A graded distribution of antidots in superconducting a-Mo₇₉Ge₂₁ thin films has been investigated by magnetization and magneto-optical imaging measurements. The pinning landscape has maximum density at the sample border, decreasing linearly towards the center. Its overall performance is noticeably superior than that for a sample with uniformly distributed antidots: For high temperatures and low fields, the critical current is enhanced, whereas the region of thermomagnetic instabilities in the field-temperature diagram is significantly suppressed. These findings confirm the relevance of graded landscapes on the enhancement of pinning efficiency, as recently predicted by Misko and Nori [Phys. Rev. B 85, 184506 (2012)].

The morphology of abrupt bursts of magnetic flux into superconducting films with engineered periodic pinning centers (antidots) has been investigated. Guided flux avalanches of thermomagnetic origin develop a treelike structure, with the main trunk perpendicular to the borders of the sample, while secondary branches follow well-defined directions determined by the geometrical details of the underlying periodic pinning landscape. Strikingly, we demonstrate that in a superconductor with relatively weak random pinning the morphology of such flux avalanches can be fully controlled by proper combinations of lattice symmetry and antidot geometry. Moreover, the resulting flux patterns can be reproduced, to the finest details, by simulations based on a phenomenological thermomagnetic model. In turn, this model can be used to predict such complex structures and to estimate physical variables of more difficult experimental access, such as the local values of temperature and electric field.

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.

We image the motion of superconducting vortices in niobium film covered with a regular array of thin permalloy strips. By altering the magnetization orientation in the strips using a small in-plane magnetic field, we can tune the strength of interactions between vortices and the strip edges, enabling acceleration or retardation of the superconducting vortices in the sample and consequently introducing strong tunable anisotropy into the vortex dynamics. We discuss our observations in terms of the attraction/repulsion between point magnetic charges carried by vortices and lines of magnetic charges at the strip edges and derive analytical formulas for the vortex-magnetic strips coupling. Our approach demonstrates the analogy between the vortex motion regulated by the magnetic strip array and electric carrier flow in gated semiconducting devices. Scaling down the geometrical features of the proposed design may enable controlled manipulation of single vortices, paving the way for Abrikosov vortex microcircuits and memories.

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 ${\text{MgB}}_2$ and Nb.

We have observed the occurrence of dendritic flux avalanches in an amorphous film of Mo ₈₄ Si ₁₆ . These events are understood to have a thermomagnetic origin and involve the abrupt penetration of bursts of magnetic flux taking place within a limited window of temperatures and magnetic fields. While dc-magnetometry allows one to determine the threshold fields for the occurrence of the thermomagnetic instabilities, magneto-optical imaging reveals the spatial distribution of magnetic flux throughout the sample. Conducting appropriate experiments, typical for this goal, avalanches were confirmed to be a characteristic of this material, ruling out the otherwise admissible possibility of an experimental artifact or a feature related to defects in the film. After the present observation, amorphous MoSi alloys, or a-MoSi, can be included in the gallery of superconducting materials exhibiting flux avalanches when in the form of thin films, a characteristic that must be carefully taken into consideration when one plans to employ films of those materials in applications.