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 NdBa₂Cu₃O_7-δ (NdBCO) films doped with BaZrO₃ (BZO) nanoparticles to form columnar defects. The dependence of J_c 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) ∼ U₀(J₀/J)^μ with fitted values for the pinning energy scale U₀ and the glassy exponent μ. 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 J₀ 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 U₀ increases with vortex density and J₀ decreases, with both following simple power law dependences on the field.