Characterization of the Resistive Transition of ${\rm MgB}_{2}$ Nanogranular Films by Current Noise Analysis

An original model for the interpretation of the noise produced during the resistive transition of disordered granular superconductive films, induced by a slow temperature change, has been recently developed and tested on MgB 2 films. Both the amplitude and frequency behavior of the noise power spectrum, simulated on the basis of this model, are in very good agreement with the experimental data, practically without the introduction of adjustable parameters. The model is based on the onset of correlated transitions of large sets of grains, forming resistive layers through the film cross-section area during the transition process. The strong non-linear behavior and correlation of the grains produces abrupt resistance variations, giving rise to the large noise, of the 1/f 3 type, observed in experiments. Presently this model is tested under more general conditions, producing the resistive transition under an external magnetic field. The results show that the field reduces the grain critical current density but does not change the mechanism of the transition process. An alternative transition model, based on fluxoids depinning and motion, which would produce a much lower, 1/f type transition noise, can thus be excluded by the present analysis.