27,080 research outputs found
Spin glass behavior in an interacting gamma-Fe2O3 nanoparticle system
In this paper we investigate the superspin glass behavior of a concentrated
assembly of interacting maghemite nanoparticles and compare it to that of
canonical atomic spin glass systems. ac versus temperature and frequency
measurements show evidence of a superspin glass transition taking place at low
temperature. In order to fully characterize the superspin glass phase, the
aging behavior of both the thermo-remanent magnetization (TRM) and ac
susceptibility has been investigated. It is shown that the scaling laws obeyed
by superspin glasses and atomic spin glasses are essentially the same, after
subtraction of a superparamagnetic contribution from the superspin glass
response functions. Finally, we discuss a possible origin of this
superparamagnetic contribution in terms of dilute spin glass models
FReD: The floral reflectance spectra database
Floral reflectance measurements are of great value to researchers who need consider the real colour of flowers, for example in the context of how the flowers appear to their pollinators. We have thus developed the Floral Reflectance Database (FReD) to assist these researchers, gathering together floral reflectance data in a publicly available, searchable online database. The first version of the database is now available online at "http://www.reflectance.co.uk":http://www.reflectance.co.uk. We anticipate that this resource will be of interest to researchers working on flower colour and animal vision
Aging, rejuvenation and memory effects in re-entrant ferromagnets
We have studied the slow dynamics of the ferromagnetic phases of the
re-entrant CdCr_{2x}In_{2-2x}S_4 system for 0.85<x<=1 by means of low frequency
ac susceptibility and magnetization measurements. Experimental procedures
widely used in the investigation of the out-of-equilibrium dynamics of spin
glasses (such as the x=0.85 compound) have been applied to search for aging,
rejuvenation and memory effects, and to test their dependence on the disorder
introduced by dilution of the magnetic ions. Whereas the rejuvenation effect is
found in all studied samples, the memory effect is clearly enhanced for
increasing dilutions. The results support a description of aging in both
ferromagnetic and re-entrant spin-glass phases in terms of hierarchical
reconformations of domain walls pinned by the disorder.Comment: Service de Physique de l'Etat Condense, DRECAM, DSM, CEA Saclay,91191
Gif sur Yvette Cedex, France, 9 pages, including 7 figures, To appear in Eur.
Phys. J. B (2002
Memory interference effects in spin glasses
When a spin glass is cooled down, a memory of the cooling process is
imprinted in the spin structure. This memory can be disclosed in a continuous
heating measurement of the ac-susceptibility. E.g., if a continuous cooling
process is intermittently halted during a certain aging time at one or two
intermediate temperatures, the trace of the previous stop(s) is recovered when
the sample is continuously re-heated [1]. However, heating the sample above the
aging temperature, but keeping it below Tg, erases the memory of the thermal
history at lower temperatures. We also show that a memory imprinted at a higher
temperature can be erased by waiting a long enough time at a lower temperature.
Predictions from two complementary spin glass descriptions, a hierarchical
phase space model and a real space droplet picture are contested with these
memory phenomena and interference effects.
[1] K. Jonason, E. Vincent, J. Hammann, J. P. Bouchaud and P. Nordblad, Phys.
Rev. Lett. 31, 3243 (1998).Comment: 7 pages, 1 LaTex file + 5 figures in EPS Revised version of June 17,
1999 (minor changes), to appear in EPJ B around November 9
Spin Anisotropy and Slow Dynamics in Spin Glasses
We report on an extensive study of the influence of spin anisotropy on spin
glass aging dynamics. New temperature cycle experiments allow us to compare
quantitatively the memory effect in four Heisenberg spin glasses with various
degrees of random anisotropy and one Ising spin glass. The sharpness of the
memory effect appears to decrease continuously with the spin anisotropy.
Besides, the spin glass coherence length is determined by magnetic field change
experiments for the first time in the Ising sample. For three representative
samples, from Heisenberg to Ising spin glasses, we can consistently account for
both sets of experiments (temperature cycle and magnetic field change) using a
single expression for the growth of the coherence length with time.Comment: 4 pages and 4 figures - Service de Physique de l'Etat Condense CNRS
URA 2464), DSM/DRECAM, CEA Saclay, Franc
Method of forming thin window drifted silicon charged particle detector Patent
Method of forming thin window drifted silicon charged particle detecto
Aging phenomena in spin glass and ferromagnetic phases: domain growth and wall dynamics
We compare aging in a disordered ferromagnet and in a spin glass, by studying
the different phases of a reentrant system. We have measured the relaxation of
the low-frequency ac susceptibility, in both the ferromagnetic and spin-glass
phases of a CdCr_{1.9}In_{0.1}S_4 sample. A restart of aging processes when the
temperature is lowered (`chaos-like' effect) is observed in both phases. The
memory of previous aging at a higher temperature can be retrieved upon
re-heating, but in the ferromagnetic phase it can rapidly be erased by the
growth of ferromagnetic domains. We interpret the behaviour observed in the
ferromagnetic phase in terms of a combination of domain growth and pinned wall
reconformations, and suggest that aging in spin glasses is dominated by such
wall reconformation processes.Comment: SPEC, CEA Saclay, 91191 Gif sur Yvette Cedex, France, to appear in
Europhys. Lett. (2000
Development of advanced techniques for rotorcraft state estimation and parameter identification
An integrated methodology for rotorcraft system identification consists of rotorcraft mathematical modeling, three distinct data processing steps, and a technique for designing inputs to improve the identifiability of the data. These elements are as follows: (1) a Kalman filter smoother algorithm which estimates states and sensor errors from error corrupted data. Gust time histories and statistics may also be estimated; (2) a model structure estimation algorithm for isolating a model which adequately explains the data; (3) a maximum likelihood algorithm for estimating the parameters and estimates for the variance of these estimates; and (4) an input design algorithm, based on a maximum likelihood approach, which provides inputs to improve the accuracy of parameter estimates. Each step is discussed with examples to both flight and simulated data cases
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