Toshio Umemura

Texture of Nd–Fe–B thin films prepared by magnetron sputtering

We have investigated the texture and magnetic properties of Nd12Fe88−xBx sputtered films with various boron contents, x. The c-axis orientation of Nd2Fe14B phase in the films was higher and the perpendicular coercive force became greater with increment of x. With more increment of x than 15, however, the coercive force decreased owing to formation of a large amount of amorphous soft magnetic phase. The transmission electron microscope observations revealed that highly c-axis oriented Nd12Fe71B17 film has a unique microstructure; oriented Nd2Fe14B phase and amorphous phase with widths of about 200 and 50 A, respectively, are alternately developed. This two-phase structure has realized an exchange coupled nanostructured magnet composed of aligned hard magnetic phase and soft magnetic phase. The maximum energy product of Nd12Fe71B17 film was estimated at 270 kJ/m3 with micromagnetic analysis assuming that the film was composed of complete regular structure.

Irreversibility line in superconducting Bi-2212 single grain with fine normal particles

Abstract The irreversibility line was obtained in terms of the imaginary a.c. susceptibility measurement for a melt-processed superconducting Bi-2212 single grain with fine non-superconducting particles. This line showed different temperature dependences in high and low temperature regions. The one in the low temperature region agrees approximately with the prediction from the flux creep theory with experimentally observed pinning parameters. On the other hand, the one in the high temperature region is close to the well known characteristics in Y-based superconductors, suggesting flux pinning by boundaries such as the surfaces of non-superconducting particles. A quantitative discussion is also given on flux bundle size.

Isotropic Jc‐B properties of YBa2Cu3O7−x thin films containing fine precipitates by metalorganic chemical vapor deposition

Y1Ba2Cu3O7−x superconducting thin films which have isotropic Jc‐B properties were prepared onto SrTiO3 (100) substrates at 700 °C by metalorganic chemical vapor deposition. The Jc did not depend on directions of the field either perpendicular or parallel to the c axis of the films and the films had a high Jc of 6.1×104 and 7.1×104 A/cm2 at 77 K and 5 T perpendicular and parallel to the c axis, respectively. Transmission electron microscopy observation and x‐ray energy dispersive spectroscopy analysis indicated that the films contained many fine Y2BaCuO5‐like precipitates with diameter of ∼10 nm and they were distributed uniformly with the density Np as high as 1023/m3.

YBa2Cu3Ox Thin Films with Yttria Stabilized Zirconia Buffer Layer on Metal Substrate by Liquid Source Chemical Vapor Deposition Using Tetrahydrofuran Solution of β-Diketonates

Textured YBa2Cu3Ox (YBCO) thin films with c-axis orientation on Ni-based metal substrate with yttria stabilized zirconia (YSZ) and Pt buffer layers were formed by liquid source chemical vapor deposition (LS-CVD) using tetrahydrofuran (THF) solution of β-diketonates. The YSZ buffer layer with a-axis orientation was formed by the similar LS-CVD on prebuffer layers of amorphous YSZ and Pt deposited by sputtering at room temperature. The CVD-YSZ buffer layer was mechanically polished before the deposition of YBCO. By transmission electron microscopy (TEM) and analytical electron microscopy (AEM), it was ascertained that YBCO and YSZ layers had platelike and column-shaped grains, respectively. An intermetallic compound of the Pt-Ni-Cr system was produced in the vicinity between the YSZ layer and metal substrate. The critical temperature (T c) of 91 K and the transport critical current densities (J c) of 1.4×105 A/cm2 at 77 K, zero field, and 2.0×104 A/cm2 at 1.5 T were achieved.

Critical Current Density and Flux Creep in Melt-Processed Bi-Pb-Sr-Ca-Cu-O Superconductors

We investigated the critical current density and the flux creep in melt-processed Bi-Pb-Sr-Ca-Cu-O for the purpose of studying the existence of strong flux pinning forces due to normal conducting precipitates. We fabricated samples with a similar microstructure to melt-processed Y-Ba-Cu-O and Tl-Sr-Ca-Cu-O having a strong pinning force. When the matrix was (Bi, Pb)2Sr2Ca1Cu2Oy (2212) phase, the irreversibility field Birr was expressed as Birr\propt(1-T/Tc)3/2, which suggested that the expected flux pinning was due to normal conducting precipitates. A part of the matrix was transformed to (2223) phase by the postannealing of the sample; however, the superconducting properties were inferior to those observed in (2223) polycrystals.

Preparation and Piezoelectric Property of Lead Titanate Thin Films for GHz-Band Resonators

Film bulk acoustic wave resonators (FBAWRs) at the GHz band were successfully fabricated using piezoelectric lead titanate, PbTiO 3 (PT) films on GaAs substrates by a process similar to that used in microwave integrated circuit (MIC) fabrication. PT films deposited on Pt/Ti bottom electrodes by RF magnetron sputtering oriented preferentially along the (111) crystal direction. Resonances of the bulk acoustic mode were observed around 1.9 GHz. The effective electromechanical coupling coefficient (k t 2 ) and electrical Q were estimated to be ∼9.4% and ∼65, respectively. The relationship between residual stress in the PT films and Q was investigated. It was found that stress in the PT film exerted a large influence on Q.

Absorption and Desorption of Oxygen in Y1Ba2Cu3O7-δ

The absorption and desorption velocities of oxygen in a Y1Ba2Cu3O7-δ sintered sample were measured by a thermogravimetric analysis when the atmospheric gas of nitrogen was exchanged for oxygen and oxygen for nitrogen. The weight change of the sample could be expressed by an exponential function of time in each process. In the absorption process of oxygen, the time constant was a complex function of the temperature, while in the desorption process, it was the monotonous temperature dependence. The diffusion constant and the activation energy were estimated at the temperature range of the tetragonal phase.

Microstructure and superconducting properties of Bi-Sr-Ca-Cu-O system prepared by a melt process

The microstructure and superconducting properties for samples with the nominal composition of Bi2Sr2Ca1+2xCu2+xOy (x=0, 0.2, 1.0) prepared by a melt process have been examined. The magnetic field dependence of the intragrain critical current density was improved by enriching the Ca and Cu contents. Such samples included two kinds of precipitates, (Ca, Sr)2CuOy and Sr-Pt-O compounds. The latter precipitates were much smaller in size than the former, and this suggests that they may be more effective in enhancing the pinning force.

Introduction of pinning center in Bi-based oxides prepared using melt process

Abstract To strengthen the pinning force by introducing extrinsic pinning centers, Bi 2 Sr 2 Ca 1+2n Cu 2+n O y superconductors which contained dispersed precipitates with high density were synthesized by using the melt process. (Ca, Sr) 2 Cu 1 O y and PtSrO seemed to be candidates for the pinning centers through XRD and EPMA observations. Critical currents flowing perpendicular to the c-axis were evaluated through magnetization measurements. Those at 4.2K were scarcely influenced by the existence of such precipitates. But, with the n =0.15 those at 20K were five times higher in the magnetic field below 0.1 teslas, and three hundred times higher above 3 teslas, and the degradation by increasing the magnetic field from 0.1 to 3 teslas was only one unit.

Superconducting Fault-Current Limiter for 200-Vrms-Class Circuits Using YBa2Cu3Ox Thin Film by Chemical Vapor Deposition

A superconducting fault-current limiter (FCL) for 200-Vrms-class circuits using YBa2Cu3Ox (YBCO) thin film fabricated by chemical vapor deposition (CVD) was investigated. The FCL consisted of a 400-nm-thick YBCO film with meander pattern of 1 mm wide×115 mm long. The film was coated with a protective silver film having a thickness varying between 110 and 130 nm from place to place. Half-cycle alternating voltage (50 Hz) was applied to the FCL device which was immersed in liquid nitrogen (77 K), and a typical current-limiting waveform was measured. When a voltage of 200 Vrms was applied, the fault current with a peak of 400 A was limited to the peak let-through current of 11 A. The beginning time of the current limiting was about 0.1 ms.