The anisotropy of the irreversible magnetizations of YBCO and LaSrCuO: Evidence for a transition from Abrikosov to Josephson vortices in LaSrCuO

Abstract

The anisotropy of the magnetization and the critical current density of the high temperature superconductors YBa 2Cu 3O 7 − δ and La 1.85Sr 0.15CuO 4 are investigated as a function of the orientation θ of the applied field with respect to the crystallographic c-axis. The results are analyzed in the framework of the recent model by Bulaevskii et al. In agreement with this model, there is a threshold angle θ 0(∼ 50°) beyond which the vortex line lattice enters the sample parallel to the ab planes and then undergoes a lock-unlock transition at a field H 1 of the order of H c1, c /cos θ. In addition, at high fields the vortex lattice of La 1.85Sr 0.15CuO 4 exhibits a sharp change in a narrow angle of less than one degree around the ab basal planes. No anomalous behavior of this kind is seen in YBa 2Cu 3O 7 − δ . In this angular domain the critical density of La 1.85Sr 0.15CuO 4 increases slightly with temperature while that of YBa 2Cu 3O 7 − δ drops sharply in the same reduced temperature range. These data are hard to interpret with existing pinning theories and are tentatively ascribed to a gradual transformation from Josephson-like to Abrikosov-like vortices in the LaSr based compound which is much more anisotropic than YBCO material.