Enhanced magnetic properties of FeCo ribbons nanocrystallized in magnetic field (original) (raw)
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Magnetic cobalt nanocrystals organized in patches and chains
IEEE Transactions on Magnetics, 2002
Co nanocrystals have been prepared via two different chemical preparation routes. Microstructural characterization has been carried out by conventional and high-resolution transmission electron microscopy. Magnetic characterization reveals that Co nanocrystals with mean particle size distributions about 6 nm are ferromagnetic at room temperature. Superlattices of about 1 m 1 m can be reproduced. For the second preparation route the formation of an -Co phase was detected by X-ray diffraction. Whereas the pure Co nanocrystals form domains with a mixture of simple cubic and hexagonal packing, FeCo alloyed nanoparticles form long chain-like structures.
Assemblies of cigar-shaped ferrite nanocrystals: orientation of the easy magnetization axes
Colloids and Surfaces A-physicochemical and Engineering Aspects, 2003
Mesoscopic structures made of cigar-shaped maghemite (␥-Fe 2 O 3 ) nanocrystals differing by their sizes are described. The structures were prepared by slow evaporation of a dilute suspension of nanocrystals, to which could be applied a magnetic field parallel to the substrate. If a magnetic field was applied, the nanocrystals rotated their long axis along the magnetic field direction to form ribbons whereas without a field the nanocrystals remained deposited on the substrate with a random orientation. The aligned nanocrystals is responsible of the anisotropy of the ribbons as evidence by the hysteresis loops. Moreover, a high demagnetizing field is observed when the magnetization measurements are made with an applied field normal to the ribbons. (M.P. Pileni). but none for 3D superlattices. For non-magnetic material, 3D "supra" crystals of nanoparticles are produced [3]. By controlling substrate temperature and evaporation rate "supra" crystals made of more than a thousand layers of nanocrystals are obtained. With magnetic nanocrystals, until now, it was impossible to produce "supra" crystals. However very recently [4], in our laboratory, we have been able to make large "supra" crystals. This provides great hope and a fascinating new field of physical chemistry. These artificial structures can be manipulated to achieve tailored materials for application and for exploration of physical phenomena, as the magnetic properties of this new class of materials should, as with dense films, differ both from those of individual nanoparticles and bulk materials.
Finite size and surface effects on the magnetic properties of cobalt ferrite nanoparticles
Journal of Nanoparticle Research, 2011
Cobalt ferrite, CoFe 2 O 4 , nanoparticles in the size range 2-15 nm have been prepared using a non-aqueous solvothermal method. The magnetic studies indicate a superparamagnetic behavior, showing an increase in the blocking temperatures (ranging from 215 to more than 340 K) with the particle size, D TEM . Fitting M versus H isotherms to the saturation approach law, the anisotropy constant, K, and the saturation magnetization, M S , are obtained. For all the samples, it is observed that decreasing the temperature gives rise to an increase in both magnetic properties. These increases are enhanced at low temperatures (below *160 K) and they are related to surface effects (disordered magnetic moments at the surface). The fit of the saturation magnetization to the T 2 law gives larger values of the Bloch constant than expected for the bulk, increasing with decreasing the particle size (larger specific surface area). The saturation magnetization shows a linear dependence with the reciprocal particle size, 1/D TEM , and a thickness of 3.7 to 5.1 Å was obtained for the non-magnetic or disordered layer at the surface using the dead layer theory. The hysteresis loops show a complex behavior at low temperatures (T B 160 K), observing a large hysteresis at magnetic fields H [ *1000 Oe compared to smaller ones (H B *1000 Oe). From the temperature dependence of the ac magnetic susceptibility, it can be concluded that the nanoparticles are in magnetic interaction with large values of the interaction parameter T 0 , as deduced by assuming a Vogel-Fulcher dependence of the superparamagnetic relaxation time. Another evidence of the presence of magnetic interactions is the almost nearly constant value below certain temperatures, lower than the blocking temperature T b , observed in the FC magnetization curves.
Magnetocaloric effect in FeCr soft magnetic nanocrystalline alloys
Journal of Magnetism and Magnetic Materials, 2007
In this work the magnetocaloric effect in a Fe 63.5 Cr 10 Si 13.5 B 9 CuNb 3 soft magnetic nanocrystalline alloy is analysed. High resolution transmission electron microscocopy indicates the precipitation of the desired nanocrystalline structure (grains around 10 nm in size surrounded by a residual amorphous phase) upon suitable treatments of the initial amorphous sample. The temperature dependence of the magnetic entropy variation, DS M ðTÞ, calculated from the magnetization curves displays a maximum negative value around the Curie temperature of the residual amorphous phase (T C;a % 180 K). The dependence of DS M ðTÞ on the applied field and its evolution with the nanocrystalline volume fraction indicates the main contribution of the residual amorphous phase to the magnetic entropy change. Some specific low temperature magnetization features (spin freezing) are also analysed in terms of DS M ðTÞ. r
Mechano-Synthesis and Characterization of Fe-Co Based Nanocrystalline Magnetic Materials
Materials Science Forum, 2005
Nanostructured Fe-Co based alloys are believed to be good candidates for imparting improved magnetic behavior in terms of higher permeability, lower coercivity, reduced hysteresis loss and higher Curie temperatures. In the present work, Fe-Co alloys with Ni additions were prepared using mechanical alloying (MA). Grain size and internal strain in the MA powders was measured using X-ray diffraction. It has been shown that the grain size could be reduced down to less than 5 nm in these alloys. Nanocrystalline materials thus obtained were also evaluated for magnetic behavior and the influence of grain size and internal strain on the magnetic properties has been discussed.
IEEE Transactions on Magnetics, 2012
We have studied the assembly of cobalt ferrite nanoparticles in a magnetic field. Nanoparticles of different sizes, i.e., 5-40 nm, were synthesized with coprecipitation or under hydrothermal conditions. Their saturation magnetization values ranged between 31 and 68 Am /kg, respectively, resulting in a strong magnetic attraction and agglomeration between the nanoparticles. In aqueous ferrofluids this was prevented by the adsorption of citric acid on the nanoparticles' surfaces. The estimated interaction energies show that the DLVO (Derjaguin, Landau, Verwey, Overbeek) theory fails to explain the stability of cobalt ferrite ferrofluids and that the solvation interaction cannot be neglected. Cobalt ferrite ferrofluids were deposited on alumina substrates and dried in a magnetic field of 0.03 or 0.5 T. The homogeneity of the deposits decreased with the increasing saturation magnetization of the nanoparticles and/or with the increasing magnetic field, until columnar structures were formed from the nanoparticles with Am /kg under a magnetic field of T Index Terms-Assembly, ferrites, magnetic forces, magnetic fluids.
Surfaces and local anisotropy effect in the magnetic order of Fe–Co–B nanoparticles
Physica B: Condensed Matter, 2004
3 nm (Fe x Co 1Àx ) y B 1Ày (0pxp1; y % 0:6) nanoparticles were synthesized by chemical route. XDR and TEM measurement show the amorphous nature of the samples. Size distribution was characterized by light-scattering measurement. Magnetization vs. magnetic field measurements at room temperature show hysteresis loop for all compositions, typical of blocked single domains. In powder samples, room temperature coercive field and remanent are larger for Fe-or Co-rich composition, and saturation magnetization exhibit a minimum at x ¼ 0:15: This behaviour is related to the composition of the different local anisotropy associated to Fe and Co ions. r 2004 Published by Elsevier B.V.