506 research outputs found
Metastable states influence on the magnetic behavior of the triangular lattice: Application to the spin-chain compound Ca3Co2O6
It is known that the spin-chain compound Ca3Co2O6 exhibits very interesting
plateaus in the magnetization as a function of the magnetic field at low
temperatures. The origin of them is still controversial. In this paper we study
the thermal behavior of this compound with a single-flip Monte Carlo simulation
on a triangular lattice and demonstrate the decisive influence of metastable
states in the splitting of the ferrimagnetic 1/3 plateau below 10 K. We
consider the [Co2O6]n chains as giant magnetic moments described by large Ising
spins on planar clusters with open boundary conditions. With this simple
frozen-moment model we obtain stepped magnetization curves which agree quite
well with the experimental results for different sweeping rates. We describe
particularly the out-of-equilibrium states that split the low-temperature 1/3
plateau into three steps. They relax thermally to the 1/3 plateau, which has
long-range order at the equilibrium. Such states are further analyzed with
snapshots unveiling a domain-wall structure that is responsible for the
observed behavior of the 1/3 plateau. A comparison is also given of the exact
results in small triangular clusters with our Monte Carlo results, providing
further support for our thermal description of this compound.Comment: 8 pages, 11 figures, submitted to PR
Dephasing Effects by Ferromagnetic Boundary on Resistivity in Disordered Metallic Layer
The resistivity of disordered metallic layer sandwiched by two ferromagnetic
layers at low-temperature is investigated theoretically. It is shown that the
magnetic field acting at the interface does not affect the classical Boltzmann
resistivity but causes a dephasing among electrons in the presence of the
spin-orbit interaction, suppressing the anti-localization due to the spin-orbit
interaction. The dephasing turns out to be stronger in the case where the
magnetization of the two layers is parallel, contributing to a positive
magnetoresistance close to a switching field at low temperature.Comment: 11 pages, 3 figures. Title modified in journal versio
Contact-induced spin polarization in carbon nanotubes
Motivated by the possibility of combining spintronics with molecular
structures, we investigate the conditions for the appearance of
spin-polarization in low-dimensional tubular systems by contacting them to a
magnetic substrate. We derive a set of general expressions describing the
charge transfer between the tube and the substrate and the relative energy
costs. The mean-field solution of the general expressions provides an
insightful formula for the induced spin-polarization. Using a tight-binding
model for the electronic structure we are able to estimate the magnitude and
the stability of the induced moment. This indicates that a significant magnetic
moment in carbon nanotubes can be observed.Comment: To appear in Phys. Rev. B (2003
Phase Diagram of Multilayer Magnetic Structures
Multilayer "ferromagnet-layered antiferromagnet" (Fe/Cr) structures
frustrated due to the roughness of layer interfaces are studied by numerical
modeling methods. The "thickness-roughness" phase diagrams for the case of thin
ferromagnetic film on the surface of bulk antiferromagnet and for two
ferromagnetic layers separated by an antiferromagnetic interlayer are obtained
and the order parameter distributions for all phases are found. The phase
transitions nature in such systems is considered. The range of applicability
for the "magnetic proximity model" proposed by Slonczewski is evaluated.Comment: 8 pages, 8 figure
Luttinger liquid superlattices
We calculate the correlation functions and the DC conductivity of Luttinger
liquid superlattices, modeled by a repeated pattern of interacting and free
Luttinger liquids. In a specific realization, where the interacting subsystem
is a Hubbard chain, the system exhibits a rich phase diagram with four
different phases: two metals and two compressible insulators. In general, we
find that the effective low energy description amalgamates features of both
types of liquids in proportion to their spatial extent, suggesting the
interesting possibility of `engineered' Luttinger liquids.Comment: RevTeX, 5 pages, 3 figure
Coexistence of glassy antiferromagnetism and giant magnetoresistance (GMR) in Fe/Cr multilayer structures
Using temperature-dependent magnetoresistance and magnetization measurements
on Fe/Cr multilayers that exhibit pronounced giant magnetoresistance (GMR), we
have found evidence for the presence of a glassy antiferromagnetic (GAF) phase.
This phase reflects the influence of interlayer exchange coupling (IEC) at low
temperature (T < 140K) and is characterized by a field-independent glassy
transition temperature, Tg, together with irreversible behavior having
logarithmic time dependence below a "de Almeida and Thouless" (AT) critical
field line. At room temperature, where the GMR effect is still robust, IEC
plays only a minor role, and it is the random potential variations acting on
the magnetic domains that are responsible for the antiparallel interlayer
domain alignment.Comment: 5 pages, 4 figure
Detecting Electronic States at Stacking Faults in Magnetic Thin Films by Tunneling Spectroscopy
Co islands grown on Cu(111) with a stacking fault at the interface present a
conductance in the empty electronic states larger than the Co islands that
follow the stacking sequence of the Cu substrate. Electrons can be more easily
injected into these faulted interfaces, providing a way to enhance transmission
in future spintronic devices. The electronic states associated to the stacking
fault are visualized by tunneling spectroscopy and its origin is identified by
band structure calculations.Comment: 4 pages, 4 figures; to be published in Phys. Rev. Lett (2000
Spin-dependent electrical transport in ion-beam sputter deposited Fe-Cr multilayers
The temperature dependence of the electrical resistivity and
magnetoresistance of Xe-ion beam sputtered Fe-Cr multilayers has been
investigated. The electrical resistivity between 5 and 300 K in the fully
ferromagnetic state, obtained by applying a field beyond the saturation field
(H_sat) necessary for the antiferromagnetic(AF)-ferromagnetic(FM) field-induced
transition, shows evidence of spin-disorder resistivity as in crystalline Fe
and an s-d scattering contribution (as in 3d metals and alloys). The sublattice
magnetization m(T) in these multilayers has been calculated in terms of the
planar and interlayer exchange energies. The additional spin-dependent
scattering \Delta \rho (T) = \rho(T,H=0)_AF - \rho(T,H=H_sat)_FM in the AF
state over a wide range of temperature is found to be proportional to the
sublattice magnetization, both \Delta \rho(T) and m(T) reducing along with the
antiferromagnetic fraction. At intermediate fields, the spin-dependent part of
the electrical resistivity (\rho_s (T)) fits well to the power law \rho_s (T) =
b - cT^\alpha where c is a constant and b and \alpha are functions of H. At low
fields \alpha \approx 2 and the intercept b decreases with H much the same way
as the decrease of \Delta \rho (T) with T. A phase diagram (T vs. H_sat) is
obtained for the field- induced AF to FM transition. Comparisons are made
between the present investigation and similar studies using dc magnetron
sputtered and molecular beam epitaxy (MBE) grown Fe-Cr multilayers.Comment: 8 pages, 10 figures, to appear in Phys. Rev.
Systematic Two-band Model Calculations of the GMR Effect with Metallic and Nonmetallic Spacers and with Impurities
By an accurate Green's function method we calculate conductances and the
corresponding Giant Magneto-Resistance effects (GMR) of two metallic
ferromagnetic films separated by different spacers, metallic and non-metallic
ones, in a simplified model on a sc lattice, in CPP and CIP geometries (i.e.
current perpendicular or parallel to the planes), without impurities, or with
interface- or bulk impurities. The electronic structure of the systems is
approximated by two hybridized orbitals per atom, to mimic s-bands and d-bands
and their hybridization.
We show that such calculations usually give rough estimates only, but of the
correct order of magnitude; in particular, the predictions on the impurity
effects depend strongly on the model parameters. One of our main results is the
prediction of huge CPP-GMR effects for {\it non-metallic} spacers in the
ballistic limit.Comment: Revised version; discussions and references improved; accepted by
JMM
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