Observation of laser-induced local modification of magnetic order in transition metal layers

Laser-induced local modifications of magnetic order in thin Fe-Cr layers were investigated. Local modification in the layers were induced by interfering laser beams. The results of the study give evidence for the formation of submicron-sized anisotropically shaped ferromagnetic regions with a well-defined direction of the easy magnetic axis in the interference maxima at the modification threshold. It was also found that the magnetic anisotropy of a medium is drastically reduced with changing the shapes of these local regions and distances between them. This may be due to the strengthening of the interaction between the regions through the paramagnetic matrix.     

Metastability and functional integration in anisotropically coupled map lattices

Metastability is a property of systems composed of many interacting parts wherein the parts exhibit simultaneously a tendency to function autonomously (local segregation) and a tendency to cooperate (global integration). We study anisotropically coupled map lattices and discover that for specific values of the coupling control parameters the entire system transits to a metastable regime. We show that this regime manifests a quasi-stable state in which the system can flexibly switch to another such state. We briefly discuss the relevance of our findings for information processing, functional integration, metastability in the brain, and phase transitions in complex systems.     

Spin-polarized inter-grain tunneling as the probable mechanism for low-field magnetoresistance in partially crystallized La0.5Sr0.5MnO3 thin films

Amorphous/crystalline mixed La_0.5Sr_0.5MnO₃ (LSMO) thin films on quartz wafers are prepared at different depositing temperatures using laser ablation and their low-field magnetoresistive property is investigated. It is argued that the insulating amorphous layers separating the magnetic microcrystalline grains may act as the barriers for electron tunneling. The rapid decay of magnetoresistance with increasing temperature is explained by the spin-polarized inter-grain tunneling. Given the spin-polarized inter-grain tunneling as the probable mechanism, it is believed that the spin flip during inter-grain tunneling reaches a minimum at the optimized depositing temperature of 600 °C and consequently the maximal low-field magnetoresistance is obtained. Received: 7 September 2000 / Accepted: 19 December 2000 / Published online: 23 March 2001     

Study of attempt frequency in Sharrock's formula for perpendicular recording hard disk media

Change of the attempt frequency f_o in Sharrock's formula as functions of magnetostatic and inter-grain exchange couplings has been investigated using micromagnetic simulation. It was found that f_o increases monotonically with increasing the magnetostatic coupling, whose mechanism is not understood yet. It was also found that f_o decreases initially with increasing the inter-grain exchange coupling, below a critical value and increases above it. The initial decrease is likely due to that the exchange coupling depresses the effect of the magnetostatic coupling, because the effective fields from the neighboring grains are competing with each other. The increase of f_o above the critical value is deeply related to the increase of the thermal activation volume V.     

Synthesis routes for the growth of complex nanostructures

The interest in research on inorganic colloidal nanoparticles has moved to more complex structures, such as anisotropically shaped particles and branched objects. Recently, schemes for the synthesis of heterostructures have also been presented. In this article we discuss the synthesis conditions for spherical and branched nanoparticles. The influence of parameters as temperature and composition of the mixture of surfactants on the shape of the growing particles is discussed. Also, an overview on different approaches for the formation of heterostructures is presented briefly.     

Study of magnetic properties of La2/3Sr1/3MnO3 nanotubes by Monte Carlo simulation

Magnetic properties of a single nanotube whose walls are constituted by nanograins of La_2/3Sr_1/3MnO₃ are studied by means of Monte Carlo (MC) simulation. The system is considered as composed by ferromagnetic grains which couple via dipolar interaction. The grain size distribution is obtained from experimental measurements and the inter-grain distance distribution is obtained from a study of distance distribution among magnetic particles distributed in the tube walls. We show the magnetization behavior for a tube of 700 nm of diameter and 3.5 μm length. We discuss the simulation for different temperatures and external fields. As a main result, we show that the distribution of inter-granular distances has a unique behavior for tubular structures of similar diameter/length aspect ratio, independent of their sizes. This scaling relation allows us to perform the simulations using a tube of smaller dimensions. We succeed in explaining the magnetization curves, finding that dipolar interaction is necessary to explain the experimental behavior and that the grains behave as having magnetic dead layers.     

Quadrupolar ordering of phospholipid molecules in narrow necks of phospholipid vesicles

Shapes of phospholipid vesicles that involve narrow neck(s) were studied theoretically. It is taken into account that phospholipid molecules are intrinsically anisotropic with respect to the membrane normal and that they exhibit quadrupolar orientational ordering according to the difference between the local principal membrane curvatures. Direct interactions between oriented molecules were considered within a linear approximation of the energy coupling with the deviatoric field. The equilibrium shapes of axisymmetric closed vesicles were studied by minimization of the free energy of the phospholipid bilayer membrane under relevant geometrical constraints. The variational problem was stated by a system of Euler-Lagrange differential equations that revealed a singularity in the derivative of the meridian curvature at points where the effect of the orientational ordering exactly counterbalances the effect of the isotropic bending. The system of Euler-Lagrange differential equations was solved numerically to yield consistently related equilibrium orientational distribution of the phospholipid molecules and vesicle shape. According to our estimation of the model constants the formation of the neck is promoted if direct interactions between the oriented molecules are taken into account. It was shown that the energy of the equilibrium shapes is considerably affected by the quadrupolar ordering of phospholipid molecules.     

Effect of radiation on dust particles in orbital resonances

The effect of electromagnetic radiation on the dynamics of arbitrarily shaped cosmic dust particles is investigated. The paper concentrates on the motion of dust grains near commensurability resonances with a planet—mean-motion resonances—and possible capture of the grains in the resonances. A particle is in resonance with a planet when the ratio of the mean motions of the two objects is a ratio of two small integers.

The most fundamental properties of the orbital evolution of spherical dust particles in the mean-motion resonances are shortly rederived: the solar wind effect is also included and the existing result is improved. The results for spherical particles are compared with the detailed numerical calculations for nonspherical particles. It is shown that the fundamental results valid for spherical grains do not hold, in general, for nonspherical particles. While spherical particles are always characterized by the secular decrease of the semi-major axes near mean-motion resonances, this may not be true for nonspherical particles. Nonspherical grains may exhibit an increase of the semi-major axes before capturing in the mean-motion resonances. This is caused by the effect of electromagnetic radiation on nonspherical dust grains. The eccentricities of spherical particles in the exterior resonances approach a limiting value, but nonspherical grains may not follow this behaviour. The interior resonances are characterized by a systematic decrease of eccentricity for spheres, but various behaviours exist in the case of irregularly shaped particles.

The motion of a nonspherical dust particle under the action of electromagnetic radiation may be characterized by a small change of the semi-major axis during a long-time interval, but the particle is not captured in any mean-motion resonance. This kind of motion does not exist for spherical grains.     

Nanomagnetism in nanocrystalline multiferroic bismuth ferrite lead titanate films

Multiferroics conventionally refer to the materials exhibiting co-existing electric, magnetic, and structure order parameters. Interplay between ferroelectricity, magnetism, and ferroelasticity in a single phase makes multiferroics truly multifunctional providing control over magnetic and electric ordering by applying electric and magnetic fields, respectively. Incorporation of multiferroic-based components into nanoscale applications will enable additional degrees of freedom in manipulating with spin and charge not easily attainable otherwise. Multiferroic bismuth ferrite lead titanate has been chemically synthesized in form of nanocrystalline films. The morphology of the films revealed a single perovskite phase confined within crystalline grains of few tens of nm in size. The films were found to exhibit ferroelectricity and ferromagnetism with characteristic electric polarization and magnetization hysteresis loops, transformations associated with spin reorientation in an external magnetic field and the spin-glassy behavior well above the room temperature. High degree of magnetic frustration and disorder in the spin system spatially confined in the nanograins, distribution of the grains anisotropy axis, inter-grain interactions, and the effects of uncompensated spins on the large effective surface/interface favored by the nanocrystalline morphology were assumed to be responsible for the anomalous magnetic properties and glassy dynamics in the films.     

Microstructure and magnetic properties of NdFeB alloys by co-doping alnico elements

Elements of alnico 8 are added into Nd-Fe-B alloys fabricated by rapid solidification method. It is observed that the magnetic properties at high temperature improved by small addition of alnico elements. The Curie temperature of the alloys increased from 580 K for standard alloy to 639 K by 20% addition of alnico 8 elements. The spin reorientation temperature decreased from 133 K to 104 K. The TEM analysis showed that elements of alnico 8 refine the microstructure of Nd-Fe-B ribbons. The STEM analysis confirmed the heterogeneous distribution of Nd, Fe, Cu, Al, Ni and homogeneous distribution of Ti, Nb and Co. The boundaries of nano grains contain more than 70% ferromagnetic elements, ensuring strong inter-grain coupling among the grains.     

Light-induced thermomechanical effect as a new mechanism of orientational optical nonlinearity in liquid crystals

A laser-induced thermomechanical effect in twist-aligned nematic liquid crystals has been revealed and experimentally studied. It has been proved that the nature of this phenomenon is orientational rather than diffraction. It has been shown that the orientational optical nonlinearity resulting from this effect can be as strong as the well-known giant optical nonlinearity. Moreover, giant optical nonlinearity does not exist at normal incidence of the laser beam, whereas thermomechanical nonlinearity remains of the same order of magnitude at any angle of incidence of the beam.     

A model for the packing of irregularly shaped grains

In this paper we will attempt to address the problem of the packing properties of granular materials composed of irregularly shaped grains (using configurational statistical mechanics). In particular, we will develop a model for a system of irregular grains based upon perturbing a packing of mono- or poly-disperse spheres. In the mono-disperse case we will show that the system packs less densely than a packing of perfect spheres, except when local correlations between configurations of grains are taken into account. The opposite is found to be true for a perturbation expansion based upon poly-disperse spheres. Finally we will show that for a bi-disperse packing of spheres phase segregation occurs for any size ratio and discuss whether this is to be expected.     

A review of the effects of light scattering on the dynamics of irregularly shaped dust grains in the Solar System

The various aspects of the interaction of electromagnetic radiation with cosmic dust particles are discussed. In particular, attention is paid to discrepancies between optical and physical behavior of realistically shaped particles and volume equivalent homogeneous spheres. The dynamical evolution of morphologically non-identical particles which are driven by gravity, electromagnetic radiation and the Lorentz forces can dramatically differ. Although spherical particles often enable analytical calculations, an orbital evolution of spheres cannot be considered as a representative evolution for real cosmic dust particles. The effect of electromagnetic radiation on the motion of dust grains plays a crucial role here. While irregularly shaped interstellar dust particles may be captured in the Solar System, the spherical particles will not survive due to close encounters with the Sun. Spherical grains can be captured almost only in the evaporation region (in the vicinity of the Sun), where they are destroyed due to high temperatures. The spherical dust particles ejected from comets will monotonously inspiral toward the Sun subject to the Poynting-Robertson effect. However, the non-spherical particles of the same origin may be temporarily stabilized at some heliocentric distances and thus their lifetime may be much longer than that for the Mie spheres. Some dust particles may also be captured in mean-motion resonances with planets (commensurability resonances). While spherical particles are always characterized by the secular decrease of the semi-major axes near mean-motion resonances, this may not be true for non-spherical particles. Resonant captures of arbitrarily shaped dust grains exist for exterior and interior mean-motion resonances with planets.     

Effect of particle shape on the stress dip under a sandpile

The results of an experimental investigation into the effects of particle shape on the stress dip formed under a 2D sandpile is reported. We find good agreement with previous results of a small dip for mixtures of disks poured from a localized source. The new finding is that the dip is significantly enhanced when elliptical particles are used. We attribute the amplification of the effect to orientational ordering induced by the shape of the grains which removes the degeneracy of circular particles.     

Simulation of polycrystalline 3D film growth: An investigation of the evolution of grain size and texture in diamond films

An analytical method for simulating gas phase film growth has been developed and used to study the growth of diamond films during prolonged deposition, i.e. the film thickness is much larger than the lateral grain size. From a model system composed of 10⁴ grains, reliable results can be evaluated for the growth of diamond films by (111) and (001) deposition under different initial conditions and with varying growth parameters. It is demonstrated that the rate of structure evolution is sensitively influenced by the aspect ratio of diamond crystal. A near-linear proportionality between the average grain size and the thickness of films can be approximately yielded for a large film thickness which is about 10 times of the average distance of the nuclei. The proportionality constant varies for a statistical nucleation from 0.0056 to 0.43 by changing the aspect ratio. Furthermore, the orientational distribution is drastically narrowed down so that the probability of coalescence of grains with a slight orientational difference is considerably increased. Received: 28 September 2000 / Accepted: 19 February 2001 / Published online: 3 May 2001     

Orientational states of C60 molecule in crystals

Local symmetry of orientational states of the C₆₀ molecule in crystals has been investigated. It was shown that the various orientational phase transitions in different crystals are related to different orientational orbits. The model of orientational phase transitions based on a sequence of orientational states with different symmetry properties has been suggested. We have found that both the local symmetry of C₆₀ molecule and the symmetry of its internal vibrations become higher after a reduction of crystal spatial symmetry at the phase transition. This effect is fairly common and can be observed in the orientational order-disorder phase transitions with wave vectors at the Brillouin zone boundary. Feasible manifestations of the predicted effect in various experiments are discussed. Zh. éksp. Teor. Fiz. 113, 1081–1093 (March 1998)     

Scaling of the electrical conductivity of granular media

We derive the scaling properties of the dependence of the macroscopic electrical conductivity of granular media (e.g., sands) with a surface mechanism of electrical conduction on the grain size, when the medium is subjected to a given mechanical stress. In order to eliminate the influence of the inter-grain junction capacity, the direct electrical current is considered. We determine the applicability restrictions on the theory which disregards the ultimate crushing compression strength, adhesion, and the effect of charge carrier tunneling at grain junctions beyond the contact surface area. Solutions for several regular packings of grains are obtained as well.     

预取向半晶态高分子片晶结构形成微观机理及其应力-应变响应特性的分子动力学模拟

通过分子动力学模拟方法对不同预取向聚乙烯醇熔体(polyvinyl alcohol,PVA)形成的半晶态高分子熔体形成核结晶及拉伸过程中的应力-应变响应特性进行了系统地研究.模拟结果显示预取向度高的PVA熔体对应更快的成核动力学.通过追踪全trans伸直链长度(dtt)、成核原子维诺体积(V)和中心对称参数(S)等序参量在不同取向度熔体下的等温成核与结晶演化过程,给出了PVA熔体成核路径及形成半晶态的分子构象;通过对形成的半晶态高分子结构进一步分析,发现随着熔体取向度的增加,晶体和无定型对应的取向度也会增加,但是当应变剪切大于5时,其对应的结晶度、晶体和无定型取向度不再发生变化;通过对无定型区链结构的定义与分析,可知取向度越高的熔体对应越高的Tie链数目,随着熔体取向度的增加,Loop链的数目也会减小;通过恒速拉伸应力测试可知,所形成半晶态高分子结构力学响应会随着取向度及Tie链数增加而增加,当取向及Tie链数目相同时,应力-应变曲线形状大小也基本保持一致.     

Orientational structure transformations caused by the electric-field-induced ionic modification of the interface in nematic droplets

The films of a polymer-dispersed nematic liquid crystal doped with an ionic surfactant were studied. The surface-anchoring modification effect caused by the local increase in the concentration of surface-active ions was observed at the polymer-liquid crystal interface under the action of an electric field. The modification of the boundary conditions leads to the transformation of the orientational structure of the nematic droplets and, as a result, to an appreciable change in their texture patterns and light-scattering efficiency at the interfaces. The monopolar director configurations (normal and curved) arising in the process of orientational structure transformations are considered and their typical textures are demonstrated. The possibility that the monopolar structure can be formed is theoretically substantiated by the computer simulation of the director field in a nematic droplet with the boundary conditions corresponding to the experiment.