130 research outputs found
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
Magnetization reversal in exchange-spring bilayer system under circularly polarized microwave field
Microwave assisted magnetization reversal are studied in the bulk bilayer
exchange coupled system. We investigate the nonlinear magnetization reversal
dynamics in a perpendicular exchange spring media using Landau-Lifshitz
equation. In the limit of the infinite thickness of the system, the propagation
field leads the reversal of the system. The reduction of the switching field
and the magnetization profile in the extended system are studied numerically.
The possibility to study the dynamics analytically is discussed and an
approximation where two P-modes are coupled by an interaction field is
presented. The ansatz used for the interaction field is validated by comparison
with the numerical results. This approach is shown to be equivalent to two
exchange coupled macrospins
Study of the first paramagnetic to ferromagnetic transition in as prepared samples of Mn-Fe-P-Si magnetocaloric compounds prepared by different synthesis routes
International audienceMagnetocaloric materials with composition of Mn 1.3 Fe 0.65 P 0.5 Si 0.5 have been prepared by ball milling and solid-state reaction methods and consolidated using powder annealing, and conventional and spark plasma sintering. Magnetic and calorimetric measurements show remarkable differences upon first cooling, and slight differences on second and further coolings between the samples prepared by different synthesis routes. Further measurements using Hall probe imaging in high magnetic field have been also carried out. As-prepared samples have been cooled down just above the critical temperature, and the first phase transition has been induced by application of a magnetic field. Bulk samples show staircase isothermal magnetization curves whereas powders show smoother transition curves
Nitrogenation and sintering of (Nd-Zr)Fe10Si2 tetragonal compounds for permanent magnets applications
International audienceNd(1-x)Zr(x)Fe10Si2 alloys have been prepared in the tetragonal ThMn12-type structure by arc-melting and melt-spinning and then nitrogenated to improve their magnetic properties. For x = 0.4 and 0.6 the Curie temperature and magnetic anisotropy fields increase from 280-300 ºC to about 390 ºC and from 2.8-3 T to 4.5-5 T respectively. The saturation magnetization remains almost unchanged. The nitrogenated powders were processed by spark plasma sintering (SPS) leading to compact pellets, which retain the full nitrogen content and magnetic properties up to 600 ºC, but segregated Fe-Si at elevated temperatures. Nitrogenation and SPS processing are, therefore, appropriate for sintering metastable materials such as (Nd,Zr)Fe10Si2 into compact material without loosing functional properties. This opens a way towards a new family of permanent magnets, lean of critical raw materials
Synthesis and characterization of core-shell structure silica-coated Fe29.5Ni70.5 nanoparticles
In view of potential applications of magnetic particles in biomedicine and
electromagnetic devices, we made use of the classical Stober method
base-catalysed hydrolysis and condensation of tetraethoxysilane (TEOS) to
encapsulate FeNi nanoparticles within a silica shell. An original stirring
system under high power ultrasounds made possible to disperse the otherwise
agglomerated particles. Sonication guaranteed particles to remain dispersed
during the Stober synthesis and also improved the efficiency of the method. The
coated particles are characterized by electron microscopy (TEM) and
spectroscopy (EDX) showing a core-shell structure with a uniform layer of
silica. Silica-coating does not affect the core magnetic properties. Indeed,
all samples are ferromagnetic at 77 K and room temperature and the Curie point
remains unchanged. Only the coercive force shows an unexpected non-monotonous
dependence on silica layer thickness.Comment: Regular paper submited to international peer-reveiwed journa
High-performance liquid chromatographic enantioseparation of unusual amino acid derivatives with axial chirality on polysaccharide-based chiral stationary phases
The successfulenantioseparationofaxiallychiralaminoacidderivativescontainingacyclohexylidene moiety onan an alyticalandsemipreparativescalewasachievedforthefirsttimebyHPLCusing polysaccharide-based chiral stationary phases. Racemicmethyl N-benzoylamino esters,easilyobtained by methanoly sisof the corresponding 5(4H)-oxazolones, were subject edtochiralHPLCresolutionusing chiral stationaryphasesbasedonimmobilized3,5-dimethylphenylcarbamatederivativesofamylose (Chiralpak® IA column)orcellulose(Chiralpak® IB column). The behaviour of both selectors under dif-ferent elution conditions was evaluated and compared. The amy los ecolumns howed better performance than the cellulose column for allenantiomers tested. These mi preparative resolution of axially chiral amino acidderivatives with different side chains has been achievedona 250mm×20mmIDChiralpak® IA columnusingtheappropriatemixtureof n-hexane/chlorofom/ethanol aseluent by successive injec-tions of asolution of the sample in chloroform. Using this protocol up to 120 m go feachenan tiomer of the correspondingaxiallychiralaminoacidderivativewereobtainedfrom300mgofracemate.[(Sa)-2a, 105 mg;(Ra)-2a, 60mg,[(Sa)-2b, 105mg;(Ra)-2b, 90mg,[(Sa)-2c, 120mg;(Ra)-2c, 100mg]
Thermal energy harvesting: thermomagnetic versus thermoelectric generator
International audienceWe compare the efficiency and the power density of thermoelectric and thermomagnetic generators at maximum power. The performances of thermomagnetic generator are computed using an equation of state, either extrapolated from experimental data for 2nd order transition or deduced using a phenomenological Landau model on measured data for 1st order transition. The performances of thermoelectric generator are computed using the Onsager model. Moreover, the heat exchange in finite time is estimated using a simple model of thermal conductance. According to the results, thermomagnetic generator is more efficient and have slightly higher power density than thermoelectric for temperature difference lower than 10 K. Therefore low grade heat thermal energy harvesting could consider thermomagnetic generator
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