Analysis of current noise during the resistive transition Of MgB 2 thin films produced by the application of an external magnetic field

The excess noise observed during the resistive transition of a superconducting material can be used to shed light on the microscopic processes underlying the transition itself. In a previous paper it has been proposed a model to explain the large noise observed during the resistive transition of MgB2 superconducting films, obtained by slowly increasing the specimen temperature across its critical value when a d.c. bias current is applied. The amplitude and frequency behaviour of the noise power spectrum, simulated with this model, are in good agreement with the experimental data. The model is based on the onset of correlated transition of large sets of grains forming resistive layers through the specimen cross section, giving rise to resistance steps. It is assumed that these events produce the large noise, of 1/f3 type in the low frequency range, observed in the experiments. To compare this model with alternative ones, based on dissipative effects produced by fluxoid dynamics, new results, obtained by producing the transition by varying an applied magnetic field, are reported in this paper. The fact that the transition noise remains practically unchanged, even if the fluxoid density is much increased by the magnetic field, suggests that the transition mechanism proposed in the described model is more appropriate than the one based on fluxoid dynamics