222 research outputs found
Visualizing Vortex Dynamics in Py/Nb Thin Film Hybrids by Low Temperature Magnetic Force Microscopy
No full textWe have analyzed the vortex dynamics in Py(1 μm)/SiO2(10 nm)/Nb(360 nm) thin film heterostructures as resulted from Magnetic Force Microscopy (MFM) frequency shift maps at low temperatures. The Nb film thickness has been chosen larger than the superconducting London penetration depth of about λ L=68 nm at the measuring temperatures. Above the Nb T c, the stripe-like Py stray field is visualized with half-period w Py=520 nm. Below the Nb T c, we have found that in a zero applied field, the supercurrents established in the Nb layer completely screen the out-of-plane component of the Py stray field. However, when the samples are cooled in a uniform external magnetic field, vortices are formed in chain-like configurations along the stripes with the same polarity. By decreasing and reversing the applied field, for low intensities, we have found a rigidity of the vortex array that remains “frozen” in the configuration as determined after the first field cooling run. The observed symmetry is then broken for higher values of the applied field, when an antivortex “avalanche” enters the Ferromagnetic/Superconducting (FM/SC) system
Vortex Confinement in Planar Superconductor/FerromagnetHybrid Structures
No full textWe use low temperature magnetic force microscopy and global magnetometry measurements to study the influence of magnetic domains on the Abrikosov vortex pinning in planar superconducting/ferromagnet bilayers. The superconducting/ferromagnet bilayers
consist of a 200 nm superconducting Nb film covering a Permalloy film, with an insulating layer in between to avoid proximity effect.
The periodic stripe domain in the Permalloy film produces a potential for directing vortex motion in the adjacent superconducting film.
We observed an enhancement of vortex pinning by a factor of 3 that occurs in bilayers with a magnetic stripe domains nm,
close to the superconducting critical temperature . At lower temperatures, when the channeled vortex motion
and the intrinsic pinning favor vortex avalanches
Visualizing vortex dynamics in Nb based Superconductor/ferromagnet thin film heterostructures
No full text
Magnetic pinning in a superconducting film by a ferromagnetic layer with stripe domains
No full textA magnetic study of superconductor/ferromagnet bilayers was performed by hysteresis loops and temperature-dependent magnetization measurements. The superconductor/ferromagnet bilayers consist of a Nb film deposited on a Py film with weak perpendicular magnetic anisotropy. By comparing the temperature-dependent magnetization data obtained on samples with different ferromagnetic layer thickness, a decrease of the magnetic pinning with increasing thickness of the ferromagnetic layer has been found. This is confirmed by the reduction of the Nb film critical current density at low fields extracted by using the magnetic irreversibility of the hysteresis loops. The reduction of the pinning can be related to the increase of the stripe width in the ferromagnetic layer observed by magnetic force microscopy (MFM) measurements
Observation of superconducting vortex clusters in S/F hybrids
No full textWhile Abrikosov vortices repel each other and form a uniform vortex lattice in bulk type-II superconductors, strong confinement potential profoundly affects their spatial distribution eventually leading to vortex cluster formation. The confinement could be induced by the geometric boundaries in mesoscopic-size superconductors or by the spatial modulation of the magnetic field in superconductor/ferromagnet (S/F) hybrids. Here we study the vortex confinement in S/F thin film heterostructures and we observe that vortex clusters appear near magnetization inhomogeneities in the ferromagnet, called bifurcations. We use magnetic force microscopy to image magnetic bifurcations and superconducting vortices, while high resolution scanning tunneling microscopy is used to obtain detailed information of the local electronic density of states outside and inside the vortex cluster. We find an intervortex spacing at the bifurcation shorter than the one predicted for the same superconductor in a uniform magnetic field equal to the thermodynamical upper critical field H c2. This result is due to a local enhanced stray field and a competition between vortex-vortex repulsion and Lorentz force. Our findings suggest that special magnetic topologies could result in S/F hybrids that support superconductivity even when locally the vortex density exceeds the thermodynamic critical threshold value beyond which the superconductivity is destroyed
Vortex-antivortex coexistence in Nb-based superconductor/ferromagnet heterostructures
No full textLow-temperature magnetic force microscopy was used to study the threshold of nucleation of spontaneous vortex-antivortex structures in superconductor/ferromagnet (S/F) hybrid systems. We investigated S/F heterostructures composed of Py as the magnetic material and Nb as the superconductor, with different thicknesses of Py and Nb. The condition for nucleation of spontaneous vortex-antivortex structures depends on fundamental parameters such as the superconducting penetration depth and the coherence length, as well as on the thickness of the superconducting film and the magnetic domain width. We compare our experimental results with those of existing theoretical models and provide an estimate of the threshold of the local out-of-plane component of the magnetization for different Py film thicknesses
Optical leakage mitigation in ortho-mode transducer detectors for microwave applications
No full textPlanar ortho-mode transducers (OMTs) are a commonly used method of coupling
optical signals between waveguides and on-chip circuitry and detectors. While
the ideal OMT-waveguide coupling requires minimal disturbance to the waveguide,
when used for mm-wave applications the waveguide is typically constructed from
two sections to allow the OMT probes to be inserted into the waveguide. This
break in the waveguide is a source of signal leakage and can lead to loss of
performance and increased experimental systematic errors. Here we report on the
development of new OMT-to-waveguide coupling structures with the goal of
reducing leakage at the detector wafer interface. The pixel to pixel optical
leakage due to the gap between the coupling waveguide and the backshort is
reduced by means of a protrusion that passes through the OMT membrane and
electrically connects the two waveguide sections on either side of the wafer.
High frequency electromagnetic simulations indicate that these protrusions are
an effective method to reduce optical leakage in the gap by ~80% percent, with
a ~60% filling factor, relative to an standard OMT coupling architecture.
Prototype devices have been designed to characterize the performance of the new
design using a relative measurement with varying filling factors. We outline
the simulation setup and results, and present a chip layout and sample box that
will be used to perform the initial measurements.Comment: 6 pages, 4 figures, conference proceedin
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