Flux creep inY(GD)Ba2Cu3O7−δ: Magnetic field dependence

The time-dependent decay of the diamagnetic magnetization was measured after zero-field cooling (ZFC) over a broad range of fields and temperatures. The time dependence was found to be linear for short times and logarithmic thereafter within the experimental time intervals for all fields and temperatures. The relaxation rates $A(H_0, T)$ and $A(H, T_0)$ show maxima at $T_m$ and $H_m$, respectively (where $T_0$ and $H_0$ are constant values). The shape of $A(H, T_0)$ is more complicated than that of $A(H_0, T)$, which is nearly symmetrical around $T_m$. Increasing $H_0$ or $T_0$ decreases $H_m$ or $T_m$. The activation parameters were found to be strongly field dependent, whereas they agree well for both investigated compositions. The activation energy decreases with increasing field, following the empirical law: $Q=a+\frac{b}{H^2}$. The frequency factor distribution broadens with increasing $H$, which can be described by the following empirical expression: $\ln \left(\frac{{\tau }_{02}}{{\tau }_{01}}\right)=d-\frac{g}{H}$. Using the activation parameters as determined from $A(H_0, T)$ and interpolating by the above expressions, the relaxation rate $A(H, T_0)$ can be calculated and agrees well with the experimental data within the experimental error. The data are interpreted using the theoretical model of Beasley, Labusch, and Webb on thermally activated flux creep over pinning centers assisted by the field-dependent driving force.