Masaru Itakura

Magnetic properties and microstructures of the Nd‐Fe‐B magnet powder produced by hydrogen treatment

The hydrogen treatment of the Nd‐Fe‐B alloy ingots was found to produce magnet powders with good magnetic properties. Typical magnetic properties of these powders are as follows; 4πIs = 9.5 kG, Br = 7.7 kG, iHc = 9.4 kOe, and (BH)max = 12.2 MGOe. Microstructural studies of these powders showed that they are made of fine crystalline grains of ∼0.3 μm diameter and that these crystalline grains in the individual magnet powder are not necessarily enclosed with boundary phase(s), which is quite different from previously known Nd‐Fe‐B magnets, i.e., the sintered magnet (the nucleation type magnet) or the amorphous ribbon magnet (the pinning type magnet). It is also noted that the size of these crystalline grains is comparable to that of the single magnetic domain of the tetragonal Nd2Fe14B intermetallic compound and the coercive force of these powders appears to be related to their fine crystalline grain size.

Ferromagnetism and electronic structures of nonstoichiometric Heusler-alloy Fe3-xMnxSi Epilayers grown on Ge(111).

For the study of ferromagnetic materials which are compatible with group-IV semiconductor spintronics, we demonstrate control of the ferromagnetic properties of Heusler-alloy Fe3-xMnxSi epitaxially grown on Ge(111) by tuning the Mn composition x. Interestingly, we obtain L2(1)-ordered structures even for nonstoichiometric atomic compositions. The Curie temperature of the epilayers with x approximately 0.6 exceeds 300 K. Theoretical calculations indicate that the electronic structures of the nonstoichiometric Fe3-xMnxSi alloys become half-metallic for 0.75 < or = x < or = 1.5. We discuss the possibility of room-temperature ferromagnetic Fe(3-x)Mn(x)Si/Ge epilayers with high spin polarization.

Microstructures and crystallographic orientation of crystalline grains in anisotropic Nd‐Fe‐Co‐B‐(Ga or Zr) magnet powders produced by the hydrogenation‐decomposition‐desorption‐recombination process

The microstructures of anisotropic Nd‐Fe‐Co‐B‐(Ga, or Zr) magnet powders produced by the hydrogenation‐decomposition‐desorption‐recombination (HDDR) process and the resulting crystallographic c‐axis orientation of Nd2(Fe,Co)14B crystals in the powder particles have been studied. It was found that the powder particles consist of fine Nd2(Fe,Co)14B crystals of the size of about 0.3 μm, and any boundary‐layer phase between the Nd2(Fe,Co)14B crystalline grains is almost absent. The morphology of fine Nd2(Fe,Co)14B crystalline grains in anisotropic magnet powders is the same as that of isotropic magnet powders produced by the HDDR process. In anisotropic powders, it was found that there is a strong correlation among the a axes, b axes, and c axes of the fine Nd2(Fe,Co)14B crystal grains. The dispersion in solid angles made by the c‐axis direction is less than ±18° in the case of Nd‐Fe‐Co‐B‐Ga magnet powder particle. The c‐axis direction of fine grains in magnet powders produced by the HDDR process is associated with that of large Nd2(Fe,Co)14B grains in the original cast or the homogenized alloy. The microstructure of anisotropic magnet powders before the hydrogen desorption step in the HDDR process is very complicated. It consists of five distinct regions, and among these, two regions are made of NdH2 and Fe.

Room-temperature epitaxial growth of ferromagnetic Fe3Si films on Si(111) by facing target direct-current sputtering

Ferromagnetic Fe3Si thin films with an extremely smooth surface morphology can be epitaxially grown on Si(111) at room temperature by facing target direct-current sputtering. The epitaxial relationship is Fe3Si(111)‖Si(111) with Fe3Si[11¯0]‖Si[1¯10]. By the application of the extinction rule of x-ray diffraction, the generated Fe3Si was confirmed to possess a B2 structure and not a DO3 one. The film showed a saturation magnetization value of 960emu∕cm3, which was slightly lower than that of bulk DO3-Fe3Si. It was observed that the magnetization easy axis was along the [11¯0] direction in the film plane.

Semiconducting nanocrystalline iron disilicide thin films prepared by pulsed-laser ablation

Amorphous iron silicide was reported to be semiconducting as well as β-FeSi2, and it has received considerable attention from both the physical and engineering points of view. However, there have been few studies and its basic properties are still unknown. We could grow the semiconducting nanocrystalline iron disilicide thin films by pulsed-laser deposition using an FeSi2 target. They consist of crystallites with diameters ranging from 3 to 5 nm. The carrier density and the mobility at 300 K were 1.5×1019 cm−3 and 35 cm/V s, respectively.

Spectral Absorption Properties of Ultrananocrystalline Diamond/Amorphous Carbon Composite Thin Films Prepared by Pulsed Laser Deposition

The spectral absorption properties of ultrananocrystalline diamond (UNCD)/amorphous carbon composite films grown by pulsed laser deposition with graphite were experimentally investigated. The indirect optical band gaps estimated from the absorption spectrum were 1.0 eV and approximately 5.65 eV, which correspond to those for the amorphous carbon surrounding the UNCDs and for UNCDs themselves, respectively. In addition, the spectrum revealed a direct gap having a value of approximately 2.2 eV. One probable origin can be the presence of grain boundaries between UNCDs and amorphous carbon since they are specific to UNCD/amorphous carbon; further, its appearance is consistent with the theoretical predictions.

Near-Edge X-ray Absorption Fine-Structure, X-ray Photoemission, and Fourier Transform Infrared Spectroscopies of Ultrananocrystalline Diamond/Hydrogenated Amorphous Carbon Composite Films

The chemical bonding structure of ultrananocrystalline diamond (UNCD)/hydrogenated amorphous carbon (a-C:H) composite films prepared by pulsed laser deposition was examined by near-edge X-ray absorption fine-structure (NEXAFS), X-ray photoemission, and Fourier transform infrared (FTIR) spectroscopies. An intense sp3-CH peak was observed in the FTIR spectrum. This implies that the sp3-CH peak originates from the grain boundaries between UNCD crystallites, wherein dangling bonds are terminated with hydrogen atoms. The presence of an intense σ*C–C peak in the NEXAFS spectrum and a narrow sp3 peak in the photoemission spectrum was specific to UNCD/a-C:H films; these confirm the existence of UNCD crystallites.

Coercivity enhancement of Dy-coated Nd-Fe-B flakes by crystallization

The coercivity of isotropic Dy-coated Nd-Fe-B flakes was enhanced by crystallization and simultaneous diffusion of Dy from their surfaces. Amorphous Dy-coated Nd-Fe-B flakes were crystallized by heating them to 923 K 2over a 2 min period followed by rapid cooling. During crystallization, the Dy on the surface diffused into the flakes. This low-temperature rapid annealing produced flakes with fine grains and the Dy diffusion enhanced their coercivity. The coercivity after crystallization increased with increasing Dy layer thickness, although the remanence decreased when the layer thickness exceeded 3 μm. Thick coatings of over 6 μm resulted in the formation of DyFe2, which degraded the magnetic properties of the crystallized flakes. Flakes with a 3-μm-thick coating exhibited excellent magnetic properties after annealing: They had a coercivity of 1880 kA/m and a remanence of 78 emu/g. This coercivity is approximately 500 kA/m higher than that of uncoated flakes, whereas the remanence is comparable to that o...

Review of Fabrication and Characterization of Nd–Fe–B Thick Films for Magnetic Micromachines

Isotropic Nd-Fe-B thick film magnets were prepared by a high-speed pulsed laser deposition method followed by a post annealing. The deposition rate of 90 mum/h could be successfully achieved, and a pulse annealing was adopted as the post annealing process in order to enhance coercivity. Use of a substrate heating system under the high deposition rate enabled us to obtain anisotropic thick films with (BH) max of approximately 120 kJ/m3, which show the potential for an improvement in the properties of the micromachines. Novel micromachines comprising the isotropic films were introduced

Short-range order in Al-rich γ-TiAl alloys studied by high-resolution transmission electron microscopy with image processing

The atomic arrangements of short-range order (SRO) in two Al-rich γ-TiAl alloys with 62.5 and 60.0at.%Al respectively have been investigated by high-resolution transmission electron microscopy with image processing. For both alloys, SRO structures are formed in Ti-rich (002) layers of the L10 ordered γ-TiAl matrix, in the course of the phase transformation from the Al5 Ti3 long-period superstructure phase at lower temperatures to the h-Al2 Ti phase at higher temperatures. The SRO structures are composed of three types of ordered cluster: Ti4 Al type (square shaped), Ti3 Al type (fat rhombus shaped) and Ti2 Al type (lean rhombus shaped). The ordered clusters form local microdomains of the Al5 Ti3 and Al3 Ti2 superstructures in addition to the Al5 Ti3 and h-Al2 Ti superstructures, by different methods of tiling the ordered clusters. Such ordered clusters or microdomains tend to be in contact with each other through common {310) boundaries. This results in peculiar diffuse streaks in diffraction connecting intensity maxima arising from the long-period superstructures.