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Santander, Spain

Barquin L.F.,CITIMAC | Martinez A.Y.,CITIMAC | Fernandez L.R.,SCTI SERMET | Rojas D.P.,Charles III University of Madrid | And 7 more authors.
Journal of Nanoscience and Nanotechnology | Year: 2012

The structural and magnetic properties of a collection of nanoparticles coated by Poly(methyl methacrylate) through a wet chemical synthesis have been investigated. The particles display either an amorphous (M=Fe, Co) M-B arrangement or a mixed structure bcc-Fe and fcc-Co+amorphous M-B. Both show the presence of a metal oxi-hydroxide formed in aqueous reduction. The organic coating facilitates technological handling. The cost-effective synthesis involves a reduction in a Poly(methyl methacrylate) aqueous solution of iron(II) or cobalt(II) sulphates (<0 .5 M) by sodium borohydride (<0 .5 M). The particles present an oxidized component, as deduced from X-ray diffraction, Mössbauer and Fe-and Co K-edge X-ray absorption spectroscopy and electron microscopy. For the ferrous alloys, this Fe-oxide is α-goethite, favoured by the aqueous solution. The Poly(methyl methacrylate) coating is confirmed by Fourier transform infrared spectroscopy. In pure amorphous core alloys there is a drastic change of the coercivity from bulk to around 30 Oe in the nanoparticles. The mixed structured alloys also lie in the soft magnetic regime. Magnetisation values at room temperature range around 100 emu/g. The coercivity stems from multidomain particles and their agglomeration, triggering the dipolar interactions. Copyright © 2012 American Scientific Publishers. Source

Marcos C.,University of Oviedo | Arguelles A.,University of Oviedo | Khainakov S.A.,University of Oviedo | Rodriguez Fernandez J.,CITIMAC | Blanco J.A.,University of Oviedo
Journal of Physics Condensed Matter | Year: 2012

We report on the magnetic properties of a Ni 2+-vermiculite intercalation compound from Santa Olalla, Huelva (Spain). This modified vermiculite was studied by means of DC and AC magnetic measurements. The existence of two maxima in magnetic susceptibility below 10K was interpreted in terms of the Cole-Cole formalism as being due to spin-glass freezing in this material. The temperature, frequency and external magnetic field dependences of these anomalies located at temperatures around 2-3K and 8-10K in the imaginary part of the magnetic susceptibility, χ, seem to suggest the existence of spin-relaxation phenomena between the magnetic moments of the Ni 2+ ions. A dynamic study of the relaxation processes associated with these phenomena considering the Cole-Cole formalism allows us to interpret the anomaly found at 2-3K according to a law of activated dynamics, obtaining values for the critical exponent, ψν<1, characteristic of a d=2 spin-glass-like system, while the maximum observed in χ at 8-10K can be described by means of a law of standard dynamics with a value of the exponent z of around 5, representative of a d=3 spin-glass-like system. © 2012 IOP Publishing Ltd. Source

Arguelles A.,University of Oviedo | Khainakov S.A.,University of Oviedo | Rodriguez-Fernandez J.,CITIMAC | Leoni M.,University of Trento | And 2 more authors.
Physics and Chemistry of Minerals | Year: 2011

Herein we report on synthetic iron-intercalated vermiculites prepared from a Mg-vermiculite mineral from Santa Olalla, Huelva, Spain, by means of an ion exchange process from aqueous solutions of FeCl2 and FeCl3. Thermogravimetric, X-ray diffraction, magnetic susceptibility and Mössbauer spectral studies have been used to characterize the synthetic iron-intercalated vermiculites. The results suggest that the intercalation process employed induces modifications in both the interlayer spacing and in the octahedral sheet; the disordered structure of the Mg-vermiculite mineral is not altered. The presence of solvated Fe(H2O)n 2+ ions has been shown by the Mössbauer spectroscopy. No magnetic order has been observed between 2 and 300 K neither in the Mg-vermiculite mineral nor in the two synthetic iron-intercalated vermiculites. © 2011 Springer-Verlag. Source

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