Vortex properties of YBa₂Cu₃O₇ superconducting thin films: Irreversible magnetization and the effect of thickness

Magnetic properties of epitaxial thin films of YBa₂Cu₃O₇ have been studied in the superconducting state in fields perpendicular to the sample, along the c crystallographic axis. In the low-temperature state, susceptibility, hysteresis loop, and remanent magnetization (trapped flux) studies show that a critical state is established in very low applied fields. The flux penetration process and the related thickness properties are well accounted for by simple electromagnetic models for the critical current. It is shown that the evaluation of the critical current J_c by the classical Bean formula is roughly valid, in spite of the curvature of the vortices. A decrease in the low-temperature value of J_c with increasing thickness is found above 100 nm. In the temperature range close to T_c, the irreversibility line moves towards higher field with increasing thickness according to a d^1/2 law. This is discussed in relation with the theories of vortex-lattice and vortex-glass melting.