Guiding thermomagnetic avalanches with soft magnetic stripes

In realistic situations, a typical scenario for applications of type II superconductors necessarily includes vortices, usually in the presence of currents. Under these circumstances, a Lorentz force acts on each individual vortex, propelling its dissipative movement across the superconducting medium, what increases the local temperature and, eventually, endangers the preservation of the superconducting state. Thus, preventing vortices from moving or, at least, designing strategies to minimize undesirable effects from their inevitable presence, is on the list of the most important demands. Vortex dynamics in superconducting films is a fundamental and complex subject in which efficiency is mainly represented by the ability of pinning vortices and thus increasing the current-carrying capability of the specimen. This Topical Review, focused on the strategies developed to control magnetic flux entry and restrict vortex mobility in low-T C superconducting films, compiles the most relevant literature on this subject, starting from a revision of the physical response of a plain film to externally applied magnetic fields and, subsequently, discussing specific changes caused to such a response when obstacles devoted to refrain vortices from moving around are cast into the scene, like arrays of antidots, conducting or magnetically active cap layers, and in-plane magnetic fields. An additional method of control discussed here is provided by the so-called Superconducting Flux Injector, through which one can decide where and when to feed the film with magnetic flux. It should also be mentioned that superconducting films are likely to exhibit avalanches at certain ranges of the applied magnetic field and the temperature, a feature that can be sharpened or mitigated by each of the above approaches, which is also discussed in this paper.

Section: Introductionmentioning

confidence: 99%

Controlling magnetic flux penetration in low-T_C superconducting films and hybrids

Colauto

Motta

Ortiz

2020

“…These abrupt events can be externally induced by different ways: varying the perpendicular applied field either slowly [19][20][21][22][23][24][25]; or fast [26,27]; applying an electrical current [28,29]; or stimulating with microwaves [30,31]. Several attempts have been made to control the instabilities: coating the film with a metallic layer [32,33], inserting macroscopic holes into the sample [34,35]; adding magnetic stripes [36]; quenching via microwave using proper frequencies [37]; and field-cooling the film under an in-plane field [14,38].…”

Section: Introductionmentioning

confidence: 99%

Active control of thermomagnetic avalanches in superconducting Nb films with tunable anisotropy

Carmo

Colauto

Andrade

et al. 2018

Active triggering and manipulation of ultrafast flux dynamics in superconductors are demonstrated in films of Nb. Controlled amounts of magnetic flux were injected from a point along the edge of a square sample, which at 2.5 K responds by nucleation of a thermomagnetic avalanche. Magneto-optical imaging was used to show that when such films are cooled in the presence of in-plane magnetic fields they become anisotropic, and the morphology of the avalanches change systematically, both with the direction and magnitude of the field. The images reveal that the avalanching dendrites consistently bend towards the direction perpendicular to that of the in-plane field. The effect increases with the field magnitude, and at 1.5 kOe the triggered avalanche becomes quenched at the nucleation stage. The experimental results are explained based on a theoretical model for thermomagnetic avalanche nucleation in superconducting films, and by assuming that the frozenin flux generates in-plane anisotropy in the film thermal conductance. The results demonstrate that applying in-plane magnetic fields to film superconductors can be a versatile external tool for controlling their ultrafast flux dynamics.

“…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].…”

Section: Introductionmentioning

confidence: 99%

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

Baruch-El

Baziljevich

Johansen

et al. 2018

Magneto-optical imaging of dendritic flux instability is reported for NbN films exposed to magnetic fields ramped at a fast rate (0.1-3.2 kT s −1). The results show that as the magnetic ramp rate increases, the temperature and field range of the instability extends significantly. In particular, the lower and upper threshold fields (H th 1 and H , th 2 respectively) that bound the field range for dendritic instability are affected. The upper field is found to increase linearly with the applied field sweep rate, a behavior which is discussed in terms of a recent theoretical work (Vestgarden et al 2016 Phys. Rev. B 73 174511). The extended instability range should be taken into account in applications in which the superconducting films are exposed to rapid changes in the magnetic field.