Low magnetic relaxation in a single crystal and a melt processed NdBa₂Cu₃O_7-δ sample

Induced current densities j_s and magnetic relaxation rates S ≡ -d In j_s/d In t were measured over a wide temperature (5 ≤ T ≤ 90 K) and field range (0 ≤ μ₀H_a ≤ 7 T) on a single crystal and a melt processed sample of NdBa₂Cu₃O_7-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 μ₀H_a = 1.0 T. Furthermore, these samples show a well developed fishtail effect, leading to a current density j_s(77 K, μ₀H_a = 2 T) = 3 x 10⁴ A m^-2. This demonstrates that the fishtail effect is closely related to the low relaxation rates observed here.