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

We measured the ac susceptibility of sintered Y₁Ba₂Cu₃O_7-δ pellets as a function of temperature and ac magnetic field amplitude and frequency. The imaginary part of the susceptibility χ’’ 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±1.0 eV in the zero-field limit [0.8 A m^r-r1(0.01 Oe)] to 1.2±0.3 eV at 800 A m^-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.