438 research outputs found
Effects of Ferromagnetic Magnetic Ordering and Phase Transition on the Resistivity of Spin Current
It has been shown experimentally a long time ago that the magnetic ordering
causes an anomalous behavior of the electron resistivity in ferromagnetic
crystals. Phenomenological explanations based on the interaction between
itinerant electron spins and lattice spins have been suggested to explain these
observations. We show by extensive Monte Carlo simulation that this behavior is
also observed for the resistivity of the spin current calculated as a function
of temperature () from low- ordered phase to high- paramagnetic phase
in a ferromagnet. We show in particular that across the critical region, the
spin resistivity undergoes a huge peak. The origin of this peak is shown to
stem from the formation of magnetic domains near the phase transition. The
behavior of the resistivity obtained here is compared to experiments and
theories. A good agreement is observed.Comment: 7 pages, 3 figures, accepted, to appear in J. Appl. Phy
A new method of measuring center-of-mass velocities of radially pulsating stars from high-resolution spectroscopy
We present a radial velocity analysis of 20 solar neighborhood RR Lyrae and 3
Population II Cepheids variables. We obtained high-resolution, moderate-to-high
signal-to-noise ratio spectra for most stars and obtained spectra were covering
different pulsation phases for each star. To estimate the gamma
(center-of-mass) velocities of the program stars, we use two independent
methods. The first, `classic' method is based on RR Lyrae radial velocity curve
templates. The second method is based on the analysis of absorption line
profile asymmetry to determine both the pulsational and the gamma velocities.
This second method is based on the Least Squares Deconvolution (LSD) technique
applied to analyze the line asymmetry that occurs in the spectra. We obtain
measurements of the pulsation component of the radial velocity with an accuracy
of 3.5 km s. The gamma velocity was determined with an accuracy
10 km s, even for those stars having a small number of spectra.
The main advantage of this method is the possibility to get the estimation of
gamma velocity even from one spectroscopic observation with uncertain pulsation
phase. A detailed investigation of the LSD profile asymmetry shows that the
projection factor varies as a function of the pulsation phase -- this is a
key parameter which converts observed spectral line radial velocity variations
into photospheric pulsation velocities. As a byproduct of our study, we present
41 densely-spaced synthetic grids of LSD profile bisectors that are based on
atmospheric models of RR Lyr covering all pulsation phases.Comment: 17 pages, 16 figures, accepted for publication in MNRAS;
doi:10.1093/mnras/stx294
The Origin of Tunneling Anisotropic Magnetoresistance in Break Junctions
First-principles calculations of electron tunneling transport in Ni and Co
break junctions reveal strong dependence of the conductance on the
magnetization direction, an effect known as tunneling anisotropic
magnetoresistance (TAMR). The origin of this phenomenon stems from resonant
states localized in the electrodes near the junction break. The energy and
broadening of these states is strongly affected by the magnetization
orientation due to spin-orbit coupling, causing TAMR to be sensitive to bias
voltage on a scale of a few mV. Our results bear a resemblance to recent
experimental data and suggest that TAMR driven by resonant states is a general
phenomenon typical for magnetic broken contacts and other experimental
geometries where a magnetic tip is used to probe electron transport.Comment: 4 pages, 3 figure
Electrically driven magnetism on a Pd thin film
Using first-principles density functional calculations we demonstrate that
ferromagnetism can be induced and modulated on an otherwise paramagnetic Pd
metal thin-film surface through application of an external electric field. As
free charges are either accumulated or depleted at the Pd surface to screen the
applied electric field there is a corresponding change in the surface density
of states. This change can be made sufficient for the Fermi-level density of
states to satisfy the Stoner criterion, driving a transition locally at the
surface from a paramagnetic state to an itinerant ferromagnetic state above a
critical applied electric field, Ec. Furthermore, due to the second-order
nature of this transition, the surface magnetization of the ferromagnetic state
just above the transition exhibits a substantial dependence on electric field,
as the result of an enhanced magnetoelectric susceptibility. Using a linearized
Stoner model we explain the occurrence of the itinerant ferromagnetism and
demonstrate that the magnetic moment on the Pd surface follows a square-root
variation with electric field consistent with our first-principles
calculations.Comment: 8 pages, 7 figure
Tunnelling magnetoresistance anomalies of a Coulomb blockaded quantum dot
We consider quantum transport and tunneling magnetoresistance (TMR) through
an interacting quantum dot in the Coulomb blockade regime, attached to
ferromagnetic leads. We show that there exist two kinds of anomalies of TMR,
which have different origin. One type, associated with TMR sign change and
appearing at conductance resonances, is of single particle origin. The second
type, inducing a pronounced increase of TMR value far beyond 100%, is caused by
electron correlations. It is manifested in-between Coulomb blockade conductance
peaks. Both types of anomalies are discussed for zero and finite bias and their
robustness to the temperature increase is also demonstrated. The results are
presented in the context of recent experiments on semiconductor quantum dots in
which similar features of TMR have been observed.Comment: 10 pages, 7 figures, Revtex style, to appaear in Phys. Rev. B
extended discussion added, some typographic errors correcte
Spin Resistivity in Frustrated Antiferromagnets
In this paper we study the spin transport in frustrated antiferromagnetic FCC
films by Monte Carlo simulation. In the case of Ising spin model, we show that
the spin resistivity versus temperature exhibits a discontinuity at the phase
transition temperature: an upward jump or a downward fall, depending on how
many parallel and antiparallel localized spins interacting with a given
itinerant spin. The surface effects as well as the difference of two degenerate
states on the resistivity are analyzed. Comparison with non frustrated
antiferromagnets is shown to highlight the frustration effect. We also show and
discuss the results of the Heisenberg spin model on the same lattice
Tunnel magnetoresistance and interfacial electronic state
We study the relation between tunnel magnetoresistance (TMR) and interfacial
electronic states modified by magnetic impurities introduced at the interface
of the ferromagnetic tunnel junctions, by making use of the periodic Anderson
model and the linear response theory. It is indicated that the TMR ratio is
strongly reduced depending on the position of the -levels of impurities,
based on reduction in the spin-dependent -electron tunneling in the majority
spin state. The results are compared with experimental results for Cr-dusted
ferromagnetic tunnel junctions, and also with results for metallic multilayers
for which similar reduction in giant magnetoresistance has been reported.Comment: 5 pages, 4 figures, 2 column revtex4 format, ICMFS 2002 (Kyoto
Spin Dependence of Interfacial Reflection Phase Shift at Cu/Co Interface
The spin dependent reflection at the interface is the key element to
understand the spin transport. By completely solving the scattering problem
based on first principles method, we obtained the spin resolved reflectivity
spectra. The comparison of our theoretical results with experiment is good in a
large energy scale from Fermi level to energy above vacuum level. It is found
that interfacial distortion is crucial for understanding the spin dependence of
the phase gain at the CuCo interface. Near the Fermi level, image state
plays an important role to the phase accumulation in the copper film.Comment: 6 papges, 3 figures, accepted by Physical Review
Magnetism and superconductivity at LAO/STO-interfaces: the role of Ti 3d interface electrons
Ferromagnetism and superconductivity are in most cases adverse. However,
recent experiments reveal that they coexist at interfaces of LaAlO3 and SrTiO3.
We analyze the magnetic state within density functional theory and provide
evidence that magnetism is not an intrinsic property of the two-dimensional
electron liquid at the interface. We demonstrate that the robust ferromagnetic
state is induced by the oxygen vacancies in SrTiO3- or in the LaAlO3-layer.
This allows for the notion that areas with increased density of oxygen
vacancies produce ferromagnetic puddles and account for the previous
observation of a superparamagnetic behavior in the superconducting state.Comment: 5 pages, 4 figures, to appear in Physical Review B (Rapid
Communications
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