Magnetization relaxation in superconducting YBa₂Cu₃O₇ films with embedded nanorods and nanoparticles

Vortex pinning on natural and artificial defects is essential for large scale applications of superconducting materials. One of the most promising solutions for the creation of efficient pinning structures is to combine the strong pinning supplied by columnar defects (with the radius of the order of the superconducting coherence length) and the presence of random quenched disorder, which inhibits the detrimental vortex kink formation. A strong pinning is revealed by high values of the vortex activation energy in the magnetic relaxation process. We present a critical analysis of the interpretation of the relaxation data at long- and short time scales, by extracting the so called normalized vortex-creep activation energy. This allowed us to find the actual temperature interval for the characteristic vortex excitations in YBa₂Cu₃O₇ films with embedded BaZrO₃ nanorods (preferentially oriented along the c axis), and to unambiguously determine the characteristic vortex pinning energy. The observed drastic change of magnetic relaxation at short time scales (attained in standard AC measurements) is attributed to a large contribution of the pinning enhanced viscosity to the vortex hopping activation energy.