Origin of the fast magnetic relaxation close toTcinYBa2Cu3O7−δ

Abstract

There have been many reports on magnetization measurements of the normalized logarithmic decay rate S(T,H), which was observed to increase rapidly with temperature T close to ${\mathrm{T}}_{\mathrm{c}}$ in ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ (YBCO) single crystals, epitaxial thin films, and granular bulk samples (grain aligned and with random orientation of the grains). We investigated the origin of S(T,H) by performing the following measurements: (1) the relaxation of the persistent current from its critical value, as a function of temperature T and magnetic field H; (2) the temperature dependence of the critical current at different magnetic fields. The measurements were carried out on granular YBCO and RBCO (where R stands for rare-earth elements) ring-shaped samples with randomly oriented grains, which always exhibit an increase of S(T,H) with temperature close to ${\mathrm{T}}_{\mathrm{c}}$. The results revealed the correlation between the temperature dependence of the unperturbed pinning potential, ${\mathrm{U}}_{0}$=kT/S, and that of the critical current. Such correlation suggests that variations of the Josephson coupling energy, or the order parameter, along the grain boundaries are responsible for weak flux pinning and consequently the increase of S close to ${\mathrm{T}}_{\mathrm{c}}$. The analysis of these results was made using the Tinkham-Lobb [Solid State Phys. 42, 91 (1989)] model of flux pinning in a granular superconductor and their description of the conceptual equivalence between the weak-link (Ambegaokar-Baratoff) and continuum (Ginzburg-Landau) flux-pinning models. According to the results of these studies, we conclude that the divergence of S(T,H) close to ${\mathrm{T}}_{\mathrm{c}}$ observed in YBCO single crystals and epitaxial thin films could stem from filamentary superconducting structures caused, for instance, by phase separation.