Dynamic vortex instabilities in Ta/Ge-based multilayers and thin films with various pinning strengths

We report measurements of a dynamic instability in the moving vortex system of Ta/Ge-based thin films and multilayers. The instability results from a nonequilibrium distribution of quasiparticles induced by motion of the vort ex cores, as described theoretically by Larkin and Ovchinnikov in the regim e where there is no pinning. For the Ta/Ge system, where the critical vorte x velocity is relatively small, we observe the instability in cases where e ither the viscosity or pinning dominate the vortex motion. We develop a sim ple model to describe the effects of pinning on the flux motion near the in stability, and show that it gives a good description of the experimental da ta, especially in fields not too close to ten. The temperature dependence o f the critical vortex velocity extracted from the analysis indicates that e lectron-electron scattering provides the dominant contribution to the energ y relaxation.