Magnetic field and temperature dependence of magnetic flux creep in c-axis-oriented YBa2Cu3O7 powder

Creep of magnetic flux lines in a c-axis-oriented powder specimen of YBa2Cu3O7 was measured at temperatures of 5<T<50 K for fields in the range 0.1<H<4.0 T. Under most conditions, the magnetization M(T,H) was found, after sufficiently long times t, to decrease linearly with lnt. But at higher temperatures, this linearity was not observed up to t≅1.2x104 sec. An apparent flux-pinning potential U0* can be calculated using the relationship U0*=kT{dM/[M d(lnt)]}-1. The value of U0* at 1.0 T was found to vary from ∼20 to ∼130 meV at 5 and 35 K, respectively, and also to increase with H. These results are not consistent with the expected temperature dependence of the true pinning potential Up, and the difficulties in the use of the above relationship to determine Up for YBa2Cu3O7 are identified