Gwangseo Park

Serial array high T/sub c/ SQUID magnetometer

SQUID magnetometers have been fabricated from series arrays of YBa/sub 2/Cu/sub 3/O/sub 7/ dc SQUIDs by pulsed laser film deposition and ion mill patterning techniques. The array was designed either in linear chain with flux focusers or in meander structure. The arrays consisted of 50-130 step-edge junction SQUIDs and each SQUID had inductance of 20-40 pH with 2-5 /spl mu/m junction width. A linear array of 50 SQUIDs showed a modulation amplitude of 150 /spl mu/V which corresponds to a gain of /spl sim/10. One of 80 SQUID meander arrays had a gain of as high as 50. These results provide feasibility of the series array of SQUIDs in the direct readout SQUID amplifier circuitry with wide bandwidth. Larger number SQUID arrays and processes to increase the gain to the theoretical value need to be studied.

Raman Spectra Study of K0:5Na0:5NbO3 Ferroelectric Thin Films

We have fabricated K0.5Na0.5NbO3 (KNN) thin films on Pt substrates by a chemical solution deposition method and investigated the effect of K and Na excess (0–30 mol %) for KNN thin film. The KNN film with 20 mol % excess K and Na was an optimized KNN perovskite phase. From the Raman spectra, a change in the scattering mode was observed due to the chemical composition fluctuations of the excess K and Na. The peak of the (v1 + v5) internal vibrational mode in the NbO6 octahedra was split into two vibration modes which were shifted toward lower or higher wavenumbers depending on the K and Na cation deficiencies or redundancies.

Fabrication of a Josephson junction using an atomic force microscope

A Josephson junction was fabricated by inducing a selective surface modification on a YBa2Cu3O7−y strip with an atomic force microscope (AFM). The surface modification in the field of conductive AFM tip results in the controlled growth of protrusions across the entire strip. By properly regulating the extent of AFM modification, we achieved a Josephson junction. The self-radiation power of about 50 pW at a resonant frequency of 22 GHz was detected from this junction, which is in excellent agreement with the Josephson frequency-voltage relationship.

Pyroelectric properties of AH2PO4(A═H, K) and L-alanine doped TGS

Abstract AH2PO4(A═H or K) doped triglycine sulfates (TGSs) were grown from water solutions by the cooling method and their ferroelectric properties were studied by measuring the pyroelectric coefficients and the dielectric constants. It was found that KDP doped TGS grown with 10 wt% L-alanine exhibited a single poled state in the ferroelectric phase and showed the best pyroelectric figure of merit in comparison to those doped with other compounds.

Ferromagnetic nanospheres of perovskite manganite La0.7Ca0.3MnO3 prepared by template replication in porous carbon framework

Nanospheres of perovskite manganite La0.7Ca0.3MnO3 (LCMO) were prepared using porous carbon replica as a template. Close-packed arrays of silica spheres were infiltrated with a sucrose solution, which was then carbonized. After removal of the silica template, pores in the replica were filled with metal acetate precursors. The filled replica was sintered at 800 °C. The resulting hollow LCMO nanospheres were characterized by magnetization, electrical transport, scanning electron microscope, and transmission electron microscope measurements. The magnetization data of LCMO show a ferromagnetic transition near 250 K.

Crossover between weak anti-localization and weak localization by Co doping and annealing in gapless PbPdO2 and spin gapless Co-doped PbPdO2

The magnetotransport properties of Pb(Pd,Co)O2 and PbPdO2 thin films were investigated. In magnetoconductance curves, we observed a crossover between weak anti-localization (WAL) and weak localization (WL) depending on the annealing and Co doping in PbPdO2 thin films. For the Pb(Pd,Co)O2 case showing WAL signals, the ex-situ annealing weakens the Pd-O hybridization by stabilizing Co3+ states and generating Pd1+ states, instead of Pd2+, so that the spin-orbit coupling (SOC) strength is significantly reduced. It causes the dominant magnetotransport mechanism change from WAL to WL. This annealing effect is compared with the PbPdO2 case, which possesses WL signals. The annealing process stabilizes the oxygen states and enhances the Pd-O hybridization, and consequently the SOC strength is enhanced. Our experimental results are well explained by the Hikami-Larkin-Nagaoka theory in terms of two important physical parameters; SOC strength-related α and inelastic scattering length lϕ.

Patterned Y1Ba2Cu3Oy films on Y2BaCuO5 substrates by a surface diffusion process

A technique which combines the fabrication and the patterning of 10‐μm‐thick films of the high Tc superconducting Y1Ba2Cu3Oy directly on the Y2BaCuO5 ceramic substrates has been developed. The Ba3Cu5Oy(3BaCuO2+2CuO) powders were patterned on the substrate by silk‐screen printing and then heat treated at 930 °C for 3 h in an oxygen atmosphere. Measurements of resistivity and critical current density at 77 K for a patterned Y1Ba2Cu3Oy film with a 250‐μm width and 10‐μm thickness show zero resistivity at 88 K and 103 A/cm2, respectively. The microwave loss measured on the Y2BaCuO5 substrate is found to be negligibly low below 77 K.

Influence of mobile space charges on the ferroelectric properties of (K0.50Na0.50)2(Sr0.75Ba0.25)4Nb10O30 ceramics

We have measured ac complex impedance and thermal depolarization current for aged (K0.50Na0.50)2(Sr0.75Ba0.25)4Nb10O30 ceramic samples to investigate the influence of mobile space charges on the ferroelectric properties. The impedance behavior was analyzed using an equivalent circuit composed of a capacitor connected in series with a parallel resistor-capacitor circuit. The dielectric relaxation is found to involve two mechanisms; one is attributed to the hopping process of the space charges bound inside grains and the other is associated both with the intrinsic polarization of the ferroelectric material and with the space charges at the grain boundary layer. The experimental results are also discussed in terms of the aging mechanism. It is suggested that the space charges bound inside grains play an important role in the aging process of the ferroelectric ceramics.

Preparation of Bi2Sr2CaCu2Oy films on alumina substrates with a CuAl2O4 buffer layer

High‐Tc Bi2Sr2CaCu2Oy films have been prepared using the surface diffusion process, with the screen printing of Bi2O3, SrCO3, and CaCO3 (Bi:Sr:Ca=2:2:2) mixed powders on Cu‐deposited alumina substrates. Through the heating at temperatures above 400 °C, CuAl2O4 buffer layers were formed via an interaction between the Cu layer and alumina. The Bi2Sr2CaCu2Oy films, heat treated at 860 °C for 30 min in air, have a zero resistance at 72 K. The x‐ray diffraction, scanning electron microscope, and energy dispersive x‐ray analysis studies show that the Bi2Sr2Ca1Cu2Oy films are strongly c‐axis oriented along the direction normal to the alumina substrate, and the CuAl2O4 buffer layer acted as a barrier to suppress the interdiffusion of Al ions into the superconducting films.

Fabrications and electrical properties of ferroelectric Bi3.25La0.75Ti3O12 thin films using a indium-tin-oxide conductive layer as the bottom electrode

Systematic effort was made to optimize the quality of ferroelectric Bi3.25La0.75Ti3O12 thin films, that were pulsed laser deposited onto indium–tin-oxide coated glass plates, by varying the conditions of laser fluence density, partial oxygen pressure and post-annealing temperature. Characterizations from the dielectric and ferroelectric measurements concluded that the films deposited with a laser power of 2.5 J cm−2, a deposition pressure of 200 mTorr and an annealing temperature of 650 °C showed the finest electrical properties, consisting of remnant polarization (2Pr) and coercive field (Ec) values of 28–32 µC cm−2 and 90–100 kV cm−1, respectively. No significant reduction in switching polarization magnitudes were observed until 1 × 1011 switching cycles, indicating that these electrical properties and the relevancy of the film with metallic-oxide ITO electrodes have satisfied the necessary requirements for applications in nonvolatile ferroelectric memory devices.