20 research outputs found
Universal conductance fluctuations and low temperature 1/f noise in mesoscopic AuFe spin glasses
We report on intrinsic time-dependent conductance fluctuations observed in
mesoscopic AuFe spin glass wires. These dynamical fluctuations have a 1/f-like
spectrum and appear below the measured spin glass freezing temperature of our
samples. The dependence of the fluctuation amplitude on temperature, magnetic
field, voltage and Fe concentration allows a consistent interpretation in terms
of quantum interference effects which are sensitive to the slowly fluctuating
spin configuration.Comment: 4 pages, 4 figure
Spin-Orbit-Induced Kondo Size Effect in Thin Films with 5/2-spin Impurities
Recently, for spin impurities quite different size dependence of the
Kondo contribution to the resistivity was found experimentally than for S=2.
Therefore previous calculation about the effect of the spin-orbit-induced
magnetic anisotropy on the Kondo amplitude of the resistivity is extended to
the case of impurity spin which differs from the integer spin case as
the ground state is degenerated. In this case the Kondo contribution remains
finite when the sample size goes to zero and the thickness dependence in the
Kondo resistivity is much weaker for Cu(Mn). The behavior of the Kondo
coefficient as a function of the thickness depends on the Kondo temperature,
that is somewhat stronger for larger . Comparing our results with a recent
experiment in thin Cu(Mn) films, we find a good agreement.Comment: 8 pages, ReVTeX + 4 figures (Postscript
Spin-Orbit-Induced Magnetic Anisotropy for Impurities in Metallic Samples I. Surface Anisotropy
Motivated by the recent measurements of Kondo resistivity in thin films and
wires, where the Kondo amplitude is suppressed for thinner samples, the surface
anisotropy for magnetic impurities is studied. That anisotropy is developed in
those cases where in addition to the exchange interaction with the impurity
there is strong spin-orbit interaction for conduction electrons around the
impurity in the ballistic region. The asymmetry in the neighborhood of the
magnetic impurity exhibits the anisotropy axis which, in the case of a
plane surface, is perpendicular to the surface. The anisotropy energy is
for spin , and the anisotropy constant is
inversionally proportional to distance measured from the surface and
. Thus at low temperature the spin is frozen in a singlet or doublet of
lowest energy. The influence of that anisotropy on the electrical resistivity
is the subject of the following paper (part II).Comment: 28 pages, RevTeX (using epsfig), 8 eps figures included, submitted to
PR
Spin-Orbit-Induced Magnetic Anisotropy for Impurities in Metallic Samples II. Finite Size Dependence in the Kondo Resistivity
The electrical resistivity including the Kondo resistivity increase at low
temperature is calculated for thin films of dilute magnetic alloys. Assuming
that in the non-magnetic host the spin-orbit interaction is strong like in Au
and Cu, the magnetic impurities have a surface anisotropy calculated in part I.
That anisotropy hinders the motion of the spin. Including that anisotropy the
effective electron-impurity coupling is calculated by using the second order
renormalization group equations. The amplitude of the Kondo resistivity
contribution is reduced as the position of the impurity approaches the surface
but the increase occurs approximately at the bulk Kondo temperature. Different
proximity effects observed by Giordano are also explained qualitatively where
the films of magnetic alloys are covered by pure second films with different
mean free path. The theory explains the experimental results in those cases
where a considerable amount of impurities is at the surface inside the
ballistic region.Comment: 39 pages, RevTeX (using epsfig), 15 eps figures included, submitted
to PR
Phase Dependent Thermopower in Andreev Interferometers
We report measurements of the thermopower S of mesoscopic Andreev
interferometers, which are hybrid loops with one arm fabricated from a
superconductor (Al), and one arm from a normal metal (Au). S depends on the
phase of electrons in the interferometer, oscillating as a function of magnetic
flux with a period of one flux quantum (= h/2e). The magnitude of S increases
as the temperature T is lowered, reaching a maximum around T = 0.14 K, and
decreases at lower temperatures. The symmetry of S oscillations with respect to
magnetic flux depends on the topology of the sample.Comment: 4 pages, 4 figure
Theory of magnetoresistance in films of dilute magnetic alloys
Earlier a magnetic anisotropy for magnetic impurities nearby the surface of
non-magnetic host was proposed in order to explain the size dependence of the
Kondo effect in dilute magnetic alloys. Recently Giordano has measured the
magnetoresistance of dilute Au(Fe) films for different thicknesses well above
the Kondo temperature . In this way he verified the existence of that
anisotropy even for such a case where the Kondo effect is not dominating. For
detailed comparison of that suggestion with experiments, the magnetic field
dependence of the magnetoresistance is calculated in the lowest approximation,
thus in the second order of the exchange coupling. The strength of the
anisotropy is very close to earlier estimates deduced from the size dependence
of the Kondo resistivity amplitude.Comment: (11 pages, 8 figures, essential changes compared to the old version
Cross-Over between universality classes in a magnetically disordered metallic wire
In this article we present numerical results of conduction in a disordered
quasi-1D wire in the possible presence of magnetic impurities. Our analysis
leads us to the study of universal properties in different conduction regimes
such as the localized and metallic ones. In particular, we analyse the
cross-over between universality classes occurring when the strength of magnetic
disorder is increased. For this purpose, we use a numerical Landauer approach,
and derive the scattering matrix of the wire from electron's Green's function.Comment: Final version, accepted for publication in New Journ. of Physics, 27
pages, 28 figures. Replaces the earlier shorter preprint arXiv:0910.427
Chirality driven anomalous Hall effect in weak coupling regime
Anomalous Hall effect arising from non-trivial spin configuration (chirality)
is studied based on the - model. Considering a weak coupling case, the
interaction is treated perturbatively. Scattering by normal impurities is
included. Chirality is shown to drive locally Hall current and leads to overall
Hall effect if there is a finite uniform chirality. This contribution is
independent of the conventional spin-orbit contribution and shows distinct low
temperature behavior. In mesoscopic spin glasses, chirality-induced anomalous
Hall effect is expected below the spin-glass transition temperature.
Measurement of Hall coefficient would be useful in experimentally confirming
the chirality ordering
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
Kondo Effect on Mesoscopic Scale (Review)
Following the discovery of the Kondo effect the bulk transport and magnetic
behavior of the dilute magnetic alloys have been successfully described. In the
last fifteen years new directions have been developed as the study of the
systems of reduced dimensions and the artificial atoms so called quantum dots.
In this review the first subject is reviewed starting with the scanning
tunneling microscope (STM) study of a single magnetic impurity. The next
subject is the reduction of the amplitude of the Kondo effect in samples of
reduced dimension which was explained by the surface magnetic anisotropy which
blocks the motion of the integer spin nearby the surface. The electron
dephasing and energy relaxation experiments are discussed with the possible
explanation including the surface anisotropy, where the situation in cases of
integer and half-integer spins is very different. Finally, the present
situation of the theory of dynamical structural defects is briefly presented
which may lead to two-channel Kondo behavior.Comment: 8 pages, submitted to the JPSJ Special Issue "Kondo effect -- 40
years after the Discovery