Reconfigurable superconducting vortex pinning potential for magnetic disks in hybrid structures

High resolution scanning Hall probe microscopy has been used to directly visualise the superconducting vortex behavior in hybrid structures consisting of a square array of micrometer-sized Py ferromagnetic disks covered by a superconducting Nb thin film. At remanence the disks exist in almost fully flux-closed magnetic vortex states, but the observed cloverleaf-like stray fields indicate the presence of weak in-plane anisotropy. Micromagnetic simulations suggest that the most likely origin is an unintentional shape anisotropy. We have studied the pinning of added free superconducting vortices as a function of the magnetisation state of the disks, and identified a range of different phenomena arising from competing energy contributions. We have also observed clear differences in the pinning landscape when the superconductor and the ferromagnet are electron ically coupled or insulated by a thin dielectric layer, with an indication of non-trivial vortex-vortex interactions. We demonstrate a complete reconfiguration of the vortex pinning potential when the magnetisation of the disks evolves from the vortex-like state to an onion-like one under an in-plane magnetic field. Our results are in good qualitative agreement with theoretical predictions and could form the basis of novel superconducting devices based on reconfigurable vortex pinning sites

First vortex entry into a perpendicularly magnetized superconducting thin film

In type-II superconductors, the flux-free state (Meissner state) may be invaded by vortices bearing quantized flux once H is above the lower critical field Hc1. However, the actual first flux penetration does not occur at Hc1 due to the presence of a surface barrier and the fact that the Meissner state may also exist as a metastable state up to a larger (superheating) field. In this work we determine the field for the first vortex penetration in superconductors by directly imaging the first vortex threading a superconducting Pb film with antidots. We find that the first vortex penetration occurs when the surface superconducting currents reach the depairing current, locally breaking superconductivity and allowing a vortex to nucleate.status: publishe

Quantitative magneto-optical investigation of superconductor/ferromagnet hybrid structures

We present a detailed quantitative magneto-optical imaging study of several superconductor/ ferromagnet hybrid structures, including Nb deposited on top of thermomagnetically patterned NdFeB and permalloy/niobium with erasable and tailored magnetic landscapes imprinted in the permalloy layer. The magneto-optical imaging data are complemented with and compared to scanning Hall probe microscopy measurements. Comprehensive protocols have been developed for calibrating, testing, and converting Faraday rotation data to magnetic field maps. Applied to the acquired data, they reveal the comparatively weaker magnetic response of the superconductor from the background of larger fields and field gradients generated by the magnetic layer.status: publishe

Imprinting superconducting vortex footsteps in a magnetic layer

Local polarization of a magnetic layer, a well-known method for storing information, has found its place in numerous applications such as the popular magnetic drawing board toy or the widespread credit cards and computer hard drives. Here we experimentally show that a similar principle can be applied for imprinting the trajectory of quantum units of flux (vortices), travelling in a superconducting film (Nb), into a soft magnetic layer of permalloy (Py). In full analogy with the magnetic drawing board, vortices act as tiny magnetic scribers leaving a wake of polarized magnetic media in the Py board. The mutual interaction between superconducting vortices and ferromagnetic domains has been investigated by the magneto-optical imaging technique. For thick Py layers, the stripe magnetic domain pattern guides both the smooth magnetic flux penetration as well as the abrupt vortex avalanches in the Nb film. It is however in thin Py layers without stripe domains where superconducting vortices leave the clearest imprints of locally polarized magnetic moment along their paths. In all cases, we observe that the flux is delayed at the border of the magnetic layer. Our findings open the quest for optimizing magnetic recording of superconducting vortex trajectories

Iron (Fe) speciation in xylem sap by XANES at a high brilliant synchrotron X-ray source: opportunities and limitations

The development of highly brilliant synchrotron facilities all around the world is opening the way to new research in biological sciences including speciation studies of trace elements in plants. In this paper, for the first time, iron (Fe) speciation in xylem sap has been assessed by X-ray absorption near-edge structure (XANES) spectroscopy at the highly brilliant synchrotron PETRA III, beamline P06. Both standard organic Fe-complexes and xylem sap samples of Fe-deficient tomato plants were analyzed. The high photon flux provided by this X-ray synchrotron source allows on one side to obtain good XANES spectra in a reasonable amount of time (approx. 15 min for 200 eV scan) at low Fe concentrations (sub parts-per-million), while on the other hand may cause radiation damage to the sample, despite the sample being cooled by a stream of liquid nitrogen vapor. Standard Fe-complexes such as Fe(III)-succinate, Fe(III)-α-ketoglutarate, and Fe(III)-nicotianamine are somehow degraded when irradiated with synchrotron X-rays and Fe(III) can undergo photoreduction. Degradation of the organic molecules was assessed by HPLC-UV/Vis analyses on the same samples investigated by X-ray absorption spectroscopy (XAS). Fe speciation in xylem sap samples revealed Fe(III) to be complexed by citrate and acetate. Nevertheless, artifacts created by radiation damage cannot be excluded. The use of highly brilliant synchrotrons as X-ray sources for XAS analyses can dramatically increase the sensitivity of the technique for trace elements thus allowing their speciation in xylem sap. However, great attention must be paid to radiation damage, which can lead to biased results