Dendritic flux instabilities inYBa2Cu3O7−xfilms: Effects of temperature and magnetic

Abstract

Our recent success in triggering dendritic flux instabilities in YBa 2 Cu 3 O 7−δ (YBCO) films by applying magnetic fields at ultrahigh rates is followed here by a detailed study of the effect as a function of the field ramp rate,Ḃ a , and temperature, T. We trace the borderline in theḂ aT plane separating regions of smooth, gradual flux penetration and dendritic flux avalanches. In addition, we describe the changes in the dendritic morphology in the instability region as a result of changes in eitherḂ a or T. Our experimental results, showing a monotonic increase of the avalanche threshold field ramp rate with temperature, are discussed in the framework of existing theories. On the basis of these theories we also explain the high stability of YBCO to dendritic avalanches as compared to, e.g., MgB 2 , identifying the flux flow resistivity, rather than any of the thermal parameters, as the main parameter governing the film stability.

Dendritic flux instability in MgB2films above liquid hydrogen temperature

“…The observed monotonic increase of the threshold ramp rate with temperature is similar to that reported for YBCO [17], and it agrees also qualitatively with theoretical predictions [18,19]. These theories propose that the dendritic instability is controlled by the magnetic flux diffusion coupled to the thermal diffusion in the sample.…”

“…It should also be mentioned that a recent theoretical work [28] emphasizes the enhancement of the thermomagnetic instability by AC magnetic fields. Also note that some ultrapure MgB2 films, grown with the hybrid physical-chemical vapor deposition (HPCVD) method [9,10], do not produce dendrites, even at our very fast ramp rates [17,29]. As suggested in our recent study [17], the flux flow resistivity, ρF, is the key parameter in the stability of the film against avalanches.…”

“…These theories propose that the dendritic instability is controlled by the magnetic flux diffusion coupled to the thermal diffusion in the sample. At high temperatures, as the sample becomes more susceptible to flux entry, faster application of the external field is required to obtain sufficient heat and trigger the instability [17].…”

Anisotropic thermomagnetic avalanche activity in field-cooled superconducting films

“…For a long time the only known way to trigger avalanche events in this material was by applying a laser pulse to perturb the flux-filled film locally [1]. More recently, Baziljevich et al [13,14] showed that also YBa 2 Cu 3 O x films become unstable in magnetic fields when applied at rates near 3000 T/s. They also found that during these avalanches, the YBa 2 Cu 3 O x decomposed and evaporated, leaving ditches in the film as permanent traces of the advancing avalanche front-a unique evidence for the thermomagnetic nature of the instability [15][16][17][18].…”

Magnetic flux instability in NbN films exposed to fast field sweep rates

“…This phenomenon appears as flux jumps in wires and bulk superconductors [7][8][9], and as dendritic flux formations in thin films. The latter has been observed in a large number of superconductors important for practical applications, such as MgB 2 [10,11], Nb [12,13], YBCO [14], and NbN [15].…”