Flux creep and activation energies at the grain boundaries of Y-Ba-Cu-O superconductors

Abstract

We measured the ac susceptibility of sintered ${\mathrm{Y}}_{1}$${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ pellets as a function of temperature and ac magnetic field amplitude and frequency. The imaginary part of the susceptibility \ensuremath{\chi}'' exhibits two peaks. A narrow peak is located at the critical temperature of the grains. A broad peak at lower temperature is attributed to hysteresis losses at the grain boundaries. There is a small shift in this coupling peak to higher temperature as the frequency increases from 10 to 1000 Hz. We explain the shift in terms of Anderson flux creep on a time scale of milliseconds. The shift depends on the amplitude of the measuring field. The activation energy for flux creep ranges from 11.9\ifmmode\pm\else\textpm\fi{}1.0 eV in the zero-field limit [0.8 A ${\mathrm{m}}^{\mathrm{r}}$-r1(0.01 Oe)] to 1.2\ifmmode\pm\else\textpm\fi{}0.3 eV at 800 A ${m}^{\ensuremath{-}}$1(10 Oe). We extrapolate our data to find the value for an intergrain decoupling field of 1--2 kA m-1 (13--25 Oe), above which flux creep presumably becomes flux flow at the grain boundaries.