Understanding magnetic force microscopy

Magnetic force microscopy is a new method for imaging ferromagnetic domains with a high lateral resolution (10 nm). In this paper we give the basic tip parameters that have to be taken into account to achieve a quantitative image interpretation. For the electrochemically otched polycrystalline iron, nickel and cobalt wires, the tip-apex domain is found to be oriented along the tip axis, because of shape anisotropy. The stray field emerging from the tip apex is comparable to the size of the tip saturation field. The effective domain lengthL determines the image formation: the force due to magnetization patterns of scales which are large compared toL follow the point-dipole approximation. In the opposite case, a single-pole model is more appropriate. While a cobalt tip can be treated as an isolated domain, for nickel and iron a net polarization in the tip wire induced by the front apex-domain has to be considered. A new analytical theory provides an overall understanding of the image formation and allows the determination of the magnetic field vector and the estimation of its magnitude from measurements.     

Influence of corona on strike probability of grounded electrodes by high voltage discharges

This paper presents the results of an investigation into the influence of corona formation processes at the tips of grounded rod electrodes on the probability of those rods being “struck” by high-voltage discharges. Experiments simulating the final stage of the attachment process were carried out with a composite voltage comprising a simultaneously applied impulse and DC potential of different levels to grounded rod electrodes simulating lightning rods, featuring either a spherical or pointed (conical) tip. The experiments show that corona does not influence the probability of the electrode being struck until a critical electric field (EF) strength is reached.     

3D atom probe assisted by femtosecond laser pulses

The 3DAP allows to image a material in 3D on a nearly atomic scale. It is based on the field evaporation occurring at the surface of a biased tip like shape specimen with an end radius of 50 nm. Surface atoms are removed one by one from the tip by means of fs laser pulses so that the physical process involved in this laser enhanced field evaporation might correspond to the very early stages of the ablation process. This technique makes possible to distinguish between different regimes of material removal such as thermal evaporation or in the case of metals or semiconductors an evaporation assisted by the rectification of the optical field at the surface. In this paper the principle of the 3DAP is presented and the underlying physics involved in the field evaporation assisted by femtosecond laser pulses is discussed.     

Investigation of the structural state of potassium polytitanate replaced by iron

Investigations were carried out for iron ion-substituted potassium polytitanates (PPTs). The mechanism of iron incorporation into the interlayer space of the PPTs, the chemical purity of the polytitanate powders, and the particle morphology were studied. The replacement of potassium ions in the polytitanate structure was experimentally observed and the degree of this substitution was numerically evaluated at different stages of preparation. It was shown that the iron incorporation into the interlayer space of polytitanate was accompanied by the formation of lepidocrocite, which follows from Mössbauer spectroscopic data. It was found that in the iron ions incorporation process used in the experiment polytitanate particles agglomerate, which may be associated with the formation of lepidocrocite.     

Phase separation in binary systems with internal multiplicative noise

A study of a phase separation process in stochastic systems with a field dependent kinetic coefficient and an internal multiplicative noise is presented. Dynamics of spinodal decomposition at early and late stages is investigated by computer simulations where the domain growth law is generalized. A mean field approach was carried out in order to obtain the stationary probability, bifurcation and phase diagrams displaying reentrant phase transitions. We relate our approach to entropy driven phase transitions theory.     

Magnetization reversal of single domain permalloy nanowires

Magnetization reversal process and magnetoresistance (MR) hysteresis of single domain permalloy nanowires are numerically investigated by using OOMMF. It is shown that the abrupt jumps in the magnetoresistance are due to the domain formation and domain wall propagation so that a magnetic domain suddenly switches from one state into another. A nonmonotonic angular dependence of the jump (switching) field is found. Coherent rotation mode is responsible for the smooth variation of MR curves. The nucleation pattern of newly born domains depends on the tilted angle of external field.     

Field evaporation of silicon surfaces

Quantitative measurements on field evaporation of Si(111) surfaces in hydrogen imaging gas have been carried out by field ion microscopy. The field evaporation rate is found to increase exponentially with increase of the reciprocal of tip temperature in the range 80–103 K. The evaporation field strength increases with increase of tip temperature in the investigated range, 80–300 K. Within the applied pressure range, 5× 10^?6 to 2 × 10^?4 Torr of hydrogen gas, the evaporation rate linearly increases with the gas pressure. Similar effects of temperature and gas pressure on field evaporation of Si(111) surfaces have been observed also in silane imaging gas. A model, based on a field-induced formation of surface hydrides as a rate-determining step, is proposed, which accounts for all the experimental results of the field evaporation process.     

Second-harmonic generation of semiconductor quantum dots studied by near-field optical microscopy

We present a theoretical study of optical second-harmonic generation(SHG) of symmetric semiconductor quantum dots (QDs) excited by the near field of the tip in a near-field scanning optical microscope. We show that the usual optical transition selection rules for the SH nonlinear interaction between the tip field and the QD are broken when the tip is scanned over the QD, because the tip field varies rapidly over the QD domain. It is also demonstrated that the tip-position dependence of the SH signal essentially maps the spatial distribution of the tip field.     

Microwave plasma CVD-grown graphene–CNT hybrids for enhanced electron field emission applications

The growth and electron emission characteristics were investigated from a hybrid structure of multiwalled carbon nanotubes (MWCNTs) and multilayer layer graphene (MLG) deposited on silicon substrate coated with iron catalyst and an interlayer of aluminium. The hybrid structures were synthesized in a two-step process by microwave plasma-enhanced chemical vapour deposition technique. The formation of MWCNTs takes place by absorption and precipitation of carbon radicals into the catalyst particles. Thereafter, ample carbon forms MLG on tip of the MWCNTs resulting in a MLG-MWCNTs hybrid nanostructure. MLG was observed to grow branching out of the tips and sidewalls of the MWCNTs and is expected to attach by Van der Walls bonds. Transmission electron microscopy and micro-Raman spectroscopy confirmed the crystalline nature of the hybrid structures. Electron emission studies were carried out using a diode-type field emission setup. The enhancement factor was found to be ~3,500 for bare MWCNTs, ~4,070 to ~5,000 for hybrid structures and ~6,500 for N-doped MLG-MWCNTs hybrid structures. Modification in the defects structure and enhancement of emission sites are suggested to be responsible for the increase of the field emission characteristics.     

基于区域分解算法的叶顶间隙流场的数值模拟

本文采用Marini－Quarteroni的不重叠区域分解算法和“镶嵌”式间隙网格技术,开发了可计算叶项间隙流场的N－S方程求解程序。通过计算结果同实验显示结果的比较表明,本文的计算方法不仅可以很好地描述叶顶间隙流动的细节,而且在复杂区域流场计算中是非常有效的。本文指出在叶栅存在间隙时,前缘产生什么样的拓扑结构不仅取决于相对间隙的大小,同时也取决于叶栅的负荷情况。     

The role of nucleation field of media in heat-assisted magnetic probe recording

We characterize a method of heat-assisted magnetic probe recording on perpendicular media. Heating source is field emission current from a scanning tunneling microscope (STM) tip. Recording media are three kinds of magnetic films, Co/Pt, CoNi/Pt, and Co/Pd multilayers with different nucleation fields. Pulses with amplitude of 5 V were applied between the STM tip and the recording medium. Experiments show that magnetic marks with an average size of 180 nm were formed on both Co/Pt and CoNi/Pt films whose nucleation fields are greater than their saturation magnetization. No marks were observed on the Co/Pd film whose nucleation field is smaller than its saturation magnetization. A model is built to simulate the dynamic process of domain formation in probe-based magnetic recording system. Simulation results agree with experiments and it explains the effect of the nucleation field of medium in perpendicular recording.     

叶尖小翼对扩压叶栅气动特性影响的数值研究

通过在叶片顶端加装小翼来降低叶顶二次流的叶尖小翼技术在叶轮机械领域受到关注。本文对具有不同叶尖小翼方案的压气机叶栅进行了全三维数值模拟,并详细分析了叶尖小翼对叶顶间隙流场的影响.结果表明,合理选择叶尖小翼的安装位置及自身宽度可以在一定程度上降低叶顶泄漏损失,在叶顶吸力面侧加装宽度为5 mm的小翼可以较好的削弱泄漏流动的强度,减少泄漏涡卷吸起更多的吸力面/端壁角区的低能流体及较早地阻止上通道涡的形成和发展。     

Ferroelectric domain breakdown

We observe a stringlike domain penetration from a ferroelectric surface deep into the crystal bulk induced by a high voltage atomic force microscope tip. The domains, which resemble channels of an electrical breakdown, nucleate under an electric field of around 10(7) V/cm at the ferroelectric surface, and grow throughout the crystal bulk where the external electric field is practically zero. A theory explaining the shape of the formed domains is presented. It shows that the driving force for the domain breakdown is the decrease of the total free energy of the system with increasing domain length.     

Axially symmetrical domain structures in garnet ferrite films

The process of domain structure formation and evolution after magnetization of a small area of a garnet ferrite film to saturation was examined with the use of modified two-step high-speed photography. It was found that the radial deformation caused by magnetizing coil breaks the uniaxial anisotropy of the specimen and results in formation of the axially oriented stripe domain structure inside the magnetization reversal area. The period of this domain structure decreases with the increasing amplitude of the pulsed field. The formation of the axially oriented stripe domain structure occurs under the axially symmetrical magnetostatic field.     

Transient response in flexure to general uni-directional loads of variable cross-section beam with concentrated tip inertias immersed in a fluid

This paper presents a method for solving problems of transient response in flexure due to general unidirectional dynamic loads of beams of variable cross section with tip inertias. An elastodynamic theory which includes effects of continuous mass and rigidity of the beam has been applied. In the analysis the general dynamic load is expanded into a Fourier series and the beam is divided into many small uniform thickness segments. The equation of motion of each segment is mapped onto the complex domain by use of the Laplace transform method. The solutions of each set of adjoining segments are related to each other at the boundaries by the use of the transfer matrix method. The displacement, the bending slope, the bending moment and the shearing force at each boundary and at arbitrary time are obtained from the Laplace transform inversion integral by using the residue theorem. The theoretical results given in this paper are applicable to problems of dynamic response due to arbitrary loads varying with time of beams of arbitrary shape with concentrated tip inertias. As applications of the present theoretical results, numerical calculations have been carried out for two cases: a uniform beam with a tip inertia and a non-uniform beam (a truncated cone) with a tip inertia. Both are immersed in a fluid and subjected to large waves such as cnoidal waves.     

Nonlinear constitutive behavior of ferroelectric materials

The ferroelectric specimen is considered as an aggregation of many randomly oriented domains. According to this mechanism, a multi-domain mechanical model is developed in this paper. Each domain is represented by one element. The applied stress and electric field are taken to be the stress and electric field in the formula of the driving force of domain switching for each element in the specimen. It means that the macroscopic switching criterion is used for calculating the volume fraction of domain switching for each element. By using the hardening relation between the driving force of domain switching and the volume fraction of domain switching calibrated, the volume fraction of domain switching for each element is calculated. Substituting the stress and electric field and the volume fraction of domain switching into the constitutive equation of ferroelectric material, one can easily get the strain and electric displacement for each element. The macroscopic behavior of the ferroelectric specimen is then directly calculated by volume averaging. Meanwhile, the nonlinear finite element analysis for the ferroelectric specimen is carried out. In the finite element simulation, the volume fraction of domain switching for each element is calculated by using the same method mentioned above. The interaction between different elements is taken into account in the finite element simulation and the local stress and electric field for each element is obtained. The macroscopic behavior of the specimen is then calculated by volume averaging. The computation results involve the electric butterfly shaped curves of axial strain versus the axial electric field and the hysteresis loops of electric displacement versus the electric field for ferroelectric specimens under the uniaxial coupled stress and electric field loading. The present theoretical prediction agrees reasonably with the experimental results. Supported by the National Natural Science Foundation of China (Grant No. 10572138)     

Influence of Template Geometry on Polymer Micro-Structure Duplication in Electrohydrodynamics Patterning Process

Electrohydrodynamics patterning is a process for electrically duplicating micro–nano patterns on polymer films by applying an electrical field. An investigation into the hydraulic behavior of the polymer as affected by the template-modulated electrical field is critical for understanding the process capability and template design. A coupling model is presented to characterize the influence of template geometry on the pattern duplication. The transient simulation is carried out to study the formation of a pattern. It is proposed that the electrical field modulated by the template can be divided into a spatially nonhomogeneous component which generates the periodic pillars undesirably and a spatially nonhomogeneous component which generates a duplication of the template pattern as desired. We find that the template geometry has a significant effect on the template-modulated electrohydrodynamics. A high-fidelity duplication of pattern is better suited to a low projection ratio, a decreased template spacer height, and an increased pattern aspect ratio.     

Field-induced realignment of a smectic nanodroplet in an external magnetic field: A numerical investigation

The field-induced realignment of a smectic-A phase is in principle a complicated process involving the director rotation via the interaction with the field and the layer rotation via the molecular interactions. Time-resolved X-ray scattering experiments have revealed major phenomena concerning the maintenance of the integrity of the smectic-A layer structure during the alignment process. In order to obtain a deeper insight into this process, we have carried out a dissipative particle dynamics study of the realignment kinetics of a nanodroplet of a smectic-A liquid crystal suspended in an isotropic fluid following a switch in the direction of an applied magnetic field. The strength of the mesogen-field interaction is small compared to the inter-molecular interactions. The reaction of the smectic configuration to the field switch was found to depend on the balance between the inter-molecular interactions stabilising the formation of the smectic layering and the interaction of the mesogens with the external field. It is found that the rotational behaviour of the smectic layers under the influence of an external magnetic field arises from a combination of stochastic translational displacements and rotational motions of the centres of mass of the mesogens in the nanodroplets. The simulations indicate that X-ray scattering and NMR experiments monitoring the orientational order are sensitive to different aspects of the realignment process.     

Domain wall thickness variations of ferroelectric BaMgF4 single crystals in the tip fields of an atomic force microscope

The ferroelectric domain wall thickness of a fluoride BaMgF₄ single crystal was investigated by piezoresponse force microscopy. It was found that the domain wall thickness shows a strong spatial variation in the as‐grown crystal and the polarization reversal process. The original wall thickness is greater (about two to seven times) than that switched by the tip fields of the atomic force microscope. A significantly narrower domain wall was obtained in the higher tip‐field. The trapped defects at the domain wall play an important role in the spatial variation of the polarization width of 180° domain wall in the BaMgF₄ single crystal. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

风力机叶尖加小翼流场的试验研究

叶尖加小翼可以提高风力机的功率系数.本文在风洞开口试验段利用PIV技术,对旋转风力机叶尖加装V型小翼时叶片和小翼周围的流场进行了实验研究.比较加小翼和不加小翼时的流场图,发现加上小翼后,叶尖部分的流场得到了改善,小翼对风力机的主要影响范围是从截面r/R=0.86到1.04之间,约占主叶片长度的14%左右.