Field and frequency dependences of ac magnetic measurements as a probe of nonlinear flux diffusion in high-temperature superconductors

We studied the frequency and field dependences of harmonic magnetic suscept ibilities by numerically solving the diffusion equation for the magnetic fl ux, incorporating both creep and flow processes in a collective pinning app roach. In particular the irreversibility (H, T) line is analysed and discus sed in terms of the de magnetic field dependence of the onset temperatures of the first and the third harmonics respectively. It is shown that, in our conditions, the Taff resistivity is the dominant process for the flux diff usion at the onset temperature of the imaginary part of the first harmonic. On the contrary, the onset of the third harmonic is determined by the cros s-over between processes dominated by the Taff and the creep resistivities.