Masae Kikuchi

Modulated Structure of the Thermoelectric Compound [Ca2CoO3]0.62CoO2

We have determined the crystal structure of the composite crystal [Ca 2 CoO 3 ] 0.62 CoO 2 , known as Ca 3 Co 4 O 9 , by a superspace group approach. Structural parameters were refined with a super...

Tunnel magnetoresistance for junctions with epitaxial full-Heusler Co2FeAl0.5Si0.5 electrodes with B2 and L21 structures

The tunnel magnetoresistance (TMR) effect has been investigated for magnetic tunnel junctions with epitaxial Co2FeAl0.5Si0.5 Heusler electrodes with B2 and L21 structures on a Cr-bufferd MgO substrate. The epitaxially grown Co2FeAl0.5Si0.5 has B2 structure when annealed below 400°C, and has L21 structure for annealing above 450°C. The TMR ratio of 76% at room temperature and 106% at 5K were obtained for a MgO(001)∕Cr∕B2-type Co2FeAl0.5Si0.5∕Al oxide/Co75Fe25∕IrMn∕Ta. The TMR ratio is larger than that of magnetic tunnel junction with an L21-type electrode, which may be due to the smoother surface of the B2 structure and disordered L21 structure due to the Cr atom interdiffusion.

Structural and magnetic properties and tunnel magnetoresistance for Co2(Cr,Fe)Al and Co2FeSi full-Heusler alloys

We have investigated the structure and magnetization of Co2(Cr1−xFex)Al (0 ≤ x ≤ 1) and Co2FeSi full-Heusler alloy films deposited on thermally oxidized Si (SiO2) and MgO (001) single crystal substrates by ultra-high vacuum sputtering at various temperatures. The films were also post-annealed after deposition at room temperature (RT). Magnetic tunnel junctions with a full-Huesler alloy electrode were fabricated with a stacking structure of Co2YZ (20 nm)/Al (1.2 nm)-oxide/Co75Fe25 (3 nm)/IrMn (15 nm)/Ta (60 nm) and microfabricated using electron beam lithography and Ar ion etching with a 102 µm2 junction area, where Co2YZ stands for Co2(Cr1−xFex)Al or Co2FeSi. The tunnel barriers were formed by the deposition of 1.2 nm Al, followed by plasma oxidization in the chamber. The x-ray diffraction revealed the A2 or B2 structure depending on heat treatment conditions and the substrate, but not L21 structure for the Co2(Cr1−xFex)Al (0 ≤ x ≤ 1) films. The L21 structure, however, was obtained for the Co2FeSi films when deposited on a MgO (001) substrate at elevated temperatures above 473 K. The maximum tunnelling magnetoresistance (TMR) was obtained with 52% at RT and 83% at 5 K for a junction using a Co2(Cr0.4Fe0.6)Al electrode. While the junction using a Co2FeSi electrode with the L21 structure exhibited the TMR of 41% at RT and 60% at 5 K, which may be improved by using a buffer layer for reducing the lattice misfit between the Co2FeSi and MgO (001) substrate.

Shock behavior of zircon: phase transition to scheelite structure and decomposition

Abstract Both single-crystal and powdered specimens of zircon (ZrSiO4) were shocked to peak pressures between 30 and 94 GPa using the gun method, and specimens recovered were studied by means of X-ray diffraction analysis, transmission electron microscopy and infrared spectroscopy. Transformation to the scheelite structure started above 30 GPa, and was completed above 53 GPa in the case of single crystal specimens. Tetragonal unit cell parameters of the scheelite type ZrSiO4 at room condition are measured to bea = 4.7341(1)A, c = 10.51(1)A, c/a = 2.219(2) andV = 235.5(2)A3, which is smaller than that of the zircon type by 9.9%. The recovered scheelite-type ZrSiO4 reverts to the zircon type after rapid heating to 1200°C at room pressure. This transformation from the zircon type to the scheelite type is unique in that it is fast, displacive-like, but does not reverse. Tetragonal ZrO2 was detected as decomposition product in the single-crystal specimen shocked to 94 GPa, and further confirmed in a powdered specimen shocked to 53 GPa where enhancement of temperature is expected because of high porosity. Decomposition behavior of zircon observed in natural shock events is discussed on the basis of present experimental results.

Preparation of fine silicon particles from amorphous silicon monoxide by the disproportionation reaction

Fine Si particles have been prepared by the disproportionation reaction of silicon monoxide (SiO), that is: 2SiO → Si + SiO 2 . Amorphous powders of SiO are heated between 900°C and 1400°C in a flow of Ar and the obtained specimens are analyzed by X-ray powder diffraction and high-resolution transmission electron microscopy. The treatments between 1000°C and 1300°C for more than 0.5 h result in origination of Si particles dispersed in amorphous oxide media. The particle size varies from 1-3 to 20-40 nm, depending on the heating temperature. Kinetic analyses of the reaction reveal that the activation energy is 1.1 eV (82.1 kJ mol -1 ). The specimens annealed above 1350°C changes into a mixture of Si and cristobalite, suggesting a solid state transformation in the surrounding oxides from the amorphous to crystalline states.

Crystal Structures of YBa2Cu3-δAδO9-γ (A=Ni, Zn and Co)

A structural study of orthorhombic YBa2Cu3O6.94, YBa2Cu2.8Ni0.2O6.85 and YBa2Cu2.7Zn0.3O6.75, and tetragonal YBa2Cu2CoO7.16 by means of X-ray and time-of-flight type (TOF) neutron diffractometers was carried out to determine the locations of the dopant atoms, Ni, Zn and Co, in the 1-2-3 (Y/Ba/Cu) oxide. The nickel atoms occupy only the Cu2 site 2q(00z) with z=0.36. The zinc atoms occupy the Cu1 site 1a(000) and the Cu2 site with occupancies of 0.20(5) and 0.05(2). In the tetragonal YBa2Cu2CoO7.16, cobalt atoms were found at both the Cu1 and Cu2 sites with occupancies of 0.83(2) and 0.08(1).

X-Ray and electron microscopic study of a high temperature superconductor Y0.4Ba0.6CuO2.22

A superconducting oxide Y0.4Ba0.6CuO2.22 with Tc higher than 88 K was synthesized. The crystal structure was found to be a derivative of the perovskite structure with orthorhombic unit cell dimensions of a=3.818 (1) A, b=3.888 (1) A, c=11.667 (4) A, and V=173.19 (8) A3. Basic unit cell of the cubic perovskite structure was tripled along the c axis, probably due to the ordering of Y3+ and Ba2+ ions and/or of oxygen vacancy. Apparent short bond length for Cu-O may partly be explained by oxygen vacancy amounting to a quarter, but the observed high Tc suggests actual contraction of Cu-O bonds.

Spin polarization of Co2FeSi full-Heusler alloy and tunneling magnetoresistance of its magnetic tunneling junctions

The authors report spin polarization (P) and tunneling magnetoresistance (TMR) of epitaxially grown Co2FeSi thin films on a MgO (001) substrate. A Heusler-type L21 structure was observed in the samples sputter deposited at 473K or above. The P value of the ordered film was measured as 0.49±0.02 by the point contact Andreev reflection (PCAR) technique. The TMR values obtained from the magnetic tunneling junction (MTJ) using the Co2FeSi electrode and Al-oxide barrier were 67.5% at 5K and 43.6% at 298K, respectively. The P value estimated from the TMR using Julliere’s model matches the spin polarization measured by the PCAR very well, indicating that the TMR value from the MTJ is governed by the intrinsic value of P of the electrode material for incoherent tunneling.

Preparation and Chemical Composition of Superconducting Oxide Tl2Ba2Can-1CunO2n+4 with n=1, 2 and 3

Superconducting oxides of Tl2Ba2Can-1CunO2n+4 with n=1, 2 and 3 were synthesized, and their structure and chemical composition were examined by X-ray powder diffraction, EPMA, Rietveld analysis and electron microscopy. Partial substitution of Tl with Ca was confirmed, but the production of holes in Cu layers expected from this substitution was rather small for a 120 K superconductor. Charge transfer of Tl3-δ-Cu2+δ would provide both the hole in the Cu-O bond required for high Tc and s-like electron of Tl atoms for stabilization of layered structures.

Structural considerations on the mechanism of the shock-induced zircon-scheelite transition in ZrSiO4

Abstract The crystal structure of the scheelite-type ZrSiO 4 , which was transformed from the zircon-type under shock compression, was studied using an X-ray powder diffraction analysis. Positional parameters of O 2- in the 16 f site are determined to be u = 0.28, v = 0.14 and w = 0.07 (space group, I4 1 /a). The X-ray powder diffraction lines were broad, indicating that the scheelite-type ZrSiO 4 has considerable residual strain in the crystal. The mechanism of this shock-induced phase transition is discussed in terms of the displacive mechanism, where the [110] direction of the zircon-type is converted to the [001] direction of the scheelite-type. This model can explain why this zircon-scheelite transition occurs so fast under shock compression.