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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.

Two types of MgB2 thin films produced by pulsed laser deposition with different in situ and ex situ sintering routes have been studied. Using the magneto-optical (MO) imaging technique, the magnetic flux penetration behaviour in the films has been investigated. In the case of the in situ film, the MO observations reveal conventional flux jumps below the corresponding threshold temperature, whereas in the case of the ex situ film the flux jumps appear to take the form of unusual, structurally driven 'blob'-like patterns. The underlying structural features of the films have been investigated by scanning electron and atomic force microscopy. The critical current density dependence on applied magnetic field obtained from magnetization measurements is consistent with the local behaviour of the magnetic flux, as well as with flux pinning properties expected from the obtained crystalline structure of the ex situ film and the amorphous-like surface of the in situ film. On the basis of these structural and electromagnetic observations, a mechanism for the structurally driven, spatially reproducible flux jumps is proposed.

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.

Polycrystalline MgB2 films with a thickness of about 1 µ mw ere prepared at ambient pressure. The boron layers were deposited onto single-crystalline and polycrystalline Al2O3 by screen printing, by airbrush techniques or by painting, followed by a reaction with Mg vapour. Grinding, mechanical and electrolytic polishing in non-aqueous dimethyl sulfoxide significantly improve dt he surface smoothness of the samples. The films had transition temperatures (Tc(0) )o f3 5K an di rreversibility fields of up to 15 T at 5K .T ransport critical current densities of 3 × 10 8 Am −2 were measured

Flux-pinning-induced stress and strain distributions in a thin disk superconductor in a perpendicular magnetic field is analyzed. We calculate the body forces, solve the magneto-elastic problem and derive formulas for all stress and strain components, including the magnetostriction $\Delta R/R$. The flux and current density profiles in the disk are assumed to follow the Bean model. During a cycle of the applied field the maximum tensile stress is found to occur approximately midway between the maximum field and the remanent state. An effective relationship between this overall maximum stress and the peak field is found., Comment: 8 pages, 6 figures, submitted to Supercond. Sci. Technol., Proceed. of MEM03 in Kyoto

Magneto-optical imaging was used to study a zero-field-cooled MgB2 film at 9.6K where in a slowly increasing field the flux penetrates by abrupt formation of large dendritic structures. Simultaneously, a gradual flux penetration takes place, eventually covering the dendrites, and a detailed analysis of this process is reported. We find an anomalously high gradient of the flux density across a dendrite branch, and a peak value that decreases as the applied field goes up. This unexpected behaviour is reproduced by flux creep simulations based on the non-local field-current relation in the perpendicular geometry. The simulations also provide indirect evidence that flux dendrites are formed at an elevated local temperature, consistent with a thermo-magnetic mechanism of the instability, 5 pages, 5 figures, submitted to Supercond. Sci. Technol

Magnetic flux instability limits potential applications of superconductors such as MgB2 in practical devices. Previous studies in MgB2 films exposed to magnetic fields revealed the occurrence of dendritic flux avalanches at temperatures below T ~ 10 K. In the present work it is shown that films of MgB2 exposed to a fast-ramped magnetic field display a dendritic flux instability at elevated temperatures, up to 23 K. Such instability can therefore cause malfunctioning of practical devices based on MgB2 films even when operating at liquid hydrogen temperature.

We investigate the effects of magnetic impurities on the upper critical field (μ 0 H c2) in La-doped CaFe2As2 (LaCa122) single crystals. The magnetic field dependency of the superconducting transition temperature (T c) for LaCa122 is rapidly suppressed at low fields up to ~1 kOe despite its large μ 0 H c2(0) value on the order of tens of Tesla, resulting in a large positive curvature of μ 0 H c2(T) near T c. The magnetization hysteresis (M–H) loop at temperatures above T c shows a ferromagnetic-like signal and the M(H) value rapidly increases with increasing magnetic field up to ~1 kOe. Taken together with the linear suppression of T c with the magnetization in the normal state, these results suggest that the large upward curvature of μ 0 H c2(T) near T c in La-doped CaFe2As2 mainly originates from the suppression of superconductivity due to the presence of magnetic impurities. © 2017 IOP Publishing

The development of bulk high-temperature superconductors (HTSs) and their applications has today come to a point where the mechanical response to high magnetic fields may be more important than their critical-current density and large-grain property. Reviewed in this article are the recent studies of the magneto-elastic effects which are caused by flux pinning in the superconductors. This includes the work on the giant irreversible magnetostriction and internal stress, which often cause fatal cracking of the HTS bulks as they become magnetized. The cracking is a problem that today accompanies the quest for the highest trapped field values, and the latest development in this area is also presented. While the first part is an overview of experimental efforts, the second summarizes the work done to model the pinning-induced stress and strain under various magnetic and geometrical conditions.

The magnetically induced stresses in a circular superconducting bulk are calculated for two magnetization processes: (i) during removal of the applied field after field-cooling, and (ii) during a field pulse applied to a zero-field-cooled superconductor. As these processes are commonly used to activate bulks as trapped-field magnets, the tensile stresses are analysed to shed light on the habit of fracturing during activation. The calculations, which are based on the critical-state model for the magnetic part, and three-dimensional linear elasticity theory (plane strain approach), determine the maximum tensions and also where they occur in the material. A comparison between stresses in the two activation processes is presented.

The field-cooled (FC) magnetization M is measured as a function of the applied external field, Ha, for various bulk, Nd-based 123 superconductors which exhibit different magnetization-field characteristics (i.e. with and without secondary peak effect). We find that the FC magnetization data reproduce directly the M(Ha) behaviour measured after zero-field cooling. Based on the critical state model, we derive expressions for the field-cooled magnetization, and show that a large M(0) is essential to achieve large trapped fields in superconducting permanent magnets.

An analysis of how the detectivity and lifetime depend on the fabrication process of superconducting antenna-coupled microbolometers has been carried out. The temperature dependences of responsivity and noise equivalent power (NEP) have been estimated in terms of the thermal model. To reveal the main degradation mechanism, 1/f -noise characterization has been used. Monte-Carlo simulation of the annealing procedure of YBa2 Cu3 O7 (YBCO) films for the operating ranges of frequency and temperature has shown that prevailing sources of flicker noise in superconducting microstrips are associated with transitions of oxygen atoms situated close to low-angle boundaries of the film blocks. The magnetron sputtering technique has been optimized to reduce the Hooge parameter for flicker noise to a record-breaking low value for YBCO films of about 10-4 at 93 K. Comparative analysis of chemical, ion and laser etching techniques by low-temperature scanning electron microscopy and magneto-optics allowed the fabrication of microstrips with uniform current distribution characterized by critical current density higher than 106 A cm-2 at 77 K and long-time stability. The process of low-energy ion milling of YBCO films with an Ar+ beam generated in a duopigatron ion source was used to reach a width resolution at the topology edge better than 0.2 µm. The antenna-coupled bolometers fabricated from the superconducting microstrips were used to register microwave radiation at a frequency of 70.3 GHz and temperature of 93 K. It is demonstrated that the developed technology makes possible the fabrication of long-lived YBCO-based antenna microbolometers with electrical NEPe = 1.5 × 10-12 W Hz-1/2 . The calculated response time of the antenna is about 30-150 ns. Further development is associated with fabrication of coupling microbolometers with immersed lens, with predicted optical detectivity D * = (4 × 109 - 4 × 1010 ) cm Hz1/2 W-1 in the wavelength range 100-1000 µm.

Coupling through random superconducting bridges between filaments in a multifilamentary Ag-sheathed Bi2Sr2Ca2Cu3O10+ tape has been investigated by magneto-optical imaging at temperatures from 20 K up to Tc. Magnetic flux distributions have been measured on the surface of an intact tape in the remanent state on applying a strong perpendicular magnetic field. The flux distributions observed at low temperatures reflect the arrangement of individual filaments. At high temperatures, the distribution becomes more similar to that for a uniform monocore tape, indicating that superconducting connections appear between the filaments. To discuss the relative contributions of the intra- and inter-filament currents, a simple model based on the Bean critical state was proposed and applied to analyse the temperature dependent behaviour. The inter-filament coupling, increasing with temperature, reaches at 77 K a point where the currents flowing in large inter-filament loops are roughly equal to the intra-filament currents.

Vortex avalanches are known to occur in MgB2 films within a certain range of temperatures and magnetic fields. These events, resulting from a thermomagnetic instability, were first revealed by real-time magneto-optical imaging, which exposed dendritic paths of abrupt flux propagation. This very powerful technique has, however, a practical limitation, since sensors that are currently available cannot be used at high magnetic fields. This letter shows that results obtained using dc magnetometry are in good correspondence with those furnished by magneto-optical imaging, demonstrating that the two techniques can be efficiently used as complementary tools to map vortex avalanches in superconducting films.

The irreversible behaviour of the pinning-induced magnetostriction of a cylindrical superconductor placed in a parallel magnetic field, , is analysed. An exact solution of the magneto-elastic problem is found and we present formulas for the deformation, , and the stress distributions, and , valid for any critical-state model . For the Bean model, explicit results are derived for the various stages during a field cycle. A simple method to infer from measurements of versus is pointed out. When decreases, the stresses become tensile, and the probability of generating cracks in the material is discussed.

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.

The effect of bending on silver-sheathed Bi-2223 tape is investigated by means of magneto-optical (MO) visualization of flux distributions. One single piece of rolled tape is used throughout all experiments starting from as-prepared, and subsequently the tape is bent to 4 mm diameter and then down to 1 mm diameter in 1 mm steps. Flux patterns are visualized using an intact tape, i.e. the visualization is done through the silver sheath after removing the bending strain. It is shown that bending of the tape causes a series of cracks running perpendicular to the tape axis (rolling direction). This crack pattern remains practically unchanged down to 1 mm bending diameter. The flux penetration changes from nearly homogeneous (as-prepared) to entirely crack-governed. Vortices are found to penetrate the tape preferentially along the cracks and from there into the remaining bulk. In bent samples, a very homogeneous flux distribution is observed when the external field is decreased after having reached the full-penetration field. A further decrease of the field forces vortices to leave the sample along the cracks, and trapped vortices remain within the Bi-2223 grains (grain clusters). With each step of bending, the values for the first flux penetration and full-penetration field are decreased. The observations of flux patterns at 1 mm bending diameter clearly demonstrate that intergranular currents are still flowing in the sample. Our observations demonstrate the capability of the MO technique for non-destructive testing of superconducting tapes.

A superconducting flux injector (SFI) has been designed to allow for controlled injections of magnetic flux into a superconducting film from a predefined location along the edge. The SFI is activated by an external current pulse, here chosen to be 200 ms long, and it is demonstrated on films of Nb that the amount of injected flux is controlled by the pulse height. Examples of injections at two different temperatures where the flux enters by stimulated flux-flow and by triggered thermomagnetic avalanches are presented. The boundary between the two types of injection is determined and discussed. The SFI opens up for active use of phenomena which up to now have been considered hazardous for a safe operation of superconducting devices.

We demonstrate here real-time imaging of individual vortices in a NbSe2 single crystal using polarized light microscopy. A new high-sensitivity magneto-optical (MO) imaging system enables observation of the static vortex lattice as well as single vortex motion at low flux densities.

Magneto-opitcal studies of a c-oriented epitaxial MgB2 film with critical current density 10^7 A/cm^2 demonstrate a breakdown of the critical state at temperatures below 10 K [cond-mat/0104113]. Instead of conventional uniform and gradual flux penetration in an applied magnetic field, we observe an abrupt invasion of complex dendritic structures. When the applied field subsequently decreases, similar dendritic structures of the return flux penetrate the film. The static and dynamic properties of the dendrites are discussed., Comment: Accepted to Supercond. Sci. Technol

The interaction between a magnet and a high Tc superconductor (HTSC) is investigated by a novel method. A permanent magnet mounted at one end of a stiff rod hanging by a knife's edge forms a mechanical pendulum. When a HTSC is placed below the magnet the frequency v and damping factor Gamma will be changed due to the interaction between the magnet and the HTSC. The experiments show that, as the separation between the magnet and the HTSC decreases, the flux lines start to penetrate the sample via intergranules and grain boundaries, which results in a horizontal restoring force even for the field range B<

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

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.

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.

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

An effective method for suppressing dendritic flux avalanches in MgB2 films is demonstrated by coating the films with a metal rim along the edges, which is where the avalanches nucleate. The effect of the partial coating has been investigated by means of magneto-optical imaging on a series of samples with golden rims of different width. Measurements of the onset field of instability reveal that such rims can substantially improve the thermo-magnetic stability of the superconducting films.

In contrast to the detailed study on macroscopic vortex avalanches, the development of microscopic small flux jumps and dendritic branches during the growth of an avalanche in a MgB2 thin film with changing temperature and magnetic field is not well studied. Our study of the two phenomena shows that the number and the size of the small flux jumps and the maximum flux density critically depend on the width of the rectangular thin film with dimensions 3 mm × width for film width ranging from 0.2 to 1.6 mm. We also found that when the ratio between the maximum field density and the applied flux was below 1, dendrites do not exist.

The magnetization of a type-II superconductor obeying the power-law critical state model J c = K B -n is discussed for the case of an infinitely long rectangular bar placed in a parallel field. Explicit formulas are derived for all branches of the high-field hysteresis loop. Both the infinite slab and circular cylinder geometries are included as special cases. It is shown that the vertical width of the hysteresis loop has the asymptotic high-field behavior ΔM(B a) ∼J c(B a), with a prefactor given by the geometrical aspect ratio. Also, two more terms in the functional expansion of ΔM { J c} are derived. These terms are shown to vanish rapidly when the applied field exceeds the full penetration value. This allows for a new way of extracting J c(B) from magnetization data.

When a permanent magnet, placed above a high temperature superconductor (HTSC), is moved by an external force, the flux lines in the HTSC will follow the magnet and perform a collective motion due to the interaction between the flux lines and the magnet. Here, the collective motion of flux lines in an Y1Ba2Cu3O7-δ (Y123) HTSC is studied by a mechanical pendulum method for various transport currents I through the sample. The magnet is set in low frequency (typically 1 Hz) oscillations with a mechanical pendulum, and the current I is flowing perpendicular to the direction of the magnet motion. The experiments show that the shift of the stiffness (corresponding the shift of the frequency squared) exhibits a cross-over phenomenon: from independence to power law-like dependence for increasing currents. The results are compared with the theories of collective flux creep in HTSCs.