Induced current densities js and magnetic relaxation rates S ≡ -d In js/d In t were measured over a wide temperature (5 ≤ T ≤ 90 K) and field range (0 ≤ μ0Ha ≤ 7 T) on a single crystal and a melt processed sample of NdBa2Cu3O7-δ using a SQUID magnetometer. The temperature dependence of the magnetic relaxation rate shows a pronounced minimum at elevated temperatures. This leads to very high values of the effective pinning energy U0(77 K) = 4500 K and U0(77 K) = 2900 K for the single crystal and the melt processed sample respectively, for an applied magnetic field μ0Ha = 1.0 T. Furthermore, these samples show a well developed fishtail effect, leading to a current density js(77 K, μ0Ha = 2 T) = 3 x 104 A m-2. This demonstrates that the fishtail effect is closely related to the low relaxation rates observed here.
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