Magnetostatic bias in multilayer microwires: theory and experiments

The hysteresis curves of multilayer microwires consisting of a soft magnetic nucleus, intermediate non-magnetic layers, and an external hard magnetic layer are investigated. The magnetostatic interaction between magnetic layers is proved to give rise to an antiferromagnetic-like coupling resulting in a magnetostatic bias in the hysteresis curves of the soft nucleus. This magnetostatic biasing effect is investigated in terms of the microwire geometry. The experimental results are interpreted considering an analytical model taking into account the magnetostatic interaction between the magnetic layers.Comment: 6 pages, 7 figure

Fabrication and magnetic properties of hard/soft magnetostatically coupled FePt/FeNi multilayer microwires

3 pages, 5 figures.-- PACS: 75.70.Cn; 75.60.Ej; 75.50.Ww; 81.15.Pq; 68.65.Ac; 81.40.EfA family of multilayer microwires with hard/soft biphase magnetic behavior is here introduced. The microwires consist of a Fe63Pt27Si10 hard magnetic nucleus and a Fe20Ni80 soft outer shell separated by an intermediate insulating Pyrex glass microtube. The precursor FePtSi glass-coated microwire is fabricated by quenching and drawing technique, and its L10 hard magnetic phase is grown by postannealing treatment technique. The polycrystalline FeNi soft magnetic outer shell has been deposited by electroplating. The analysis of the low-field hysteresis loops of the FeNi soft phase after premagnetizing until near magnetic saturation provides information about the magnetostatic coupling between phases. The FeNi magnetization curve is shifted toward positive field when the FePt remanent magnetization is positive and vice versa. A systematic analysis of the magnetostatic coupling and the corresponding bias field arising from uncompensated poles of the premagnetized FePt hard phase has been performed. The strength of the bias field is shown to increase with the reduction of thickness of the FeNi layer. These magnetostatically coupled biphase systems are thought to be of large potential interest as sensing elements in sensor devices.The work has been developed under the Project No. MAT2004-00150 supported by the Spanish Ministry of Education and Science, MEC. J. T. acknowledges a FPU program fellowship from MEC.Peer reviewe

Investigation of porous glass-like substrates for use in fibre-optic chemical sensors

Fibre optic chemical sensors (FOCSs) are reviewed and a general scheme of a typical sensor is given with details of the various component sections. The interest in FOCSs is presented from the stand-point of the potential advantages which they can offer, but reasons are also proffered for the hitherto small presence of these instruments in the market place. The subject is further developed by considering the types of optical sensors which have been described in the literature. Attention is focussed on the types of sensing materials which have been investigated with particular reference to pH sensing. A description is given of two opto-electronic configurations which were developed specifically for this investigation. The first consists of a fibre-optic link to a commercial spectrophotometer. The second involved the use of an argon ion laser and was constructed to illustrate some of the other design principles which feature in FOCS. The arrangement of fibres in a simple fibre optic bundle is considered and its optimum configuration is illustrated by experiment. A data acquisition program which was designed specifically for this work is outlined and, in practice, the use of this program greatly facilitated the acquisition and evaluation of data from the opto-electronic configurations described. The fluorophore fluorescein isothiocyanate (FITC) is discussed and its fluorescence properties are assessed. The suitability of FITC as a pH sensitive reagent is confirmed using the opto-electronic configurations developed for this study. The use of porous glass (PG) as a support for the analyte-sensitive reagent in FOCS is investigated in order to gain further understanding regarding the advantages and disadvantages of the use of PG for such applications. Microanalysis was used in an attempt to assess the effect of certain reaction parameters on the efficiency of each of the two steps involved in immobilising FITC onto the glass surface. The low organic content of the derivatised samples limits the overall usefulness of this analytical approach Nevertheless, a model is presented which, with some modification, can explain most of the observations concerning the two step immobilisation reaction involving 3-aminopropyltriethoxysilane (3APTSJ and FITC. It is found that refluxing in 3APTS in the first step significantly improves the loading of the fluorophore on the porous glass, but that the type of porous glass pretreatment and the concentration of the FITC solution do not result in differences which can be measured by conventional microanalysis. The fluorescence response of the derivative porous glass, both dry and in solution, was recorded. The fibre optic configuration described resulted in strong fluorescent signals but a longer than desirable response time. Generally, it was found that the fluorescence intensity of the dry PG was greater when lower concentrations of FITC had been used in the immobilisation step. The sol-gel process is introduced and its potential applicability to preparing glass-like structures which can be used in FOCS is reviewed and further explored. The main features of sol-gel processing which are potentially relevant to the preparation of a high surface area solid support for use as a sensor substrate are presented. From the literature reviewed it was not possible clearly to define an ideal sol composition which would lead to a material with properties that could best be tailored to FOCS. Nevertheless, there was much evidence that such a material could be prepared and work was conducted in order to investigate this approach with a view of the possible application of such materials in FOCS. FITC containing monoliths were prepared by two routes; impregnation of the FITC into already formed gels and the incorporation of FITC into the gel at the sol stage. The properties of these materials are discussed with respect to the applicability in FOCS and their stability, both from the point of view of a wet-dry-wet cycle and in terms of storage stability over a number of years. The preparation of FITC containing coatings is also discussed and appears to offer the best opportunity for use of this technology in FOCS

Magnetic anisotropy in ordered textured Co nanowires

The following article appeared in Applied Physics Letters 100.25 (2012): 252405 and may be found at http://scitation.aip.org/content/aip/journal/apl/100/25/10.1063/1.4729782The magnetization reversal in ordered arrays of Co nanowires with tailored hcp-phase texture, controlled by pH synthesis and nanowires length, has been investigated. The angular dependence of coercivity has been experimentally determined for different crystal textures, and the corresponding magnetization reversal mode is interpreted by analytical modelling. The results show that reversal takes place by propagation of a transverse-like domain wall mode. The fitting of experimental and calculated data allows us the quantitative evaluation of the magnetocrystalline anisotropy constant strength whose magnetization easy direction evolves from parallel to the wires toward in-plane orientation with the change of hcp-phase textureThe project has been performed under bilateral Chile-Spain project 2010CL0018. Additional financial support is acknowledged from the Spanish Ministry of Science and Innovation, MICINN, and MAT2010-20798-C05-01. Financial support from FONDECYT No. 1110784, Grant ICM P10-061-F by Fondo de Innovación para la Competitividad-MINECON and Financiamiento Basal para Centros Científicos y Tecnológicos de Excelencia, under project FB0807

Selective Laser Sintering versus Multi Jet Fusion: A Comprehensive Comparison Study Based on the Properties of Glass Beads-Reinforced Polyamide 12

Selective laser sintering (SLS) and multi jet fusion (MJF) are the most widespreadpowder bed fusion additive manufacturing techniques for fabricating polymericparts since they offer great designflexibility, productivity, and geometricalaccuracy. However, these technologies differ in the thermal energy source usedto melt the powders as well as the innovative use of printing agents featured inthe latter one to promote material consolidation and to avoid thermal bleeding atthe part contours. The use of a single powder made of glass beads-reinforcedpolyamide 12 (PA12/GB) for the fabrication of MJF and SLS samples makespossible a systematic comparison of the printed parts properties. A thoughtfulanalysis of the microstructure and mechanical properties of the samples revealsdifferences and peculiarities between the two technologies. SLS exhibits lowerporosity and higher mechanical performances when the parts are printed alongthe build plane thanks to the powerful heating ensured by the laser. In contrast,MJF samples show higher mechanical isotropy with greatflexural and tensilebehavior for vertically oriented parts. The role of glass beads in the materialbehavior is defined by their mechanical properties, meaning higher rigidity andlower strength compared to neat PA12, and fracture mechanism

Novel 3D printable bio-based and biodegradable poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) microspheres for selective laser sintering applications

Selective laser sintering (SLS) has become the most popular additive manufacturing process due to its high accuracy, productive efficiency, and surface quality. However, currently there are still very few commercially available polymeric materials suitable for this technique. This research work focused on the fabrication and characterization of bio-based and biodegradable microspheres obtained by oil-in-water emulsion solvent evaporation, starting from a poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBH) biopolymer matrix. First, the fabrication parameters were optimized to improve the morphological, thermal, and flowability properties of the synthetized microspheres. Once the best production conditions were established, the PHBH microspheres were further used to study their effective 3D printability on an SLS 3D printer using geometries varying from simple shapes to architectures with more complex internal patterns. The results of this research revealed that PHBH has promising applicability for the SLS technique. This study undertook the first step toward broadening the range of polymeric materials for this additive manufacturing technology. These findings will contribute to a greater and wider dissemination of the SLS technique in the future, as well as they will bring this manufacturing process closer to applications, such as the biomedical sector, where the use of biodegradable and biocompatible materials can add value to the final application

Growth modes of nanoparticle superlattice thin films

We report about the fabrication and characterization of iron oxide nanoparticle thin film superlattices. The formation into different film morphologies is controlled by tuning the particle plus solvent-to-substrate interaction. It turns out that the wetting vs. dewetting properties of the solvent before the self-assembly process during solvent evaporation plays a major role to determine the resulting film morphology. In addition to layerwise growth also three-dimensional mesocrystalline growth is evidenced. The understanding of the mechanisms ruling nanoparticle self-assembly represents an important step toward the fabrication of novel materials with tailored optical, magnetic or electrical transport properties

How ethnic are African parties really? Evidence from Francophone Africa

Though African party systems are said to be ethnic, there is little evidence for this claim. The few empirical studies rarely rely on individual data and are biased in favour of Anglophone Africa. This paper looks at four Francophone countries, drawing on representative survey polls. Results reveal that ethnicity matters, but that its impact is generally rather weak and differs with regard to party systems and individual parties. ‘Ethnic parties’ in the strict sense are virtually absent. In particular, the voters’ location seems more important than ethnic affiliation. Other determinants such as regional ties, elite strategies, cross-cutting cleavages, and rational preferences deserve more attention in the future study of voting behaviour in Africa

Comprehensive Modeling of Multimode Fiber Sensors for Refractive Index Measurement and Experimental Validation

We propose and develop a comprehensive model for estimating the refractive index (RI) response over three potential sensing zones in a multimode fiber. The model has been developed based on a combined ray optics, Gaussian beam, and wave optics analysis coupled to the consideration of the injected interrogating lightwave characteristics and validated experimentally through the realization of three sensors with different lengths of stripped cladding sections as the sensing region. The experimental results highly corroborate and validate the simulation output from the model for the three RI sensing zones. The sensors can be employed over a very wide dynamic RI range from 1.316 to over 1.608 at a wavelength of 1550 nm, with the best resolution of 2.2447 × 10−5 RI unit (RIU) obtained in Zone II for a 1-cm sensor length