1,175 research outputs found
Temperature dependent magnetization dynamics of magnetic nanoparticles
Recent experimental and theoretical studies show that the switching behavior
of magnetic nanoparticles can be well controlled by external time-dependent
magnetic fields. In this work, we inspect theoretically the influence of the
temperature and the magnetic anisotropy on the spin-dynamics and the switching
properties of single domain magnetic nanoparticles (Stoner-particles). Our
theoretical tools are the Landau-Lifshitz-Gilbert equation extended as to deal
with finite temperatures within a Langevine framework. Physical quantities of
interest are the minimum field amplitudes required for switching and the
corresponding reversal times of the nanoparticle's magnetic moment. In
particular, we contrast the cases of static and time-dependent external fields
and analyze the influence of damping for a uniaxial and a cubic anisotropy.Comment: accepted by Journal of Physics: Condensed Matte
Wireless-based identification and model updating of a skewed highway bridge for structural health monitoring
Vibration-based monitoring was performed on a short-span skewed highway bridge on the basis of wireless measurements. By means of operational modal analysis, highly accurate modal results (frequencies and mode shapes) were extracted by using a self-developed wireless acquisition system, for which the performance was verified in the field. In order to reproduce the experimental modal characteristics, a refined finite element model was manually tuned to reduce the idealization errors and then updated with the sensitivity method to reduce the parametric errors. It was found that to build a reliable Finite element (FE) model for application in structural health monitoring, the effects of superelevation and boundary conditions of a skewed bridge should be taken into account carefully
Observation of a Griffiths-like phase in the paramagnetic regime of ErCo_2
A systematic x-ray magnetic circular dichroism study of the paramagnetic
phase of ErCo2 has recently allowed to identify the inversion of the net
magnetization of the Co net moment with respect to the applied field well above
the ferrimagnetic ordering temperature, Tc. The study of small angle neutron
scattering measurements has also shown the presence of short range order
correlations in the same temperature region. This phenomenon, which we have
denoted parimagnetism, may be related with the onset of a Griffiths-like phase
in paramagnetic ErCo2. We have measured ac susceptibility on ErCo2 as a
function of temperature, applied field, and excitation frequency. Several
characteristics shared by systems showing a Griffiths phase are present in
ErCo2, namely the formation of ferromagnetic clusters in the disordered phase,
the loss of analyticity of the magnetic susceptibility and its extreme
sensitivity to an applied magnetic field. The paramagnetic susceptibility
allows to establish that the magnetic clusters are only formed by Co moments as
well as the intrinsic nature of those Co moments
Kinetic Modelling of ELM-induced Fast-ion Transport and Acceleration in the ASDEX Upgrade Tokamak
Prospects for studying the solar CNO cycle by means of a lithium neutrino detector
Lithium detectors have a high sensitivity to CNO neutrinos from the Sun. The
present experimental data and prospects for future experiments on the detection
of CNO neutrinos are discussed. A nonstationary case is considered when the
flux of 13N neutrinos is higher than the standard solar model predicts due to
the influx of fresh material from the peripheral layers to the solar core.Comment: 12 pages, 5 figures, a thoroughly revised version, reported at
International Symposium "Physics of Massive Neutrinos" at MILOS (Greece)
19-23 May 200
Anisotropic Superparamagnetism of Monodispersive Cobalt-Platinum Nanocrystals
Based on the high-temperature organometallic route (Sun et al. Science 287,
1989 (2000)), we have synthesized powders containing CoPt_3 single crystals
with mean diameters of 3.3(2) nm and 6.0(2) nm and small log-normal widths
sigma=0.15(1). In the entire temperature range from 5 K to 400 K, the
zero-field cooled susceptibility chi(T) displays significant deviations from
ideal superparamagnetism. Approaching the Curie temperature of 450(10) K, the
deviations arise from the (mean-field) type reduction of the ferromagnetic
moments, while below the blocking temperature T_b, chi(T) is suppressed by the
presence of energy barriers, the distributions of which scale with the particle
volumes obtained from transmission electron microscopy (TEM). This indication
for volume anisotropy is supported by scaling analyses of the shape of the
magnetic absorption chi''(T,omega) which reveal distribution functions for the
barriers being also consistent with the volume distributions observed by TEM.
Above 200 K, the magnetization isotherms M(H,T) display Langevin behavior
providing 2.5(1) mu_B per CoPt_3 in agreement with reports on bulk and thin
film CoPt_3. The non-Langevin shape of the magnetization curves at lower
temperatures is for the first time interpreted as anisotropic
superparamagnetism by taking into account an anisotropy energy of the
nanoparticles E_A(T). Using the magnitude and temperature variation of E_A(T),
the mean energy barriers and 'unphysical' small switching times of the
particles obtained from the analyses of chi''(T,omega) are explained. Below T_b
hysteresis loops appear and are quantitatively described by a blocking model,
which also ignores particle interactions, but takes the size distributions from
TEM and the conventional field dependence of E_A into account.Comment: 12 pages with 10 figures and 1 table. Version accepted for
publication in Phys. Rev. B . Two-column layou
Landau-Zener tunnelling in dissipative circuit QED
We investigate the influence of temperature and dissipation on the
Landau-Zener transition probability in circuit QED. Dissipation is modelled by
coupling the transmission line to a bath of harmonic oscillators. The reduced
description for the density operator is treated within Bloch-Redfield theory. A
phase-space representation allows an efficient numerical implementation of the
resulting master equation. It provides reliable results which are valid even
for rather low temperatures. We find that the spin-flip probability as a
function of temperature and dissipation strength exhibits a non-monotonic
behaviour. Our numerical results are complemented by analytical solutions for
zero temperature and for vanishing dissipation strength.Comment: Accepted for publicatio
Compact \u3cb\u3e-300 kV\u3c/b\u3e dc Inverted Insulator Photogun With Biased Anode and Alkali-Antimonide Photocathode
This contribution describes the latest milestones of a multiyear program to build and operate a compact â300ââkV dc high voltage photogun with inverted insulator geometry and alkali-antimonide photocathodes. Photocathode thermal emittance measurements and quantum efficiency charge lifetime measurements at average current up to 4.5 mA are presented, as well as an innovative implementation of ion generation and tracking simulations to explain the benefits of a biased anode to repel beam line ions from the anode-cathode gap, to dramatically improve the operating lifetime of the photogun and eliminate the occurrence of micro-arc discharges
Lithium abundances and extra mixing processes in evolved stars of M67
Aims. We present a spectroscopic analysis of a sample of evolved stars in M67
(turn-off, subgiant and giant stars) in order to bring observational
constraints to evolutionary models taking into account non-standard transport
processes. Methods. We determined the stellar parameters (Teff, log g, [Fe/H]),
microturbulent and rotational velocities and, Lithium abundances (ALi) for 27
evolved stars of M67 with the spectral synthesis method based on MARCS model
atmospheres. We also computed non-standard stellar evolution models, taking
into account atomic diffusion and rotation-induced transport of angular
momentum and chemicals that were compared with this set of homogeneous data.
Results. The lithium abundances that we derive for the 27 stars in our sample
follow a clear evolutionary pattern ranging from the turn-off to the Red Giant
Branch. Our abundance determination confirms the well known decrease of lithium
content for evolved stars. For the first time, we provide a consistent
interpretation of both the surface rotation velocity and of the lithium
abundance patterns observed in an homogeneous sample of TO and evolved stars of
M67. We show that the lithium evolution is determined by the evolution of the
angular momentum through rotation-induced mixing in low-mass stars, in
particular for those with initial masses larger than 1.30 M_\odot at solar
metallicity.Comment: 13 pages, 8 figure
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