Dependence of flux-creep activation energy upon current density in grain-aligned ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathit{x}}$

We have investigated magnetic relaxation on a grain-aligned sample of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathit{x}}$ powder embedded in epoxy with the magnetic field along the c axis. Through a thermal-cycling procedure and through variations in the measuring temperature over the range 10–30 K, we manage to commence magnetic relaxation from a range of values of the magnetization, M, or current density J. Analysis of relaxation measurements at a magnetic field of 1.0 T over the temperature range 10–30 K reveals a nonlinear dependence of the effective activation energy ${\mathit{U}}_{\mathit{e}}$ on M or J that is consistent with the logarithmic dependence obtained by Zeldov et al. from transport measurements. This demonstration of a nonlinear dependence of ${\mathit{U}}_{\mathit{e}}$(M) is capable of resolving anomalies in the magnitudes and temperature dependence of flux-creep activation energies obtained from magnetic-relaxation studies.