LOW MAGNETIC-RELAXATION IN A SINGLE-CRYSTAL AND A MELT-PROCESSED NDBA2CU3O7-DELTA SAMPLE

Induced current densities j(s) and magnetic relaxation rates S = -d ln j(s)/d ln t were measured over a wide temperature (5 less than or equ al to T less than or equal to 90 K) and field range (O less than or eq ual to mu(0)H(a) less than or equal to 7 T) on a single crystal and a melt processed sample of NdBa(2)Cu(3)O7-delta using a SQUID magnetomet er. 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 respec tively, for an applied magnetic field mu(0)H(a) = 1.0 T. Furthermore, these samples show a well developed fishtail effect, leading to a curr ent density j(s)(77 K, mu(0)H(a) = 2 T) = 3 x 10(4) A m(-2). This demo nstrates that the fishtail effect is closely related to the low relaxa tion rates observed here.