 Fe 14 B phases with the highest anisotropies. Small additions of these elements greatly enhance the coercive force of the Nd 2 Fe 14 B base magnet. (6) Partial replacement of Fe by Co raises the Curie temperature of the Nd 2 Fe 14 B compound, which improves the temperature coefficient of the remanence of the magnet. But the intrinsic coercive force is decreased by the Co addition.

The microstructure evolution of a Fe73.5Si13.5B9Nb3Cu1 nanocrystalline soft magnetic material

The microstructure evolution in the course of crystallization of a splat-quenched Fe73,5Si13.5B9Nb3Cu1 amorphous alloy was investigated by atom probe field ion microscopy (APFIM) and high resolution transmission electron microscopy (HRTEM). All the alloying elements were found to be distributed homogeneously as an amorphous solid solution in the as-quenched state. At an initial stage of annealing, a concentration fluctuation of Cu was found to occur. Cu formed clusters of a few nanometer diameter and their composition was found to be approximately 30 at.% Cu at the beginning. In the later stage, a b.c.c. FeSi solid solution and the B and Nb enriched amorphous phase with the smaller Si content were found to coexist. In addition to these two phases, Cu enriched particles containing approximately 60 at.% Cu were found to be present in the intergranular regions, although we were not successful yet to determine whether this was a crystalline or amorphous phase. Based on these observations, we discuss the crystallization process of this alloy at 550°C which leads to the emergence of excellent soft magnetic properties.

Growth of ZnO single crystal thin films on c-plane (0 0 0 1) sapphire by plasma enhanced molecular beam epitaxy

Abstract ZnO single crystal thin films were grown by plasma enhanced molecular beam epitaxy on (0 0 0 1) sapphire. The growth modes of ZnO epilayers were investigated by reflection high-energy electron diffraction. A transition from two-dimensional nucleation to three-dimensional nucleation is found at the initial growth stage. Optical properties of the films, studied by photoluminescence spectroscopy, exhibit a dominant bound exciton emission at 3.361 eV at 4 K, and a deep level emission centered at 2.42 eV which is associated with either impurities or native defects. The deep level emission which is successfully suppressed to 1 500 of intensity of the excitonic emission. Fabrication of these high-quality ZnO epilayers had lead to observation of stimulated emission at room temperature.

Improvement of coercivity of sintered NdFeB permanent magnets by heat treatment

Abstract The purpose of this study was to better understand the metallurgical transformations that occur during heat treatment of Nd-based magnets. DSC analysis and microstructural investigations of the grain boundaries with SEM and TEM were carried out. DSC permitted to optimize the heat treatment. Microstructural investigations always showed the presence of thin and smooth RE-rich film, associated with the highest coercivity.

Transmission electron microscopy study on Nd-rich phase and grain boundary structure of Nd–Fe–B sintered magnets

The crystal structure and morphology of the Nd-rich phase in postannealed sintered Nd–Fe–B hard magnets were investigated by transmission electron microscopy (TEM). The Nd-rich phase at grain boundaries thinner than about 2nm was found to be amorphous; as the boundary region broadened, it assumed a face-centered cubic (fcc) structure with a≈0.54nm; eventually, the selected area diffraction pattern exhibited systematically allowed superlattice spots, revealing that the Nd-rich phase belongs to the space group Ia3¯ with a≈1.1nm. The elemental analysis of the Nd-rich phase showed that it contains a considerable amount of oxygen. These findings indicate that the Nd-rich phase with a≈1.1nm is in fact isostructural to cubic Nd2O3 (Mn2O3 structure). We also found Nd-rich fcc precipitates (a≈0.54nm) of several ten to hundred nm inside the Nd2Fe14B grains. Both intergranular and intragrain Nd-rich phases possess simple orientation relationships with the matrix Nd2Fe14B phase, such as (111)Nd2Fe14B‖(111)Nd‐rich [2¯...

Transmission electron microscopy study of the evolution of precipitates in aged Al–Li–Cu alloys: the θ′ and T1 phases

Abstract We have investigated the structures of the θ′ and T1 precipitates in Al–1.6wt%Li–3.2wt%Cu and Al–2.4wt%Li–3.2wt%Cu alloys aged at 220 °C. The θ′ precipitates in the 1.6 wt%-Li alloy are those known for the Al–Cu binary system (a=0.40 and c=0.58 nm); whereas those in the 2.4 wt%-Li alloy exhibited two atypical structures. One, named a type I TB′ plate in this study, is isostructural to the known θ phase with a large c value of about 0.64 nm, having a habit plane parallel to the matrix {1 0 0}α; the other, type II TB′, is characterized by a=0.41 and c=0.61 nm, having a habit plane inclined at about 20° with {1 0 0}α, while maintaining a coherent interface. Also images of {1 1 1} precipitates in the 1.6 wt%-Li alloy revealed a continuous change from the T1 phase (c=0.935 nm), to a structure with c=0.87–0.90 nm. The image and small lattice parameter suggest that this {1 1 1} precipitate is likely to be the Ω phase.

Preparation and crystal structure of Sr2CuO2(CO3)

Abstract Sr 2 CuO 2 (CO 3 ) was prepared at 1273 K and 0.01 MPa CO 2 partial pressure in a flowing gas of O 2 CO 2 using a mixture of SrCO 3 and CuO powders as a starting material. The compound has a tetragonal structure with lattice constants a = 7.8045(1), and c = 14.993(1) A , and its space group is 14. The formula per unit cell is 8 Sr 2 CuO 2 (CO 3 ), and measured and calculated densities are D m = 4.71 g/cm 3 , and D x = 4.81 g/cm 3 , respectively. The crystal structure was refined by Rietveld analysis on X-ray powder diffraction and neutron powder diffraction data. The final residuals ( R F ) were 4.31 and 4.27% for the X-ray and neutron data, respectively. The structure consists of deformed [CuO 6 ] octahedrons and layers of ordered triangular CO 3 groups. Sr atoms having eight near oxygen neighbors are between the [CuO 6 ] octahedrons and the CO 3 layers.

The crystal structure of (C0.4Cu0.6)Sr2(Y0.86Sr0.14)Cu2O7

Abstract The crystal structure of a new compound, (C0.4Cu0.6)Sr2(Y0.86Sr0.14)Cu2O7, is derived from the structure of YBa2Cu3O7. Forty percent of CuO chains in the YBa2Cu3O7 structure are replaced by CO3 groups. This new compound has a superstructure along the a-axis and the c-axis. Diffuse superlattice reflections having periods of a∗/2-a∗/3 and c∗/2 were observed in electron diffraction patterns. Locally ordered distributions of C and Cu atoms were seen high-resolution images taken by transmission electron microscopy with an incident beam parallel to [010]. The basic structure of this superstructure was determined by neutron powder diffraction, assuming orthorhombic symmetry with the space group, Pmmm (lattice constants: a=3.8278(2), b=3.8506(2) and c=11.1854(5) A ).

Structural characteristics and magnetic properties of chemically synthesized CoPt nanoparticles

CoxPt100−x nanoparticles with dimensions from approximately 2 to 5 nm were synthesized using the reverse micelle method. High-resolution electron microscopy revealed single- and poly-crystalline nanoparticle structures. Twin boundary is a common feature in the polycrystals. As-grown nanoparticles did not show any coercivity at room temperature. However, the nanoparticles became ferromagnetic after annealing at 550 °C for 4 h. Face-centered-cubic to face-centered-tetragonal phase transformation of the nanoparticles that occurred at annealing temperatures above 550 °C was confirmed by electron diffraction patterns and x-ray diffractometry. Coercivity of the annealed nanoparticles were found depending on the nanoparticle chemical compositions. Composition atomic ratio of Co to Pt at around unity gives the highest coercivity of 5500 Oe at room temperature.