Magnetic field and temperature dependence of magnetic flux creep in c-axis-oriented ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ powder

Creep of magnetic flux lines in a c-axis-oriented powder specimen of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ 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≃1.2×${10}^4$ sec. An apparent flux-pinning potential $U_0^{\mathrm{*}}$ can be calculated using the relationship $U_0^{\mathrm{*}}$=kT{dM/[M d(lnt${\left)\right]\mathrm{\}}}^{\mathrm{-}1}$. The value of $U_0^{\mathrm{*}}$ 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 $U_p$, and the difficulties in the use of the above relationship to determine $U_p$ for ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ are identified.