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When an AC magnetic field of a suitable magnitude is applied locally to a wide surface of a superconducting film, a third harmonic voltage is induced in a pick-up coil. This measurement is used for an evaluation of the critical current density of the film. Mawatari et al. analyzed the relationship between the third harmonic voltage amplitude (V3) and the driving current amplitude for the field (I0) by using the critical state model and a simple assumption that the magnetic field is locally parallel to a flat surface. In this study, this relationship is numerically analyzed by finite element method (FEM) for a condition similar to the experiment by Mawatari et al. This result is compared with the above theoretical and experimental results. Some deviations from theory are found. However, the scaling of V3–I0 curves holds and I0 at which V3 starts to appear is proportional to the critical current density. [Copyright &y& Elsevier]

The AC-current loss in a quench melt growth (QMG)-processed Y-123 bulk superconductor was found to fit well with the Norris’s formula for a superconductor with an ellipsoidal cross-section based on the Bean’s model, in spite of a rectangular cross-section and the magnetic field dependence of critical current density. The reason for this coincidence is generally argued by a theoretical analysis with respect to the effects of the magnetic field dependence of Jc, the geometry of effect of superconductor and the effect of n-value on the AC loss. In this paper, finite element method is used for the numerical analysis of the AC-current loss of the QMG bulk sample with taking into account of the latter two effects. It is found that when the aspect ratio of rectangular superconductor increases, the loss becomes small. The AC loss takes on a larger value when the n-value decreases, suggesting a large effect of flux creep. [Copyright &y& Elsevier]

The AC transport current loss of a meander-shaped fault current limiting device of quench melt growth (QMG) Y-based superconductor was numerically analyzed by finite element method. The magnetic field and the field-angle dependencies of the critical current density Jc were taken into account as well as a shape of a rectangular cross-section. The numerical result shows a good agreement with the experimental result. The reason why Norris’s formula fits to the experimental result seems to be because that the effect of the magnetic field dependence of Jc on the AC current loss is compensated by the geometrical effect of the superconductor and the field-angle dependence of Jc. [Copyright &y& Elsevier]

It is known that the peak effect in melt-processed Y-123 bulk superconductors is caused by the disorder transition of flux lines pinned by oxygen deficient regions with lower Tc. In order to study the effect of dimensionality of superconductor on the peak effect, a change in the pinning characteristics by heat treatment at 550 °C was measured. Jc decreased and the peak effect became less pronounced. At the same time, the peak field and the dip field reduced. This shows that the disorder transition of flux lines took place much easier than before the heat treatment. It is speculated that the pinning energy and the transverse flux bundle size reduced due to a change of specimens to more two-dimensional underdoped state. [Copyright &y& Elsevier]

It is known that the peak effect in melt-processed Y-123 bulk superconductors is caused by the disorder transition of flux lines pinned by oxygen deficient regions with lower Tc. In order to study the effect of dimensionality of superconductor on the peak effect, a change in the pinning characteristics by heat treatment at 550 °C was measured. Jc decreased and the peak effect became less pronounced. At the same time, the peak field and the dip field reduced. This shows that the disorder transition of flux lines took place much easier than before the heat treatment. It is speculated that the pinning energy and the transverse flux bundle size reduced due to a change of specimens to more two-dimensional underdoped state. [ABSTRACT FROM AUTHOR]

Abstract: Ag added bulk FeSe0.5Te0.5 samples Ag x (FeSe0.5Te0.5)1− x were prepared by solid-state reaction. The x, nominal concentration of Ag, was ranged between 0 and 0.08. The mixed powder was first put into an alumina tube and sealed in an evacuated quartz tube that was heated at 1323K for 20h. Subsequently, it was cooled to 673K and annealed for 200h to stabilize the superconductive layers. The pure Ag or Ag compound was not observed in the X-ray diffraction patterns of Ag doped samples. It indicates that the Ag is doped into the lattice site of FeSe0.5Te0.5. The T c is about 14K up to x =0.04, but the T c decreases gradually to 11K above x =0.05. The magnetization decreased with increase of x, but a fishtail-like hump was observed for x =0.05 sample. The magnetic hysteresis of x =0.05 sample is larger than that of x =0 sample above 2T. [Copyright &y& Elsevier]

Abstract: The intrinsic pinning properties of FeSe0.5Te0.5, which is a superconductor with a critical temperature T c of approximately 14K, were studied through the analysis of magnetization curves obtained using an extended critical state model. For the magnetization measurements carried out with a superconducting quantum interference device (SQUID), external magnetic fields were applied parallel and perpendicular to the c-axis of the sample. The critical current density J c under the perpendicular magnetic field of 1 T was estimated using the Kimishima model to be equal to approximately 1.6×104, 8.8×103, 4.1×103, and 1.5×103 A/cm2 at 5, 7, 9, and 11K, respectively. Furthermore, the temperature dependence of J c was fitted to the exponential law of J c(0)×exp(−αT/T c) up to 9K and the power law of J c(0)×(1− T/T c) n near T c. [Copyright &y& Elsevier]

Abstract: The 5d transition metal W was added into the MgB2 superconductor. The Mg, B and W were sintered at 1173K for 30min under H2/Ar atmosphere in the electric furnace. The W x (MgB2)1− x samples were prepared in the W concentration range of 0⩽ x ⩽0.05. Temperature and field dependences of magnetization were measured by the SQUID magneto-meter. The field and x dependences of J c at 20K were analyzed by the extended critical state model. The enhancement of J c became maximum for the x =0.02 sample. [Copyright &y& Elsevier]