Vortex dynamics in amorphous MoxSi1-x films detected by voltage noise

We have measured the current-induced voltage noise SV for both the thick an d thin films of amorphous MoxSi1-x with strong pinning, over a broad freque ncy range, to study the effects of both the current I and magnetic field B on the vortex dynamics. The results show that the vortex dynamics probed by S-V strikingly depends on the static vortex states. Irrespective of the fi lm thickness, noise is largest at B similar to 0, while small vortex shot n oise is observed at high B in the vortex-liquid phase. The origin of large noise at B similar to 0 is due mainly to density fluctuations of the therma lly excited and subsequently grown vortex loops and dissociated vortex-anti vortex pails for three dimensions (3D) and 2D, respectively, in the presenc e of an applied current. In the three-dimensional vortex-solid phase, the 1 /f(beta)(beta<0.6)-like noise spectra resulting from a plastic-flow motion of vortices are observed at low I over the broad field region, which is att ributed to high concentration of pinning centers. With increasing I in the nonlinear regime, both the amplitude and spectral exponent beta of S-V decr ease and eventually approach the values in the vortex-liquid phase.