Giant coercivity of dense nanostructured spark plasma sintered barium hexaferrite

Due to the limited rare-earth elements resources, ferrite magnets need to be improved drastically. Ideally, for a true hard magnet, the coercive field should be larger than the saturation magnetization, which is not yet realized for ferrites. Thus, an alternative can be found in making very fine grain ferrite magnets, but it is usually impossible to get small grains and dense material together. In this paper, it is shown that the spark plasma sintering method is able to produce approximately 80% of dense material with crystallites smaller than 100 nm. The as-prepared bulk sintered anisotropic magnets exhibits coercive field of 0.5 T which is approximately 60% of the theoretical limit and only a few percentage below that of loose nanopowders. As a result, the magnets behave nearly ideal (-1.18 slope in the BH plane second quadrant) and the energy product reaches 8.8 kJ m-3, the highest value achieved in the isotropic ferrite magnet to our knowledge

Giant Barkhausen jumps in exchange biased bulk nanocomposites sinterd fom core-shell Fe3O4-CoO nanoparticles

International audienceThe magnetic behavior of spark plasma sintered Fe3O4-CoO nanoparticles is studied. The samples sintered at 500°C exhibit density over 90% and average magnetite grain size about 100 nm. When the nanocomposite is field cooled below the Néel temperature (TN=291 K for CoO), hysteresis loops shows the expected shift with an exchange field of 80 mT at 100 K that drops down to zero approaching TN. The coercivity at 100 K reaches 0.4 T, ten times larger than nanostructured magnetite prepared in the same conditions. When the sample is zero field cooled down to 90 K, the hysteresis loops exhibits giant Barkhausen jumps, an anomalous feature never observed before to our knowledge. The density of jumps gradually decrease on heating and disappear between 150 and 170 K. The stochastic character of the jumps is visible in the plot of the differential permeability. This new phenomenon is thought that it could be related to self-field cooling

Enhancement of the magnetoelectric effect in multiferroic CoFe2_2O4_4/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

First vs second order magnetocaloric material for thermomagnetic energy conversion

International audienceWe estimate the power and efficiency of a thermal energy harvesting thermodynamic Brayton cycle using a first and second order magnetocaloric materials as active substance. The thermodynamic cycle was computed using a simple thermal exchange model and an equation of state deduced from a phenomenological Landau model. For the first and second order materials, narrow and high frequency cycles are optimum and give similar performances. Considering technological issues hindering the increase of frequency, we introduced a more detailed approach where we take into account the time needed to switch the material between two heat reservoirs. We show that the first order material equation of state leads thermodynamic cycle shape keeping it closer to the optimum cycle. Conditions to improve the performance of second order materials are discussed. In addition, we infer key remarks for prototype design regarding the power density and efficiency reachable in different configurations

Magnetocaloric effect at the reorientation of the magnetization in ferromagnetic multilayers with perpendicular anisotropy

We investigate the magnetocaloric effect obtained by the rotation of a magnetic field applied to an exchange-coupled multilayer system composed of two different ferromagnetic (FM) materials. We specifically consider a system in which the two FMs have perpendicular uniaxial anisotropy axes and utilise conditions which yield a reorientation of the total magnetization when compensation between the anisotropies of the two layers occurs. We calculate the consequent entropy change associated with the "artificial" reorientation. By using known parameters from MnBi and Co we predict an entropy change of $\Delta s = 0.34$ Jkg$^{-1}$K$^{-1}$ for perfect coupling. Lastly, we study the behavior of the multilayer under a rotating magnetic field via a micromagnetic model. When the layer thicknesses are of the order of the local domain wall width, the magnetic field-induced entropy change can be obtained with magnetic fields one order of magnitude lower than in the uncoupled case.Comment: 7 pages, 7 figure

Hard Ferromagnets as a New Perspective on Materials for Thermomagnetic Power Generation Cycles

We consider the ways in which magnetically hard materials can be used as the working materials in thermomagnetic power generation (TMG) cycles in order to expand the area in the magnetisation vs. applied field ($M-H$) plane available for energy conversion. There are 3 parts to this Perspective. First, experiments on commercially available hard ferrites reveal that, while these materials are not yet good TMG candidates, hard ferromagnets with higher thermal conductivity and a greater change of magnetization with temperature could outperform existing TMG materials. Second, computational results indicate that biasing a soft magnet with a hard ferromagnet is essentially equivalent to shifting the $M-H$ loop by an amount proportional to the field of the biasing magnet. Work outputs under biased conditions show a substantial improvement over unbiased cycles, but experimental verification is needed. Third, we discuss the rationale for exploring artificial spin reorientation materials as novel TMG working materials.Comment: 13 pages, 7 figure

Système de récupération d'énergie thermique à base de matériaux magnétocaloriques

International audience-Les générateurs thermomagnétiques convertissent le flux de chaleur en énergie électrique. Le matériau magnétocalorique (MMC) réalise un cycle thermodynamique entre deux sources de chaleur ce qui produit une variation d'aimantation du matériau. Cette énergie (variation d'aimantation) est ensuite transformée en énergie mécanique via les forces magnétiques et enfin en énergie électrique via un transducteur électromécanique. Le dimensionnement du cantilever permettant l'auto-oscillation du MMC entre les deux sources de chaleur nous a permis de déduire la vitesse au cours des déplacements. Ainsi à partir du modèle où le transducteur est découplé de la partie mécanique, nous avons à l'aide de simulation par éléments finis estimé l'aptitude d'un transducteur piézoélectriques (PZT 5a) et de bobines à convertir l'énergie mécanique en énergie électrique. Le système à base de piézoélectriques et de bobines récupèrent seulement 0,025 % et 0,018% respectivement de l'énergie mécanique disponible (116 mJ/cm 3). Finalement quelques pistes seront soulevées pour expliquer les faibles valeurs obtenues et les stratégies possibles pour y remédier Mots-clés-Matériaux Magnétocalorique, Thermomagnétique, Récupération d'énergie thermique, Energie

Optimizing Magnetocaloric Material for Thermomagnetic Energy Harvesting

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Finite time thermodynamics of thermomagnetic generation

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