134 research outputs found
Vortex rectification effects in films with periodic asymmetric pinning
We study the transport of vortices excited by an ac current in an Al film
with an array of nanoengineered asymmetric antidots. The vortex response to the
ac current is investigated by detailed measurements of the voltage output as a
function of ac current amplitude, magnetic field and temperature. The
measurements revealed pronounced voltage rectification effects which are mainly
characterized by the two critical depinning forces of the asymmetric potential.
The shape of the net dc voltage as a function of the excitation amplitude
indicates that our vortex ratchet behaves in a way very different from standard
overdamped models. Rather, as demonstrated by the observed output signal, the
repinning force, necessary to stop vortex motion, is considerably smaller than
the depinning force, resembling the behavior of the so-called inertia ratchets.
Calculations based on an underdamped ratchet model provide a very good fit to
the experimental data.Comment: 5 pages, 4 figure
Self organized mode locking effect in superconductor / ferromagnet hybrids
The vortex dynamics in a low temperature superconductor deposited on top of a
rectangular array of micrometer size permalloy triangles is investigated
experimentally. The rectangular unit cell is such that neighboring triangles
physically touch each other along one direction. This design stabilizes
remanent states which differ from the magnetic vortex state typical of
individual non-interacting triangles. Magnetic Force Microscopy images have
revealed that the magnetic landscape of the template can be switched to an
ordered configuration after magnetizing the sample with an in-plane field. The
ordered phase exhibits a broad flux flow regime with relatively low critical
current and a highly anisotropic response. This behavior is caused by the
spontaneous formation of two separated rows of vortices and antivortices along
each line of connected triangles. The existence of a clear flux flow regime
even for zero external field supports this interpretation. The density of
induced vortex-antivortex pairs is directly obtained using a high frequency
measurement technique which allows us to resolve the discrete motion of
vortices. Strikingly, the presence of vortex-antivortex rows gives rise to a
self organized synchronized motion of vortices which manifests itself as field
independent Shapiro steps in the current-voltage characteristics.Comment: 9 pages, 11 figure
Dipole-induced vortex ratchets in superconducting films with arrays of micromagnets
We investigate the transport properties of superconducting films with
periodic arrays of in-plane magnetized micromagnets. Two different magnetic
textures are studied: a square array of magnetic bars and a close-packed array
of triangular microrings. As confirmed by MFM imaging, the magnetic state of
both systems can be adjusted to produce arrays of almost point-like magnetic
dipoles. By carrying out transport measurements with ac drive, we observed
experimentally a recently predicted ratchet effect induced by the interaction
between superconducting vortices and the magnetic dipoles. Moreover, we find
that these magnetic textures produce vortex-antivortex patterns, which have a
crucial role on the transport properties of this hybrid system.Comment: 4 pages, 4 figure
Determination of the magnetic penetration depth in a superconducting Pb film
Under the terms of the Creative Commons Attribution (CC BY) license to their work.By means of scanning Hall probe microscopy technique, we accurately map the magnetic field pattern produced by Meissner screening currents in a thin superconducting Pb stripe. The obtained field profile allows us to quantitatively estimate the Pearl length ¿ without the need of pre-calibrating the Hall sensor. This fact contrasts with the information acquired through the spatial field dependence of an individual flux quantum where the scanning height and the magnetic penetration depth combine in a single inseparable parameter. The derived London penetration depth λL coincides with the values previously reported for bulk Pb once the kinetic suppression of the order parameter is properly taken into account.This work was partially supported by the Fonds de la Recherche Scientique-FNRS, the Methusalem Funding of the Flemish Government, the Fund for Scientic Research-Flanders (FWO-Vlaanderen), and the ARC Grant 13/18-08 for Concerted Research Actions, financed by the Wallonia-Brussels Federation. J.B. acknowledges support from FRS-FNRS (Research Fellowship). J.V.d.V. acknowledges support from FWO-Vl. The work of A.V.S. was partially supported by “Crédit de demurrage,” U.Lg.Peer Reviewe
Metastable states and hidden phase slips in nanobridge SQUIDs
We fabricated an asymmetric nanoscale SQUID consisting of one nanobridge weak
link and one Dayem bridge weak link. The current phase relation of these
particular weak links is characterized by multivaluedness and linearity. While
the latter is responsible for a particular magnetic field dependence of the
critical current (so-called vorticity diamonds), the former enables the
possibility of different vorticity states (phase winding numbers) existing at
one magnetic field value. In experiments the observed critical current value is
stochastic in nature, does not necessarily coincide with the current associated
with the lowest energy state and critically depends on the measurement
conditions. In this work, we unravel the origin of the observed metastability
as a result of the phase dynamics happening during the freezing process and
while sweeping the current. Moreover, we employ special measurement protocols
to prepare the desired vorticity state and identify the (hidden) phase slip
dynamics ruling the detected state of these nanodevices. In order to gain
insights into the dynamics of the condensate and, more specifically the hidden
phase slips, we performed time-dependent Ginzburg-Landau simulations.Comment: 10 pages, 4 figures, 1 supplementary vide
Local mapping of dissipative vortex motion
We explore, with unprecedented single vortex resolution, the dissipation and
motion of vortices in a superconducting ribbon under the influence of an
external alternating magnetic field. This is achieved by combing the phase
sensitive character of ac-susceptibility, allowing to distinguish between the
inductive-and dissipative response, with the local power of scanning Hall probe
microscopy. Whereas the induced reversible screening currents contribute only
inductively, the vortices do leave a fingerprint in the out-of-phase component.
The observed large phase-lag demonstrates the dissipation of vortices at
timescales comparable to the period of the driving force (i.e. 13 ms). These
results indicate the presence of slow microscopic loss mechanisms mediated by
thermally activated hopping transport of vortices between metastable states.Comment: 5 pages, 2 figure
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