S. Treiber

The formation and propagation of flux avalanches in tailored MgB2 films

The applications of superconducting magnesium diboride are substantially limited by the presence of magnetic flux avalanches at low temperatures. Here, quickly moving magnetic vortices create large amounts of heat and magnetic noise. Such avalanches can be suppressed by evaporating metal layers to the surface of the superconductor, which acts both as a heat sink and as an electromagnetic drag by induced eddy currents. We show that it is necessary to distinguish between the mechanisms that are responsible for the formation and the propagation of avalanches. A high critical current favors avalanche formation but avalanche propagation is suppressed. The diverse consequences for creation and propagation explain the preference of avalanches for inhomogeneous superconductors.

Gold nanocrystals in high-temperature superconducting films: creation of pinning patterns of choice

One of the major drawbacks for applications of high-temperature superconducting films is that magnetic flux is not completely expelled but penetrates the film in the form of flux vortices. Any motion of these vortices is accompanied by magnetic noise and prevents larger supercurrents. Thus, an effective pinning of flux vortices is a prerequisite for applications based on thin films of high-temperature superconductors such as coated conductors or magnetic sensor devices. However, particular superconducting structures such as tunnel barriers or flux guides suffer from increased pinning forces. We report that adding thin gold layers to the production process of epitaxial films of the high-temperature superconductor YBa2Cu3O7−δ allows the fabrication of superconducting films with spatially varying flux pinning properties. This paves the way for an easy realization of custom tailored current carrying capabilities in arbitrary patterns. Critical current densities of already strong pinning films can be locally enhanced up to 150% to prepare a material of choice at a position of choice for the realization of high-quality electronic devices with improved performance.

Increased flux pinning in YBa2Cu3O7-δ thin-film devices through embedding of Au nano crystals

We prepared direct-current superconducting quantum interference device (dc-SQUID) gradiometers consisting of a single YBa2Cu3O7- δ (YBCO) layer on SrTiO3 (STO) bicrystal substrates. The superconducting thin film was modified by embedding crystalline gold nanoparticles. We investigated the growth conditions of these particles as well as their influence on the properties of the YBCO thin film. In our magneto-optical measurements we found that the presence of embedded nano crystals results in a distinct enhancement of jc over the whole investigated temperature range. We attribute the higher critical current density to an increased pinning, which also results in a reduction of the flux noise of our investigated gradiometers.

Using magnetic coupling in bilayers of superconducting YBCO and soft-magnetic CoFeB to map supercurrent flow

Bilayers of high-temperature superconducting YBa2Cu3O7−δ and amorphous soft-magnetic CoFeB have been prepared by pulsed-laser deposition and subsequent ion beam sputtering. In such structures magnetic coupling phenomena are found between the superconducting component and the ferromagnetic component. First, a significant increase of the critical current in the superconductor at temperatures close to Tc is found which is attributed to magnetic flux line pinning. Second, magnetic coupling across the interface allows a transfer of the current pattern of the superconductor into the ferromagnet via magnetic stray fields. This creates a map of the current flow of the superconductor inside the ferromagnet which is persistent when heating the bilayer up to room temperature. If it can be realized that the ferromagnet does not harm the superconductor too much, this could offer an easy (and novel) way of characterizing the current transport in superconductors.

The avalanche process in gold covered MgB2 films

Below a certain threshold temperature, the critical state of superconducting thin films can become unstable, which results in large magnetic flux jumps, called flux avalanches. The phenomenon of magnetic flux avalanches is strongly connected to the thermal and electrical conductivity in the material. Enhancing both by metallic cover layers on the superconducting film can suppress the magnetic avalanches. It was found that it is feasible to subdivide the avalanche process into a formation and a propagation step. In this work we want to address the question regarding the stage at which the metallic cover layer influences the avalanche mechanism. The investigations are carried out on thin MgB2 films with an inhomogeneous current density distribution, where both the formation and propagation of magnetic flux avalanches are highly supported. Evaporating a gold cover layer on top of the inhomogeneous MgB2 films leads to a suppression of the instabilities in this case. Magnetization measurements and magneto-optical imaging are used to observe the instabilities in pure MgB2 films and MgB2 films covered with gold layers. We investigate how the two steps of the avalanche process, nucleation and propagation, are influenced by the additional gold layer. We find a particular influence on the initial phase of the avalanche formation.

The temperature-dependent magnetization profile across an epitaxial bilayer of ferromagnetic La2/3Ca1/3MnO3 and superconducting YBa2Cu3O7−δ

Epitaxial bilayers of ferromagnetic (FM) La2/3Ca1/3MnO3 (LCMO) and superconducting YBa2Cu3O7?? (YBCO) have been grown on single-crystalline SrTiO3 (STO) substrates by pulsed laser deposition. The manganese magnetization profile across the FM layer has been determined with high spatial resolution at low temperatures by x-ray resonant magnetic reflectivity (XRMR) performed at the BESSY II synchrotron light source of the Helmholtz Zentrum Berlin. It is found that not only the adjacent superconductor but also the substrate underneath influences the magnetization of the LCMO film at the interface at low temperatures. Both effects can be investigated individually by XRMR.

Unusual flux jumps above 12 K in non-homogeneous MgB2 thin films

Flux jumps are well known and widespread in superconducting thin films. Low temperature superconductors in particular show flux jumping below a certain threshold temperature. MgB2 thin films exhibit large flux jumps, so-called flux avalanches, at temperatures below about 10 K. Above this temperature, the vortex state in MgB2 should be stable. Magneto-optical investigations of non-homogeneous MgB2 films show that large flux jumps can also occur in the temperature range considered as stable. However, their behavior is different from the jumps below 10 K. This means in particular that the critical state is most probably not destroyed during the propagation of these jumps.

Increasing the sensor performance using Au modified high temperature superconducting YBa2Cu3O7-delta thin films

We prepared planar, galvanically coupled gradiometers whereby the antenna structures of some of them were modified by incorporating Au nanoparticles. Gradiometers with gold modified antennas were compared with conventional ones to investigate the influence of gold induced pinning on the performance of superconducting sensor devices. We found that a local inclusion of gold nanoparticles offers the possibility of increasing the pinning of flux lines in the antenna regions, thus significantly reducing flux noise, especially in the low-frequency range. Since also the properties of grain boundary Josephson Junctions are strongly affected by Au particles, SQUID and antenna regions in gradiometric sensor devices can be separately optimized.