169 research outputs found
Transition from the macrospin to chaotic behaviour by a spin-torque driven magnetization precession of a square nanoelement
We demonstrate (using full-scale micromagnetic simulations) that the spin
injection driven steady-state precession of a thin magnetic nanoelement exhibit
a complicate transition from the quasi-macrospin to the chaotic behaviour with
the increasing element size. For nanoelement parameters typical for those used
experimentally we have found that the macrospin approximation becomes invalid
already for very small nanoelement sizes (~ 30 nm), in contrast to the
previously reported results (Li and Zhang, Phys. Rev. B, vol. B68, 024404-1
(2003))Comment: Submitted to Phys. Rev.
Magnetization precession due to a spin polarized current in a thin nanoelement: numerical simulation study
In this paper a detailed numerical study (in frames of the Slonczewski
formalism) of magnetization oscillations driven by a spin-polarized current
through a thin elliptical nanoelement is presented. We show that a
sophisticated micromagnetic model, where a polycrystalline structure of a
nanoelement is taken into account, can explain qualitatively all most important
features of the magnetization oscillation spectra recently observed
experimentally (S.I. Kiselev et al., Nature, vol. 425, p. 380 (2003), namely:
existence of several equidistant spectral bands, sharp onset and abrupt
disappearance of magnetization oscillations with increasing current, absence of
the out-of-plane regime predicted by a macrospin model and the relation between
frequencies of so called small-angle and quasichaotic oscillations. However, a
quantitative agreement with experimental results (especially concerning the
frequency of quasichaotic oscillations) could not be achieved in the region of
reasonable parameter values, indicating that further model refinement is
necessary for a complete understanding of the spin-driven magnetization
precession even in this relatively simple experimental situation.Comment: Submitted to Phys. Rev. B; In this revised version figure positions
on the page have been changed to ensure correct placements of the figure
caption
Cosmological Signatures of Interacting Neutrinos
We investigate signatures of neutrino scattering in the Cosmic Microwave
Background (CMB) and matter power spectra, and the extent to which present
cosmological data can distinguish between a free streaming or tightly coupled
fluid of neutrinos. If neutrinos have strong non-standard interactions, for
example, through the coupling of neutrinos to a light boson, they may be kept
in equilibrium until late times. We show how the power spectra for these models
differ from more conventional neutrino scenarios, and use CMB and large scale
structure data to constrain these models. CMB polarization data improves the
constraints on the number of massless neutrinos, while the Lyman--
power spectrum improves the limits on the neutrino mass. Neutrino mass limits
depend strongly on whether some or all of the neutrino species interact and
annihilate. The present data can accommodate a number of tightly-coupled
relativistic degrees of freedom, and none of the interacting-neutrino scenarios
considered are ruled out by current data -- although considerations regarding
the age of the Universe disfavor a model with three annihilating neutrinos with
very large neutrino masses.Comment: 17 pages, 14 figures, minor changes and references added, published
in Phys. Rev.
Spin-transfer torque induced reversal in magnetic domains
Using the complex stereographic variable representation for the macrospin,
from a study of the nonlinear dynamics underlying the generalized
Landau-Lifshitz(LL) equation with Gilbert damping, we show that the
spin-transfer torque is effectively equivalent to an applied magnetic field. We
study the macrospin switching on a Stoner particle due to spin-transfer torque
on application of a spin polarized current. We find that the switching due to
spin-transfer torque is a more effective alternative to switching by an applied
external field in the presence of damping. We demonstrate numerically that a
spin-polarized current in the form of a short pulse can be effectively employed
to achieve the desired macro-spin switching.Comment: 16 pages, 6 figure
Magnetic relaxation in finite two-dimensional nanoparticle ensembles
We study the slow phase of thermally activated magnetic relaxation in finite
two-dimensional ensembles of dipolar interacting ferromagnetic nanoparticles
whose easy axes of magnetization are perpendicular to the distribution plane.
We develop a method to numerically simulate the magnetic relaxation for the
case that the smallest heights of the potential barriers between the
equilibrium directions of the nanoparticle magnetic moments are much larger
than the thermal energy. Within this framework, we analyze in detail the role
that the correlations of the nanoparticle magnetic moments and the finite size
of the nanoparticle ensemble play in magnetic relaxation.Comment: 21 pages, 4 figure
'Hole-digging' in ensembles of tunneling Molecular Magnets
The nuclear spin-mediated quantum relaxation of ensembles of tunneling
magnetic molecules causes a 'hole' to appear in the distribution of internal
fields in the system. The form of this hole, and its time evolution, are
studied using Monte Carlo simulations. It is shown that the line-shape of the
tunneling hole in a weakly polarised sample must have a Lorentzian lineshape-
the short-time half-width in all experiments done so far should be
, the half-width of the nuclear spin multiplet. After a time
, the single molecule tunneling relaxation time, the hole width begins
to increase rapidly. In initially polarised samples the disintegration of
resonant tunneling surfaces is found to be very fast.Comment: 4 pages, 5 figure
Diversity of universes created by pure gravity
We show that a number of problems of modern cosmology may be solved in the
framework of multidimensional gravity with high-order curvature invariants,
without invoking other fields. We use a method employing a slow-change
approximation, able to work with rather a general form of the gravitational
action, and consider Kaluza-Klein type space-times with one or several extra
factor spaces. A vast choice of effective theories suggested by the present
framework may be stressed: even if the initial Lagrangian is entirely fixed,
one obtains quite different models for different numbers, dimensions and
topologies of the extra factor spaces. As examples of problems addressed we
consider (i) explanation of the present accelerated expansion of the Universe,
with a reasonably small cosmological constant, and the problem of its fine
tuning is considered from a new point of view; (ii) the mechanism of closed
wall production in the early Universe; such walls are necessary for massive
primordial black hole formation which is an important stage in some scenarios
of cosmic structure formation; (iii) sufficient particle production rate at the
end of inflation; (iv) it is shown that our Universe may contain spatial
domains with a macroscopic size of extra dimensions. We also discuss chaotic
attractors appearing at possible nodes of the kinetic term of the effective
scalar field Lagrangian.Comment: 14 pages, 8 figures, revtex4. Final version, some considerations
added in response to referee remark
FeCo Nanowire-Strontium Ferrite Powder Composites for Permanent Magnets with High-Energy Products
Due to the issues associated with rare-earth elements, there arises a strong
need for magnets with properties between those of ferrites and rare-earth
magnets that could substitute the latter in selected applications. Here, we
produce a high remanent magnetization composite bonded magnet by mixing FeCo
nanowire powders with hexaferrite particles. In the first step, metallic
nanowires with diameters between 30 and 100 nm and length of at least 2 {\mu}m
are fabricated by electrodeposition. The oriented as-synthesized nanowires show
remanence ratios above 0.76 and coercivities above 199 kA/m and resist core
oxidation up to 300 {\deg}C due to the existence of a > 8 nm thin oxide
passivating shell. In the second step, a composite powder is fabricated by
mixing the nanowires with hexaferrite particles. After the optimal nanowire
diameter and composite composition are selected, a bonded magnet is produced.
The resulting magnet presents a 20% increase in remanence and an enhancement of
the energy product of 48% with respect to a pure hexaferrite (strontium
ferrite) magnet. These results put nanowire-ferrite composites at the forefront
as candidate materials for alternative magnets for substitution of rare earths
in applications that operate with moderate magnet performance
Theoretical study of the magnetization dynamics of non-dilute ferrofluids
The paper is devoted to the theoretical investigation of the magnetodipolar
interparticle interaction effect on remagnetization dynamics in moderately
concentrated ferrofluids. We consider a homogeneous (without particle
aggregates) ferrofluid consisting of identical spherical particles and employ a
rigid dipole model, where magnetic moment of a particle is fixed with respect
to the particle itself. In particular, for the magnetization relaxation after
the external field is instantly switched off, we show that the magnetodipolar
interaction leads to the increase of the initial magnetization relaxation time.
For the complex ac-susceptibility we find that the this interaction leads to an
overall increase of the imaginary susceptibility part and shifts the peak on
its frequency dependence towards lower frequencies. Comparing results obtained
with our analytical approach (second order virial expansion) to numerical
simulation data (Langevin dynamics method), we demonstrate that the employed
virial expansion approximation gives a good qualitative description of the
ferrofluid magnetization dynamics and provides a satisfactory quantitative
agreement with numerical simulations for the dc magnetization relaxation - up
to the particle volume fraction c ~ 10% and for the ac-susceptibility - up to c
~ 5 %.Comment: 12 pages, 6 figures, submitted to PR
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