1,235 research outputs found
Integral Relaxation Time of Single-Domain Ferromagnetic Particles
The integral relaxation time \tau_{int} of thermoactivating noninteracting
single-domain ferromagnetic particles is calculated analytically in the
geometry with a magnetic field H applied parallel to the easy axis. It is shown
that the drastic deviation of \tau_{int}^{-1} from the lowest eigenvalue of the
Fokker-Planck equation \Lambda_1 at low temperatures, starting from some
critical value of H, is the consequence of the depletion of the upper potential
well. In these conditions the integral relaxation time consists of two
competing contributions corresponding to the overbarrier and intrawell
relaxation processes.Comment: 8 pages, 3 figure
Magnetization reversal of ferromagnetic nanodisc placed above a superconductor
Using numerical simulation we have studied a magnetization distribution and a
process of magnetization reversal in nanoscale magnets placed above a
superconductor plane. In order to consider an influence of superconductor on
magnetization distribution in the nanomagnet we have used London approximation.
We have found that for usual values of London penetration depth the ground
state magnetization is mostly unchanged. But at the same time the fields of
vortex nucleation and annihilation change significantly: the interval where
vortex is stable enlarges on 100-200 Oe for the particle above the
superconductor. Such fields are experimentally observable so there is a
possibility of some practical applications of this effect.Comment: 8 pages, 9 figure
Evolution and stability of a magnetic vortex in small cylindrical ferromagnetic particle under applied field
The energy of a displaced magnetic vortex in a cylindrical particle made of
isotropic ferromagnetic material (magnetic dot) is calculated taking into
account the magnetic dipolar and the exchange interactions. Under the
simplifying assumption of small dot thickness the closed-form expressions for
the dot energy is written in a non-perturbative way as a function of the
coordinate of the vortex center. Then, the process of losing the stability of
the vortex under the influence of the externally applied magnetic field is
considered. The field destabilizing the vortex as well as the field when the
vortex energy is equal to the energy of a uniformly magnetized state are
calculated and presented as a function of dot geometry. The results (containing
no adjustable parameters) are compared to the recent experiment and are in good
agreement.Comment: 4 pages, 2 figures, RevTe
Effective anisotropy of thin nanomagnets: beyond the surface anisotropy approach
We study the effective anisotropy induced in thin nanomagnets by the nonlocal
demagnetization field (dipole-dipole interaction). Assuming a magnetization
independent of the thickness coordinate, we reduce the energy to an
inhomogeneneous onsite anisotropy. Vortex solutions exist and are ground states
for this model. We illustrate our approach for a disk and a square geometry. In
particular, we obtain good agreement between spin-lattice simulations with this
effective anisotropy and micromagnetic simulations.Comment: ReVTeX, 14 pages, 6 figure
A Spin-Mechanical Device for Detection and Control of Spin Current by Nanomechanical Torque
We propose a spin-mechanical device to control and detect spin currents by
mechanical torque. Our hybrid nano-electro-mechanical device, which contains a
nanowire with a ferromagnetic-nonmagnetic interface, is designed to measure or
induce spin polarized currents. Since spin carries angular momentum, a spin
flip or spin transfer process involves a change in angular momentum--and hence,
a torque--which enables mechanical measurement of spin flips. Conversely, an
applied torque can result in spin polarization and spin current.Comment: 6 pages, 2 figure
Tverberg-type theorems for intersecting by rays
In this paper we consider some results on intersection between rays and a
given family of convex, compact sets. These results are similar to the center
point theorem, and Tverberg's theorem on partitions of a point set
Easy collective polarization switching in ferroelectrics
The actual mechanism of polarization switching in ferroelectrics remains a
puzzle for many decades, since the usually estimated barrier for nucleation and
growth is insurmountable ("paradox of the coercive field"). To analyze the
mechanisms of the nucleation we consider the exactly solvable case of a
ferroelectric film with a "dead" layer at the interface with electrodes. The
classical nucleation is easier in this case but still impossible, since the
calculated barrier is huge. We have found that the {\em interaction} between
the nuclei is, however, long range, hence one has to study an {\em ensemble} of
the nuclei. We show that there are the ensembles of small (embryonic) nuclei
that grow {\em without the barrier}. We submit that the interaction between
nuclei is the key point for solving the paradox.Comment: 5 pages, REVTeX 3.1 with one eps-figure. Corrected discussion of
single stripe and cylindrical nuclei, and their interaction. The estimate for
equilibrium density of embryonic nuclei is added. To appear in Phys. Rev.
Letter
Nanostratification of optical excitation in self-interacting 1D arrays
The major assumption of the Lorentz-Lorenz theory about uniformity of local
fields and atomic polarization in dense material does not hold in finite groups
of atoms, as we reported earlier [A. E. Kaplan and S. N. Volkov, Phys. Rev.
Lett., v. 101, 133902 (2008)]. The uniformity is broken at sub-wavelength
scale, where the system may exhibit strong stratification of local field and
dipole polarization, with the strata period being much shorter than the
incident wavelength. In this paper, we further develop and advance that theory
for the most fundamental case of one-dimensional arrays, and study nanoscale
excitation of so called "locsitons" and their standing waves (strata) that
result in size-related resonances and related large field enhancement in finite
arrays of atoms. The locsitons may have a whole spectrum of spatial
frequencies, ranging from long waves, to an extent reminiscent of ferromagnetic
domains, -- to super-short waves, with neighboring atoms alternating their
polarizations, which are reminiscent of antiferromagnetic spin patterns. Of
great interest is the new kind of "hybrid" modes of excitation, greatly
departing from any magnetic analogies. We also study differences between
Ising-like near-neighbor approximation and the case where each atom interacts
with all other atoms in the array. We find an infinite number of "exponential
eigenmodes" in the lossless system in the latter case. At certain "magic"
numbers of atoms in the array, the system may exhibit self-induced (but linear
in the field) cancellation of resonant local-field suppression. We also studied
nonlinear modes of locsitons and found optical bistability and hysteresis in an
infinite array for the simplest modes.Comment: 39 pages, 5 figures; v2: Added the Conclusions section, corrected a
typo in Eq. (5.3), corrected minor stylistic and grammatical imperfection
Conservation of miRNA-mediated silencing mechanisms across 600 million years of animal evolution
Our current knowledge about the mechanisms of miRNA silencing is restricted to few lineages such as vertebrates, arthropods, nematodes and land plants. miRNA-mediated silencing in bilaterian animals is dependent on the proteins of the GW182 family. Here, we dissect the function of GW182 protein in the cnidarian Nematostella, separated by 600 million years from other Metazoa. Using cultured human cells, we show that Nematostella GW182 recruits the CCR4-NOT deadenylation complexes via its tryptophan-containing motifs, thereby inhibiting translation and promoting mRNA decay. Further, similarly to bilaterians, GW182 in Nematostella is recruited to the miRNA repression complex via interaction with Argonaute proteins, and functions downstream to repress mRNA. Thus, our work suggests that this mechanism of miRNA-mediated silencing was already active in the last common ancestor of Cnidaria and Bilateria
Superconductivity in the Ferroquadrupolar State in the Quadrupolar Kondo Lattice PrTiAl
The cubic compound PrTiAl is a quadrupolar Kondo lattice system
that exhibits quadrupolar ordering due to the non-Kramers ground
doublet and has strong hybridization between and conduction electrons. Our
study using high-purity single crystal reveals that PrTiAl exhibits
type-II superconductivity at mK in the nonmagnetic
ferroquadrupolar state. The superconducting critical temperature and field
phase diagram suggests moderately enhanced effective mass of
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