345 research outputs found
Magnetic-domain-controlled vortex pinning in a superconductor/ferromagnet bilayer
Vortex pinning in a type-II superconducting Pb film covering a Co/Pt
multilayer with perpendicular magnetic anisotropy is investigated. Different
stable magnetic domain patterns like band and bubble domains can be created in
the Co/Pt multilayer, clearly influencing the vortex pinning in the
superconducting Pb layer. Most effective pinning is observed for the bubble
domain state. We demonstrate that the pinning properties of the
superconductor/ferromagnet bilayer can be controlled by tuning the size,
density and magnetization direction of the bubbles.Comment: 4 pages, 3 figures, accepted for AP
Dynamic Regimes in Films with a Periodic Array of Antidots
We have studied the dynamic response of Pb thin films with a square array of
antidots by means of ac susceptibility chi(T,H) measurements. At low enough ac
drive amplitudes h, vortices moving inside the pinning potential give rise to a
frequency- and h-independent response together with a scarce dissipation. For
higher amplitudes, the average distance travelled by vortices surpasses the
pinning range and a critical state develops. We found that the boundary h*(H,T)
between these regimes smoothly decreases as T increases whereas a step-like
behavior is observed as a function of field. We demonstrate that these steps in
h*(H) arise from sharp changes in the pinning strength corresponding to
different vortex configurations. For a wide set of data at several fields and
temperatures in the critical state regime, we show that the scaling laws based
on the simple Bean model are satisfied.Comment: 7 pages, 5 figure
Confinement and Quantization Effects in Mesoscopic Superconducting Structures
We have studied quantization and confinement effects in nanostructured
superconductors. Three different types of nanostructured samples were
investigated: individual structures (line, loop, dot), 1-dimensional (1D)
clusters of loops and 2D clusters of antidots, and finally large lattices of
antidots. Hereby, a crossover from individual elementary "plaquettes", via
clusters, to huge arrays of these elements, is realized. The main idea of our
study was to vary the boundary conditions for confinement of the
superconducting condensate by taking samples of different topology and, through
that, modifying the lowest Landau level E_LLL(H). Since the critical
temperature versus applied magnetic field T_c(H) is, in fact, E_LLL(H) measured
in temperature units, it is varied as well when the sample topology is changed
through nanostructuring. We demonstrate that in all studied nanostructured
superconductors the shape of the T_c(H) phase boundary is determined by the
confinement topology in a unique way.Comment: 28 pages, 19 EPS figures, uses LaTeX's aipproc.sty, contribution to
Euroschool on "Superconductivity in Networks and Mesoscopic Systems", held in
Siena, Italy (8-20 september 1997
Creation and pinning of vortex-antivortex pairs
Computer modeling is reported about the creation and pinning of a magnetic
vortex-antivortex (V-AV) pair in a superconducting thin film, due to the
magnetic field of a vertical magnetic dipole above the film, and two antidot
pins inside the film. For film thickness , , and no pins,
we find the film carries two V-AV pairs at steady state in the imposed flux
range , and no pairs below. With two antidot
pins suitably introduced into the film, a single V-AV pair can be stable in the
film for . At pin separation , we find the
V-AV pair remains pinned after the dipole field is removed, and, so can
represent a 1 for a nonvolatile memory.Comment: 8 pages, 6 figure
Phase diagram of a superconductor / ferromagnet bilayer
The magnetic field (H) - temperature (T) phase diagram of a superconductor is
significantly altered when domains are present in an underlying ferromagnet
with perpendicular magnetic anisotropy. When the domains have a band-like
shape, the critical temperature Tc of the superconductor in zero field is
strongly reduced, and the slope of the upper critical field as a function of T
is increased by a factor of 2.4 due to the inhomogeneous stray fields of the
domains. Field compensation effects can cause an asymmetric phase boundary with
respect to H when the ferromagnet contains bubble domains. For a very
inhomogeneous domain structure, Tc~H^2 for low H and Tc~H for higher fields,
indicating a dimensional crossover from a one-dimensional network-like to a
two-dimensional behavior in the nucleation of superconductivity.Comment: 6 pages, 7 figure
Experimental ratchet effect in superconducting films with periodic arrays of asymmetric potentials
A vortex lattice ratchet effect has been investigated in Nb films grown on
arrays of nanometric Ni triangles, which induce periodic asymmetric pinning
potentials. The vortex lattice motion yields a net dc-voltage when an ac
driving current is applied to the sample and the vortex lattice moves through
the field of asymmetric potentials. This ratchet effect is studied taking into
account the array geometry, the temperature, the number of vortices per unit
cell of the array and the applied ac currents.Comment: 15 pages, figures include
Magnetic nanoparticles as efficient bulk pinning centers in type-II superconductors
Enhancement of flux pinning by magnetic nanoparticles embedded into the bulk
of type-2 superconductor is studied both theoretically and experimentally.
Magnetic part of the pinning force associated with the interaction between a
spherical magnetic inclusion and an Abrikosov vortex was calculated in the
London approximation. Calculations are supported by the experimental results
obtained on sonochemically modified MgB2 superconductor with embedded magnetic
Fe2O3 nanoparticles and compared to MgB2 with nonmagnetic Mo2O5 pinning centers
of similar concentration and particle size distribution. It is shown that
ferromagnetic nanoparticles result in a considerable enhancement of vortex
pinning in large-kappa type-2 superconductors.Comment: PDF, 14 page
- …