Induced current densities j s and magnetic relaxation rates S ≡ −d ln j s /d ln t were measured over a wide temperature (5 ≤ T ≤ 90 K) and field range (0 ≤ µ 0 H a ≤ 7 T) on a single crystal and a melt processed sample of NdBa 2 Cu 3 O 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 U 0 (77 K) = 4500 K and U 0 (77 K) = 2900 K for the single crystal and the melt processed sample respectively, for an applied magnetic field µ 0 H a = 1.0 T. Furthermore, these samples show a well developed fishtail effect, leading to a current density j s (77 K, µ 0 H a = 2 T) = 3 × 10 4 A m −2 . This demonstrates that the fishtail effect is closely related to the low relaxation rates observed here.

Induced current densities j s (T,B) were measured in a wide temperature ͑5 рTр 90 K͒ and field range ͑0 р 0 H a р7 T͒ on single crystals of NdBa 2 Cu 3 O 7Ϫ␦ using a superconducting quantum interference device magnetometer. The samples exhibit the fishtail or peak effect at temperatures above 30 K. In a small temperature window between 50 and 74 K, three peaks are seen in the j s (T, 0 H a ) curves. The temperature and field behavior of the induced current densities is analyzed using a model describing a magnetization curve as composed of two independent contributions at low and high fields. By means of this analysis, it is demonstrated that the third peak corresponds to the fishtail peak, whereas the position of the second peak is practically independent of temperature. It is shown that the appearance of the second peak is due to the large values of the position of the fishtail peak. ͓S0163-1829͑96͒52334-9͔

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