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

TLDR: The ac susceptibility of sintered sintering pellets is measured as a function of temperature and ac magnetic field amplitude and frequency to find the value for an intergrain decoupling field, above which flux creep presumably becomes flux flow at the grain boundaries.

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.