K.B. Li

Critical current density and flux pinning in vortex liquid regime for YBa2Cu3O7−δ epitaxial thin films

Abstract The critical current density J c for epitaxial YBa 2 Cu 3 O 7− δ thin films as a function of applied magnetic field, temperature and angle between the magnetic field and the ab plane has been measured in detail. We observed an evidence for the dramatic change of the critical current density J c above and below the vortex–glass transition line. In the vortex solid state, the field dependence of J c follows an exponential dependence was argued to be from the intrinsic pinning in layered structural superconductors. But in the pinned vortex–liquid state, the J c decreases more rapidly with increasing magnetic field. It was suggested that the nonlinear temperature dependence of J c ( T ) near T c was from the vortex–liquid pinning. The temperature dependence of the critical current density exhibited an exponential law, J c ∝exp(− T / T 0 ), in the pinned vortex liquid regime.

Pinning by planar defects along the c-axis in epitaxial YBa2Cu3O7-δ thin films

Abstract We have measured the resistive broadening and the critical current density of an epitaxial YBa 2 Cu 3 O 7− δ thin film at several fixed magnetic fields as a function of the angle between applied magnetic field and surface of the film. The results show that the angular dependencies of the characteristic temperature T * determined by different reduced resistivity criteria, the effective pinning potential U and the critical current density J c follow the prediction of the intrinsic pinning model for the angle changing from 0° to 75°, showing the strong planar pinning mechanism. The anomalous peaks of the characteristic temperature T * , the effective pinning potential U and the critical current density J c were observed near 90°. The twin planes and 2D defects along the c -axis are responsible for the effect of flux line pinning, indicating the pinning by planar defects for H ‖ c -axis.

A phenomenological description of temperature dependence of magnetoresistance in La2/3Ca1/3MnO3-δ thin films

AbstractThe electrical resistance in zero magnetic field and magnetoresistance in different external magnetic fields have been measured in a temperature range of 77—300 K. It is found that the temperature dependence of magnetoresistance can be well described by a phenomenofogical formula of

The vortex-glass transition and irreversibility line in epitaxial YBa2Cu3O7−δ thin film

\rho (T) = \frac{1}{{\sigma (T)}} = \frac{1}{{(M/M_s )^2 + \beta ( - E_0 /k_B T)}}

U(J,H,α) dependence in epitaxial YBa2Cu3O7−δ thin film

where the fitting parameters α, β vary as the external magnetic fieldH changes,E0 is the activation energy,E0/kB = 1160 K,Ms is the saturation magnetization, the temperature and magnetic field dependence ofM/Ms is obtained by the mean-field expression.

Effective activation energy U(J, H) in epitaxial YBa2Cu3O7-δ thin films

Abstract The resistive transition of (YBCO) 24 /(PrBCO) 2 multilayer was measured as a function of magnetic field up to 8 T at different fixed angles. It is found that the broadening of the resistive transition was mainly determined by the orientation of magnetic field with respect to the c -axis of multilayer rather than the direction of transport current. The H – T relation showed a strong anisotropy, γ = H ‖ * / H ⊥ * ≈15. The magnetic field dependence of the activation energy suggested dimensional crossover in different circumstances, two-dimension (2D) for H ‖ c , three-dimension (3D) for H ‖ ab . The phenomenon may be attributed to the flux lines that are cut by the insulated PrBCO layers when magnetic field is perpendicular to the ab -plane, and the YBCO layer is in the range of thickness which could induce a dimensional crossover from 3D to 2D.

Field dependence of magnetoresistance in epitaxial single-crystal La 2/3 Ca 1/3 Mn

Copyright (c) 1997 Elsevier Science B.V. All rights reserved. The comparative study of the irreversibility line from the broadening of resistive transition with the vortex-glass line from the isothermal I-V curves was made in identical epitaxial YBa 2 Cu 3 O 7−δ thin film. The results show that vortex-glass line lies below the irreversibility line. The two lines follow the same power law, H (1 − t) n , and n = 4/3 for the vortex-glass line, n = 3/2 for the irreversibility line. The possible origin of the difference between such two lines was discussed.

U(J,H,~) dependence in epitaxial YBa2Cu3O7-delta thin film

AbstractEpitaxial La2/3Ca1/3MnO3 thin films were prepared on NdGaO3(1lO) substrates by d.c. magnetron sputtering method. The measurements of magnetoresistance ρ(H) upon magnetic field at different temperatures were carried out in the field range of 0–8 T. It is found that ρ(H) obeys the following relations: when the temperature (T) is higher than the Curie temperature