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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.

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

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.

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.

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.

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 introduce a new choice of material for the creation of microscopic magnetic potentials for the trapping and guiding of ultracold neutral atoms. The potentials are created above a ferrimagnetic, transparent (BiYTmGd)3(FeGa)5O12 film by patterning the magnetic-domain structure in the film with a magneto-optical recording method. Patterns with linewidth down to 2 µm have been achieved, enabling trap frequencies of the order of 100 kHz for 87 Rb atoms in the state |F =1 ,m F = −1� .T he main advantages of the material are: 1) magnetic-field noise is suppressed due to the dielectricity of the material and the absence of electric currents, 2) trapped atoms can be addressed optically through the transparent film, and 3) the film can be repatterned, which enables different experiments with the same component. PACS. 39.25.+k Atom manipulation (scanning probe microscopy, laser cooling, etc.) - 75.50.Ss Magnetic recording materials - 75.70.-i Magnetic properties of thin films, surfaces, and interfaces

The time dependence of flux patterns obtained on an untwinned Y Ba2Cu3O7−δ single crystal showing the 'meandering instability' is observed at T = 65 K using magneto-optical imaging. When applying a reversed external field to a remanent state, along the front of invading antiflux, macrovortices or droplets of flux are formed, and eventually separate from the flux front in a spiral-like motion. The time-dependent behaviour of these macrovortices is investigated in detail.

Magneto-optical imaging has become a powerful technique for the measurement of local magnetic fields. The technique consists in measuring the rotation in the light polarization plane when light travels through a transparent sensitive garnet (Ytrium Iron Garnet, YIG). The rotation angle is a function of the magnetic field at the YIG location. We have studied commercial audio tapes in which computer generated functions were recorded. We present a study of the stray field of periodically magnetized tapes with square and sawtooth waveforms. Observations are described modeling the magnetized tapes as an array of coaxial circular coils where the current in each coil reproduces the recorded functions. The effect of the magnetic field components, normal and parallel to the YIG surface, is discussed.

We discuss how electrical current wires and micromagnetic elements can be used to trap or propagate paramagnetic beads, and consider their strengths and limitations. In particular, it is shown that the interactions can be effectively tuned by applying an external bias field. We demonstrate experimentally how to transport and structure a colloidal system using a domain wall, which could find use in microfluidic systems.

The effect of ion bombardment on the growth of Nb films and their crystal structure is investigated. Epitaxial niobium films with the (001) orientation are grown on (01i2) Al2O3 substrates heated to 600°C and biased at −20 V. Niobium films with pronounced axial texture in the [110] direction are grown on water-cooled Sitall (devitrified glass such as Pyroceram) substrates. In Nb films biased at −50 V, which are in the superconducting state, the motion of individual magnetic vortices is observed with a magnetooptic indicator.

The stability of the anisotropic flux flow in the vortex-antivortex system of a superconductor is theoretically studied. The vortex motion is assumed to be governed by the power current-voltage (I-V) characteristic, E ∝ Jm. It is shown that the instability of the interface between vortices and antivortices occurs even at relatively small anisotropy of the current capability of a sample if the exponent m is large. The dispersion equation defining the dependence of the increment of the instability on the wave number is obtained and analyzed. The instability leads to the macroturbulence of the flux flow which is observed in superconductors of the system Y-123.

A recently developed high-resolution magneto-optical imaging (MOI) setup is reviewed. It is the first MOI system capable of resolving the individual vortices in a type-II superconductor. We give a detailed description of the whole setup, and discuss its measured properties in terms of magnetic sensitivity and signal-noise characteristics. A simple model for the image intensity distribution due to a vortex lattice is developed, and for the intensity profile across a single vortex, we find good agreement between model calculations and experimental data. The minimum vortex spacing resolved experimentally is 1.3 μm. Our analysis shows that increased resolution can most easily be achieved by increasing the light input intensity, but maximum resolution is ultimately limited by the effective extinction ratio through the optical system and mechanical vibrations in the setup.

Flux avalanches are known to occur as a consequence of thermomagnetic instabilities. Some of their fingerprints are jumps in magnetization curves, or a paramagnetic reentrance in AC susceptibility measurements. In this work we have studied flux avalanches triggered by an AC field cycle by means of AC susceptibility and residual magnetization after an applied AC field measured as a function of an AC excitation field (h). These measurements allow comparing both results with magneto-optical imaging carried out in similar conditions. The results show a correspondence for the onset of the avalanche activity, as well as between the residual magnetic moment and the mean gray value calculated from the magneto-optical images in the remanent state.

The mechanism of instability of the front of magnetization reversal, i.e., of the boundary separating the regions of existence of vortices with mutually opposite orientations of the magnetic flux (vortex and anti-vortex regions) in type II superconductors, is suggested. The instability is associated with the anisotropy of flow of a vortex fluid, caused by planar defects in the ab plane of high-temperature superconducting single crystals of the 1-2-3 system. The anisotropy of the dynamic properties of vortex matter brings about a jump of the tangential component of the vortex velocity at the front of magnetization reversal; as is known from the classical hydrodynamics of viscous fluids, this leads to the turbulization of flow. It is demonstrated that the hydrodynamic approach to the description of vortex flow helps explain the emergence of a positive increment of the rise of vortex density fluctuations under conditions of a fairly strong anisotropy. The results of magnetooptical investigations of macroturbulence in the vortex system of Y-123 single crystal with a high density of the twinning boundaries lend qualitative support to the theoretical inferences.

The instability of a magnetic flux flow in a system of vortices and antivortices in superconductors with a power (exponent m) anisotropic current-voltage characteristic was studied theoretically. It was shown that instability arose even at a comparatively weak anisotropy of the current-carrying properties of a superconductor if m≫1. The dispersion equation determining the dependence of the increment of instability growth on the wave number was derived and analyzed.

We investigate the detection of magnetic data using magnetooptical indicators with in-plane magnetization. A simple model for the magnetooptical detection system is presented. We find that the signal to noise ratio changes strongly with the bit size, the polarization noise, and the distance between the magnetic carrier and the indicator. In particular, it is found that our model a signal to noise ratio of 30 is possible for a bit size of 200 nm. We also estimate the resolution of the system, and find that a spot size of ∼200 nm can be obtained using a suitably designed solid immersion lens. Finally, we discuss detection of several magnetic tracks simultaneously.

An electrodynamic model that describes the dispersion of magnetostatic surface waves in ferrite/superconductor structures is suggested. On its basis, a new approach to determining the microwave sheet resistance RS of superconducting films in a magnetic field is elaborated. The values calculated (RS=0.20–0.96 mΩ) agree with results obtained by the Tauber method. For YIG/YBCO structures, the controllable phase shift is about 1.5π when the depth of magnetostatic wave penetration into the YBCO film varies from 2.0 to 0.8 µm.

The mechanical stress caused by flux pinning in a cylindrical superconductor with a concentric hole is investigated theoretically. Exact expressions for the radial and hoop stress are derived using the critical-state model. Stress profiles during a magnetization process often used to activate high-Tc superconductors as strong trapped-field magnets are presented and analyzed in detail. It is shown that due to the hole the tensile hoop stress is enhanced by a factor of 2 or more, depending on the hole diameter. The dramatic increase in cracking probability is emphasized.

Superconducting joints between monofilamentary, Ag-sheathed tapes were investigated by means of magneto-optic imaging. Two types of joint were studied; one joint with direct contact between the tape cores, and the other one with an Ag layer between them. The local flux distributions directly reveal the obstacles hindering the current flow through the joints. The direct contact of the tape cores provides joints which can carry about 80% of the current of the original tape, whereas the joints with the Ag layer are considerably worse. This difference becomes even more drastic in applied magnetic fields.

Using magneto-optic imaging, the flux entry and exit into a piece of Ag-sheathed multifilamentary tape are studied in an applied magnetic field. In low fields the shielding currents are seen to flow mostly in the outermost filaments. With increasing external magnetic field, the inner filaments also contribute to the current flow. In the remanent state, the flux is confined to the tape centre and is not pinned in each filament independently. The magneto-optic flux patterns obtained reveal that the filaments in the tape are coupled together, which enables the currents to flow around defects within the filaments.

The time dependence of flux patterns of an untwinned YBa2Cu3O7 ? ? single crystal showing the meandering instability is observed at T = 65 K using magneto-optical imaging. When applying a reversed external field to a remanent state, along the front of penetrating antivortices, flux droplets are formed which can separate from the flux front and move in a spiral-like fashion. It is shown that the vortices along the flux front are in fast motion. These observations are discussed using a newly developed theory. Furthermore, the flux patterns are analyzed in terms of flux creep.

Magnetization loops in the critical state for the critical current density\(J_c = J_1 + J_2 e^{ - |B|/B_0 } \) and\(J_c = J_1 + J_2 /(1 + |B|/B_0 )\) are calculated. The geometry considered is an infinitely long superconductor with a finite cross-section in the shape of a rectangle or circle. Explicit formulas are derived for the various branches of the complete high-field hysteresis loop. In the discussion emphasis is put on the behavior of the width, δM, of the hysteresis loop, and a simple graphical method to fit model parameters to experimental data is outlined.

A microscopic magnetic squeezer based on two magnetic domain walls moving along a one-dimensional potential well generated by a stress line in ferrite garnet films is demonstrated. The squeezer can operate on magnetic objects of size 1–200μm and exert compressive forces up to 10pN. The squeezer operation, i.e., the relative motion of the two domain walls, is well controlled by a small external magnetic field modulation. The squeezer has potential applications in microfluidics and also as sensitive pressure gauges for microbiological systems.

A scheme is described for analytical calculation of critical‐state magnetization M of superconductors in the geometry of long rectangular slabs and cylindrical specimens in a parallel magnetic field. The simplicity of the general scheme is demonstrated by deriving compact expressions for the ascending and descending field branches of M in the exponential model Jc=jc0 exp(−B/B0) and in the Kim, Hempstead, and Strnad model [Phys. Rev. 129, 528 (1963)], Jc=jc0/(1+B/B0). The analyses focus on the vertical width ΔM of large field magnetization hysteresis loops. While Bean’s result [Phys. Rev. Lett. 8, 250 (1962)], Jc∝ΔM, today is used extensively to infer the critical current, it is well known that the method lacks consistency when a field dependence is seen in ΔM. For the two models it is shown explicitly that in the expansion of the functional relation ΔM(Jc), Bean’s result corresponds to the lowest‐order term. Also to the next order in the functional expansion we find a unifying form of expressing the model...

In a type-II superconductor, the magnetic field penetrates in the form of thin filaments called vortices. The controlled behavior of these vortices may provide the basis for a new generation of nanodevices. We present here a series of experiments showing simultaneous manipulation and imaging of individual vortices in a NbSe2 single crystal. The magnetic field from a Bloch wall in a ferrite garnet film (FGF) is used to manipulate the vortices. High-resolution magneto-optical imaging enables real-time observation of the vortex positions using the Faraday effect in the same FGF. Depending on the thickness of the sample, the vortices are either swept away or merely bent with the Bloch wall.

We propose a critical‐state model based on the empirical fact that the critical current density Jc in high‐Tc superconductors in some cases is found to decay to an essentially field independent level at large inductions B. The new critical current function, Jc(‖B‖), consists of a constant plus a field dependent term of the Kim model type. Analytical expressions for internal induction profiles and the entire shape of major magnetization loops, as well as the initial branch, are derived for the infinite slab geometry. As an illustration it is shown that the new model gives a precise overall description of a magnetization loop measured for melt processed YBa2Cu3Ox.

A new experimental setup has been designed to measure with high‐resolution the horizontal force between a permanent magnet levitating over a high‐temperature superconductor. We have studied the lateral stability as well as elastic stiffness associated with small‐amplitude (

A quantitative first‐order theory for the lateral force between a permanent magnet and a type‐II superconductor is presented. The levitation configuration discussed is that of a long rectangular bar magnet placed above a finite‐sized rectangular superconductor. The central issues of stability and stiffness (elastic spring constant) associated with lateral vibrations are discussed. Closed‐form expressions for both the force and stiffness are derived, thus bringing out clearly the significance of both geometrical dimensions and the magnetic response of the superconductor. It is assumed that the superconductor is either a sintered granular or consists of grains embedded in a nonactive matrix (composite) so that only intragranular shielding currents are important. The predicted behavior as a function of levitation height agrees very well with existing experimental results.

To further understand the relationship between the residual loss and the deviations of the magnetic field from perfect axisymmetry in a superconductor magnetic bearing, the drag torque acting on a high temperature superconductor slowly rotating in a nonaxisymmetric magnetic field was studied. A circular YBaCuO disk is rotated about its central axis, in the presence of a nonaxisymmetric magnetic field created from a pair of coils placed in its vicinity. This nonaxisymmetric field is either a uniform, or a uniform gradient magnetic field perpendicular to the axis of rotation of the YBaCuO disk. The torque exerted by the coils on the YBaCuO disk was monitored as a function time as the rotation proceeds, for different values of the current being passed through the coil. This torque exhibits periodic variations with a nonzero mean drag component which can be related to power loss, while the periodic variations were found to be connected to initial conditions and/or nonuniformity of the YBaCuO disk itself.

Flux patterns of multifilamentary Ag-sheathed (Pb,Bi)2Sr2Ca2Cu3O10+δ tapes comprising 19 filaments are visualized by means of magneto-optic imaging. In low fields, the shielding currents are seen to flow mainly in the outermost filaments. With increasing external magnetic field, the inner filaments also contribute to the current flow. To compare the local flux distribution with the integral magnetization values, magnetization loops are measured by a SQUID magnetometer on the same sample following the fields used in the magneto-optic imaging (± 120 mT) and covering fields up to ±5 T at various temperatures. The magnetization loops also reveal that the multifilamentary tapes show the anomalous position of the central peak, but always less pronounced than in monofilamentary tapes.

Slits patterned into a YBa2Cu3O7−δ thin film were observed to obstruct Meissner sheet currents leading to an imbalance in the local Meissner screening properties. The new phenomenon was studied with magneto‐optic imaging where twin lobes of opposite flux polarity were seen to form near the slits and inside the Meissner region. The lobe closest to the sample edge is always polarized opposite to the applied field. At weak fields, the anomalous flux generation is reversible. At higher fields, but still sufficiently small to keep the vortex penetration front away from the slits, the anomalous current starts nucleating flux lines which become trapped when the field is removed.

Based on a simple mechanical pendulum design, we developed a sensor for measurement of the horizontal force on a permanent magnet placed at any distance above a high‐Tc superconductor. In this fully computerized system, which employs closed‐loop feedback control, the lateral interaction with its restoring and hysteretic nature can be studied under conditions of constant vertical separation. Results are presented for the case of a small Nd‐Fe‐B magnet at a distance of 1 mm above a large planar disk of a YBa2Cu3Ox superconductor.

The stabilizing effect of placing a normal metal layer adjacent to a thermomagnetically unstable superconducting film is investigated. By solving the coupled Maxwell and heat transfer equations numerically it is shown that the metal, via inductive braking of the rapidly propagating flux avalanches, strongly reduces their impact. It is found that with increasing thickness and/or electrical conductivity of the metal layer, the number of avalanche events increases, while the amount of flux involved in each avalanche is strongly reduced, resulting in an overall more stable thermomagnetic system. The numerical results provide detailed insight into the braking process, and explain both previous dc magnetometry measurements and new magneto-optical imaging results obtained for a superconducting NbN film coated with a Cu-layer.

Little is known on the electrical properties of superconducting tapes and coatings in the direction transverse to the long dimension of the composite. However, transverse dissipation can eventually determine the fate of a transmission line in the case of failure due to the presence of transversal cracks, and is also crucial in the AC regime. In this paper we present a detailed experimental study of the electrical transport properties along the transverse direction of Bi2Sr2Ca2Cu3O10+x-Ag tapes, and compare them with those measured along the long axis of the material. We study in detail the influence of the tape's microstructure on electrical properties along both directions by using sliding electrodes. Our measurements suggest that there is always dissipation in the transverse direction for any value of the current. We also demonstrate that the local dissipation in the transverse direction has a nontrivial correlation with the local density of superconducting filaments.

It is demonstrated that circular holes in superconducting films of Nb can arrest the propagation of thermomagnetic avalanches. The effect was found over a range of temperatures where the material is susceptible to this instability. For other hole shapes, like square and triangular, the sharp corners provoke secondary avalanches, thus extending the breakdown. Making use of circular stop-holes can become a practical way to limit thermomagnetic breakdown in superconducting films.

Appearance of dendritic magnetic flux avalanches in superconducting films, which are associated with thermo-magnetic instability (TMI), very often indicates serious limitations for the ultimate performance of superconducting devices made of type-II superconducting thin films. We demonstrate that the stability can be controlled by a thorough adjustment of samples morphology at nano-scale, which affects internal material parameters. By this, the metal coating, commonly used as for stabilization, becomes redundant. Most importantly, we directly show by the mean of magneto-optical imaging that introduction of nano-scaled disorder dramatically changes the mode of magnetic flux propagation in the superconductors, from uniform motion of individual vortices to correlated jumps of relatively large vortex bundles, revealing the triggering mechanism of TMI.

This paper presents the magnetic flux distributions in Fe-based superconducting materials including single crystal of Ba(Fe1.9Ni0.1)As2 and Ba(Fe1.8Co0.2)As2, as well as polycrystalline SmFeO0.75F0.2As by means of magneto-optical imaging (MOI) technique. The single crystals were grown out of FeAs flux while polycrystalline sample was grown by hot-press. A MOI film with in-plan magnetization was used to visualize flux distributions at the sample surface. A series of magneto-optical images was taken when the samples were zero-field cooled and field cooled. The flux behavior, including penetration into and expelling from the samples, as well as pinning properties were studied. When external fields increase, flux is completely shielded from the crystals, then, gradually penetrates toward the crystal center from the edge. For polycrystalline sample, Meissner state was observed at very low field. With increasing the field further, flux penetrates into the sample easily along grain boundary, then into grain. Compared with high-Tc cuprates, it is found that the flux distributions in Fe-based superconducting materials are very similar to that in high-Tc cuprates with strong pinning strength.

Using fluorescent microthermal imaging we have investigated the origin of "two-step" behavior in I-V curves for a current-carrying YBa_2Cu_3O_x superconducting bridge. High resolution temperature maps reveal that as the applied current increases the first step in the voltage corresponds to local dissipation (hot spot), whereas the second step is associated with onset of global dissipation throughout the entire bridge. A quantitative explanation of the experimental results is provided by a simple model for an inhomogeneous superconductor, assuming that the hot spot nucleates at a location with slightly depressed superconducting properties., 4 pages, 4 figures

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 MgB2 and Nb.

We use a uniaxial garnet film with a magnetic bubble lattice to sort paramagnetic colloidal particles with different diameters, i.e., 1.0 and 2.8μm. We apply an external magnetic field which precesses around an axis normal to the film with a frequency Ω=62.8s−1 and intensity 3120A∕m

We report how one-dimensional nanomagnets can be used to manipulate paramagnetic particles located in liquid systems. It is shown that by using an asymmetric magnetic potential well, single particles can be efficiently sorted out from a one-dimensional array and moved along the phase boundary confining the array.