Flux pinning and magnetic relaxation in melt-processed YBa2Cu3O7−δ doped with Li

Abstract

Melt-processed samples of YBa2Cu3O7−δ (YBCO) doped with Li additions substituting at Cu(2) plane sites were used to study the effects of doping on the superconducting temperature, critical current density, irreversibility field, upper critical field, coherence length, and magnetic relaxation as a function of temperature and magnetic field. The intrinsic superconducting properties were found only slightly influenced by Li additions at some optimal concentration (0.6at.%). At this Li concentration the volume pinning force of doped material is about five times that of undoped YBCO in intermediate magnetic fields at 77K. No significant change of the maximum volume pinning force with oxygen defects after long-time oxygenation experiment (∼450h) has been observed. The normalized pinning force of the sample optimally doped with Li obeys the same scaling law as a function of magnetic field and temperature before and after additional heat treatment, indicative of a single type of pinning centers. The time dependence of the magnetization in the standard and in the Li-doped sample has been analyzed in the framework of collective pinning theory. The magnetic relaxation measurements combined with the B-T phase diagram data provide evidence of an elastic-plastic transition. The pinning parameters estimated show a considerable enhancement of the critical current density in the optimally doped sample, but only an insignificant change in the effective activation energy, pointing to a weak pinning of pointlike pinning centers, created by Li additions.