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 superconducting critical current of corner-shaped Al superconducting microstrips has been investigated. We demonstrate that the sharp turns lead to asymmetric vortex dynamics, allowing for easier penetration from the inner concave angle than from the outer convex angle. This effect is evidenced by a rectification of the voltage signal otherwise absent in straight superconducting strips. At low magnetic fields, an enhancement of the critical current with increasing magnetic field is observed for a particular combination of field and current polarity, confirming a theoretically predicted competing interplay of superconducting screening currents and applied currents at the inner side of the turn.

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 present a detailed quantitative magneto-optical imaging study of several superconductor/ferromagnet hybrid structures, including Nb deposited on top of thermomagnetically patterned NdFeB and permalloy/niobium with erasable and tailored magnetic landscapes imprinted in the permalloy layer. The magneto-optical imaging data are complemented with and compared to scanning Hall probe microscopy measurements. Comprehensive protocols have been developed for calibrating, testing, and converting Faraday rotation data to magnetic field maps. Applied to the acquired data, they reveal the comparatively weaker magnetic response of the superconductor from the background of larger fields and field gradients generated by the magnetic layer.

We present a thorough investigation by magneto-optical imaging of the magnetic flux penetration in Nb thin films with lithographically defined border indentations. We demonstrate that discontinuity lines ($d$ lines), caused by the abrupt bending of current streamlines around the indentations, depart from the expected parabolic trend close to the defect and depend on the shape and size of the indentation as well as on the temperature. These findings are backed up and compared with theoretical results obtained by numerical simulations and analytical calculations highlighting the key role played by demagnetization effects and the creep exponent $n$. In addition, we show that the presence of nearby indentations and submicrometer random roughness of the sample border can severely modify the flux front topology and dynamics. Strikingly, in contrast to what has been repeatedly predicted in the literature, we do not observe that indentations act as nucleation spots for flux avalanches, but they instead help to release the flux pressure and avoid thermomagnetic instabilities.