306 research outputs found
Dendritic flux avalanches in superconducting Nb3Sn films
The penetration of magnetic flux into a thin superconducting film of Nb3Sn
with critical temperature 17.8K and critical current density 6MA/cm^2 was
visualized using magneto-optical imaging. Below 8 K an avalanche-like flux
penetration in form of big and branching dendritic structures was observed in
response to increasing perpendicular applied field. When a growing dendritic
branch meets a linear defect in the film, several scenarios were observed: the
branch can turn and propagate along the defect, continue propagation right
through it, or "tunnel" along a flux-filled defect and continue growth from its
other end. The avalanches manifest themselves in numerous small and random
jumps found in the magnetization curve.Comment: 3 pages, 4 figures, submitted to Cryogenics. Revision: M(H) data
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Single vortices observed as they enter NbSe
We observe single vortices as they penetrate the edge of a superconductor
using a high-sensitivity magneto-optical microscope. The vortices leap across a
gap near the edge, a distance that decreases with increasing applied field and
sample thickness. This behaviour can be explained by the combined effect of the
geometrical barrier and bulk pinning.Comment: 2 pages, 1 figure, M2S-Rio proceeding
Local threshold field for dendritic instability in superconducting MgB2 films
Using magneto-optical imaging the phenomenon of dendritic flux penetration in
superconducting films was studied. Flux dendrites were abruptly formed in a 300
nm thick film of MgB2 by applying a perpendicular magnetic field. Detailed
measurements of flux density distributions show that there exists a local
threshold field controlling the nucleation and termination of the dendritic
growth. At 4 K the local threshold field is close to 12 mT in this sample,
where the critical current density is 10^7 A/cm^2. The dendritic instability in
thin films is believed to be of thermo-magnetic origin, but the existence of a
local threshold field, and its small value are features that distinctly
contrast the thermo-magnetic instability (flux jumps) in bulk superconductors.Comment: 6 pages, 6 figures, submitted to Phys. Rev.
Buckling instability in type-II superconductors with strong pinning
We predict a novel buckling instability in the critical state of thin type-II
superconductors with strong pinning. This elastic instability appears in high
perpendicular magnetic fields and may cause an almost periodic series of flux
jumps visible in the magnetization curve. As an illustration we apply the
obtained criteria to a long rectangular strip.Comment: Submitted to Phys. Rev. Let
Vortex microavalanches in superconducting Pb thin films
Local magnetization measurements on 100 nm type-II superconducting Pb thin
films show that flux penetration changes qualitatively with temperature. Small
flux jumps at the lowest temperatures gradually increase in size, then
disappear near T = 0.7Tc. Comparison with other experiments suggests that the
avalanches correspond to dendritic flux protrusions. Reproducibility of the
first flux jumps in a decreasing magnetic field indicates a role for defect
structure in determining avalanches. We also find a temperature-independent
final magnetization after flux jumps, analogous to the angle of repose of a
sandpile.Comment: 6 pages, 5 figure
Visualization of Spin Polarized States in Biologically-Produced Ensembles of Ferromagnetic Palladium Nanoparticles
We report visualization of spin polarized states in macroscopic ensembles of biologically-produced
ferromagnetic palladium nanoparticles using the Faraday effect-based technique of magneto-optical
imaging. The ferromagnetic palladium only exists in the form of nanoparticles. Large quantities of
palladium nanoparticles may be synthesized via biomineralization from a Pd2+ solution. The ferromagnetic
Pd nanoparticles are formed in the periplasmic space of bacteria during the hydrogen-assisted reduction of
Pd2+ ions by hydrogenases. The ferromagnetism in Pd comes from itinerant electrons. A high Curie
temperature of ferromagnetic palladium, about 200 degrees centigrade above room temperature, would
allow for a range of room-temperature magnetic applications. The processes of the isolation of electron
spins in separate nanoparticles, spin hopping, spin transport and spin correlations may even form a basis
of quantum computing. So far, measurements of the magnetic properties of Pd nanoparticles (PdNP) have
been limited by integral techniques such as SQUID magnetometry, magnetic circular dihroism and muon
spin rotation spectroscopy ( SR). In the present study, ferromagnetic Pd nanoparticles are characterized
using the technique of magneto-optical imaging. This allows visualization of the spin polarization by the
variations in the intensity of polarized light. To perform measurements at relatively low magnetic fields, a
spin injection from a colossal magnetoresistive material has been used.
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Dendritic flux avalanches in superconducting films
Thermomagnetic instability in general, and dendritic flux avalanches in particular, have attracted considerable attention of both scientists and engineers working on superconductor applications. Though being harmful for the performance of many superconducting devices, the avalanches provide a fruitful playground for experimental and theoretical studies of complex dynamics of the vortex matter. In this paper, we report on the progress in understanding the mechanisms responsible for the development of the giant magnetic avalanches. We review recent results on magneto-optical imaging of the fingering instability in superconducting films and analyze them basing on the recent theoretical model that establishes criteria for onset of the dendritic avalanches
Magneto-optical investigations of Ag-sheathed Bi-2223 tapes with ferromagnetic shielding
An increase in the critical current and suppression of AC losses in
superconducting wires and tapes with soft magnetic sheath have been predicted
theoretically and confirmed experimentally. In this work we present the results
of magneto-optical investigations on a series of Ag-sheathed Bi-2223 tapes with
Ni coating. We visualize distributions of magnetic field at increasing external
field and different temperatures, demonstrating a difference between the flux
propagation in the superconductor with Ni rims and a reference sample without
Ni coating.Comment: 2 page
Enhanced critical current density of YBa2Cu3Ox films grown on Nd1/3Eu1/3Gd1/3Ba2Cu3Ox with nano-undulated surface morphology
We report a simple and easily controllable method where a nano-undulated
surface morphology of Nd1/3Eu1/3Gd1/3Ba2Cu3Ox (NEG) films leads to a
substantial increase in the critical current density in superconducting
YBa2Cu3Ox (YBCO) films deposited by pulsed laser deposition on such NEG layers.
The enhancement is observed over a wide range of fields and temperatures.
Transmission electron microscopy shows that such YBCO films possess a high
density of localized areas, typically 20 x 20 nm2 in size, where distortion of
atomic planes give rotational (2 to 5 degrees) moire patterns. Their
distribution is random and uniform, and expected to be the origin of the
enhanced flux pinning. Magneto-optical imaging shows that these films have
excellent macroscopic magnetic uniformity.Comment: 4 pages, 4 figure
Macroturbulent Instability of the Flux Line Lattice in Anisotropic Superconductors
A theory of the macroturbulent instability in the system containing vortices
of opposite directions (vortices and antivortices) in hard superconductors is
proposed. The origin of the instability is connected with the anisotropy of the
current capability in the sample plane. The anisotropy results in the
appearance of tangential discontinuity of the hydrodynamic velocity of vortex
and antivortex motion near the front of magnetization reversal. As is known
from the classical hydrodynamics of viscous fluids, this leads to the
turbulization of flow. The examination is performed on the basis of the
anisotropic power-law current-voltage characteristics. The dispersion equation
for the dependence of the instability increment on the wave number of
perturbation is obtained, solved, and analyzed analytically and numerically. It
is shown that the instability can be observed even at relatively weak
anisotropy.Comment: 10 pages, 5 figures, submitted to Physical Review
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