F Colauto

Enhanced pinning in superconducting thin films with graded pinning landscapes

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.

Boundaries of the instability region on the HT diagram of Nb thin films

Catastrophic penetration of magnetic fields in the form of dendrites has been reported for some specimens of superconducting thin films in the perpendicular geometry. Such behavior is related essentially to the capability of the sample and substrate to assimilate heat generated by vortex motion. In order to map the region where instabilities occur, we have employed dc magnetometry, which has been shown to be an efficient technique for this purpose. This catastrophic regime ceases at temperatures above 4 K. For the sample studied, fluctuations have been detected on virgin curves of the magnetic moment as a function of the applied field. Our systematic study shows that there is a threshold line on the HT diagram, which encompasses the region where the magnetic response fluctuates.

Cascade dynamics of thermomagnetic avalanches in superconducting films with holes

Vestgarden, Jorn Inge; Colauto, Fabiano; de Andrade, AMH; Oliveira, AAM; Ortiz, W. A.; Johansen, Tom Henning. Cascade dynamics of thermomagnetic avalanches in superconducting films with holes. Physical Review B. Condensed Matter and Materials Physics 2015 ;Volum 92.(14) s. -

First Observation of Flux Avalanches in a-MoSi Superconducting Thin Films

We have observed the occurrence of dendritic flux avalanches in an amorphous film of Mo84Si16. 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.

Classical analogy for the deflection of flux avalanches by a metallic layer

Sudden avalanches of magnetic flux bursting into a superconducting sample undergo deflections of their trajectories when encountering a conductive layer deposited on top of the superconductor. Remarkably, in some cases the flux is totally excluded from the area covered by the conductive layer. We present a simple classical model that accounts for this behaviour and considers a magnetic monopole approaching a semi-infinite conductive plane. This model suggests that magnetic braking is an important mechanism responsible for avalanche deflection.

Trapping Flux Avalanches in Nb Films by Circular Stop-Holes of Different Size

Dendritic flux avalanches triggered by thermomagnetic instabilities in Nb superconducting films have been limited by inclusion of circular holes of different diameters produced by optical lithography. We have observed a compromise between dendrite and hole sizes, for which avalanches are effectively arrested. For holes much smaller than the dendrites, only individual branches are stopped. Large holes are not a good solution, since they change the film geometry. A noteworthy trapping is observed at the holes with diameter between the width of an individual branch and the size of a whole dendrite. The present work shows the trend to optimize stop-holes for limiting thermomagnetic avalanches in superconducting films.

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

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 frozen-in 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.

Spin texture on top of flux avalanches in Nb/Al2O3/Co thin film heterostructures

We report on magneto-optical imaging, magnetization, Hall effect, and magneto-resistance experiments in Nb/Al2O3/Co thin film heterostructures. The magneto-transport measurements were performed in samples where electrical contacts were placed on the Co layer. The magnetic field is applied perpendicularly to the plane of the film and gives rise to abrupt flux penetration of dendritic form. A magnetization texture is imprinted in the Co layer in perfect coincidence with these ramifications. The spin domains that mimic the vortex dendrites are stable upon the field removal. Moreover, the imprinted spin structure remains visible up to room temperature. In the region of the field-temperature diagram where flux instabilities are known to occur in bare Nb films, irregular jumps are observed in the magnetic hysteresis and large amplitude noise is detected in the magneto-resistance and Hall resistivity data when measured as a function of the field.

The role of demagnetizing factors in the occurrence of vortex avalanches in Nb thin films

Under specific circumstances, magnetic flux penetrates into superconducting thin films as dendritic flux jumps. The phenomenon has a thermomagnetic origin, where flux motion generates heat that suppresses flux pining and facilitates further flux motion. We have studied the thickness influence on the flux stability for very thin Nb films, 20, 40, 60, and 80 nm, through dc-magnetomerry. The thicker the film; the higher is the threshold field where instabilities first take place. Due to the demagnetizing factor in a perpendicular geometry, the effective magnetic field at the border of the film is largely amplified. For thin specimens, a linear dependence between the threshold field and the thickness is expected and has been actually observed. When normalized by the sample aspect ratio, the effective threshold magnetic field is nearly the same for all specimens studied.