Vortex dynamics in amorphous ${\mathrm{Mo}}_x{\mathrm{Si}}_{1-x}$ films detected by voltage noise

We have measured the current-induced voltage noise $S_V$ for both the thick and thin films of amorphous ${\mathrm{Mo}}_x{\mathrm{Si}}_{1-x}$ with strong pinning, over a broad frequency 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 film thickness, noise is largest at $B\sim 0,$ while small vortex shot noise is observed at high B in the vortex-liquid phase. The origin of large noise at $B\sim 0$ is due mainly to density fluctuations of the thermally excited and subsequently grown vortex loops and dissociated vortex-antivortex pairs for three dimensions (3D) and 2D, respectively, in the presence 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 attributed to high concentration of pinning centers. With increasing I in the nonlinear regime, both the amplitude and spectral exponent β of $S_V$ decrease and eventually approach the values in the vortex-liquid phase.