Mitsugu Sohma

Fabrication and magnetoresistance of tunnel junctions using half-metallic Fe3O4

Magnetite (Fe3O4) is believed to be half metal, providing 100% spin-polarized conduction electrons. The half-metallic nature of magnetic electrodes for tunneling junction devices is expected to induce a large magnetoresistance. We investigated the structural and chemical properties of interfaces in ferromagnet–insulator–ferromagnet (Fe3O4/MgO/Fe) tunnel junctions. Al/Ag/Fe3O4/MgO multilayers for magnetic tunnel junction have been fabricated on α-Al2O3 (001) and MgO (100) substrates by a molecular beam epitaxy system. The Fe3O4 quality was examined by reflection high-energy electron diffraction (RHEED), x-ray diffraction (XRD), superconducting quantum interference device magnetometry, atomic force microscopy (AFM), and in situ x-ray photoelectron spectroscopy. RHEED and XRD results showed that the epitaxial Fe3O4 layer with a smooth surface was successfully grown on substrates. The stoichiometric Fe3O4 was confirmed by Verway transition in temperature dependence of magnetization. AFM data showed relatively...

Three Types of Ternary Selenides with Layered Composite Crystal Structures Formed in the Pb-Nb-Se System

Three types of ternary selenides with layered composite crystal structures, (PbSe)1.14NbSe2[A], (PbSe)1.14(NbSe2)2[B] and (PbSe)1.14(NbSe2)3[C], have been prepared and characterized by powder X-ray diffraction, electron diffraction, intercalation and electric resistivity measurement. Results suggest that each selenide consists of PbSe layers(two-atoms-thick) and NbSe2 sandwiches(three-atoms-thick) stacked alternately with the ratios of 1:1 for A, 1:2 for B and 1:3 for C, respectively. Electric resistivity measurements suggest that B and C are superconductors whose transition temperatures are 3.4 K and 4.8 K, respectively.

Preparation and Characterization of Epitaxial VO2 Films on Sapphire Using Postepitaxial Topotaxy Route via Epitaxial V2O3 Films

Epitaxial VO2 films were prepared on the C-planes of α-Al2O3 substrates by a metal organic deposition (MOD) process. It was difficult to obtain the single phase of (010)M-oriented VO2 films, in which the subscript M refers to the monoclinic indices, by the heat treatment of amorphous precursor films in the VO2-stable region after the pyrolysis of the coating solution. The product films consisted of discontinuous circular grains of 1–2 µm size on the substrate surface. Therefore, we prepared the (010)M-oriented epitaxial VO2 films using postepitaxial topotaxy (PET), that is, topotactic oxidation of (0001)-oriented epitaxial V2O3 films. First, epitaxial V2O3(0001) films were obtained by MOD starting with a vanadium naphthenate solution. Second, the epitaxial V2O3(0001) films were topotactically oxidized at 500 °C in an Ar–O2 gas mixture with pO2 = 10-4 atm to obtain (010)M-oriented epitaxial VO2 films. The epitaxial relationships were VO2(010)M ∥ α-Al2O3(0001) and VO2[100]M ∥ α-Al2O3[0110], [1010], [1100]. The VO2(010)M films exhibited metal–semiconductor transitions with hysteresis loops at 60–80 °C. The resistivity change before and after the transition of the VO2(010)M film oxidized for 6 h was three orders of magnitude.

Automatic Measurement of the Distribution of

Nondestructive, ac inductive methods are widely used to measure critical current densities Jc of large-area superconducting films. However, conventional methods are not accurate because they have not considered the electric-field E criterion of the Jc measurement. In these methods, ac magnetic fields are generated with ac currents Io cos wt in a small coil mounted just above the film. Jc is calculated from the threshold coil current Ith, at which full penetration of magnetic field is achieved. We recently estimated the induced electric fields in the superconducting film at Io = Ith, which are proportional to the frequency, and proposed a method to determine the E - J characteristics by measuring Ith at various frequencies. In this paper we propose a standard method to precisely measure Jc with an electric-field criterion by detecting third-harmonic voltages. We also demonstrate a machine that can automatically measure the distribution of Jc and the n-values of the power-law E - J characteristics. We show several examples of measurements of co-evaporated YBCO films and low-cost metal organic deposition YBCO films, which were used in switching experiments for resistive fault-current limiters.

J_{\rm c}

Epitaxial (YBCO) films were prepared on cerium oxide-buffered (40 nm)-cut single-crystal sapphire substrates by a novel excimer-laser-assisted metalorganic deposition (ELAMOD). Laser irradiation after application of a fluorine-free ingredient solution onto the substrates revealed to have the advantages of shorter heating (prefiring and heat-treatment) time and higher superconducting current density . The in excess of 5 was measured by an inductive method at 77.3 K. The laser irradiation is considered to produce a desirable precursor, in which three metal components are well-mixed in short time. The X-ray diffraction measurement of the irradiated area clearly showed a complete -axis oriented YBCO and no impurity phase, whereas -axis oriented YBCO was also observed in the area without irradiation. The ratio of the -axis component decreased with an increase of the laser irradiation.

and n-Values in Large-Area Superconducting Films Using Third-Harmonic Voltages

Epitaxial cerium oxide (CeO/sub 2/; thickness: 40 nm) buffer layers were deposited on large area (maximum size 10/spl times/30 cm/sup 2/) R-cut single-crystal sapphire (/spl alpha/-Al/sub 2/O/sub 3/) substrates by double electron-beam guns. Substrate heater-and-holder system was modified to maintain the substrate temperature uniform over the large area during deposition. Oxygen gas of a pressure 4/spl times/10/sup -2/ Pa was introduced to keep radio frequency plasma. When the substrate temperature was controlled in the range 640/spl deg/-700/spl deg/C, the CeO/sub 2/ buffer showed a complete [001] orientation and very smooth surfaces by x-ray diffraction analysis and atomic force microscopic observations, respectively, in the whole area of the large size substrates. These buffer layers without post-deposition annealing were quite suitable for preparing YBa/sub 2/Cu/sub 3/O/sub 7/ (YBCO) films by a metal-organic deposition (MOD) using a metal acetylacetonate-based solution. The average critical current density of the 210-nm-thick MOD-YBCO film was in excess of 2.6 MA/cm/sup 2/ measured by an inductive method at 77.3 K.

Preparation of Epitaxial YBCO Films by a Novel Excimer-Laser-Assisted MOD

Magnetic-field-angle θ dependence of critical current density Jc was measured in YBa2Cu3O7 films irradiated with 3-MeV gold ions. Such films were recently found to show large pinning force arising from point-like irradiation defects. A dimpled line-shape of Jc(θ) and its variation with magnetic field strength were revealed that could be well described by a simple model based on flux-lattice shear and Blatter scaling. Our results strongly suggest that vortex elasticity and electron-mass anisotropy coordinately produce the characteristic Jc anisotropy, called “shoulders,” in cuprate and iron-pnictide films.

Preparation of CeO/sub 2/-buffer layers for large-area MOD-YBCO films (10/spl times/30 cm/sup 2/) with high-J/sub c/

Epitaxial YBa/sub 2/Cu/sub 3/O/sub 7/ (Y123) films were obtained by metal-organic deposition (MOD) using a simple, low-cost vacuum technique without gas flowing. The total pressure and oxygen partial pressure in the furnace were controlled to be 10 kPa and 10 Pa, respectively, by evacuation with a rotary pump followed by refilling with a mixture of O/sub 2/ and N/sub 2/ (O/sub 2/ content: 1000 ppm). XRD analyses exhibited that c-axis-oriented epitaxial Y123 films have successfully been obtained on CeO/sub 2/-buffered YSZ (100) (CbZ) and CeO/sub 2/-buffered sapphire (012) (CbS) substrates. In-plane alignments of these films were as high as that of the CeO/sub 2/ buffer layer. The film on the CbZ demonstrated a high critical current density J/sub c/ of /spl sim/2.1 MA/cm/sup 2/ at 77 K all over the film; the fluctuation of J/sub c/ being within /spl plusmn/10% of the average. Inductive T/sub c/ measurement showed a very sharp peak with T/sub c/=90.5 K. Inductive-J/sub c/ and T/sub c/ of the Y123 film on CbS were 1.1 MA/cm/sup 2/ and 89.6 K, respectively, whereas the T/sub c/ of the Y123 film on LaAlO/sub 3/ was lower than 77 K owing to the occurrence of the a-axis grains.

Dimpling in critical current density vs. magnetic field angle in YBa2Cu3O7 films irradiated with 3-MeV gold ions

NiO/Pd epitaxial superlattices were successfully grown by a reactive evaporation method and their structures were studied by various X-ray diffraction (XRD) methods. We prepared (111) oriented films and (100) oriented films on α-Al2O3(00·1) and MgO(100) substrates, respectively. The XRD patterns of 2θ/θ scans were analyzed by an extended step model calculation. In-plane alignment was investigated by pole figure method and reciprocal space mapping. The specimens can be epitaxially grown up to a certain thickness of the NiO layer tNiO [tNiO≤20 A for (111) oriented film and tNiO≤10 A for (100) oriented film] with the lattice shrinking of the NiO of approximately 10%. On the other hand, those beyond the critical thickness have texture structure with the lattice spacing of bulk NiO.

Metal organic deposition of epitaxial Y123 films using a low-cost vacuum technique

Abstract Single phase copper oxide (CuO) films were prepared by a radio frequency (RF) plasma assisted reactive vapor deposition method. Magnetic susceptibility of the films was measured by a SQUID susceptometer in the temperature range between 4.3K and 300K. The susceptibility has an almost constant value against temperature. Moessbauer measurements of 57 Fe-doped CuO films indicate that the magnetic transition temperature is a little lower than that of the reported bulk CuO.