187 research outputs found
Low-temperature structural transition in FeCr_2S_4
Transmission electron microscopy studies of [110] and [111] oriented
FeCr_2S_4 single crystals at different temperatures reveal a structural
transition at low temperatures indicating a cubic-to-triclinic symmetry
reduction within crystallographic domains. The overall crystal symmetry was
found to be reduced from Fd3m to F-43m. The triclinic distortions were
suggested to result from the combined actions of tetragonal distortions due to
the Jahn-Teller active Fe^2+ ions and trigonal distortions due to a
displacement of the Cr^3+ ions in the direction.Comment: 4 pages, 8 figure
Negative magnetoresistance and phase slip process in superconducting nanowires
We argue that the negative magnetoresistance of superconducting nanowires,
which was observed in recent experiments, can be explained by the influence of
the external magnetic field on the critical current of the phase slip process.
We show that the suppression of the order parameter in the bulk superconductors
made by an external magnetic field can lead to an enhancement of both the first
and the second critical currents of the phase slip process in
nanowires. Another mechanism of an enhancement of can come from
decreasing the decay length of the charge imbalance at weak
magnetic fields because is inversely proportional to . The
enhancement of the first critical current leads to a larger intrinsic
dissipation of the phase slip process. It suppresses the rate of both the
thermo-activated and/or quantum fluctuated phase slips and results in
decreasing the fluctuated resistance.Comment: 7 pages, 4 figure
Phonon anomalies and charge dynamics in Fe_{1-x}Cu_{x}Cr_{2}S_{4} single crystals
A detailed investigation of phonon excitations and charge carrier dynamics in
single crystals of Fe_{1-x}Cu_{x}Cr_{2}S_{4} (x = 0, 0.2, 0.4, 0.5) has been
performed by using infrared spectroscopy. In FeCr_{2}S_{4} the phonon
eigenmodes are strongly affected by the onset of magnetic order. Despite
enhanced screening effects, a continuous evolution of the phonon excitations
can be observed in the doped compounds with x = 0.2 (metallic) and x = 0.4, 0.5
(bad metals), but the effect of magnetic ordering on the phonons is strongly
reduced compared to x = 0. The Drude-like charge-carrier contribution to the
optical conductivity in the doped samples indicates that the colossal
magneto-resistance effect results from the suppression of spin-disorder
scattering.Comment: 8 pages, 6 figure
Multiple phase slips phenomena in mesoscopic superconducting rings
We investigate the behavior of a mesoscopic one-dimensional ring in an
external magnetic field by simulating the time dependent Ginzburg-Landau
equations with periodic boundary conditions. We analyze the stability and the
different possible evolutions for the phase slip phenomena starting from a
metastable state. We find a stability condition relating the winding number of
the initial solution and the number of flux quanta penetrating the ring. The
analysis of multiple phase slips solutions is based on analytical results and
simulations. The role of the ratio of two characteristic times u is studied for
the case of a multiple phase slips transition. We found out that if u>>1,
consecutive multiple phase slips will be more favorable than simultaneous ones.
If u>1 is often a necessary
condition to reach the ground state. The influence of the Langevin noise on the
kinetics of the phase transition is discussed.Comment: 8 pages, 6 figure
Spin-lattice coupling in the ferrimagnetic semiconductor FeCr2S4 probed by surface acoustic waves
Using surface acoustic waves, the elastomagnetic coupling could be studied in
thin single crystalline plates of the ferrimagnetic semiconductor FeCr2S4 by
measuring the attenuation and the frequency tracking in the temperature range
4.2 K to 200 K. The data clearly display the anomalies found in low-field
magnetization measurements.Comment: 15 pages, 3 figures. To appear in J. Appl. Phys., 99 (2006
Memory Effect and Triplet Pairing Generation in the Superconducting Exchange Biased Co/CoOx/Cu41Ni59/Nb/Cu41Ni59 Layered Heterostructure
We fabricated a nanolayered hybrid superconductor-ferromagnet spin-valve
structure, the resistive state of which depends on the preceding magnetic field
polarity. The effect is based on a strong exchange bias (about -2 kOe) on a
diluted ferromagnetic copper-nickel alloy and generation of a long range odd in
frequency triplet pairing component. The difference of high and low resistance
states at zero magnetic field is 90% of the normal state resistance for a
transport current of 250 {\mu}A and still around 42% for 10 {\mu}A. Both logic
states of the structure do not require biasing fields or currents in the idle
mode.Comment: 9 pages, 4 figures, Accepted to Applied Physics Letter
Experimental and theoretical analysis of the upper critical field in FSF trilayers
The upper critical magnetic field H_{c2} in thin-film FSF trilayer spin-valve
cores is studied experimentally and theoretically in geometries perpendicular
and parallel to the heterostructure surface. The series of samples with
variable thicknesses of the bottom and of the top Cu_{41}Ni_{59} F-layers are
prepared in a single run, utilizing a wedge deposition technique. The critical
field H_{c2} is measured in the temperature range K and for magnetic
fields up to 9 Tesla. A transition from oscillatory to reentrant behavior of
the superconducting transition temperature versus F-layers thickness, induced
by an external magnetic field, has been observed for the first time. In order
to properly interpret the experimental data, we develop a quasiclassical
theory, enabling one to evaluate the temperature dependence of the critical
field and the superconducting transition temperature for an arbitrary set of
the system parameters. A fairly good agreement between our experimental data
and theoretical predictions is demonstrated for all samples, using a single set
of fit parameters. This confirms adequacy of the
Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) physics in determining the unusual
superconducting properties of the studied Cu_{41}Ni_{59}/Nb/Cu_{41}Ni_{59}
spin-valve core trilayers.Comment: 16 pages, 7 figures; published versio
Evidence for two-dimensional nucleation of superconductivity in MgB
According to the crystal structure of MgB and band structure
calculations quasi-two-dimensional (2D) boron planes are responsible for the
superconductivity. We report on critical fields and resistance measurements of
30 nm thick MgB films grown on MgO single crystalline substrate. A linear
temperature dependence of the parallel and perpendicular upper critical fields
indicate a 3D-like penetration of magnetic field into the sample. Resistivity
measurements, in contrast, yield a temperature dependence of fluctuation
conductivity above T which agrees with the Aslamazov-Larkin theory of
fluctuations in 2D superconductors. We consider this finding as an experimental
evidence of two-dimensional nucleation of superconductivity in MgB.Comment: 5 RevTex pages, 3 PostScript Figures ZIPed in archive Sidoren.zip.
Submitted to EuroPhys. Lett. December 3, 200
Reentrant Superconductivity and Superconducting Critical Temperature Oscillations in F/S/F trilayers of Cu41Ni59/Nb/Cu41Ni59 Grown on Cobalt Oxide
Ferromagnet/Superconductor/Ferromagnet (F/S/F) trilayers constitute the core
of a superconducting spin valve. The switching effect of the spin valve is
based on interference phenomena occurring due to the proximity effect at the
S/F interfaces. A remarkable effect is only expected if the core structure
exhibits strong critical temperature oscillations, or most favorable, reentrant
superconductivity, when the thickness of the ferromagnetic layer is increased.
The core structure has to be grown on an antiferromagnetic oxide layer (or such
layer to be placed on top) to pin by exchange bias the
magnetization-orientation of one of the ferromagnetic layers. In the present
paper we demonstrate that this is possible, keeping the superconducting
behavior of the core structure undisturbed.Comment: 22 pages, 12 figures, 1 tabl
Reentrant superconductivity in superconductor/ferromagnetic-alloy bilayers
We studied the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) like state
establishing due to the proximity effect in superconducting Nb/Cu41Ni59
bilayers. Using a special wedge-type deposition technique, series of 20-35
samples could be fabricated by magnetron sputtering during one run. The layer
thickness of only a few nanometers, the composition of the alloy, and the
quality of interfaces were controlled by Rutherford backscattering
spectrometry, high resolution transmission electron microscopy, and Auger
spectroscopy. The magnetic properties of the ferromagnetic alloy layer were
characterized with superconducting quantum interference device (SQUID)
magnetometry. These studies yield precise information about the thickness, and
demonstrate the homogeneity of the alloy composition and magnetic properties
along the sample series. The dependencies of the critical temperature on the Nb
and Cu41Ni59 layer thickness, Tc(dS) and Tc(dF), were investigated for constant
thickness dF of the magnetic alloy layer and dS of the superconducting layer,
respectively. All types of non-monotonic behaviors of Tc versus dF predicted by
the theory could be realized experimentally: from reentrant superconducting
behavior with a broad extinction region to a slight suppression of
superconductivity with a shallow minimum. Even a double extinction of
superconductivity was observed, giving evidence for the multiple reentrant
behavior predicted by theory. All critical temperature curves were fitted with
suitable sets of parameters. Then, Tc(dF) diagrams of a hypothetical F/S/F
spin-switch core structure were calculated using these parameters. Finally,
superconducting spin-switch fabrication issues are discussed in detail in view
of the achieved results.Comment: 34 pages, 9 figure
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