Flux motion and pinning in superconductors through flux flow noise measurements

The motion of fluxoids in type II superconducting foils has been studied by analyzing the time dependent voltage through measurements of its associated noise power spectrum. A noise power spectrum is characterized by three parameters: (1) its high frequency behavior, (2) its value in the limit of zero frequency, and (3) a so-called critical frequency. Information regarding the temporal behavior of flux motion is obtained primarily from the critical frequency and high frequency characteristic of the noise power spectrum; whereas, the spatial behavior of moving fluxoids determines the zero frequency value of the spectrum. Over 300 noise power spectra have been measured on foils of Pb 80 at. percent In 20 at. percent. Measurements have been performed as a function of the metallurgical condition of the foils, applied magnetic field value and direction, dc voltage level, and sample and voltage probe geometry. Data from this research indicate that flux pinning due to material inhomogeneities, such as grain boundaries, dislocations and surface irregularities, is fundamentally important in determining both the temporal and spatial behavior of flux motion in type II superconductors. In particular a new model of flux flow noise power spectra is proposed in which fluxoids are momentarily detained at pinning centers during their transit across the sample. Using this model much better agreement with the data is obtained on both the critical frequency and high frequency behavior of the spectra than with previous models. Analysis of the zero frequency limit of the noise power spectra enables information to be obtained regarding the number of fluxoids that move together as a unit