Critical currents, magnetic relaxation and pinning in NdBa2Cu3O7−δ films with BaZrO3-generated columnar defects

The critical current density J(c) and the magnetic relaxation ('creep') properties have been studied for a set of NdBa2Cu3O7-delta (NdBCO) films doped with BaZrO3 (BZO) nanoparticles to form columnar defects. The dependence of Jc on the magnitude and orientation of the applied magnetic field H-app (0-6.5 T) and temperature T (5 K-T-c) was investigated. The normalized flux-creep rate S = -dln(J)/dln(t) was determined as a function of T. The current dependence of the effective activation energy U-eff(J) was derived using the formalism developed by Maley. The results are well described by an inverse power law type barrier of the form U-eff(J) similar to U-0(J(0)/J)(mu) with fitted values for the pinning energy scale U-0 and the glassy exponent mu. When comparing values for these parameters in the BZO-doped samples with those for their undoped control counterparts, the most striking difference is the larger scale of current density J(0) in the doped samples (a factor of 2.4 higher), while the other pinning parameters do not differ strongly. In the BZO-doped materials, the pinning energy scale U-0 increases with vortex density and J(0) decreases, with both following simple power law dependences on the field.