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Iron sheathed superconducting MgB2 wires were investigated with the help of magneto-optical (MO) imaging technique in order to visualize local effects induced by ferromagnetic iron screen (sheath) into the superconducting MgB2 core. MO experiments were also carried out in the wires with transport currents applied. The magnetic flux distribution within the superconducting core of the wire has been shown to be unusually modified in the case when the current is applied in the field-cooled state. The observed phenomenon is discussed in terms of magnetic interaction between the ferromagnetic screen and the superconducting core. This interaction enables a super-current redistribution within the core, so that the super-current paths are pushed to the middle of the core. This redistribution is consistent with the explanation of experimentally observed overcritical currents in the wires, which are higher than the conventional critical current density.

Magneto-optical imaging was used to study flux penetration into MgB2 films in a slowly increasing perpendicular applied field. A variety of flux jumps and avalanches have been observed at temperatures below 10 K. At small fields, jumps with typical size of 20 μm and regular shape occur at random locations along the flux front. Above some threshold field of 2–10 mT, big dendritic jumps with dimensions comparable to the sample size (mm scale) take place. The jumps are developing extremely fast, result in highly-branching irreproducible flux patterns, and effectively suppress the apparent critical current. Both types of jumps are believed to result from thermo-magnetic instability.

The penetration of magnetic flux into a thin superconducting film of Nb3Sn and NbN with critical temperature 17.8 K and 14.5 K respectively were visualized using magneto-optical imaging. Below 8 K for Nb3Sn and 6 K for NbN an avalanche-like flux penetration in form of big and branching dendritic structures was observed in response to increasing perpendicular applied field. When a growing dendritic branch meets a linear defect in the film, several scenarios were observed: the branch can turn and propagate along the defect, continue propagation right through it, or “tunnel ” along a flux-filled defect and continue growth from its other end. The avalanches manifest themselves in numerous small and random jumps found in the magnetization curve.

A theory of the macroturbulent instability in the system containing vortices of opposite directions (vortices and antivortices) in hard superconductors is proposed. The origin of the instability is connected with the anisotropy of the current capability in the sample plane. The anisotropy results in the appearance of tangential discontinuity of the hydrodynamic velocity of vortex and antivortex motion near the front of magnetization reversal. As is known from the classical hydrodynamics of viscous fluids, this leads to the turbulization of flow. The examination is performed on the basis of the anisotropic power-law current-voltage characteristics. The dispersion equation for the dependence of the instability increment on the wave number of perturbation is obtained, solved, and analyzed analytically and numerically. It is shown that the instability can be observed even at relatively weak anisotropy., 10 pages, 5 figures, submitted to Physical Review B

Paramagnetic beads electrostatically stabilized in aqueous solution are attracted toward domain walls in magnetic films. The position above the domain wall can be destabilized by realigning the beads magnetic moment with an external magnetic field. The destabilization may result in a steady state dissipative mode, where the beads surf on the slope of the moving domain wall. The technique could be an alternative route to probe electrostatic and hydrodynamic interactions between particles and interfaces, and could also serve as a model system for studying motion in a one-dimensional potential.

We demonstrate a method for manipulation of single magnetic microparticles based on a domain wall tip displaced in a controlled manner. By applying an external magnetic field, the tip can either drag or push magnetic particles. This kind of tweezers has potential applications in probing and manipulating colloidal systems.

We investigate the adsorption and diffusion of colloidal particles at an interface containing a one-dimensional potential well. It is observed how the adsorption kinetics onto the wall is altered with time, and one observes the formation of a particle chain. We find that the time it takes for a bead to penetrate into the chain depends strongly on the particle density, and beyond a critical value this time diverges. We also study diffusion within the well, and find that at low particle densities the short time behavior is governed by normal Fickian diffusion.

During systematic studies of the structural phase diagram of ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6+x}$ (YBCO), a YBCO single crystal of oxygen concentration $x=0.62$ turned out to have a tetragonal crystal structure. By magneto-optical imaging of the expulsion of an applied magnetic field, it was shown that the sample was bulk superconducting. This makes the sample highly anomalous in two respects: with a stoichiometry of ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6.62}$ the sample should have an orthorhombic symmetry, and a tetragonal undoped sample should not be superconducting at all. Our results corroborate previous findings of Topnikov et al. [JETP Lett. 46, 577 (1987)] of a tetragonal superconducting YBCO crystal with $x=0.62.$

We observe single vortices as they penetrate the edge of a superconductor using a high-sensitivity magneto-optical microscope. The vortices leap across a gap near the edge, a distance that decreases with increasing applied field and sample thickness. This behaviour can be explained by the combined effect of the geometrical barrier and bulk pinning., Comment: 2 pages, 1 figure, M2S-Rio proceedings

The height, h, of the meniscus bridging the bulk melt and the crystal during Czochralski growth depends on the crystal radius, r, and the angle of contact, θL, at the three-phase boundary. Based on a comparison with numerical calculations, we modify an approximate analytical meniscus relation, h = h(r, θL), due to Boucher and Jones. The modified relation significantly improves the overall agreement over a wide range of r and θL relevant for Czochralski growth.

Using magneto-optical imaging the phenomenon of dendritic flux penetration in superconducting films was studied. Flux dendrites were abruptly formed in a 300 nm thick film of MgB2 by applying a perpendicular magnetic field. Detailed measurements of flux density distributions show that there exists a local threshold field controlling the nucleation and termination of the dendritic growth. At 4 K the local threshold field is close to 12 mT in this sample, where the critical current density is 10^7 A/cm^2. The dendritic instability in thin films is believed to be of thermo-magnetic origin, but the existence of a local threshold field, and its small value are features that distinctly contrast the thermo-magnetic instability (flux jumps) in bulk superconductors., 6 pages, 6 figures, submitted to Phys. Rev. B

The flux creep problem for a superconductor slab placed in a constant or time-dependent magnetic field is considered. Logarithmic dependence of the activation energy on the current density is assumed, U=U0 ln(J/Jc), with a field dependent Jc. The density B of the magnetic flux penetrating into the superconductor, is shown to obey a scaling law, i.e., the profiles B(x) at different times can be scaled to a function of a single variable. We found exact solution for the scaling function in some specific cases, and an approximate solution for a general case. The scaling also holds for a slab carrying transport current I resulting in a power-law V(I) with exponent p~1. When the flux fronts moving from two sides of the slab collapse at the center, the scaling is broken and p crosses over to U0/kT., Comment: RevTex, 10 pages including 6 figures, submitted to Phys.Rev.B

The interaction between a magnetic thin film and a superconductor is studied. In particular, the equilibrium width of a Bloch wall is estimated with and without the superconductor. It is shown that the Bloch wall experiences a small shrinkage on cooling through the critical temperature of the superconductor. Furthermore, the interaction between the Bloch wall and a single vortex is estimated, and a simple expression for the force is found., 12 pages, 4 figures Corrected the units. Published in PRB

We have investigated the Faraday rotation of in-plane magnetized bismuth - substituted ferrite garnet films grown by liquid phase epitaxy on (100) oriented gadolinium gallium garnet substrates. The Faraday spectra were measured for photon energies between 1.7 - 2.6 eV. To interprete the spectra, we use a model based on two electric dipole transitions, one tetrahedral and one octahedral. Furthermore, the Faraday rotation sensitivity was measured at 2.3 eV, and found to be in good agreement with the theoretical predicitions. In particular, we find that the sensitivity increases linearly with the bismuth content and nonlinearly with the gallium content., 17 pages, 8 figures. Submitted to PRB

The interest in the physics of the vortex state in type-II superconductors increased significantly after the discovery of high-Tc superconductivity (HTS). A main reason for the renewed attention is the observation of many novel nontrivial phenomena occurring in the vortex matter of the HTS materials. The perhaps most dramatic of these phenomena is the turbulence instability of the vortex-antivortex interface, which was observed in 1-2-3 systems using magneto-optical (MO) imaging [1, 2, 3]. It consists in the following. When magnetic flux is trapped in the superconductor and a moderate field of reverse direction is subsequently applied, a boundary of zero flux density will separate regions containing flux and antiflux. In some temperature and field range this stable flux-antiflux distribution can display unstable behavior characterized by an irregular time-dependent propagation of the boundary where finger-like patterns develop. This contrasts strongly the stationary and reproducible propagation of the flux front during virgin field penetration, when only one flux polarity is present in the sample. It

We calculate the temperature and magnetic field distributions in a bulk superconductor during the process of pulsed-field magnetic activation. The calculations are based on the heat diffusion equation with account of the heat produced by flux motion, and the critical state model with temperature dependent critical current density. For a given activation time the total amount of trapped flux is maximum for an optimal value B_opt of the maximal applied field. We analyze how B_opt and the trapped flux depend on the material parameters and the field ramp rate., Comment: 6 pages, 6 figures, submitted to J.Appl.Phys., will be presented at ASC-2002

Magneto-optical imaging reveals that in superconducting films of MgB2 a transport current creates avalanche-like flux dynamics where highly branching dendritic penetration patterns are formed. The instability is triggered when the current exceeds a threshold value, and the superconductor, shaped as a long strip, is initially in the critical state. The instability exists up to 19 K, which is a much wider temperature range than in previous experiments, where dendrites were formed by applying a magnetic field. The instability is believed to be of thermo-magnetic origin indicating that thermal stabilization may become crucial in applications of MgB2., Comment: 3 pages, 3 figures, resubmitted to Appl.Phys.Lett

We propose a model for a superconductor-metal composite that allows to derive intrinsic transport properties of the superconducting phase based on 2D images of its cross section, and a minimal set of parameters. The method is tested experimentally by using, as model composite, a “transversal bridge” made on a Bi2Sr2Ca2Cu3O10+x (BSCCO)-Ag multi-filamentary tape. It is shown that the approach allows to predict the measured I−〈E〉 curves of the filaments. In addition, one can determine the critical current anisotropy between the longitudinal and transverse directions of the Ag-BSCCO tape, and also of its superconducting filaments separately, which emphasizes the role of the morphology of the composite in the transport properties.

Superconducting films of YBa2Cu3O7–δ are shown to become thermomagnetically unstable when experiencing a time-varying perpendicular magnetic field. Using magneto-optical imaging and ramping the applied field at rates up to 3000 T/s, dendritic flux avalanches were observed in two different films, one grown by evaporation on sapphire and one by laser ablation on SrTiO3. The unstable behavior occurs over a wide temperature range limited by an upper threshold value of 40 K for the film on sapphire, and 20 K for the one on SrTiO3. At 7 K for the same films, the threshold ramping rates are 1000 T/s and 3000 T/s, respectively. The avalanches are causing permanent damage by leaving a micron wide track where the superconductor melted during the thermomagnetic runaway.

Single crystalline films of bismuth-substituted ferrite garnets have been synthesized by the liquid phase epitaxy method where GGG substrates are dipped into the flux. The growth parameters are controlled to obtain films with in-plane magnetization and virtually no domain activity, which makes them excellently suited for magnetooptic imaging. The Faraday rotation spectra were measured across the visible range of wavelengths. To interprete the spectra we present a simple model based on the existence of two optical transitions of diamagnetic character, one tetrahedral and one octahedral. We find excellent agreement between the model and our experimental results for photon energies between 1.77 and 2.53 eV, corresponding to wavelengths between 700 and 490 nm. It is shown that the Faraday rotation changes significantly with the amount of substituted gallium and bismuth. Furthermore, the experimental results suggest that the magnetooptic response changes linearly with the bismuth substitution., 15 pages, 6 figures, published in Phys. Rev. B

The macroturbulence instability observed in fluxline systems during remagnetization of superconductors is explained. It is shown that when a region with flux is invaded by antiflux the interface can become unstable if there is a relative tangential flux motion. This condition occurs at the interface when the viscosity is anisotropic, e.g., due to flux guiding by twin boundaries in crystals. The phenomenon is similar to the instability of the tangential discontinuity in classical hydrodynamics. The obtained results are supported by magneto-optical observations of flux distribution on the surface of a YBCO single crystal with twins., 12 pages, 3 figures, submitted to Physical Review Letters

Systematic quantitative measurements have been performed in order to study the effect of proton irradiation on the critical current density Jc(B,T) for YBa2Cu3O7−δ (YBCO) thin films using magneto-optic imaging (MOI) technique. Results for external magnetic fields up to 400 Oe and temperature ranging from 5 to 80 K are presented for a 600 nm thick YBCO film irradiated by proton up to a dosage of 1016 cm−2 at 500 keV. While no significant differences were observed for T>50 K, the Jc values consistently increased with the increasing external field for T

Magneto-opitcal studies of a c-oriented epitaxial MgB2 film show that below 10 K the global penetration of vortices is dominated by complex dendritic structures abruptly entering the film. We suggest that the observed behavior is due to a thermo-magnetic instability which is supported by vortex dynamics simulations. The instability is also responsible for large fluctuations in the magnetization curves in MgB2 at low temperatures., Comment: Extended version: 4 pages, 7 figures, submitted to PRL, for high-quality MO images see http://www.fys.uio.no/faststoff/ltl/results/biology/index.html

The dynamic response of the levitation force between a permanent magnet and a bulk YBCO superconductor when subjected to a sudden motion‐induced step in the magnetic field has been measured. It is found that the levitation force relaxes logarithmically with time. Assuming that force and magnetization behave similarly, the rate of decay can be interpreted in terms of a thermally activated flux creep with a depinning activation energy of U0=0.20 eV. A replot of previously published results for melt‐quenched YBCO also demonstrates that also in this material the force relaxation is characterized by a logarithmic decay.

The critical-state in a thin strip of YBaCuO is studied by magneto-optical imaging. The distribution of magnetic flux density is shown to have a specific symmetry in the remanent state after a large applied field. The symmetry was predicted [PRL 82, 2947 (1999)] for any Jc(B), and is therefore suggested as a simple tool to verify the applicability of the critical-state model. At large temperatures we find deviations from this symmetry, which demonstrates departure from the critical-state behavior. The observed deviations can be attributed to an explicit coordinate dependence of $j_c$ since both a surface barrier, and flux creep would break the symmetry in a different way., 5 pages including 5 eps figures, submitted to PRB

Measurements of the levitation force, F z , and magnetic stiffness, κ =│δF/z δɀ│, functions of the separation z between a thin-film disk of YBa2Cu3O7-δ and cylindrical NdFeB magnet are presented. The magneto-mechanical behavior is fully explained from the magnetic response of thin disks within the critical-state model. It is shown that a scaled plot of F z (z) allows the critical current, and its field dependence to be inferred. The stiffness is shown to be essentially independent of the j c.

Magnetization hysteresis loops of type-II superconductors are characterized by a peak occuring at an applied field B pnear zero. This central peak is formed due to a field-dependence of the critical current density, J c (B), monotonously decreasing at small fields. The present work focuses on how the peak position is influenced by (i) the sample thickness, and (ii) the sample granularity. Theoretically we prove that in the limit of an infinitely thin and long strip placed in a perpendicular field, the central peak occurs at B p = 0, for any function J c (B). This result is shown to be in excellent agreement with experimental data.

The critical state in a superconducting thin circular disk in perpendicular applied magnetic field is analysed. Assuming an arbitrary field dependence of the critical sheet current, J c(B), an exact solution in the form of coupled integral equations for the flux and current distributions is derived. The equations are solved numerically, and flux and current proflles are presented graphically for several commonly used J c(B) dependences. Also the effect of a B-dependence in J c on the ac susceptibility is analysed.

The lateral force interaction between a permanent bar magnet and a large slab of high Tc superconductor has been investigated at 77 K, under conditions of a constant vertical separation of 2 mm. The restoring force as a function of lateral displacement rises very steeply, and reaches 90% of its saturation value, 5.1 mN, after 1.8 mm. The profile of the force‐displacement curve is in qualitative agreement with the existing theory. A significant quantitative discrepancy is interpreted as due to a theoretical penetration depth exceeding the sample thickness. The lateral magnetic stiffness or spring constant, is found to be independent of displacement away from lateral equilibrium.

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.

A YBa2Cu3O7-δ (YBCO) thin film is patterned into a hexagonal close packed lattice of disks which are touching each other at the circumferences in order to enable the flow of an intergranular current. Such a sample was suggested by Koblischka et al. [Appl Phys Lett 70, 514 (1997)] as a model for a layered granular structure like in a Bi2Sr2Ca2Cu3O10+δ tape. Magnetization measurements reveal an anomalous position of the low field peak (central peak) similar to the tapes. Magneto-optic imaging is employed to visualize the local field distributions.

Magnetization hysteresis loops and the ac susceptibility of a superconducting thin disk are calculated in the critical-state model assuming a field-dependent critical current density, Jc(B). The results are obtained by solving numerically the set of coupled integral equations for the flux and current distributions [PRB 60(17) p.13112 (1999)] for a disk placed in a perpendicular applied field Ba. From the magnetization curves the range of fields where the vertical width of the loop relates directly to Jc(Ba) is determined. Comparing our results on susceptibility with experimental data for parametric plot Im(chi) vs. Re(chi) shows that by taking the B-dependence of Jc into account the agreement improves dramatically. We show that the asymptotic behavior for large ac-field amplitudes Bam changes from Re(chi) ~ 1/Bam^{3/2} and Im(chi) ~ 1/Bam for the Bean model, to Re(chi) ~ 1/Bam^3 and Im(chi) ~ 1/Bam^2 for Jc decreasing with |B| as 1/|B| or faster. For small Bam the behavior can always be described by an effective Bean model with a renormalized Jc. We also find that in the Im(chi) vs. Re(chi) plot the peak of Im(chi) increases in magnitude and shifts to lower |Re(chi)|. This allows an easy experimental discrimination between a Bean model behavior, one with Jc(B), and one where flux creep is an ingredient., Comment: RevTex, 8 pages including 11 figures

Properties of YBa 2 Cu 3 O x (YBCO) thin films deposited on Nd 1/3 Eu 1/3 Gd 1/3 Ba 2 Cu 3 O x (NEG) sublayers with a nano-undulated surface morphology were investigated. We found a strong influence of the NEG sublayer on the microstructure and superconducting properties of such YBCO films. The correlation between microstructural features and superconducting properties is discussed.

Magnetic flux distribution across a high-temperature superconductor strip is measured using magneto-optical imaging at 15 K. Both the current-carrying state and the remanent state after transport current are studied up to the currents 0.97 Ic where Ic is the critical current. To avoid overheating of the sample current pulses with duration 50 ms were employed. The results are compared with predictions of the Bean model for the thin strip geometry. In the current-carrying state, reasonable agreement is found. However, there is a systematic deviation -- the flux penetration is deeper than theoretically predicted. A much better agreement is achieved by accounting for flux creep as shown by our computer simulations. In the remanent state the Bean model fails to explain the experimental results. The results for the currents I < 0.7 Ic can be understood within the framework of our flux creep simulations. However, after the currents I > 0.7 Ic the total flux trapped in a strip is substantially less than predicted by the simulations. Furthermore, it decreases with increasing current. Excessive dissipation of power in the annihilation zone formed in the remanent state is believed to be the source of this unexpected behavior., RevTeX, 11 pages with 10 Postscript figures

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.

Electrically conducting films in a time-varying transverse applied magnetic field are considered. Their behavior is strongly influenced by the self-field of the induced currents, making the electrodynamics nonlocal, and consequently difficult to analyze both numerically and analytically. We present a formalism which allows many phenomena related to superconducting and Ohmic films to be modelled and analyzed. The formalism is based on the Maxwell equations, and a material current-voltage characteristics, linear for normal metals, and nonlinear for superconductors, plus a careful account of the boundary conditions. For Ohmic films, we consider the response to a delta function source-field turned on instantly. As one of few problems in nonlocal electrodynamics, this has an analytical solution, which we obtain, in both Fourier and real space. Next, the dynamical behaviour of a square superconductor film during ramping up of the field, and subsequently returning to zero, is treated numerically. Then, this remanent state is used as initial condition for triggering thermomagnetic avalanches. The avalanches tend to invade the central part where the density of trapped flux is largest, forming dendritic patterns in excellent agreement with magneto-optical images. Detailed profiles of current and flux density are presented and discussed. Finally, the formalism is extended to multiply connected samples, and numerical results for a patterned superconducting film, a ring with a square lattice of antidots, are presented and discussed., 25 pages, 10 figures

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.

Magneto-optic imaging was used for a detailed study of the flux and current distribution of a long thin strip of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ placed in a perpendicular external magnetic field. The inverse magnetic problem, i.e., that of deriving from a field map the underlying current distribution, is formulated and solved for the strip geometry. Applying the inversion to the magneto-optically found field map we find on a model-independent basis the current distribution across the strip to be in remarkable agreement with the profile predicted by the Bean model. The paper also presents results on the behavior of the Bi-doped YIG film with in-plane anisotropy which we use as field indicator, explaining why previous measurements of flux density profiles have displayed surprisingly large deviations from the expected behavior. \textcopyright{} 1996 The American Physical Society.

Inhomogeneity of the properties of YBa2Cu3O7 epitaxial c-oriented films has been investigated. Using a low - temperature SEM the lateral distribution of local critical temperature has been determined with spatial resolution of 2 µm. The technique provides a direct observation of the percolative character of the superconducting transition on a microbridge scale. It is shown for the first time that near the percolation threshold the current channel has a very rough, irregular configuration and the fractal dimensionality of the superconducting cluster is estimated as 1.55. The magnetic field imaging of samples is carried out and a quantitative correlation between Tc-map and the map of critical density of shielding currents is observed. The transition curves R(T) over a wide range of resistances and bias currents are described in terms of the network model. The model takes into account both the spatial distribution of Tc and the presence of random weak links with a wide distribution of their critical currents.

We report on the first spatially resolved observation of mesoscopic flux jumps in superconducting films. Magneto-optical imaging was used to visualize the flux penetration in MgB2 films subjected to a slowly varying perpendicular field. Below 10 K, flux jumps with typical size 10-20 microns and regular shape are found to occur at random locations along the flux front. The total number of vortices participating in one jump is varying between 50 and 10000. Simultaneously, big dendritic jumps with dimensions comparable to the sample size (10^6-10^8 vortices) are also found in this temperature range. We believe that both types of jumps result from thermo-magnetic instability., Comment: 2 pages, 1 figure, Proceedings of M2S-Rio

We consider magnetic flux penetration in a superconducting film with a concave corner. Unlike convex corners, where the current flow pattern is easily constructed from Bean’s critical state model, the current flow pattern at a concave corner is highly nontrivial. To address the problem, we do a numerical flux creep simulation, where particular attention is paid to efficient handling of the non-local electrodynamics, characteristic of superconducting films in the transverse geometry. We find that the current stream lines at the concave corner are close to circular, but the small deviation from exact circles ensures that the electric field is continuous inside the sample. Yet, the electric field is, as expected, very high at the concave corner. At low fields, the critical state penetration is deeper from the concave corner than from the straight edges, which is a consequence of the electrodynamic non-locality. A magneto-optical experiment on Y Ba2Cu3Ox displays an almost perfect match with the magnetic flux distribution from the simulation, hence verifying the necessity of including electrodynamic non-locality in the modeling of superconducting thin films.

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.

A technique for joining superconducting MgB2 pellets has been developed. For the first time, magneto-optical imaging has been used to visualize the superconducting behaviour of an MgB2 joint. No significant difference was found between the penetration of the magnetic flux in the joint and in the bulk at low magnetic fields and at temperatures less than 35 K. The critical current density of the joint in high magnetic field was determined by measuring the transversal and longitudinal magnetic moment of a bar cut from the sample. A description was developed to extract the critical current density from magnetization measurements. The magnetization measurements reveal a critical current anisotropy in the pressed MgB2 and a high critical current in the joint at low temperatures. A possibility to shape the superconductor by pressing, and the conditions preventing development and spread of fractures have been investigated. The joint technique, which utilizes high pressure, high temperature and high electrical current could be scaled up providing a basis for the wide use of superconductors at liquid hydrogen temperatures.

Paramagnetic colloidal particles move in the potential energy landscape of a magnetically modulated bubble lattice of a magnetic garnet film. The modulation causes the energy minima to alternate between positions above the centres of the bubbles and interstitial positions. The particles deterministically follow the time-dependent positions of the energy minima until the minima become unstable in one or several directions and allow the particles to hop to a new minimum. We control the time delay between instabilities of the minima in alternative directions by the angle of the external magnetic field with the crystallographic directions of the bubble lattice. When the time delay is large, the particles deterministically hop to the next minimum along the direction that becomes unstable first. When the time delay is short, diffusion of the particle in the marginal potential randomizes the choice of the hopping directions or the choice of the transport network. Gradual changes of the external field direction from 0° to 30° lead to a continuous crossover from a deterministic to a fully stochastic path of the colloids.

We study the collective dynamics of an ensemble of paramagnetic particles organized as a 1D chain and driven by an a-thermal ratchet effect on a uniaxial garnet film. The centre of mass of the chain shows diffusive behavior with mean square displacement ∼t, while its end-to-end distance shows anomalous (sub-diffusive) kinetics growing as . We extract the potential of mean force between the particles within the chain by invoking the Pope–Ching equation. We interpret the experimental data using the Rouse model, originally developed for polymers, and experimentally extract all relevant parameters.

We have performed systematic quantitative measurements of the critical current density Jc(B, T) for YBCO thin films proton-irradiated to various dosages using the MOI technique. Results for external magnetic fields up to 400Oe and temperature ranging from 5 to 80K are presented for a 600nm thick YBCO film irradiated by proton up to a dosage of 1016 cm -2 at 500 keV. While no significant differences were observed for T>50K, the Jc values consistently increased with the increasing external field for T