Coupling at the interface of core/shell magnetic nanoparticles is known to be responsible for exchange bias (EB) and the relative sizes of core and shell components are supposed to influence the associated phenomenolo...
详细信息
Coupling at the interface of core/shell magnetic nanoparticles is known to be responsible for exchange bias (EB) and the relative sizes of core and shell components are supposed to influence the associated phenomenology. In this work, we have prepared core/shell structured nanoparticles with a total average diameter around ∼27 nm and a wide range of shell thicknesses through the controlled oxidation of Co nanoparticles well dispersed in an amorphous silica host. Structural characterizations give compelling evidence of the formation of Co3O4 crystallite phase at the shells surrounding the Co core. Field cooled hysteresis loops display nonmonotonous dependence of the exchange bias HE and coercive HC fields, that become maximum for a sample with an intermediate shell thickness, at which lattice strain is also maximum for both phases. The EB effects persist up to temperatures above the ordering temperature of the oxide shell. Results of our atomistic Monte Carlo simulations of particles with the same size and composition as in experiments are in agreement with the experimental observations and have allowed us to identify a change in the contribution of the interfacial surface spins to the magnetization reversal, giving rise to the observed maximum in HE and HC.
We study the magnetic properties of the adatom systems on a semiconductor surface Si(111):{C,Si,Sn,Pb}-(3×3). On the basis of all-electron density functional theory calculations we construct effective low-energy ...
详细信息
We study the magnetic properties of the adatom systems on a semiconductor surface Si(111):{C,Si,Sn,Pb}-(3×3). On the basis of all-electron density functional theory calculations we construct effective low-energy models taking into account spin-orbit coupling and electronic correlations. The Hartree-Fock simulations for the unit cell with nine correlated orbitals put forward insulating ground states with the noncollinear 120∘-Néel (for C, Si, Sn monolayer coverages) and 120∘-row-wise (for Pb adatom) antiferromagnetic orderings. The corresponding spin Hamiltonians with anisotropic exchange interactions are derived by means of the superexchange theory and the calculated Dzyaloshinskii-Moriya interactions in the systems with Sn and Pb adatoms are revealed to be very strong and compatible with the isotropic exchange couplings. To simulate the excited magnetic states we solve the constructed spin models by means of the Monte Carlo method, where at low temperatures and zero magnetic field we observe complex spin spiral patterns in Sn/Si(111) and Pb/Si(111). On this basis the formation of antiferromagnetic skyrmion lattice states at high magnetic fields in the adatom sp electron systems is discussed.
We consider a system defined as a collection of two types of components. The number of failures of each component is described as a stochastic process, with one of the processes depending on the other. None of the pro...
详细信息
We consider a system defined as a collection of two types of components. The number of failures of each component is described as a stochastic process, with one of the processes depending on the other. None of the processes is observed directly. The only available information is the number of type 1 components at risk in the system. Because of this missing data situation, different algorithms relying on an Expectation Maximization (EM) strategy are proposed to obtain the MLE of the intensity parameters for both processes so we can assess the reliability of type 1 and type 2 components. To overcome the computational limits of EM, a Monte Carlo EM (MCEM) algorithm using a metropolis procedure is presented. Stochastic EM (SEM) algorithms including a Bayesian approach are also described. The methods are applied to simulated data to demonstrate their efficiency.
In this work we suggest a novel model for automatic noise estimation and image denoising. In particular, we investigate the useful affinity which arises between statistical mechanics and image processing, and describe...
详细信息
In this work we suggest a novel model for automatic noise estimation and image denoising. In particular, we investigate the useful affinity which arises between statistical mechanics and image processing, and describe a framework from which novel denoising algorithms can be derived: Ising-like models and simulated annealing techniques. This is the first time such algorithms are used for colored images and video denoising. Results, as well as benchmarks, suggest a significant gain in PSNR and SSIM in comparison to other filters, mainly in cases of low impulse noise. When hybridizing our models with other image processing techniques they are shown to be even more effective. Their major disadvantages- high complexity and limited applicability, are also discussed. (C) 2015 Elsevier B.V. All rights reserved.
Some multiple-point-based sampling algorithms, such as the snesim algorithm, rely on sequential simulation. The conditional probability distributions that are used for the simulation are based on statistics of multipl...
详细信息
Some multiple-point-based sampling algorithms, such as the snesim algorithm, rely on sequential simulation. The conditional probability distributions that are used for the simulation are based on statistics of multiple-point data events obtained from a training image. During the simulation, data events with zero probability in the training image statistics may occur. This is handled by pruning the set of conditioning data until an event with non-zero probability is found. The resulting probability distribution sampled by such algorithms is a pruned mixture model. The pruning strategy leads to a probability distribution that lacks some of the information provided by the multiple-point statistics from the training image, which reduces the reproducibility of the training image patterns in the outcome realizations. When pruned mixture models are used as prior models for inverse problems, local re-simulations are performed to obtain perturbed realizations. Consequently, these local re-simulations lead to additional pruning in the set of conditioning data, which further deteriorates the pattern reproduction. To mitigate this problem, it is here suggested to combine the pruned mixture model with a frequency matching model. The multiple-point statistics of outcome realizations from this combined model has improved degree of match with the statistics from the training image. An efficient algorithm that samples this combined model is suggested. Finally, a tomographic cross-borehole inverse problem with prior information expressed by the combined (prior) model is used to demonstrate the effect of pattern reproducibility on the resolution of an inverse problem.
We describe a metropolis Monte Carlo algorithm for random sampling of freely reduced words equal to the identity in a finitely presented group. The algorithm samples from a stretched Boltzmann distribution pi(w) = (ve...
详细信息
We describe a metropolis Monte Carlo algorithm for random sampling of freely reduced words equal to the identity in a finitely presented group. The algorithm samples from a stretched Boltzmann distribution pi(w) = (vertical bar w vertical bar + 1)(alpha)beta(vertical bar w vertical bar) . Z(-1) where vertical bar w vertical bar is the length of a word w, alpha and beta are parameters of the algorithm, and Z is a normalizing constant. It follows that words of the same length are sampled with the same probability. The distribution can be expressed in terms of the cogrowth series of the group, which allows us to relate statistical properties of words sampled by the algorithm to the cogrowth of the group, and hence its amenability. We have implemented the algorithm and applied it to several group presentations including the Baumslag-Solitar groups, some free products studied by Kouksov, a finitely presented amenable group that is not subexponentially amenable (based on the basilica group), the genus 2 surface group, and Richard Thompson's group F.
In the present work we use the well known Monte Carlo Renormalization Group method to study the phase transition of the Baxter-Wu model in the presence of a negative external magnetic field. We find that the critical ...
详细信息
ISBN:
(纸本)9780735412873
In the present work we use the well known Monte Carlo Renormalization Group method to study the phase transition of the Baxter-Wu model in the presence of a negative external magnetic field. We find that the critical exponent v is equal to 0.80 +/- 0.02, while the ratio gamma/v is 0.40 +/- 0.05. Our results are consistent to all scaling equalities, including the hyperscaling relation, within statistical errors. The validity of two usual order-parameters is also checked.
This paper describes sufficient conditions to ensure the correct ergodicity of the Adaptive metropolis (AM) algorithm of Haario, Saksman and Tamminen [Bernoulli 7 (2001) 223-242] for target distributions with a noncom...
详细信息
This paper describes sufficient conditions to ensure the correct ergodicity of the Adaptive metropolis (AM) algorithm of Haario, Saksman and Tamminen [Bernoulli 7 (2001) 223-242] for target distributions with a noncompact support. The conditions ensuring a strong law of large numbers require that the tails of the target density decay super-exponentially and have regular contours. The result is based on the ergodicity of an auxiliary process that is sequentially constrained to feasible adaptation sets, independent estimates of the growth rate of the AM chain and the corresponding geometric drift constants. The ergodicity result of the constrained process is obtained through a modification of the approach due to Andrieu and Moulines [Ann. Appl. Probab. 16 (2006) 1462-1505].
暂无评论