Critical currents, magnetic relaxation and pinning in NdBa ₂Cu ₃O ₇ films with BaZrO ₃-generated columnar defects

The critical current density J _c and the magnetic relaxation (creep) properties have been studied for a set of NdBa ₂Cu ₃O ₇ (NdBCO) films doped with BaZrO ₃ (BZO) nanoparticles to form columnar defects. The dependence of J _c on the magnitude and orientation of the applied magnetic field H _app (06.5T) and temperature T (5KT _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)U ₀(J ₀/J) with fitted values for the pinning energy scale U ₀ and the glassy exponent . 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.