Effect of the amplitude of the AC magnetic field on the vortex dynamics: A quantitative analysis in Nb and NbTi polycrystalline samples

The harmonics of the AC magnetic susceptibility as a function of the temperature have been measured on Nb and NbTi polycrystalline samples, at various amplitudes of the AC magnetic field, without a DC field. The data have been analysed in terms of Cole-Cole plots, i.e. the imaginary part as a function of the real part. A similar AC field dependence in the height of the maximum of the first harmonics Cole-Cole plots has been observed for both materials. The experimental results have been compared with numerical curves obtained by integrating the nonlinear diffusion equation for the magnetic field inside the sample with different flux creep models. It has been proved that the AC field amplitude dependence have to be included in the existing models in order to reproduce the behaviour of the experimental data. Finally, a quantitative agreement has been achieved by supposing different vortex dynamical regimes tuned by changing of the AC field amplitude. (c) 2007 Elsevier B.V. All rights reserved.