Las Rozas de Madrid, Spain
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Perez Alcazar G.A.,University of Valle | Zamora L.E.,University of Valle | Tabares J.A.,University of Valle | Piamba J.F.,University of Valle | And 4 more authors.
Journal of Magnetism and Magnetic Materials | Year: 2013

Powders of melted disordered Fe50Mn10Al40 alloy were separated at different mean particle sizes as well as magnetically and structurally characterized. All the samples are BCC and show the same nanostructure. Particles larger than 250 μm showed a lamellar shape compared to smaller particles, which exhibited a more regular form. All the samples are ferromagnetic at room temperature and showed reentrant spin-glass (RSG) and superparamagnetic (SP)-like behaviors between 30 and 60 K and 265 and > 280 K, respectively, as a function of frequency and particle size. The freezing temperature increases with increasing particle size while the blocking one decreases with particle size. The origin of these magnetic phenomena relies in the internal disordered character of samples and the competitive interaction of Fe and Mn atoms. The increase of their critical freezing temperature with increasing mean particle size is due to the increase of the magnetic dipolar interaction between the magnetic moment of each particle with the field produced by the other magnetic moments of their surrounding particles. © 2012 Elsevier B.V.


Zamora L.E.,University of Valle | Perez Alcazar G.A.,University of Valle | Tabares J.A.,University of Valle | Romero J.J.,CSIC - Institute of Ceramics and Glass | And 3 more authors.
Physica B: Condensed Matter | Year: 2012

Melted Fe 50Mn 10Al 40 alloy powder with particle size less than 40 μm was characterized at room temperature by XRD, SEM and XPS; and at low temperatures by Mössbauer spectrometry, ac susceptibility, and magnetization analysis. The results show that the sample is BCC ferromagnetic but with a big contribution of paramagnetic sites, and presents super-paramagnetic and re-entrant spin-glass phases with critical temperatures of 265 and 35 K, respectively. The presence of the different phases detected is due to the disordered character of the sample and the competitive magnetic interactions. The obtained values of the saturation magnetization and the coercive field as a function of temperature present a behavior which indicates a ferromagnetic phase. However, the behavior of the FC curve and that of the coercive field as a function of temperature suggest that the dipolar magnetic interaction between particles contributes to the internal magnetic field in the same way as was reported for nanoparticulate powders. © 2012 Elsevier B.V. All rights reserved.


Flores M.,Micromag | Flores M.,Institute Magnetismo Aplicado | Calo A.,Micromag | Gorriti A.,Micromag | And 5 more authors.
Journal of Electromagnetic Waves and Applications | Year: 2014

Composite materials consisting of amorphous magnetic microwires added to a matrix of paint have been proven to be very useful for electromagnetic absorption of microwave frequencies. The main goal of this article is to calculate the values of the electrical permittivity and the magnetic permeability of several such composites depending on the microwire mass fraction added. In order to analyze this dependency of electromagnetic parameters on microwire mass fraction, three composites with mass fractions 2, 3, and 4%, respectively, have been prepared. The values of the electromagnetic parameters have been extracted by S-parameter measurements using a rectangular waveguide at band X. Three different methods have been applied in the extraction of the electromagnetic parameter values: Nicolson-Ross-Wier, Impedances, and Optimization. It has been found that the real part of the electrical permittivity linearly increases with the mass fraction of microwires, while the imaginary component does not follow the same linear trend. Regarding the electrical permeability, the values calculated for both the real and imaginary components of composites with different concentrations are too similar to extract any relevant dependency. © 2013 © 2013 Taylor & Francis.


Crespo P.,Institute Magnetismo Aplicado | Crespo P.,Complutense University of Madrid | De La Presa P.,Institute Magnetismo Aplicado | De La Presa P.,Complutense University of Madrid | And 11 more authors.
Journal of Physics Condensed Matter | Year: 2013

This paper reviews the effect of organic and inorganic coatings on magnetic nanoparticles. The ferromagnetic-like behaviour observed in nanoparticles constituted by materials which are non-magnetic in bulk is analysed for two cases: (a) Pd and Pt nanoparticles, formed by substances close to the onset of ferromagnetism, and (b) Au and ZnO nanoparticles, which were found to be surprisingly magnetic at the nanoscale when coated by organic surfactants. An overview of theories accounting for this unexpected magnetism, induced by the nanosize influence, is presented. In addition, the effect of coating magnetic nanoparticles with biocompatible metals, oxides or organic molecules is also reviewed, focusing on their applications. © 2013 IOP Publishing Ltd.


Garcia-Escorial A.,CSIC - National Center for Metallurgical Research | Lieblich M.,CSIC - National Center for Metallurgical Research | Hernando A.,Institute Magnetismo Aplicado | Aragon A.,Institute Magnetismo Aplicado | Marin P.,Institute Magnetismo Aplicado
Journal of Alloys and Compounds | Year: 2012

In this work, the dependence of the coercive field of Fe 73.5Si 13.5B 9Nb 3Cu 1 gas atomized powder with the temperature for different particle sizes has been studied, observing an anomalous behavior in the under 25 powder particle size fraction. This unusual behavior is related with the microstructure of the powder, and is attributed to the presence of a multiphase magnetic system, with non-magnetic regions decoupling the ferromagnetic domains. © 2011 Elsevier B.V. All rights reserved.


Marin P.,Institute Magnetismo Aplicado | Aragon A.M.,Institute Magnetismo Aplicado | Escorial A.G.,CSIC - National Center for Metallurgical Research | Lieblich M.,CSIC - National Center for Metallurgical Research | And 2 more authors.
Journal of Applied Physics | Year: 2013

Fe73.5Si13.5B9Nb3Cu1 powder particles have been obtained by gas atomization. Magnetization curves and coercivity were studied for particles ranging in size up to 1000 μm. The overall magnetic behavior of such material is a consequence of compositional heterogeneity of the microstructure as a whole. Anomalous temperature variation of coercivity (Hc) (i.e., a decrease in Hc with decreasing temperature) together with a decrease of saturation magnetization has been observed for less than 25 μm size. The origin of this behavior has been ascribed to metastable FeCu and FeNbSi phases in combination with an Fe-rich one. Making magnetic powders with coercive fields of the order of mOe remains a challenge for researchers. Our experiment has allowed us, at low temperature, achieving a coercive field of 9 Oe, much lower than those observed so far in this type of materials. This behaviour has been related with a FeCu phase present on grain boundaries. © 2013 American Institute of Physics.


Garcia-Escorial A.,CSIC - National Center for Metallurgical Research | Lieblich M.,CSIC - National Center for Metallurgical Research | Lopez M.,Institute Magnetismo Aplicado | Marin P.,Institute Magnetismo Aplicado
Journal of Alloys and Compounds | Year: 2011

Powder particles of Fe73.5Si13.5B9Nb 3Cu1 and Fe97Si3 soft magnetic alloys have been prepared by gas atomization. The gas atomized powder was microstructurally characterized and the dependence of coercivity with the composition and powder particle size investigated. As-atomized powder particles of both compositions were constituted by a bcc α-Fe (Si) solid solution. The Fe73.5Si13.5B9Nb3Cu1 powder particles presented a grain microstructure with dendrite structure, which dendrite arms were enriched in Nb. The coercivity increased as the particle size decreased, with a minimum coercivity, of 5 Oe, measured in the Fe 97Si3 alloy in the range of 50-100 μm powder particle size. The coercive fields were quite higher in the Fe73.5Si 13.5B9Nb3Cu1 than in the Fe 97Si3 powder, due to the Nb addition, which produced a phase segregation that leads to a noticeable magnetic hardening. © 2010 Elsevier B.V. All rights reserved.

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