The Fe-based superconductors, such as LnFeAs(O,F) (Ln=lanthanide) and related compounds, represent a new class of superconductors exhibiting the highest critical temperature (Tc) apart from the cuprates. Due to relative high anisotropies of LnFeAs(O,F) combined with the intrinsic pinning, the critical current density Jc in high magnetic fields exhibits a strong angular dependence with regard to the magnetic field orientation. These unconventional superconductors are interesting not only from a fundamental research point of view, but also with regard to possible sensor applications. For such applications, however, thin films with a smooth surface and high Tc are necessary.
With this respect, we comparatively investigate the microstructure – transport property relationships of NdFeAsO1-xFx thin films and NdFeAsO/NdOF compounds, grown with several thicknesses by molecular beam epitaxy (MBE) on MgO(001) substrates. The (micro) structure was investigated by X-Ray diffraction as well as atomic force and scanning electron microscopy and atom probe tomographyic. Superconducting properties were determined by electrical transport measurements. We show how film thicknesses, structure, surface morphology and growth defects influence the resistive transition and Tc as well as temperature, field and orientation dependencies of Jc. For a 20 nm thin NdFeAsO1-xFx film with a root mean square roughness < 2 nm a high transition temperature of Tc,0 = 44.7 K and Tc,onset = 48 K could be observed which is close to values measured on single crystals. High-field measurements reveal Fp ≈ 1 TN/m3 and Jc ≈ 10 MA/cm2, which are the highest values overserved for Fe-based superconductors.