A. A. F. Olsen

Onset of dendritic flux avalanches in superconducting films

We report a detailed comparison of experimental data and theoretical predictions for the dendritic flux instability, believed to be a generic behavior of type-II superconducting films. It is shown that a thermomagnetic model published very recently [Phys. Rev. B 73, 014512 (2006)10.1103/PhysRevB.73.014512] gives an excellent quantitative description of key features like the stability onset (first dendrite appearance) magnetic field, and how the onset field depends on both temperature and sample size. The measurements were made using magneto-optical imaging on a series of different strip-shaped samples of MgB2. Excellent agreement is also obtained by reanalyzing data previously published for Nb.

Dendritic magnetic avalanches in carbon-free MgB2 thin films with and without a deposited Au layer

From magneto optic images (MOI), the dendritic magnetic avalanche is known to appear dominantly for thin films of the newly discovered MgB2. To clarify the origin of this phenomenon, we studied in detail the MOI of carbon-free MgB2 thin films with and without a deposited gold layer. The MOI indicated that carbon contamination was not the main source of the avalanche. The MOI clearly showed that the deposition of metallic gold on top of a MgB2 thin film improved its thermal stability and suppressed the sudden appearance of the dendritic flux avalanche. This is consistent with the previous observation of flux noise in the magnetization.

Manipulation of vortices by magnetic domain walls

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.

Width-dependent upper threshold field for flux noise in MgB2 strips

The authors measured magnetization hysteresis curves and used magneto-optical imaging to visualize the flux distributions in superconducting MgB2 films in order to study dendritic flux avalanches. The flux avalanches are found to disappear above some upper threshold field that is typically ∼1kOe, but strongly depends on the film width. If the film is made wider, this threshold field first increases and then tends to saturate. This behavior is quantitatively explained using a thermomagnetic model for the dendritic avalanches and taking into account the field dependence of Jc. The results demonstrate that patterning superconducting films into narrow strips substantially increases the range of magnetic fields for which they can be used for applications.

Preparation and characterization of superconducting MgB2 films on alumina

Polycrystalline MgB2 films with a thickness of about 1 µm were 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 improved the surface smoothness of the samples. The films had transition temperatures (Tc (0)) of 35 K and irreversibility fields of up to 15 T at 5 K. Transport critical current densities of 3 × 108 A m−2 were measured at 20 K and 1 T for MgB2 films on single-crystalline Al2O3, whereas 8 × 108 A m−2 was found for films on polycrystalline Al2O3 under the same conditions. Magnetization data and magneto-optical imaging indicated instabilities (flux jumps) at 4.2 K, which disappeared at around 10 K. The critical current densities of all MgB2 films on either single-crystalline or polycrystalline Al2O3 were independent of the orientation of the film with respect to the magnetic field direction.

Single vortices observed as they enter NbSe2

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.

Temperature Dependence of the Flux Jump Upper Threshold Field in MgB2 Thin Films

The origin of the magnetic flux noise in MgB 2 thin films is believed to be a thermo-magnetic instability, where the local dissipation created by vortex motion leads to reduced pinning, and forms a positive feedback loop which can develop into a thermo-magnetic runaway. However, only the lower threshold field ( H l ) for the appearance of this noise was investigated by the magneto-optics (MO) technique due to the field limit of the MO indicator. In this paper, we report the upper threshold field, ( H u ) which is successfully obtained by different technique using a vibrating sample magnetometer while varying temperature and the field interval. After the analysis of magnetic hysteresis curves, we found that H u is strongly depends on sample geometry and temperature. This is consistent with the phenomenological model mentioned above.

Mesoscopic flux jumps in MgB2 films visualized by magneto-optical imaging

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.

Microscopic flux density of small and dendritic flux jumps in MgB2 thin 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.

UPPER THRESHOLD FIELDS OF DENDRITIC FLUX JUMPS IN GOLD-COATED MgB2 THIN FILMS

We measured the magneto-optical images (MOIs) and the magnetic hysteresis (M – H) curves of c-axis-oriented MgB2 thin films to investigate the flux penetration in the form of dendritic avalanches. In order to understand the role of the thermal effects, we prepared Au-coated MgB2 thin films with different thicknesses of gold. While the MOI provides a spatially resolved flux pattern, the M – H curve presents global and average information about the flux noise associated with avalanches. These two types of measurements complement each other. The upper threshold field, above which the flux noise disappears, was determined from the M-H curves while the lower threshold field was determined from both the M – H curves and the MO images. The field range where the flux penetrates via avalanches is found to be smaller for thicker gold layers. These results are important for many superconducting applications.