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Granularity is one of the main features restricting the maximum current which a superconductor can carry without losses, persisting as an important research topic when applications are concerned. To directly observe its effects on a typical thin superconducting specimen, we have modeled the simplest possible granular system by fabricating a single artificial weak-link in the center of a high-quality Nb film using the focused ion beam technique. Then, its microstructural, magnetic, and electric properties in both normal and superconducting states were studied. AC susceptibility, DC magnetization, and magneto-transport measurements reveal well-known granularity signatures and how they negatively affect superconductivity. Moreover, we also investigate the normal state electron scattering mechanisms in the Boltzmann theory framework. The results clearly demonstrate the effect of the milling technique, giving rise to an additional quadratic-in-temperature contribution to the usual cubic-in-temperature sd band scattering for the Nb film. Finally, by analyzing samples with varying density of incorporated defects, the emergence of the additional contribution is correlated to a decrease in their critical temperature, in agreement with recent theoretical results.

The magnetic anisotropy of superconducting niobium films of different thickness was investigated using magneto-optical imaging. A wide range of anisotropic behaviors was generated by freezing in different in-plane external magnetic fields with magnitude up to 1.12 kOe. The critical fields for the parallel configuration of the different samples were calculated, taking into account the strong deviation of the superconducting properties at small thicknesses. Above their lower in-plane critical field, the films are in the mixed state containing coplanar vortices. In structurally isotropic Nb films, when applying a perpendicular magnetic field, the frozen-in vortices were seen to guide the perpendicular ones. It is shown that the anisotropy increases with both the film thickness and the magnitude of the in-plane field. The results demonstrate that the field-induced anisotropy is a highly versatile way to manipulate vortex matter in film superconductors.

The problem of determining the transparency of current flow through a planar interface connecting two superconducting parts, being bulks, tapes, or films, is analyzed within the critical-state model. An exact relation between the magnetic moment of a sample containing such an interface and its transparency is derived for rectangular specimens in fully developed Bean critical states. The result can serve as theoretical basis for using standard magnetometry to measure the quality of joints, grain boundaries, and other weak links between superconducting parts.

When developing superconducting devices patterned on thin films, one should bear in mind that flux avalanches might occur for some materials at a certain window of applied fields and temperatures. Although the A15 superconductors are well known and used in a variety of purposes, there are no studies about flux avalanches in V 3 Si thin films. In the present work, we report the observation of flux avalanches in films of V 3 Si, grown by pulsed laser deposition on a LaAlO 3 substrate. The range of temperatures and applied magnetic fields at which the avalanches take place is determined using dc-magnetometry. Magneto-Optical Imaging is employed to visualize the occurrence and spatial distribution of such flux avalanches. Images of the flux penetrated into the samples indicate a clear anisotropic distribution, which is ascribed to a thickness gradient. The observation of flux avalanches reported here implies that attention to this feature must be given when films of V 3 Si are envisaged for possible applications.

A graded distribution of pinning centers (antidots) in superconducting MoGe thin films has been investigated by magnetization and magneto-optical imaging. The pinning landscape has maximum density at the border, decreasing progressively towards the center. At high temperatures and low fields, where this landscape mimics the vortex distribution predicted by the Bean model, an increase of the critical current is observed. At low temperatures and fields, the superconducting performance of the non-uniform sample is also improved due to suppression of thermomagnetic avalanches. These findings emphasize the relevance of non-uniform pinning landscapes, so far experimentally unexplored, on the enhancement of pinning efficiency.

A method to measure the electrical connectivity between square superconducting plates joined by weak link interfaces is presented. It is based on observation of lines where the flow of critical current abruptly changes direction due to the presence of weak links, and the confinement created by the shape of the sample. The method is demonstrated using magneto-optical imaging (MOI) of systems consisting of up to 2 × 2 plates joined to form a larger square. The samples used in this study are Nb films with weak links created by focused ion beam machining. Common features are found in the current flow patterns, which allow to measure the electrical connectivity between the plates by observing an angle between pairs of lines indicating where the current abruptly changes flow direction, so-called discontinuity, or d-lines.

Quantitative magneto-optical imaging of a type-II superconductor thin film cooled under zero, homogeneous, and inhomogeneous applied magnetic fields, indicates that the latter procedure leads to an enhancement of the screening capacity. Such an observation is corroborated by both B-independent and B-dependent critical state model analyses. Furthermore, repulsive (attractive) vortex-(anti)vortex interactions were found to have a decisive role in the shielding ability, with initial states prepared with vortices resulting in a shorter magnetic flux front penetration depth than those prepared with antivortices. The proposed strategy could be implemented to boost the performance of thin superconducting devices., Comment: 6 pages and 5 figures

Considering a noncentrosymmetric pinning texture composed of a square array of triangular holes, the magnetic flux penetration and expulsion are investigated experimentally and theoretically. A direct visualization of the magnetic landscape obtained using a magneto-optical technique on a Nb film is complemented by a multiscale numerical modeling. This combined approach allows the magnetic flux dynamics to be identified from the single flux quantum limit up to the macroscopic electromagnetic response. Within the theoretical framework provided by time-dependent Ginzburg-Landau simulations, an estimation of the in-plane current anisotropy is obtained and its dependence with the radius of the curvature of hole vertices is addressed. These simulations show that current crowding plays an important role in channeling the flux motion, favoring hole-to-hole flux hopping rather than promoting interstitial flux displacement in between the holes. The resulting anisotropy of the critical current density gives rise to a distinct pattern of discontinuity lines for increasing and decreasing applied magnetic fields, in sharp contrast to the invariable patterns reported for centrosymmetric pinning potentials. This observation is partially accounted for by the rectification effect, as demonstrated by finite-element modeling. At low temperatures, where magnetic field penetration is dominated by thermomagnetic instabilities, highly directional magnetic flux avalanches with a fingerlike shape are observed to propagate along the easy axis of the pinning potential. This morphology is reproduced by numerical simulations. Our findings demonstrate that anisotropic pinning landscapes and, in particular, ratchet potentials produce subtle modifications to the critical state field profile that are reflected in the distribution of discontinuity lines.

Magneto-optical imaging of YBa2Cu3O7-δ films with high critical current density, synthesized by a cost-effective metal organic decomposition technique reveals inhomogeneous flux penetration in the specimens in the form of thin parallel lines. The origin of such a stripy pattern and its dependence on the sample preparation conditions and state of substrate is discussed. The stripes reflect accumulation of planar defects forming parallel lines of reduced in-plane critical current density, jc , perpendicular to planar defects and enhanced jc parallel to them. Such channel-like reduction and corresponding enhancement of jc is especially expressed in a sample deposited on vicinal substrate, which, as a consequence, demonstrates global temperature-dependent in-plane anisotropy with an anisotropy ratio up to 2.4. The directional enhancement of critical current density due to planar defects could be beneficial for practical use of superconducting films.

We investigate the directional locking effects that arise when a monolayer of paramagnetic colloidal particles is driven across a triangular lattice of magnetic bubbles. We use an external rotating magnetic field to generate a two dimensional traveling wave ratchet forcing the transport of particles along a direction that intersects two crystallographic axes of the lattice. We find that, while single particles show no preferred direction, collective effects induce transversal current and directional locking at high density via a spontaneous symmetry breaking. The colloidal current may be polarized via an additional bias field that makes one transport direction energetically preferred., Comment: accepted in Phys. Rev. Lett

Time-resolved magneto-optical (TRMO) imaging with ultrashort laser pulses now allows direct observation of the excitation, propagation, and relaxation dynamics of magnetization, with the details of excitation still being studied. This article shows how to image the in-plane component of magnetization, modulated by propagating spin waves, via the birefringence that arises in a transverse magnetic field. The authors use this method to resolve the contribution from magnetoelastic coupling in an out-of-plane-magnetized film. This technique extends the scope of TRMO imaging for investigating laser-induced magnetization dynamics, which will enable work in spintronics.

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 figures

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.

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, 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., Comment: Presented on ASC 2014 and submitted to IEEE TRANSACTION ON APPLIED SUPERCONDUCTIVITY (Special Issue)

Stability of the vortex matter -- magnetic flux lines penetrating into the material -- in type-II superconductor films is crucially important for their application. If some vortices get detached from pinning centres, the energy dissipated by their motion will facilitate further depinning, and may trigger an electromagnetic breakdown. In this paper, we review recent theoretical and experimental results on development of the above mentioned thermomagnetic instability. Starting from linear stability analysis for the initial critical-state flux distribution we then discuss a numerical procedure allowing to analyze developed flux avalanches. As an example of this approach we consider ultra-fast dendritic flux avalanches in thin superconducting disks. At the initial stage the flux front corresponding to the dendrite's trunk moves with velocity up to 100~km/s. At later stage the almost constant velocity leads to a specific propagation regime similar to ray optics. We discuss this regime observed in superconducting films coated by normal strips. Finally, we discuss dramatic enhancement of the anisotropy of the flux patterns due to specific dynamics. In this way we demonstrate that the combination of the linear stability analysis with the numerical approach provides an efficient framework for understanding the ultra-fast coupled non-local dynamics of electromagnetic fields and dissipation in superconductor films.

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 (Hth 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.

We have investigated optically-excited magnetoelastic waves by phase-resolved spin-wave tomography (PSWaT). PSWaT reconstructs dispersion relation of spin waves together with their phase information by using time-resolved magneto-optical imaging for spin-wave propagation followed by an analysis based on the convolution theorem and a complex Fourier transform. In PSWaT spectra for a Bi-doped garnet film, we found a 180 degree phase shift of magnetoelastic waves at around the crossing of the dispersion relations of spin and elastic waves. The result is explained by a coupling between spin waves and elastic waves through magnetoelastic interaction. We also propose an efficient way for phase manipulation of magnetoelastic waves by rotating the orientation of magnetization less than 10 degree., Comment: 5 pages, 4 figures

Using spin-wave tomography (SWaT), we have investigated the excitation and the propagation dynamics of optically-excited magnetoelastic waves, i.e. hybridized modes of spin waves and elastic waves, in a garnet film. By using time-resolved SWaT, we reveal the excitation dynamics of magnetoelastic waves through coherent-energy transfer between optically-excited pure-elastic waves and spin waves via magnetoelastic coupling. This process realizes frequency and wavenumber selective excitation of spin waves at the crossing of the dispersion relations of spin waves and elastic waves. Finally, we demonstrate that the excitation mechanism of the optically-excited pure-elastic waves, which are the source of the observed magnetoelastic waves, is dissipative in nature., 5 pages, 4 figures

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. peerReviewed

In this paper the origin of flux-jumps in Nb thin films is established during magnetization measurements using a vibrating sample magnetometer (VSM). Magnetization measurements of the flux avalanche activity show its strong dependence on frequency and amplitude of VSM vibration. In particular, under certain conditions the vibrations induce a transition from a stable superconducting critical state to an undercritical state, accompanied by the 20-fold drop in the magnetic moment. These features allow the elucidation of the origin of the flux-jumps. In contrast to the commonly assumed thermomagnetic instabilities to be responsible for the flux-jumps in Nb films, our results provide solid support for an alternative explanation being due to criticality-built instability well represented by a sandpile. Considering properties of the flux-flow during a flux avalanche regime allows us to estimate nonuniformity of a magnetic field in a VSM sample space developed as a result of vibrations.

By implementing a unique magneto-optical system with ultrafast magnetic-field ramping-rate capability (up to 3 kT/s), we have been able to routinely generate and image dendritic flux instabilities in YBa2Cu3O7-x films. In the present work we study the effect of the film thickness on the dendritic instability. Dendritic avalanches in 50 - 600 nm thick films were magneto-optically imaged at 7 K, after ramping the magnetic field from zero to 60 mT at different rates. The data reveal a remarkable change in flux morphologies between the thin and the thicker films. While the former (50-250 nm) display well-developed dendritic patterns, the latter (350-600 nm) exhibit few avalanches with favored branch directions parallel to the film's edges. Several possible explanations for this behavior are discussed.

Colloidal inclusions in lab-on-a-chip devices can be used to perform analytic operations in a non-invasive fashion. We demonstrate here a novel approach to realize fast and reversible micro-sieving operations by manipulating and transporting colloidal chains via mobile domain walls in a magnetic structured substrate. We show that this technique allows to precisely move and sieve non-magnetic particles, to tweeze microscopic cargos or to mechanically compress highly dense colloidal monolayers., Comment: 7 pages, 6 figures, published in Lab on a Chip

The propagation dynamics of spin waves are represented by their dispersion relations. Recently, we have developed a method, called spin-wave tomography (SWaT), to obtain dispersion relation of spin waves in the long wavelength regime, so-called pure magnetostatic waves. In our previous studies on SWaT, phase information of spin waves was disregarded. In this report, we demonstrate an advanced SWaT analysis, called phase-resolved spin-wave tomography (PSWaT), to realize the direct observation of the amplitude and the phase of spin waves. The PSWaT spectra are obtained by separating the real and the imaginary components of the complex Fourier transform in the SWaT analysis. We demonstrate the PSWaT spectra of spin waves excited by the photo-induced demagnetization in a Bi-doped garnet film, reflecting the characteristic features of the complex dynamical susceptibility affected by magnetostatic coupling in the film., 5 pages, 4 figures

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 changes 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.

Tapes of MgB2 with high critical current have a significant technological potential, but can experience operational breakdown due to thermomagnetic instabilities. Such events are routinely registered by magnetisation measurements, but were never observed by direct imaging techniques. Here we report magneto-optical imaging visualisation of a tape with reduced thickness and enhanced critical current density. The spatial structure of the thermomagnetic avalanche events was resolved, and the reproducibility and nucleation thresholds were determined. Avalanches appearing at low fields are small intrusions at the tape edges, while larger fields create more extensive avalanches having dendritic structures similar to those observed in thin-film superconductors. In contrast to dendritic avalanches in thin films, the avalanches in the tapes show reproducibility to a larger extent., This work was financially supported by the University of Oslo, the Spanish Ministerio de Economía y Competitividad, the European FEDER Program (Projects MAT2011-22719 and ENE-2014-52105-R) and the Gobierno de Aragón (research group T12).

The electrodynamic behavior of isotropic superconducting Nb films cooled below their critical temperature in the presence of in-plane applied magnetic fields is investigated using magneto-optical imaging. A specially designed local flux injector is used to show that the frozen-in in-plane vortices strongly guide and enhance the penetration of perpendicular vortices, whereas their penetration across the array of in-plane vortices is essentially unchanged. This result provides the key to understanding why field-cooled square superconducting films show anisotropic nucleation of flux avalanches (jumps) along the four edges. The explanation is based on an analytical model for thermomagnetic avalanche nucleation in type-II superconducting films, and allows one to understand the entire scenario of different flux dynamics observed experimentally.

We report the experimental observation of the refraction and reflection of propagating magnetostatic spin waves crossing a 90° domain wall (DW). Time-resolved magneto-optical imaging was used to observe the propagation dynamics of magnetostatic spin waves. Due to the magnetization rotation across such a DW, the dispersion relation of magnetostatic spin waves rotates by 90°. This results in a change in the propagation dynamics of spin waves on both sides of the DW. We observed the refraction and reflection of magnetostatic spin waves at the DW and reveal their characteristics that include negative refraction. The incident-angle dependence of the refraction angle is explained by the wavenumber conservation along the DW, quite similar to the case of Snell's law for light.

We have investigated the effect of substrate thermal conductivity on dendritic flux formation in thin superconducting YBa 2Cu 3 O 7−x films grown on yttrium-stabilized zirconia, SrTiO 3, MgO, and sapphire, exploiting a recently developed ultra-fast magneto-optical imaging system and its ultra-fast field ramp (3 kT/s). Dendritic flux formation triggered solely by rapid ramping of the external field is reported for the first time in YBCO on yttrium-stabilized zirconia, the substrate with the lowest thermal conductivity. For the other substrates, the dendritic instability could be generated only after introducing an artificial defect at the edge, enhancing the local induction. We find that the upper temperature threshold for the appearance of dendrites depends on the thermal conductivity of the substrate.

Smart superconducting grid is a global network for CO 2 emissions-free renewable energy economy. The grid combines delivery of liquid hydrogen and electrical energy via superconducting pipelines. The paper reviews the development of the grid concept and the efforts in material science and engineering advancing its practical implementation. Original results are presented outlining an activity that targets the challenge of joining sections of pipelines, as well as manufacture of superconducting pipes. The focus is put on testing the quality of superconducting MgB 2 joints and coatings using the magnetic flux visualization technique of magneto-optical imaging.

Electrical conductivity, Hall effect and magnetoresistance of In2O3:Sn thin films deposited on a glass substrates at different temperatures and oxygen pressures, have been investigated in the temperature range 4.2–300 K. The observed temperature dependences of resistivity for films deposited at 230 °C as well as at nominally room temperatures were typical for metallic transport of electrons except temperature dependence of resistivity of the In2O3:Sn film deposited in the oxygen deficient atmosphere. The electrical measurements were accompanied by AFM and SEM studies of structural properties, as well as by XPS analysis. It is established that changes of morphology and crystallinity of ITO films modify the low-temperature behavior of resistivity, which still remains typical for metallic transport. This is not the case for the oxygen deficient ITO layer. XPS analysis shows that grown in situ oxygen deficient ITO films have enhanced DOS between the Fermi level and the valence band edge. The extra localized states behave as acceptors leading to a compensation of n-type ITO. That can explain lower n-type conductivity in this material crossing over to a Mott-type hopping at low temperatures. Results for the low temperature measurements of stoichiometric ITO layers indicate that they do not show any trace of metal-to-insulator transition even at 4.2 K. We conclude that, although ITO is considered as a highly doped wide-band gap semiconductor, its low-temperature properties are very different from those of conventional highly doped semiconductors.

The effects of the precursor milling energy on the superconducting and microstructural homogeneity of Fe-sheathed MgB2 monocore wires and tapes have been investigated. The conductors were produced by the powder-in-tube method using drawing and rolling deformation methods and in situ reaction. Combined magneto-optical, structural and magnetization measurements were performed on MgB2 superconducting tapes prepared from differently milled precursors revealing 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 the Gobierno de Aragón (research group T12).

International audience; The influence of a conducting layer on the magnetic flux penetration in a superconducting Nb film is studied by magneto-optical imaging. The metallic layer partially covering the superconductor provides an additional velocity-dependent damping mechanism for the flux motion that helps to protect the superconducting state when thermomagnetic instabilities develop. If the flux advances with a velocity slower than w = 2/μ 0 σ t, where σ is the cap layer conductivity and t is its thickness, the flux penetration remains unaffected, whereas for incoming flux moving faster than w, the metallic layer becomes an active screening shield. When the metallic layer is replaced by a perfect conductor, it is expected that the flux braking effect will occur for all flux velocities. We investigate this effect by studying Nb samples with a thickness step. Some of the observed features, namely the deflection of the flux trajectories at the border of the thick center, as well as the favored flux penetration at the indentation, are reproduced by time-dependent Ginzburg-Landau simulations.

Our recent success in triggering dendritic flux instabilities in $\mathrm{YB}{\mathrm{a}}_{2}\mathrm{C}{\mathrm{u}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ (YBCO) films by applying magnetic fields at ultrahigh rates is followed here by a detailed study of the effect as a function of the field ramp rate, ${\stackrel{\ifmmode \dot{}\else \.{}\fi{}}{B}}_{a}$, and temperature, $T$. We trace the borderline in the ${\stackrel{\ifmmode \dot{}\else \.{}\fi{}}{B}}_{a}\ensuremath{-}T$ plane separating regions of smooth, gradual flux penetration and dendritic flux avalanches. In addition, we describe the changes in the dendritic morphology in the instability region as a result of changes in either ${\stackrel{\ifmmode \dot{}\else \.{}\fi{}}{B}}_{a}$ or $T$. Our experimental results, showing a monotonic increase of the avalanche threshold field ramp rate with temperature, are discussed in the framework of existing theories. On the basis of these theories we also explain the high stability of YBCO to dendritic avalanches as compared to, e.g., $\mathrm{Mg}{\mathrm{B}}_{2}$, identifying the flux flow resistivity, rather than any of the thermal parameters, as the main parameter governing the film stability.

We demonstrate a general and robust method to confine on a plane strongly diffusing submicrometer particles in water by using size tunable magnetic channels. These virtual conduits are realized with pairs of movable Bloch walls (BWs) located within an epitaxially grown ferrite garnet film. We show that, once inside the magnetic conduit, the particles experience an effective local parabolic potential in the transverse direction, while freely diffusing along the conduit. The stiffness of the magnetic potential is determined as a function of field amplitude which varies the width of the magnetic channel, and precise control of the degree of confinement is demonstrated by tuning the applied field. The magnetic conduit is then used to realize single files of non-passing particles and to induce periodic condensation of an ensemble of particles into parallel stripes in a completely controllable and reversible manner.

Restricted deposits of fossil fuels and ecological problems created by their extensive use require a transition to renewable energy resources and clean fuel free from emissions of CO2. This fuel is likely to be liquid hydrogen. An important feature of liquid hydrogen is that it allows wide use of superconductivity. Superconductors provide compactness, high efficiency, savings in energy and a range of new applications not possible with other materials. The benefits of superconductivity justify use of low temperatures and facilitate development of fossil-free energy economy. The widespread use of superconductors requires a simple and reliable technique to monitor their properties. Magneto-optical imaging (MOI) is currently the only direct technique allowing visualization of the superconducting properties of materials. We report the application of this technique to key superconducting materials suitable for the hydrogen economy: MgB2 and high temperature superconductors (HTS) in bulk and thin-film form. The study shows that the MOI technique is well suited to the study of these materials. It demonstrates the advantage of HTS at liquid hydrogen temperatures and emphasizes the benefits of MgB2, in particular.

Two voltage jumps were found in the current–voltage characteristic of a thin superconducting film of Bi 2 Sr 2 CaCu 2 O x at temperature 58 K. The fluorescent thermal imaging technique revealed a localized heat dissipation (hotspot) nucleated at the current causing the second jump, and the hotspots’ temperature is close to the critical temperature of the superconducting film.

Hysteretic effects are seen in the upper and lower threshold fields for the appearance of dendritic flux instabilities, first explained in Yurchenko et al. [Phys. Rev. B 76 (2007) 092504], in NbN-films. We have measured the threshold fields at increasing and decreasing applied fields at different temperatures and proposed a mechanism explaining how the hysteresis arises by analyzing the field profiles inside the sample.

Regular arrays of planar defects with a period of a few nanometers can be introduced in superconducting YBa 2 Cu 3 O 7 - δ (YBCO) thin films by depositing them on vicinal (also called miscut or tilted) substrates. This results in the anisotropy of critical currents flowing in the plane of the film. We present results of real-time magneto-optical imaging (MOI) of magnetic flux distribution and dynamics in a series of YBCO thin films deposited on NdGaO 3 substrates with different miscut angles θ . MOI allows reconstructing the current flow profiles. From the angle formed between domains with different directions of the current flow we determine the anisotropy parameter of the in-plane current, as well as its field and temperature dependences. The artificially introduced defects also have a dramatic effect on the dynamics of the flux propagation: for 10 ° θ 14 ° the magnetic flux propagates along the easy channels intermittently, i.e. in a form of flux jumps. This behavior is indicative of thermo-magnetic instability in superconductors, but we argue that this effect can be of a different nature.

Thermomagnetic instability in general, and dendritic flux avalanches in particular, have attracted considerable attention of both scientists and engineers working on superconductor applications. Though being harmful for the performance of many superconducting devices, the avalanches provide a fruitful playground for experimental and theoretical studies of complex dynamics of the vortex matter. In this paper we report on the progress in understanding the mechanisms responsible for the development of the giant magnetic avalanches. We review recent results on magneto-optical imaging of the fingering instability in superconducting films and analyze them on the basis of recent theoretical model that establishes criteria for onset of the dendritic avalanches.

Using numerical calculations, the asymmetric pinning potential caused by the asymmetric shape of the pinning centers was found to cause significant asymmetries in the condensation energy, and the magnetic pinning force generated on changing the polarity of the applied current was discussed. In order to investigate the asymmetric properties, step-shaped patterns were introduced as asymmetric artificial pinning centers (APCs) in a 0.5-μm thick Nb film evaporated on an Al 2 O 3 (1 1 0 2) single crystalline substrate by using a lithographic technique. The patterns had periodic intervals of 5.0 μm. In addition, instead of the distribution of the critical current density, the distribution of the magnetic flux density in the specimen was directly observed using the magneto-optical (MO) imaging method. The asymmetric distribution of the magnetic flux density caused by the asymmetric shape of the APCs was observed by comparing the shift between the position where the magnetic flux density is the lowest and the position where it is the highest using MO images. In this study, for the purpose of comparison, the symmetric distribution of the magnetic flux density in a specimen with symmetric groove-shaped APCs was also observed.

Hotspot formation is observed in a structured thin superconducting film of Bi2Sr2CaCu2Ox (BSCCO) using the fluorescent thermal imaging technique. The BSCCO film is deposited on SrTiO3 (STO) and has a superconducting transition at 80 K. A film of rare-earth doped polymer film deposited directly on the superconductor is used as thermal sensor down to 4 K.

Bi-substituted yttrium iron garnet, Y3-xBixFe5O12 (Bi:YIG, x = 1, 1.5), films were prepared on Gd3Ga5O12 (111) substrates by metal-organic decomposition (MOD) method to use as MO indicator films. Those films had the in-plane magnetic anisotropy and Faraday rotation as large as that of the single crystal and do not show prominent magnetic domain structure, which is often observed in single crystalline garnet films grown by liquid phase epitaxy method. Disappearance o magnetic domain structure can be attributed to a granular structure with a grain size of ∼ 50 nm in Bi:YIG thin films. The characteristics are suitable for MO indicator films to visualize a stray magnetic field strayed from a specimen. In this paper, MO imaging of Nb films with groove patterns is demonstrated.

The emergence of wave fronts in dissipative driven systems is a fascinating phenomenon which can be found in a broad range of physical and biological disciplines. Here we report the direct experimental observation of discrete fronts propagating along chains of paramagnetic colloidal particles, the latter propelled above a traveling wave potential generated by a structured magnetic substrate. We develop a rigorously reduced theoretical framework and describe the dynamics of the system in terms of a generalized one-dimensional dissipative Frenkel-Kontorova model. The front dynamics is explored in a wide range of field parameters close to and far from depinning, where the discrete and continuum limits apply. We show how symmetry breaking and finite size of chains are used to control the direction of front propagation, a universal feature relevant to different systems and important for real applications.

We report on magneto-optical imaging, magnetization, Hall effect and magneto-resistance experiments in Nb/Al_2O_3/Co thin film heterostructures. 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. Complementary magnetization, Hall effect and magneto-resistance experiments were performed in a similar sample where electrical contacts were placed on the Co layer. In the region of the field - temperature diagram where flux instabilities are known to occur in 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., Comment: 13 pages, 8 figures

We demonstrate a size sensitive experimental scheme which enables bidirectional transport and fractionation of paramagnetic colloids in a fluid medium. It is shown that two types of magnetic colloidal particles with different sizes can be simultaneously transported in opposite directions, when deposited above a stripe-patterned ferrite garnet film subjected to a square-wave magnetic modulation. Due to their different sizes, the particles are located at distinct elevations above the surface, and they experience two different energy landscapes, generated by the modulated magnetic substrate. By combining theoretical arguments and numerical simulations, we reveal such energy landscapes, which fully explain the bidirectional transport mechanism. The proposed technique does not require pre-imposed channel geometries such as in conventional microfluidics or lab-on-a-chip systems, and permits remote control over the particle motion, speed and trajectory, by using relatively low intense magnetic fields., Comment: 5 pages, 4 figures, published in Physical Chemistry Chemical Physics

Magneto-optic (MO) imaging is based on Faraday rotation of a linearly polarized incident light beam illuminating a sensitive MO layer (MOL) placed in close contact to the sample. For in-plane magnetized layers of Lu 3− x Bi x Fe 5− y Ga y O 12 ferrimagnetic garnet films, zig-zag domain formation occurs whenever the sample stray parallel field component, H ∥ , changes sign. Considering the anisotropy, exchange and magnetostatic energies in the Neel tails, and the contribution of an applied magnetic field, it is possible to describe the zig-zag walls that separate domains with opposite in-plane magnetization. The size of the walls decreases with the spatial derivative of H ∥ . We studied the evolution of these domains as we steadily forced the change in sign of H ∥ to shorter length scales, from hundreds to a few microns. We describe the samples used to control the change in sign of H ∥ at the MOL plane, and we analyze the images that evolve from zig-zag walls to much more complex closed domain structures.

The determination of inter- and intra-filament characteristics in superconducting composites such as BSCCO-Ag tapes is of great importance for material evaluation towards applications. Most attempts to separate the two contributions have relied on indirect methods based on magnetic measurements such as SQUID or magneto-optic imaging techniques. Here we show that laser patterning of superconducting BSCCO-Ag tapes constitutes a simple approach to measure local transport properties in a direct way, even able to separate inter- and intra-filament contributions to the overall transport behavior of the sample.

We measured the magneto-optical images (MOIs) and the magnetic hysteresis (M – H) curves of c-axis-oriented MgB 2 thin films to investigate the flux penetration in the form of dendritic avalanches. In order to understand the role of the thermal effects, we prepared Au -coated MgB 2 thin films with different thicknesses of gold. While the MOI provides a spatially resolved flux pattern, the M – H curve presents global and average information about the flux noise associated with avalanches. These two types of measurements complement each other. The upper threshold field, above which the flux noise disappears, was determined from the M-H curves while the lower threshold field was determined from both the M – H curves and the MO images. The field range where the flux penetrates via avalanches is found to be smaller for thicker gold layers. These results are important for many superconducting applications.