Article révisé par les pairs
Résumé : Thermally activated flux motion has been studied in c-axis oriented epitaxial YBa2Cu3O7-δ thin films with a critical current density Jc of 4 × 106 A/cm2 at 77 K in the absence of an external magnetic field. The magnetoresistivity and the critical current density were measured in applied magnetic fields H, up to 20 T, for H perpendicular and parallel to the c-axis of the films. Critical current densities at 77 K are still as high as 7 × 104 A/cm2 in 20 T field for H ⊥ c-axis and 2 × 103 A/cm2 in 10 T field for H ∥ c-axis. It was found that the magnetic-field dependence of the activation energy follows a power law, U0(H) ∼ H-α. Crossovers of the α values were observed from α1 = 0.22 to α2 = 0.5 at 0.3 T for H ⊥ c-axis and from α1 = 0.33 to α2 = 1 at 6 T for H ∥ c-axis. These crossovers were interpreted as transitions from a low-field single-vortex pinning to a collective-vortex pinning in high field as predicted by Yeshurun and Malozemoff [Phys. Rev. Lett. 60 (1988) 2202]. In the collective-pinning regime, the observed anisotropic dependences of U0(H) are assumed to be due to the intrinsic planar pinning for H ⊥ c-axis and extrinsic point pinning for H ∥ c-axis.