Nobuyoshi Sakamoto

Microwave Dielectric Properties of Ba(Mg1/2W1/2)O3?BaTiO3 Ceramics

Ceramics with the chemical formula x Ba(Mg1/2W1/2)O3–(1-x) BaTiO3 (x=0.40–1.0) are prepared for use as dielectric resonators. The specimens were investigated by X-ray diffraction analysis, scanning electron microscopy and transmission electron microscopy. As x decreased from 1.0 to 0.40, the temperature coefficient of resonant frequency increased gradually from -33 to +303.5 ppm/°C. At x=2/3, the Q×f value increased markedly. The crystal structure with a Mg–W ordered B-site sublattice changed to a crystal structure with a Mg–W–Ti ordered B-site sublattice at x=0.84–0.72. Ba(Mg1/3W1/3Ti1/3)O3 is formed at x~2/3, and has excellent dielectric properties (e r=14.5, Q×f=107000 ( GHz), τ f=-7.6 ppm/°C). The B-site cation of Ti improved the stability of the temperature coefficient of resonant frequency (TCF) for Ba(Mg1/2W1/2)O3 (τ f=-33 ppm/°C).

NbN Films Prepared by Magnetron Sputtering

NbN films on sapphire or copper substrates have been prepared by magnetron sputtering in argon-nitrogen atmospheres. The highest transition temperature, Tc=15.7 K, was achieved with a film sputtered on a sapphire substrate at a substrate temperature Ts=800°C and a partial nitrogen pressure PN2 =5×10-3 Torr. Uniform films with transition width narrower than 0.4 K were prepared with good reproducibility by this method. A short NbN tape of metallic luster stripped off from the copper substrate still showed a Tc of ~15 K even after being bent to a radius of curvature as small as 6 mm. These results indicate that magnetron sputtering is useful for preparing NbN films and long NbN tapes.

Radio frequency magnetic field effects on molecular dynamics and iron uptake in cage proteins

The protein ferritin has a natural ferrihydrite nanoparticle that is superparamagnetic at room temperature. For native horse spleen ferritin, we measure the low field magnetic susceptibility of the nanoparticle as 2.2 x 10(-6) m(3) kg(-1) and its Néel relaxation time at about 10(-10) s. Superparamagnetic nanoparticles increase their internal energy when exposed to radio frequency magnetic fields due to the lag between magnetization and applied field. The energy is dissipated to the surrounding peptidic cage, altering the molecular dynamics and functioning of the protein. This leads to an increased population of low energy vibrational states under a magnetic field of 30 microT at 1 MHz, as measured via Raman spectroscopy. After 2 h of exposure, the proteins have a reduced iron intake rate of about 20%. Our results open a new path for the study of non-thermal bioeffects of radio frequency magnetic fields at the molecular scale.

Magnetic properties of proximity-induced superconducting matrix in multifilamentary wires

Critical current densities Jcp of proximity-induced superconducting matrices in NbTi multifilamentary wires are estimated from measured twist-pitch dependence of magnetization. The values of Jcp are 2–4 orders of magnitude smaller than those of the superconducting filaments and decrease rapidly by raising temperature T and magnetic field Be. The interfilamentary spacing dN of submicrons results in zero screening length at measured temperatures ranging 4.5 to 8 K and the induced superconductivity is suggested to be type-II. The upper critical field Bc2p is obtained by applying the scaling law to Jcp data. Bc2ps at 4.5 K are 1.2–3.0 T for dN = 0.20–0.59 μm. The Cooper pair penetration length KN−1 estimated from the temperature dependence of Bc2p shows the clean limit characteristic as T−1 and 0.32 μm for dN = 0.59 μm wire at 4.5 K. KN−1 estimated from the temperature dependence of Jcp agrees consistently with those from Bc2p(T).

Magnetic susceptibility studies of grains in HgPb1223 ceramic superconductors

Abstract AC susceptibilities and DC magnetizations of Hg (1− x ) Pb x Ba 2 Ca 2 Cu 3 O y ( x =0.1 denoted as Hg-01 and x =0.2 as Hg-02) synthesized directly from the metal oxides were measured using a SQUID magnetometer and a PPMS susceptometer. The superconducting volume fractions were estimated from the temperature dependence of Meissner magnetization. Pb substitution of Hg site showed to enhance the volume fraction without sacrificing the critical temperatures of 134 K in both samples. Critical current densities J c were estimated from the magnetization width. Pinning potentials U p were evaluated from the relaxation rate of magnetic flux creep characteristics by using the Anderson–Kim model. Both samples show rapid decreases of the J c at high temperature. Hg-01 sample showed slightly higher J c and U p . Second peaks χ ″ p2 in the imaginary part of the AC susceptibilities χ ″ appeared at low magnetic fields in both samples. The peak χ ″ p2 of Hg-01 disappears gradually at high magnetic field and the χ ″ p2 of Hg-02 sample dominates at low temperatures. Dependence of the χ ″ p2 on the temperature, DC magnetic field and AC field is extensively examined. Grain characteristics in HgPb1223 ceramic superconductors are discussed by taking into account of weak-coupling effect between grains and existence of second phases in both samples.

Magnetization and irreversibility field in powdered MgB2 superconductor

Abstract Magnetizations and AC susceptibilities were measured on powdered new superconductor MgB 2 using an SQUID magnetometer and a PPMS susceptometer at temperature range 4.5–40 K under DC magnetic fields up to 14 T. Particle diameters d of powdered MgB 2 specimens are 20 d 2 -20) and 50 d 2 -50). Magnetization curve for MgB 2 -50 sample is symmetric and larger than that for MgB 2 -20 sample. The irreversibility fields are determined by the width of magnetization curves and the imaginary parts of the AC susceptibilities. Temperature dependence of the irreversibility field almost agrees for both powder samples and is smaller than that of oxide superconductors. In addition these characteristics are also discussed by comparing with the flux creep theory.

AC susceptibility in NbTi multifilamentary wires

Abstract AC susceptibilities of NbTi multifilamentary wires with copper matrix were measured using a SQUID magnetometer and a susceptometer in a range of temperatures 2.0–10.0 K. The DC (up to 6.0 T) and AC magnetic fields (0.05 or 1 mT, 10 Hz) were applied perpendicular to the wire axis. The upper critical fields of matrix Bc2p were obtained from the peak temperatures Tp appearing in the imaginary parts of the AC susceptibilities. These values were compared with those estimated from measured twist-pitch dependence of magnetization where the scaling law was applied to the critical current densities of the proximity-induced superconducting matrix Jcp. Both results are favorably compared.

Y-Ba-Cu oxide thin films prepared by DC magnetron sputtering

Superconducting Y-Ba-Cu oxide thin films were sputtered on MgO(100) substrates by DC magnetron sputtering from an oxide powder target. The films were deposited at the high rates of 9–29 nm/min and showed, at best, Tce (R=0) up to 78 K with a transition width ΔTc of 5 K, after annealing in oxygen for 1 h at 900°C. The X-ray diffraction pattern showed a highly preferred orientation of the c-axis even in the thick (2.0 µm) film. The film sputtered at a higher deposition rate had a broad transition width ΔTc=22 K and its X-ray diffraction pattern showed the coexistence of orthorhombic and tetragonal phases.

Effect of grain alignment on magnetic properties of Hg(Re)-1223 superconductors

Abstract Alignment of HgBa 2 Ca 2 Cu 3 Re 0.2 O y (Hg(Re)-1223) powders was made in epoxy resin under a high magnetic field of 10 T to be confirmed by X-ray analysis. DC magnetizations and AC susceptibilities of the grain aligned specimen were measured by SQUID and PPMS magnetometers at temperatures of 5–110 K and under the field of 0–14 T for both field directions of B parallel and perpendicular to ab -plane. The magnetization width for B parallel to the c -axis ΔM c showed high values at low field, decreased rather rapidly with the magnetic field compared to that for B parallel to the ab -plane ΔM ab and became lower than ΔM ab above a crossing field B cr . Peak-heights of the imaginary parts of the AC susceptibilities χ ″ were largest at B ∥ c -axis. Non-aligned samples always showed intermediate characteristics between B ∥ c -axis and B ∥ ab -plane. Irreversibility fields of all samples were also evaluated. Correlations of the pinning mechanism with the crystal axis orientations are discussed.

AC susceptibility and magnetic relaxation in powdered MgB2 with several diameter sizes

Abstract AC susceptibilities and magnetic relaxations were measured on powdered new superconductor MgB2 using a SQUID magnetometer at temperature range 4.5–40 K under DC magnetic fields U 0 ∗ for flux creep were also estimated from measured magnetic relaxation. Estimated values of U 0 ∗ were 383 and 482 meV at T=20 K and B=0.8 T for samples with particle diameter of 20 U 0 ∗ were also compared with those of oxide superconductors.