140 research outputs found
Magnetic flux density and the critical field in the intermediate state of type-I superconductors
To address unsolved fundamental problems of the intermediate state (IS), the
equilibrium magnetic flux structure and the critical field in a high purity
type-I superconductor (indium film) are investigated using magneto-optical
imaging with a 3D vector magnet and electrical transport measurements. The
least expected observation is that the critical field in the IS can be as small
as nearly 40% of the thermodynamic critical field . This indicates that
the flux density in the \textit{bulk} of normal domains can be
\textit{considerably} less than , in apparent contradiction with the long
established paradigm, stating that the normal phase is unstable below .
Here we present a novel theoretical model consistently describing this and
\textit{all} other properties of the IS. Moreover, our model, based the
rigorous thermodynamic treatment of observed laminar flux structure in a tilted
field, allows for a \textit{quantitative} determination of the domain-wall
parameter and the coherence length, and provides new insight into the
properties of all superconductors.Comment: 5 pages, 5 figure
Tailoring Fe/Ag Superparamagnetic Composites by Multilayer Deposition
The magnetic properties of Fe/Ag granular multilayers were examined by SQUID
magnetization and Mossbauer spectroscopy measurements. Very thin (0.2 nm)
discontinuous Fe layers show superparamagnetic properties that can be tailored
by the thickness of both the magnetic and the spacer layers. The role of
magnetic interactions was studied in novel heterostructures of
superparamagnetic and ferromagnetic layers and the specific contribution of the
ferromagnetic layers to the low field magnetic susceptibility was identified.Comment: 5 pages and 3 figure
Equilibrium properties of the mixed state in superconducting niobium in a transverse magnetic field: Experiment and theoretical model
Equilibrium magnetic properties of the mixed state in type-II superconductors
were measured with high purity bulk and film niobium samples in parallel and
perpendicular magnetic fields using dc magnetometry and scanning Hall-probe
microscopy. Equilibrium magnetization data for the perpendicular geometry were
obtained for the first time. It was found that none of the existing theories is
consistent with these new data. To address this problem, a theoretical model is
developed and experimentally validated. The new model describes the mixed state
in an averaged limit, i.e. %without detailing the samples' magnetic structure
and therefore ignoring interactions between vortices. It is quantitatively
consistent with the data obtained in a perpendicular field and provides new
insights on properties of vortices. % and the entire mixed state. At low values
of the Ginzburg-Landau parameter, the model converts to that of Peierls and
London for the intermediate state in type-I superconductors. It is shown that
description of the vortex matter in superconductors in terms of a 2D gas is
more appropriate than the frequently used crystal- and glass-like scenarios.Comment: 8 pages, 9 figure
Surface Enhancement of Superconductivity in Tin
The possibility of surface enhancement of superconductivity is examined
experimentally. It is shown that single crystal tin samples with cold-worked
surfaces represent a superconductor with a surface-enhanced order parameter (or
negative surface extrapolation length b), whose magnitude can be controlled.Comment: 8 pages, 4 figure
Spin-orbit Scattering and the Kondo Effect
The effects of spin-orbit scattering of conduction electrons in the Kondo
regime are investigated theoretically. It is shown that due to time-reversal
symmetry, spin-orbit scattering does not suppress the Kondo effect, even though
it breaks spin-rotational symmetry, in full agreement with experiment. An
orbital magnetic field, which breaks time-reversal symmetry, leads to an
effective Zeeman splitting, which can be probed in transport measurements. It
is shown that, similar to weak-localization, this effect has anomalous magnetic
field and temperature dependence.Comment: 10 pages, RevTex, one postscript figure available on request from
[email protected]
Conductance of Mesoscopic Systems with Magnetic Impurities
We investigate the combined effects of magnetic impurities and applied
magnetic field on the interference contribution to the conductance of
disordered metals. We show that in a metal with weak spin-orbit interaction,
the polarization of impurity spins reduces the rate of electron phase
relaxation, thus enhancing the weak localization correction to conductivity.
Magnetic field also suppresses thermal fluctuations of magnetic impurities,
leading to a recovery of the conductance fluctuations. This recovery occurs
regardless the strength of the spin-orbit interaction. We calculate the
magnitudes of the weak localization correction and of the mesoscopic
conductance fluctuations at an arbitrary level of the spin polarization induced
by a magnetic field. Our analytical results for the ``'' Aharonov-Bohm
conductance oscillations in metal rings can be used to extract spin and
gyromagnetic factor of magnetic impurities from existing experimental data.Comment: 18 pages, 8 figure
Magnetization reversal in long chains of submicrometric Co dots
This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics.Long chains of 400 nmâdiam Co dots prepared by combined electron-beam lithography exhibit interesting magnetotransport properties. The magnetoresistance of the chains of dots is markedly different from single Co films, indicating a strongly modified magnetization reversal process. Magnetic force microscopy(MFM) shows that, after magnetic saturation, in the remanent state the single-domain dots are all oriented with their magnetic moment along the chain. A comparison of the magnetoresistance and the MFM reveals that the magnetization reversal occurs by coherent rotation of the magnetic moment in the single-domain dots forming the chain
Superconducting proximity effect in the presence of strong spin scattering
We report measurements of the four terminal temperature dependent resistance
of narrow Au wires implanted with ~100 ppm Fe impurities in proximity to
superconducting Al films. The wires show an initial decrease in resistance as
the temperature is lowered through the superconducting transition of the Al
films, but then show an increase in resistance as the temperature is lowered
further. In contrast to the case of pure Au wires in contact with a
superconducting film, the resistance at the lowest temperatures rises above the
normal state resistance. Analysis of the data shows that, in addition to
contributions from magnetic scattering and electron-electron interactions, the
temperature dependent resistivity shows a substantial contribution from the
superconducting proximity effect, which exists even in the presence of strong
spin scattering.Comment: 4 pages, 3 figure
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