Suppression of flux avalanches in superconducting films by electromagnetic braking

Abstract

Magnetic fields perpendicular to superconducting films often trigger vortex avalanches, which always are very harmful for electronic devices and other applications. Such avalanches can be suppressed by a metal layer placed in contact with the superconductor surface, an effect that up to now has been thought to be a consequence of improved heat conduction. Here we show experimentally that the role of the metal layer is not that of a heat-sink, but rather that of an electromagnetic drag due to eddy currents induced in the metal layer during the abrupt onset of the flux avalanches. The effect is demonstrated for films of MgB2 and Nb.

Nanosecond voltage pulses from dendritic flux avalanches in superconducting NbN films

“…In this respect, the system is similar to that described in Ref. 16, where dendritic avalanches were suppressed by electromagnetic braking with clear separation between the metal and the superconductor. In that work, the voltage was not measured, but the presence of electromagnetic braking is a clear indication of the existence of a significant voltage induced in the metal by the fast dendritic flux avalanches in the superconductor.…”

“…It is already known that a normal metal layer adjacent to a superconducting film tends to suppress the avalanche activity due to electromagnetic braking, [14][15][16] an effect experienced also in the present work, where we never observed avalanches starting from an edge covered with metal film. However, the Cu layer on the present NbN sample is sufficiently thin to allow branches to partly enter under the Cu electrode.…”

“…While it was shown previously 14,16 that coating a superconductor film with a metal layer strongly suppresses the dendritic instability, we demonstrate here that when flux dendrites actually penetrate into a coated region, they may have a serious impact on the device performance. Therefore, thin superconducting devices experiencing transverse magnetic fields should be carefully designed to be robust against electrical noise of the type reported in this work.…”

Flux penetration in a superconducting film partially capped with a conducting layer

“…This phenomenon arises from the electromagnetic braking of the flux propagation, caused by the eddy currents induced in the conductive layer [5][6][7][8]. The question as to whether a single element of the flux front, i.e., a superconducting vortex, could also undergo any deflection of its trajectory when entering in the region covered by a conducting layer has recently been tackled by appealing to a classical analogy, consisting of a magnetic monopole (the vortex) moving in the vicinity of a metallic film [2].…”

Enhanced pinning in superconducting thin films with graded pinning landscapes

“…Alternative means to suppress such flux bursts have been already discussed in the literature, 28,29 the simplest of which we have applied here. By placing a metallic disk (Ag) on top of sample GRAD, one manages to inhibit, via magnetic breaking, 29 the thermomagnetic instabilities that trigger avalanches. The dotted-dashed curve in Figure 2(b) shows the hysteresis loop of sample GRAD with the Ag disk, indicating that flux avalanches are mostly suppressed in this configuration.…”