Magnetization properties and irreversibility lines of YBa2Cu3Oy/PrBa2Cu3Oy multilayered films.

Abstract

Systematic measurements were made of the dc magnetization in multilayered ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{\mathit{y}}$/ ${\mathrm{PrBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{\mathit{y}}$ (YBCO/PBCO) films. The films were fabricated using a molecular-beam-epitaxy technique and were oriented with their c axis perpendicular to the substrate. A magnetic field was applied that was both perpendicular to the films and parallel to the film's c axis. The irreversibility lines of the films were determined to be at the temperature and the magnetic field when the remanent magnetization falls below the detection limits of the instrumentation. We observed that the irreversibility line of a film strongly depends on the thickness of the YBCO layers. On the other hand, for the same thickness of YBCO layer, the irreversibility lines did not change (except for a slight change in the superconducting transition temperature ${\mathit{T}}_{\mathit{c}}$) for YBCO/PBCO multilayered films having PBCO layers that were from three to eight unit cells thick. The YBCO layers appeared to completely decouple when the PBCO layer was thicker than three unit cells. As observed in other high-${\mathit{T}}_{\mathit{c}}$ materials, the temperature dependence of the irreversibility field for these films exhibited a power-law behavior predicted by a thermally activated flux-flow (TAFF) model near ${\mathit{T}}_{\mathit{c}}$, and deviated from this behavior at temperatures below T/${\mathit{T}}_{\mathit{c}}$\ensuremath{\sim}0.6, where T is temperature.Irreversibility lines of YBCO/PBCO multilayered films are considered to be analogous to those of Bi systems, which are more anisotropic than YBCO systems. Relaxation of the magnetization for the films were also measured and the electric field vs current density (E-J) characteristics were estimated. These estimates suggest the existence of a vortex glass state at relatively low temperature. In the temperature dependence of the remanent magnetization in the films that had very thin YBCO layers, another transition was observed below the irreversibility line. The E-J characteristics, scaled using this transition temperature, suggest that this transition is the vortex glass transition and that there are two kinds of vortex states below the irreversibility line, i.e., vortex liquid and vortex glass. From these experimental results, the vortex states and anisotropy of high-${\mathit{T}}_{\mathit{c}}$ superconductors were then discussed from the standpoint of both the TAFF model and the vortex glass model.