Publications (5)11.08 Total impact
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Dataset: Control of crystallite orientation and size in Fe and FeCo nanoneedles Supplementary data
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Article: Shape anisotropy and exchange bias in magnetic flattened nanospindles with metallic/oxide core/shell structures.
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ABSTRACT: A preliminary study of the magnetic phenomenology of Fe and Fe90Co10 nanospindles with axial ratio equal to 5 is presented. These nanospindles are constituted by single-domains single-crystals coated by oxide surface layer and assembled in chains into the nanospindle. The thermal dependence of the coercive field and the saturation magnetization in the temperature range from 4 K up to room temperature indicates that the coercive field is roughly proportional to the saturation magnetization (which follows the T3/2 Bloch law) at temperatures above the blocking temperature of the oxide. This suggests that the predominant source of magnetic anisotropy in this temperature range is the shape anisotropy. However, at temperatures below the oxide blocking temperature, the magnetic coupling between the spins of the oxide and the nanocrystals is produced at the interface. This exchange coupling enhances the effective anisotropy of the nanospindles and the coercive field increases more abruptly than the saturation of magnetization as temperature decreases.Journal of Nanoscience and Nanotechnology 09/2012; 12(9):7577-81. · 1.56 Impact Factor -
Article: Morin transition in hematite nanocrystals self-assembled into three-dimensional structures.
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ABSTRACT: The Morin transition (i.e., the first-order weak ferromagnetic (WF)/antiferromagnetic (AF) transition) in tridimensional (3D) nanoarchitectures constituted by self-organized hematite nanocrystals with controlled crystal size has been investigated. These intricate structures were prepared by the thermally induced hydrolysis of iron (III) solutions in presence of urea. The variation of the aging time from 1 hour up to 7 days leads to the formation of hematite crystal aggregates with crystallite sizes ranging between 7 and 42 nm. As the crystallite size decreases, it is observed that a superparamagnetic contribution, ascribed to the spins of the crystal surface, gains importance. This emergent contribution progressively hides the abrupt change of the magnetization associated to the Morin transition which, in turn, occurs at decreasing temperatures. The Morin transition found in the bigger particles exhibits thermal hysteresis. This fact has been tentatively explained by considering that in absence of crystal defects, the nucleation of the AF --> WF transition occurs in areas near to the outer spin layers, whereas the nucleation of the WF --> AF occurs in the inner of the crystal. In the outer spin layers, the AFM order is frustrated and therefore this transition is suppressed. In fact, the uncompensated surface spins can be magnetically coupled with the core spins at low temperatures when the sample is field cooled, inducing exchange anisotropy in the system.Journal of Nanoscience and Nanotechnology 09/2012; 12(9):7571-6. · 1.56 Impact Factor -
Article: Control of crystallite orientation and size in Fe and FeCo nanoneedles.
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ABSTRACT: Uniform magnetic nanoneedles have been prepared by hydrogen reduction of elongated nanoarchitectures. These precursors are as-prepared or cobalt-coated aggregates of highly oriented haematite nanocrystals (∼5 nm). The final materials are flattened nanoneedles formed by chains of assembled Fe or FeCo single-domain nanocrystals. The microstructural properties of such nanoneedles were tailored using renewed and improved synthetic strategies. In this fashion, the needle elongation and composition, the crystallite size (from 15 up to 30 nm), the nanocrystal orientation (with the 〈110〉 or 〈001〉 directions roughly along the long axis of the nanoneedle) and their type of arrangement (single chains, frustrated double chains and double chains) were controlled by modifying the reduction time, the axial ratio of the precursor haematite and the presence of additional coatings of aluminum or yttrium compounds. The values of the coercivity H(C) found for these nanoneedles are compared with the values predicted by the chain of spheres model assuming a symmetric fanning mechanism for magnetization reversal.Nanotechnology 05/2012; 23(22):225601. · 3.98 Impact Factor -
Article: FeCo magnetic nanoneedles obtained by Co-coating haematite
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ABSTRACT: Uniform FeCo magnetic nanoneedles of nm in width and axial ratio have been obtained by Co-coating haematite and subsequent hydrogen reduction in two steps. By this method, FeCo needle-like nanoparticles were obtained with Co contents up to 10% evenly distributed inside the particles. Higher Co contents up to 30% were incorporated by coating the Co-doped magnetite nanoneedles. No segregation of a secondary phase was observed in any case and the morphology of the particles was preserved without adding any extra element. Structural changes during the reduction process have been followed by several techniques. Although protected by an oxide layer, the final metal nanoparticles behave as single crystals, but they are composed of sub-crystals with the same crystallographic orientation and a mean diameter that decreases as the Co content increases. We found the highest reported saturation magnetization values for samples of similar size (180 emu g−1). The evolution of the coercivity with the cobalt modification could arise from the changes of the microstructure and the contributions of shape and crystalline anisotropies. Time dependence magnetization measurements showed the thermal activation to be related to the presence of crystallographic sub-units within the nanoneedles.Nanotechnology 03/2005; 16(6):647. · 3.98 Impact Factor
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Institutions
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2012
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Autonomous University of Nuevo León
San Nicolás de los Garza, Nuevo Leon, Mexico
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2005
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Instituto de Ciencia de Materiales de Madrid
Madrid, Madrid, Spain
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