Scaling Behavior of Quasi-One-Dimensional Vortex Avalanches in Superconducting Films

Scaling behaviour of dynamically driven vortex avalanches in superconducting YBa$_{2}$Cu$_{3}$O$_{7-\delta}$ films deposited on tilted crystalline substrates has been observed using quantitative magneto-optical imaging. Two films with different tilt angles are characterized by the probability distributions of avalanche size in terms of the number of moving vortices. It is found in both samples that these distributions follow power-laws over up to three decades, and have exponents ranging between 1.0 and 1.4. The distributions also show clear finite-size scaling, when the system size is defined by the depth of the flux penetration front -- a signature of self-organized criticality. A scaling relation between the avalanche size exponent and the fractal dimension, previously derived theoretically from conservation of the number of magnetic vortices in the stationary state and shown in numerical simulations, is here shown to be satisfied also experimentally.Comment: 7 pages, 5 figure

Energy of dendritic avalanches in thin-film superconductors

A method for calculating stored magnetic energy in a thin superconducting film based on quantitative magneto-optical imaging is developed. Energy and magnetic moment are determined with these calculations for full hysteresis loops in a thin film of the superconductor NbN. Huge losses in energy are observed when dendritic avalanches occur. Magnetic energy, magnetic moment, sheet current and magnetic flux distributions, all extracted from the same calibrated magneto-optical images, are analyzed and discussed. Dissipated energy and the loss in moment when dendritic avalanches occur are related to each other. Calculating these losses for specific spatially-resolved flux avalanches is a great advantage, because of their unpredictable and non-reproducible nature. The relative losses in energy are much higher than the relative losses in moment

Energy of dendritic avalanches in thin-film superconductors

A method for calculating stored magnetic energy in a thin superconducting film based on quantitative magneto-optical imaging is developed. Energy and magnetic moment are determined with these calculations for full hysteresis loops in a thin film of the superconductor NbN. Huge losses in energy are observed when dendritic avalanches occur. Magnetic energy, magnetic moment, sheet current and magnetic flux distributions, all extracted from the same calibrated magneto-optical images, are analyzed and discussed. Dissipated energy and the loss in moment when dendritic avalanches occur are related to each other. Calculating these losses for specific spatially-resolved flux avalanches is a great advantage, because of their unpredictable and non-reproducible nature. The relative losses in energy are much higher than the relative losses in moment

Dendritic Flux Avalanches in High-quality NbN Superconducting Films

7th IEEE International Conference Nanomaterials - Application and Properties (NAP), Odessa, UKRAINE, SEP 10-15, 2017International audienceNiobium nitride (NbN) thin films are extensively used in superconducting devices such as single-photon detectors, hot electron bolometers, microwave resonators and kinetic inductance detectors. The operation of these devices is strongly influenced by the quality of the films, especially by their resistivity and superconducting transition temperatures (T-c). NbN films have rather high T-c of similar to 16.5 K and high resistivity of few hundreds micro-Ohm cm, which is perfect for operation of many superconducting devices. However, at low temperatures films are vulnerable to thermomagnetic instabilities in form of dendritic avalanches promoted by high resistivity in the normal state. Recently, new production route for NbN films has been established using high-temperature chemical vapor deposition (HTCVD). Transport measurements show low resistivity in normal state and suggest low level of lattice disorder. The highest for NbN T-c of 17.06 K was also reported in the films grown by HTCVD. According to previous study, these films should be thermo-magnetically stable. This work clarifies if it is the case and searches in one of them for dendritic flux avalanches. The nanoscale origin of avalanches is discussed

Scaling Behavior of Quasi-One-Dimensional Vortex Avalanches in Superconducting Films

Scaling behaviour of dynamically driven vortex avalanches in superconducting YBa2Cu3O7-δ films deposited on tilted crystalline substrates has been observed using quantitative magneto-optical imaging. Two films with different tilt angles are characterized by the probability distributions of avalanche size in terms of the number of moving vortices. It is found in both samples that these distributions follow power-laws over up to three decades, and have exponents ranging between 1.0 and 1.4. The distributions also show clear finite-size scaling, when the system size is defined by the depth of the flux penetration front - a signature of self-organized criticality. A scaling relation between the avalanche size exponent and the fractal dimension, previously derived theoretically from conservation of the number of magnetic vortices in the stationary state and shown in numerical simulations, is here shown to be satisfied also experimentally

Magneto-optical, microstructural and magnetization measurements of in situ Fe/MgB2 conductors made from ball-milled precursor

Trabajo presentado a la 13th European Conference on Applied Superconductivity (EUCAS), celebrada en Geneva (Suiza) del 17 al 21 de septiembre de 2017.Highly uniform microstructure and phase composition as well as technologically reproducible critical current densitiy, Jc, is compulsory for advanced applications of MgB conductors. Grain refinement by mechanical ball-milling of precursor powders would improve homogeneity and provide additional pinning. The increase of the milling energy transferred to the precursor causes a significant enhancement of Jc in the high field region but may also produce some undesirable effects such as a decrease of Jc values at low magnetic fields, higher oxygen content and, in some cases, degradation of the deformation properties during wire manufacture. To further explore the reasons of such a decrease in Jc in the low field range, we have analysed the local magnetic flux distribution inside MgB tapes made from different balled-milled powders using magneto-optical (MO) imaging. MO imaging performed in field-cooled samples, following a specific magnetic field sequence, highlights the distortions in the magnetic pattern so that it has been found very helpful to reveal inhomogeneities in the superconductor. The combined analysis of MO, scanning electron microscopy images and magnetization measurements has revealed hidden mechanisms controlling their critical current density. Local degradation of the superconducting properties observed in some analysed conductors has been identified and correlated with microstructural observations.This work was supported by the Spanish Ministerio de Economía y Competitividad and the European FEDER Program (Projects MAT2011-22719 and ENE-2014-52105-R) and by Gobierno de Aragón (research group T12).Peer Reviewe

Magneto-optical imaging of dendritic flux avalanches in a superconducting MgB2 tape

Resumen del trabajo presentado al Congreso Magnetism, celebrado en Manchester (UK) del 9 al 10 de abril de 2018.We report first direct imaging of dendritic avalanches in a superconducting MgB2 tape. The avalanches are caused by thermomagnetic instabilities and are known to harm the operation of superconducting devices. They are often detected in magnetisation measurements. However, their lower threshold magnetic field, Hthr, in bulk superconductors is far above the operational range of common visualisation techniques like magneto-optical imaging (MOI). In the present work, we have imaged six different MgB2 tapes and observed dendritic avalanches in one of them. Fig. 1 shows a superposition of three colour-coded MOI images of the sample after cooling to 3.7 K and applying magnetic fields within the operating range of the technique. The imaged abrupt flux jumps are of dendritic nature like those appearing in MgB2 superconducting films. The red, green and blue dendrites are irreproducible. They occur only once in three experiments. However, unlike thin films, the branches of few dendrites are partially reproducible, as shown by yellow, cyan and magenta colours for fragments overlapped twice. The temperature dependence of Hthr will be discussed. When comparing with thin films, it was found that the observed Hthr is much lower than expected, highlighting the importance of edge defects.Peer Reviewe