Effects of field-sweep rate on the magnetization of melt-textured YBa2Cu3O7- delta.

Abstract

The effects of field-sweep rate K=\ensuremath{\partial}H/\ensuremath{\partial}t on magnetization hysteresis loops M(H) and on flux-creep studies M(t) in high-temperature superconductors have been investigated both theoretically and experimentally. We find the basic relation between M and K is, to first order, the following: M=const-{[dM/d ln(t)] ln(K)}-[${\mathit{Kt}}_{\mathit{e}\mathit{f}\mathit{f}}$/10], where dM/d ln(t)=aC/30 is the flux-creep rate in a cylindrical sample of radius a, and ${\mathit{t}}_{\mathit{e}\mathit{f}\mathit{f}}$ is an effective attempt time for vortex hopping. The largest possible M, which corresponds to the critical current density ${\mathit{J}}_{\mathit{c}0}$ in the absence of thermal activation, develops when K\ensuremath{\ge}${\mathit{K}}_{\mathrm{max}}$=aC/[(1+a\ensuremath{\alpha})${\mathit{t}}_{\mathit{e}\mathit{f}\mathit{f}}$] with \ensuremath{\alpha}=\ensuremath{\partial}J/\ensuremath{\partial}H. The time origin of flux creep, which is essential in studying the initial stages of relaxation, is given by ${\mathit{t}}^{\mathrm{*}}$=aC/K(1+a\ensuremath{\alpha}). The model agrees well with experiments on a melt-textured-growth sample of ${\mathrm{Y}}_{1}$${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$, yielding ${\mathit{t}}_{\mathit{e}\mathit{f}\mathit{f}}$\ensuremath{\sim}0.24\ifmmode\pm\else\textpm\fi{}0.03 s at 27 K. By incorporating the calculated time origin into flux-creep studies of M(t), we obtain a very good description in terms of the interpolation formula from vortex-glass--collective pinning theory.