MAGNETIC-RELAXATION IN GD1-XPRXBA2CU3O7-DELTA

Magnetic relaxation in Gd1-xPrxBa2Cu3O7-delta (x = 0, 0.05, 0.1, 0.2, 0.25) was studied by means of ZFC DC susceptibility at temperatures from 4.2 K to 40 K and in fields up to 1840 Oe. For logarithmic decay of the magnetization which were observed in most cases, the shifting of the relaxation rate versus T dependences with Pr-content and with the magnetic field was investigated; the activation energy U(o) from the single barrier height model has been determined and the distribution of activation barriers calculated. The average activation energies obtained from the distributions are very low (20 meV-40 meV). Deviations from the logarithmic magnetization decays towards quicker changes, observed for x greater-than-or-equal-to 0.2 at higher magnetic fields and at low temperatures, can be explained by the rapid increase of the relaxation time. For the region of the phase diagram where magnetic interactions become very strong Anderson's thermally activated flux motion model of relaxation is no longer adequate.