Magnetic field dependence of pinning mechanisms in Bi2Sr2Ca1Cu2O8+x thin films

We performed investigation of the pinning mechanisms in Bi2Sr2CaCu2O8+x (BSCCO) thin films as a function of the temperature, magnetic field and bias current. The specimens were prepared by molecular beam epitaxy deposition technique with a subsequent ''ex-situ'' annealing in air atmosphere. Transition temperatures T-c exceeded 86 K and critical current densities J(c) were of the order of 10(5) A/cm(2) at T = 4.2 K. Analyzing the temperature dependence of the resistivity rho(T) data we show that they could be well described by an activation process. The absolute value of the activation energy U depends crucially on the elaboration procedure, while the obtained magnetic field dependence U(H) does not depend on the selected model. Introducing a pinning strength distribution we were able to describe the U(H) dependence taking into account a crossover from an individually pinned vortex regime to a collective behavior. This crossover was also confirmed by the current dependence of U. In fact, below the crossover field H-0 the curves U(J) were well described by a spatial washboard-type variation of the pinning energy, while for H > H-0 the U(J) curves showed a logarithmic behavior.