Evidence of rf-driven dendritic vortex avalanches in MgB2 microwave resonators

The influence of dendritic magnetic-flux penetration on the microwave response of superconducting magnesium diboride (MgB(2)) films is investigated by a coplanar resonator technique. The characteristic feature consists of abrupt jumps in the resonance curves, which are shown to be induced by vortex avalanches freezing magnetic flux inside the resonator. Flux shaking provided by radio frequency currents is the origin of such avalanches, since a mechanism for magnetic instability of the dendrite type is local heating produced from flux motion. Once the external magnetic field has been changed, the interaction between microwave currents and flux lines influences the vortex-relaxation process. As a consequence, the vortex system is maintained close to a nonequilibrium state during the whole frequency sweep (few seconds), resulting in the observation of several events of dendrite nucleation and branching. From the analysis of a robust statistics, it results that avalanche-size distributions before and after flux-pinning tailoring by heavy-ion irradiation are fully consistent with a thermomagnetic model and with molecular dynamics simulations reported in literature.