Shichao Ding

Thermally assisted flux flow and individual vortex pinning in Bi2Sr2Ca2Cu3O10 single crystals grown by the traveling solvent floating zone technique

Magnetoresisitivity and critical current density Jc as a function of temperature and field are studied for Bi2Sr2Ca2Cu3O10 single crystals grown using the traveling solvent floating zone technique. Below a characteristic field B∗, Jc as a function of field exhibits a field-independent plateau associated with thermally activated pinning of individual vortices. Analysis of resistive transition broadening revealed that thermally activated flux flow is found to be responsible for the resistivity contribution in the vicinity of Tc. The activation energy U0 is 800K in low field, scales as B−1∕6 for B 2T.

Vector characterization of soft magnetic materials

A three-dimensional (3D) magnetic property testing system has been completed and successfully used to measure 3D hysteresis loci of soft magnetic material. This paper presents the techniques to characterize soft magnetic materials under 3D magnetic excitations in detail. Using three couples of excitation coils controlled by a computer to generate the magnetic fields in three orthogonal axes, various types of flux density loci, such as circular or elliptical rotating vectors of flux density with any given orientations in 3D space, can be obtained. Based on 3D finite element analysis and a comparative study, a sandwich arrangement comprising a sample, guard pieces, and search coils with double-layer structure was proposed. Compared with the conventional surface search coils, this arrangement can significantly improve the accuracy of measurement. The comprehensive calibration process and the experimental results of the 3D hysteresis loci of soft magnetic composite materials are also presented.

Superconductivity and flux pinning in Y and heavily Pb codoped Bi-2212 single crystals

Studies of superconductivity and flux pinning were carried out on (Bi1.64Pb0.36)Sr2Ca1−xYxCu2O8+y (x=0, 0.05, 0.11, 0.33) single crystals grown by the self-flux method. X-ray diffraction, transport, and magnetic measurements were performed for purposes of characterization. X-ray analysis revealed that the c lattice parameter systemically decreases as the Y doping level increases. The superconducting transition temperature Tc decreases from 80 to 30 K as x increases. A strong annealing effect on Tc and superconducting volume has been observed. Resistance measurements show that x=0.33 samples are semiconductive over a wide temperature range between 4.2 and 300 K for the as-grown state, but become metallic with Tc of 65–70 K after air or oxygen annealing. Flux pinning was studied by measuring the hysteresis loop at different temperatures and different fields. A peak effect was observed in all the codoped samples. Results show that at low temperatures, the peak field is smaller than in solely Pb doped crystal...

Three-dimensional hysteresis of soft magnetic composite

Hysteresis phenomenon is one of the important characteristics of soft magnetic materials. A three-dimensional (3D) magnetic property measurement system has been developed. This system can effectively control the locus of the time-varying magnetic flux density vector B and measure the corresponding magnetic field strength vector H in a cubic sample of soft magnetic material. The sample involved in this study is a soft magnetic composite sample made of highly pure iron powder. This paper investigates the B and H loci in 3D space and the power loss when the B loci are controlled to become circles and ellipses lying in three orthogonal planes. It is found that the B and H loci lie in the same magnetization plane, but the H loci and losses depend strongly on the orientation, position, and process of magnetization. On the other hand, the H vectors evolve into a unique locus pattern, and the power losses approach a unique value when the B vectors evolve into the same loci through either a series of small circles...

Third harmonics due to surface barrier in high-temperature superconductor

The influence of surface barrier on the third harmonics ac susceptibility (χ3) is studied numerically. The surface barrier is described by a critical current density in surfaces which are higher than the inside one. The model can act as the critical state one, the flux creep one, or the flux flow one when temperature (or field) changes. The numerical results based on the model are more close to the popular experimental data probing the harmonics as a function of temperature (or field). Besides, the surface barrier will lead to new peaks in the real and imaginary parts of the third harmonics, which are the finger signature of the surface barrier. Comparison of the peaks with those of the real part of elemental harmonic shows that they are located at the same temperature where dips in χ′ caused by the surface barrier occur, which is in well agreement with experiments.