481 research outputs found
Uniaxial anisotropy and enhanced magnetostriction of CoFeO induced by reaction under uniaxial pressure with SPS
In this study, we have compared magnetic and magnetostrictive properties of
polycrystalline CoFeO pellets, produced by three different methods,
focusing on the use of Spark Plasma Sintering (SPS). This technique allows a
very short heat treatment stage while a uniaxial pressure is applied. SPS was
utilized to sinter cobalt ferrite but also to make the reaction and the
sintering (reactive sintering) of the same ceramic composition. Magnetic and
magnetostrictive measurements show that the reactive sintering with SPS induces
a uniaxial anisotropy, while it is not the case with a simple sintering
process. The induced anisotropy is then expected to be a consequence of the
reaction under uniaxial pressure. This anisotropy enhanced the magnetostrictive
properties of the sample, where a maximum longitudinal magnetostriction of
~ppm is obtained. This process can be a promising alternative to the
magnetic-annealing because of the short processing time required (22 minutes)
Analytical modeling of demagnetizing effect in magnetoelectric ferrite/PZT/ferrite trilayers taking into account a mechanical coupling
In this paper, we investigate the demagnetizing effect in ferrite/PZT/ferrite
magnetoelectric (ME) trilayer composites consisting of commercial PZT discs
bonded by epoxy layers to Ni-Co-Zn ferrite discs made by a reactive Spark
Plasma Sintering (SPS) technique. ME voltage coefficients (transversal mode)
were measured on ferrite/PZT/ferrite trilayer ME samples with different
thicknesses or phase volume ratio in order to highlight the influence of the
magnetic field penetration governed by these geometrical parameters.
Experimental ME coefficients and voltages were compared to analytical
calculations using a quasi-static model. Theoretical demagnetizing factors of
two magnetic discs that interact together in parallel magnetic structures were
derived from an analytical calculation based on a superposition method. These
factors were introduced in ME voltage calculations which take account of the
demagnetizing effect. To fit the experimental results, a mechanical coupling
factor was also introduced in the theoretical formula. This reflects the
differential strain that exists in the ferrite and PZT layers due to shear
effects near the edge of the ME samples and within the bonding epoxy layers.
From this study, an optimization in magnitude of the ME voltage is obtained.
Lastly, an analytical calculation of demagnetizing effect was conducted for
layered ME composites containing higher numbers of alternated layers (). The
advantage of such a structure is then discussed
Direct calorimetric measurements of isothermal entropy change on single crystal W-type hexaferrites at the spin reorientation transition
We report on the magnetic field induced isothermal entropy change, \Delta
s(Ha, T), of W-type ferrite with CoZn substitution. Entropy measurements are
performed by direct calorimetry. Single crystals of the composition
BaCoZnFeO, prepared by the flux method, are measured at
different fixed temperatures under an applied field perpendicular and parallel
to the c axis. At 296 K one deduces a value of K = 8.7 \times 10^{4} J
m for the first anisotropy constant, which is in good agreement with the
literature. The spin reorientation transition temperature is estimated to take
place between 200 and 220 K
Enhancement of the magnetoelectric effect in multiferroic CoFeO/PZT bilayer by induced uniaxial magnetic anisotropy
In this study we have compared magnetic, magnetostrictive and piezomagnetic
properties of isotropic and anisotropic cobalt ferrite pellets. The isotropic
sample was prepared by the ceramic method while the sample exhibiting uniaxial
anisotropy was made by reactive sintering using Spark Plasma Sintering (SPS).
This technique permits to induce a magnetic anisotropy in cobalt ferrite in the
direction of the applied pressure during SPS process. Sample with uniaxial
anisotropy revealed a higher longitudinal magnetostriction and piezomagnetism
compared to the isotropic sample, but the transversal magnetostriction and
piezomagnetism were dramatically reduced. In the case of magnetoelectric
layered composite, the magnetoelectric coefficient is directly related to the
sum of the longitudinal and transversal piezomagnetic coefficients. These two
coefficients being opposite in sign, the use of material exhibiting high
longitudinal and low transversal piezomagnetic coefficient (or vice versa) in
ME devices is expected to improve the ME effect. Hence, ME bilayer devices were
made using isotropic and anisotropic cobalt ferrite stuck with a PZT layer. ME
measurements at low frequencies revealed that bilayer with anisotropic cobalt
ferrite exhibits a ME coefficient three times higher than a bilayer with
isotropic cobalt ferrite. We also investigated the behavior of such composites
when excited at resonant frequency
Effect of temperature and time on properties of Spark Plasma Sintered NiCuZn: Co ferrite
3 pagesInternational audienceSpark Plasma Sintering is a powerfal method to produce fine grain dense ferrite at low temperature. However, the process, usually conducted in neutral atmosphere in a carbon die, yields carbon surface deposition and reduction of Fe3+ into Fe2+. It's shown that subsequent annealing in air can remove carbon and, under some conditions, produce complete oxidation of Fe2+ ions. Regular values of the resistivity and permittivity (resp. 1 MÂm, 13 "0) are recovered for most samples annealed not higher than 750±C. Relatively high value of the permeability (up to 240) and high merit factor (μs × fr > 5 GHz) have been achieved
Le Musée d’Ethnographie du Trocadéro et ses transformations, 1878-1935 : configurations, espaces muséaux et réseaux
Fruit de la relance de la politique muséale par la troisième République et de motivations circonstancielles liées à l’Exposition Universelle de 1878, le Musée d’Ethnographie du Trocadéro est dès sa création le siège d’enjeux scientifiques, politiques et culturels qui se transforment au gré des reconfigurations de chacun des champs. Dans une approche diachronique, nous nous proposons d’examiner les activités muséales et leurs transformations, avec une attention particulière portée à la muséographie. Nous privilégions une approche croisée des développements scientifiques et muséaux pour répondre à cette problématique, angle d’étude qui devrait ainsi permettre de souligner le rôle des réseaux. Les quelques repères historiographiques que nous synthétisons ou proposons devraient permettre de rendre compte de la transformation des usages du musée au cours de son premier demi-siècle d’existence
A method to decrease the harmonic distortion in Mn-Zn ferrite/PZT and Ni-Zn ferrite/PZT layered composite rings exhibiting high magnetoelectric effects
International audienceWe have investigated the magnetoelectric (ME) effect in layered composite rings subjected to circumferential AC magnetic fields and DC magnetic fields in radial, axial or circumferential directions. Bilayer samples were obtained combining different grades of commercial Mn-Zn ferrites or Ni-Zn ferrites with commercial lead zirconate titanate (PZT). Mn-Zn ferrites with low magnetostriction saturation () and low magneto-crystalline anisotropy constants show high ME capabilities when associated with PZT in ring structures. In certain conditions, these ME effects are higher than those obtained with Terfenol-D/PZT composites in the same layered ring structure. Magnetostrictive and mechanical characterizations have given results that explain these high ME performances. Nevertheless, Mn-Zn ferrite/PZT composites exhibit voltages responses with low linearity especially at high signal level. Based on the particular structure of the ME device, a method to decrease the nonlinear harmonic distortion of the ME voltages is proposed. Harmonic distortion analysis of ME voltages measured in different configurations allows us to explain the phenomenon
Mixed reproductive strategies of the Common moorhen on a microscale as revealed by genetic data
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