Critical Currents in Fe based Superconductors

The current transport in the recently discovered Fe based superconductors will be investigated systematically. The fundamental pinning properties will be assessed on single crystals and thin films. While uncorrelated defects are expected to dominate flux pinning in single crystals, growth related, correlated pinning centers usually prevail in thin films. Special emphasis will be placed on the anisotropy of the critical currents, which results from correlated disorder or from the effective mass anisotropy of the charge carriers. The influence of additional (uncorrelated) defects will be investigated by means of neutron irradiation experiments. Neutron irradiation allows the variation of the defect density in the sample samples and provides important benchmarking for material optimization. Large loss free currents have to pass grain boundaries in most applications. These intergranular currents will be investigated by micro Hall probe scanning on naturally grown grain boundaries in sintered materials. The macroscopic current flow will be modeled by a percolation model in order to estimate the application potential of these new materials and to draw up guidelines for their optimization.

The aim of the project was to explore the potential of the iron-based high-temperature superconductors, which were discovered in 2008, for applications that require high currents and/or magnetic fields, such as magnets for science (e.g. NMR and accelerator magnets), Magnetic Resonance Imaging (MRI) systems in medicine or future applications for the generation (nuclear fusion, generators in wind turbines) or distribution (cables, fault current limiters) of electric energy.Loss free currents in superconductors are limited either inside the grains (intra-granular currents) or by the grain boundaries (inter-granular currents). While intra-granular currents are determined by the strength of vortex pinning on crystallographic defects, inter-granular currents are either limited extrinsically by impurities at the grain boundaries or intrinsically by the different crystallographic orientation of neighboring grains. The latter is a central problem for the cuprate high-temperature superconductors, since technical superconductors require high texture (a nearly identical crystallographic orientation of all grains) to achieve the required currents, which is expensive and so far impedes their application.Flux pinning was explored in single crystals and thin films. Additional defects were introduced by means of neutron irradiation and the resulting changes were assessed. Two central results were obtained. 1) A quasi one-dimensional pinning behavior was observed in thin films: The current in the investigated films was found to be limited only by the pinning force directed along the film plane, while the orthogonal component had no influence. 2) The influence of the orientation of the applied magnetic field on the critical current was derived for the first time quantitatively in single crystals and could be successfully described.The inter-granular currents were explored in polycrystalline samples. A scanning Hall probe microscope was developed for this purpose. The inter-granular currents limited the macroscopic current in all investigated samples. Their magnetic field dependence was modelled successfully; their absolute values were close to the requirements in typical applications. The developed model predicts that a further reduction of the grain size and weak intra-granular flux pinning are promising ways for the conductor optimization.