1,634 research outputs found
Effect of the Si/B ratio on the magnetic anisotropy distribution of Fe73.5Si22.52xBxCu1Nb3 „x57,9,16… alloys along nanocrystallization
The effect of the Si/B ratio on the magnetic anisotropy distribution of Fe73.5Si22.52xBxCu1Nb3
(x57,9,16) alloys has been studied. The influence of isochronal annealing on the hysteresis loop of
the three studied alloys has been analyzed. They present two minima in coercivity: the first one can
be ascribed to structural relaxation, and the second one is related to the averaging of the
magnetocrystalline anisotropy, as predicted by the random anisotropy model. The relative
importance of both minima changes with Si content: the lower the Si content, the more effective the
structural relaxation and the less important the second minimum are. The mean value of the
magnetic anisotropy distribution presents a similar behavior, evidencing the growing importance of
the magnetoelastic anisotropy for the relaxed amorphous samples as the Si content is increased.
From the evolution of the magnetic anisotropy distribution along nanocrystallization and the
microstructural information obtained from transmission electron microscopy images, the behavior
of the coercivity minima with changes in Si content can be ascribed to a different degree in
compensation of magnetoelastic anisotropy due to the contributions of different signs coming from
the nanocrystals and the amorphous matrixCICYT. Gobierno Español-MAT95-0961-CO2-0
Dipole–dipole interaction in superparamagnetic nanocrystalline Fe63.5Cr10Si13.5B9Cu1Nb3
Cr-substituted Finemet-type nanocrystalline alloy (Fe63.5Cr10Si13.5B9Cu1Nb3) has been studied by
differential scanning calorimetry, x-ray diffraction, Mo¨ssbauer spectroscopy, and magnetic
measurements. The Curie temperature of the remaining amorphous phase decreases as the
crystalline volume fraction increases, reaching values below room temperature. This feature makes
the alloy adequate for studying the magnetic decoupling of the ~Fe,Si! nanocrystals at moderated
temperatures and, in particular, the superparamagnetic relaxation in broad temperature and
crystalline fraction ranges. It was shown that the anomalous dependence of the coercive field on the
annealing temperature can be satisfactorily explained assuming a dipolar-type interaction between
the crystallites.DGES del Gobierno español-PB97-1119-CO2-01Fondo de investigación del gobierno húngaro-OTKA T-030753Academia de Ciencias de Hungría-AKP 98-25 2,
X-ray Structure of a (o-ga/acto-Pentaacetoxypentyl)pyrazoline
1-(3,5-Dimethyl-3-nitro-1-pyrazolin-4-yl) penta-0-acetyl-o-galacto-pentitol, e20H29N 3012, Mr= 503·5, orthorhombic, P21212¡, a= 14·471 (9), b =
14·518 (3), e= 12·028 (2) A, V= 2527·0 (1·7) A3, z
= 4, Dx = 1·323 Mg m- 3, .A(Mo Ka)= 0·7107 A, J.L
= 0·1O mm- 1, F(OOO) = 1064, room temperature,
final wR = 0·035 for 1836 observed reflexions. Bond distances and ang1es are all within the expected ranges. The pyrazoline ring exhibits approximate mirror symmetry and a puckering amplitude of
0·244 (6) A. Torsion angles of the acetoxy groups
with the sugar chain range from 33 to 72°. Crystal
cohesion is mainly due to van der Waals interactions
but there are two intermolecular hydrogen bonds
o108-2701/89/1o1563-03$03.00 linking molecules related by screw axes along [100] and [001]
Influence of Milling Time on the Homogeneity and Magnetism of a Fe70Zr30 Partially Amorphous Alloy: Distribution of Curie Temperatures
In this work, the mechanically alloyed Fe70Zr30 (at. %) composition has been used to study the influence of milling time on its homogeneity and magnetic properties. The microstructure and Fe environment results show the formation of an almost fully amorphous alloy after 50 h of milling in a mixture of pure 70 at. % Fe and 30 at. % Zr. The soft magnetic behavior of the samples enhances with the increase of the milling time, which is ascribed to the averaging out of the magnetocrystalline anisotropy as the crystal size decreases and the amorphous fraction increases. The formation of a non-perfectly homogenous system leads to a certain compositional heterogeneity, motivating the existence of a distribution of Curie temperatures. The parameters of the distribution (the average Curie temperature, T C ¯ , and the broadening of the distribution, ∆ T C ) have been obtained using a recently reported procedure, based on the analysis of the approach towards the saturation curves and the magnetocaloric effect. The decrease of ∆ T C and the increase of T C ¯ with the milling time are in agreement with the microstructural results. As the remaining α-Fe phase decreases, the amorphous matrix is enriched in Fe atoms, enhancing its magnetic response.AEI/FEDER-UE Project MAT 2016-77265-
Dependence of exchange anisotropy and coercivity on the Fe–oxide structure in oxygen-passivated Fe nanoparticles
Ultrafine Fe particles have been prepared by the inert gas condensation method and subsequently oxygen passivated. The as-obtained particles consist in an Fe core surrounded by an amorphous Fe-oxide surface layer. The antiferromagnetic character of the Fe-oxide surface induces an exchange anisotropy in the ferromagnetic Fe core when the system is field cooled. Samples have been heat treated in vacuum at different temperatures. Structural changes of the Fe–O layer have been monitored by x-ray diffraction and transmission electron microscopy. Magnetic properties as coercivity, hysteresis loop shift, and evolution of magnetization with temperature have been analyzed for different oxide crystallization stages. A decrease of the exchange anisotropy strength is reported as the structural disorder of the surface oxide layer is decreased with thermal treatment
On the use of JMAK theory to describe mechanical amorphization: a comparison between experiments, numerical solutions and simulations
The kinetics of amorphization during ball milling is generally analyzed using two different
approaches: the classical Johnson-Mehl-Avrami-Kolmogorov (JMAK) theory and Delogu and Cocco’s
model for which a region deterministically transforms after it reaches a certain number of collisions.
The application of JMAK analysis to the latter model predicts Avrami exponents to be higher than
the experimental ones (typically close to one). We develop simulations based on the probabilistic
character of the nucleation phenomenon and concave growth of the amorphous phase in the core
of a nanocrystal. The predictions of our simulations are in good agreement with the low Avrami
exponents and with the size evolution of the remaining crystallites found experimentally. From these
values, the parameters involved in the simulated model (growth rate and probability of nucleation)
can be estimated.AEI/FEDER-UE (Project MAT-2016-77265-R)Junta de Andalucía (Grupo PAI
Dependence of exchange anisotropy and coercivity on the Fe–oxide structure in oxygen-passivated Fe nanoparticles
Ultrafine Fe particles have been prepared by the inert gas condensation method and subsequently oxygen passivated. The as-obtained particles consist in an Fe core surrounded by an amorphous Fe-oxide surface layer. The antiferromagnetic character of the Fe-oxide surface induces an exchange anisotropy in the ferromagnetic Fe core when the system is field cooled. Samples have been heat treated in vacuum at different temperatures. Structural changes of the Fe–O layer have been monitored by x-ray diffraction and transmission electron microscopy. Magnetic properties as coercivity, hysteresis loop shift, and evolution of magnetization with temperature have been analyzed for different oxide crystallization stages. A decrease of the exchange anisotropy strength is reported as the structural disorder of the surface oxide layer is decreased with thermal treatment
Microstructure and Magnetic Properties of Mo Containing Nanoperm-Type Alloys
The influence of composition changes on the devitrification process, microstructure and
magnetic properties of (Fe,Co)MoB(Cu) alloys was investigated. Amorphous and nanocrystalline
samples were studied by differential scanning calorimetry, thermomagnetic gravimetry, X-ray diffraction, transmission electron microscopy and vibrating sample magnetometry and obtained results are correlated. Cu addition increases the thermal stability and the crystalline fraction of the
nanocrystalline microstructure and improves the soft magnetic properties of the alloy. As B content
increases the thermal stability of the nanocrystalline microstructure decreases and for the 20 at.%
B alloy boride crystals are formed during the first crystallization stage. Saturation magnetization
decreases and Curie temperature of the amorphous phase increases with B content. Co addition
lowers the volume fraction of nanocrystals and as Co content increases in the alloy nanocrystals
are enriched by Co, Curie temperature of the amorphous phase increases, linearly with Co content, and saturation magnetization and coercive field of nanocrystalline alloys increase as well.Comisión Interministerial de Ciencia y Tecnología de España (CICYT) Project MAT 2004-04618Junta de Andalucía. Procesos Asistenciales Integrados (PAI
Cellular automata simulations on nanocrystallization processes: From instantaneous growth approximation to limited growth
Cellular automata simulations have been performed to simulate the crystallization process
under a limited growth approximation. This approximation resembles several characteristics
exhibited by nanocrystalline microstructures and nanocrystallization kinetics. Avrami exponent
decreases from a value n = 4 indicating interface controlled growth and constant nucleation
rate to a value n ~ 1 indicating absence of growth. A continuous change of the growth
contribution to the Avrami exponent from zero to 3 is observed as the composition of the
amorphous phase becomes richer in the element present in the crystalline phase.Ministerio de Ciencia e Innovación de España (MICINN) y Unión Europea FEDER. MAT2010-20537Junta de Andalucía. Procesos Asistenciales Integrados (PAI) FQM-646
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