Claudia Stahl

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.

High-resolution dichroic imaging of magnetic flux distributions in superconductors with scanning x-ray microscopy

The penetration of magnetic flux into high-temperature superconductors has been observed using a high-resolution technique based on x-ray magnetic circular dichroism. Superconductors coated with thin soft-magnetic layers are observed in a scanning x-ray microscope under the influence of external magnetic fields. Resulting electric currents in the superconductor create an inhomogeneous magnetic field distribution above the superconductor and lead to a local reorientation of the ferromagnetic layer. Measuring the local magnetization of the ferromagnet by x-ray absorption microscopy with circular-polarized radiation allows the analysis of the magnetic flux distribution in the superconductor with a spatial resolution on the nanoscale.

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 role of individual defects on the magnetic screening of HTSC films

The magnetic flux penetration into thin films of high-temperature superconducting YBCO is visualized with high spatial resolution via x-ray microscopy. Therefore superconductors are coated with soft-magnetic CoFeB layers that reproduce the magnetic flux density distribution in an adjacent superconducting film and exhibit at the same time a large XMCD effect. For the first time we present scanning x-ray microscopy in the total electron yield mode using polarized x-rays providing simultaneously structural and magnetic information of the surface with high spatial resolution. Correlating the images of structural and magnetic information the role of individual defects on the magnetic screening capability of the superconductor can be identified.

Low temperature X-ray imaging of magnetic flux patterns in high temperature superconductors

We present X-ray magnetic circular dichroism (XMCD) microscopy results obtained at liquid nitrogen temperatures on the high-Tc superconductor YBCO (YBa2Cu3O7–δ). The magnetic flux distribution arising from electric currents in the superconductor is detected and visualized using soft-magnetic Co40Fe40B20 (CoFeB) as sensor layer and XMCD as contrast mechanism. It has been shown that the XMCD contrast in the sensor layer directly corresponds to magnetic flux distribution of the superconductor and hence can be used to image magnetic structures in superconductors [Stahl et al., Phys. Rev. B 90, 104515 (2014)]. The existing scanning UHV X-ray microscopy setup MAXYMUS at the synchrotron BESSY II in Berlin has been upgraded for that purpose: we use a nitrogen based MMR Micro Miniature Joule-Thompson Cryostat with temperature range from 75 K to 580 K. The capability of the method is demonstrated on two different superconducting samples, an optimally doped thin film and a melt-textured block.

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.

Magnetic X-ray microscopy at low temperatures - Visualization of flux distributions in superconductors

X-ray Magnetic Circular Dichroism (XMCD) microscopy at liquid nitrogen temperature has been performed on bilayers of high-Tc superconducting YBCO (YBa2Cu3O7-δ) and soft-magnetic Co40Fe40B20. This should allow us to map the magnetic flux density distribution in the current-carrying state of the superconductor with high spatial resolution. For that purpose the UHV scanning X-ray microscope MAXYMUS has been upgraded by a MMR Micro Miniature Joule-Thompson cryostat capable of temperatures between 75 K and 580 K. Resulting XMCD images of the magnetic flux density in the superconductor with a field of view ranging from millimeters to micrometers are presented. The microscope’s unique combination of total electron yield (TEY) measurements together with low temperatures offers novel possibilities concerning the current transport in superconductors on small length scales.

Quantitative magneto-optical analysis of the role of finite temperatures on the critical state in YBCO thin films

We use quantitative magneto-optical microscopy to investigate the influence of finite temperatures on the critical state of thin YBCO films. In particular, temperature and time dependence of supercurrents in inhomogeneous and anisotropic films are analyzed to extract the role of temperature on the supercurrents themselves and the influence of thermally activated relaxation. We find that inhomogeneities and anisotropies of the current density distribution correspond to a different temperature dependence of local supercurrents. In addition, the thermally activated decay of supercurrents can be used to extract local vortex pinning energies. With these results the modification of vortex pinning introduced by substrate structures is studied. In summary the local investigation of supercurrent densities allows the full description of the vortex pinning landscape with respect to pinning forces and energies in superconducting films with complex properties under the influence of finite temperatures.