REDUCTION OF FLUX CREEP IN SPITE OF A JC DECREASE IN YBA2CU3O7-DELTA AFTER NEUTRON-IRRADIATION

YBa2Cu3O7-delta single crystals with T(c) = 90.5 K and different transition width have been irradiated with fast neutrons (E > 0.1 MeV) and a fluence of PHI-.t = 2 x 10(17) cm-2. The flux-creep dM/dlnt at B = 0.5 T is reduced at any temperature up to T = 60 K by a factor of about 2. Correspondingly, the effective pinning potential U(eff) is enhanced up to 100k(B)T at low temperatures and 50k(B)T at T = 77 K, independent of an increase or decrease of the magnetization current density after irradiation. Therefore, the enhancement of U(eff) may not in every case be related to a j(c) increase, which is also determined by the number of pinning centers or a certain defect structure, which may not necessarily show deep pinning potentials. The results can be explained within the conventional flux creep theory and are in a good agreement with recent considerations of the role of electronic anisotropy in the dissipative behavior of YBa2Cu3O7-delta.