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Journal ArticleDOI

Properties of nanocomposites of α-Fe and Fe3O4

01 Apr 2002-Journal of Magnetism and Magnetic Materials (North-Holland)-Vol. 246, pp 162-168
Abstract: Composites of α-Fe and Fe 3 O 4 having dimensions in the range 10-20 nm have been prepared by subjecting micrometer-sized α-Fe 2 O 3 powders to a reduction treatment in hydrogen at a temperature 683 K for a duration from 5 to 40 min. The specimens have been characterized by Mossbauer spectroscopy. The latter shows the presence of a substantial amount of superparamagnetic particles. The fraction of these particles increases as the reduction treatment is enhanced. The incorporation of hydrogen atoms during the reduction process appears to break down the precursor oxide particles. Magnetic susceptibility shows a maximum at around 125 K which arises due to Verwey transition in Fe 3 O 4 particles. Magnetization measurements on zero-field cooled and field cooled specimens indicate a blocking temperature close to 300 K. Coercivity variation with temperature shows an unusual behaviour. Exchange coupling between the ferro- and ferri-magnetic particles seems to be the reason behind this. The presence of such nearest neighbouring pairs of α-Fe/Fe 3 O 4 is confirmed by the DC resistivity data which show electron tunnelling between metal islands separated by Fe 3 O 4 granules. more

Topics: Superparamagnetism (53%), Magnetization (52%), Coercivity (51%) more
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Journal ArticleDOI
Josep Nogués1, Jordi Sort1, V. Langlais2, Vassil Skumryev1  +3 moreInstitutions (2)
01 Dec 2005-Physics Reports
Abstract: The phenomenology of exchange bias and related effects in nanostructures is reviewed. The types of systems discussed include: lithographically fabricated ferromagnetic (FM)—antiferromagnetic (AFM) nanostructures, chemically surface modified FM particles, FM particles embedded in an AFM matrix, controlled core–shell particles, nanoparticles with surface effects and coupled AFM–AFM systems. The main applications of exchange biased nanostructures are summarized. Finally, the implications of the nanometer dimensions on some of the existing exchange bias theories are briefly discussed. more

1,598 citations

Journal ArticleDOI
Abstract: The fabrication process and magnetic properties of three types of system consisting of ferromagnetic (FM) particles embedded in an antiferromagnetic (AFM) matrix are discussed. The preparation techniques are ball milling, H2 partial reduction of oxides and nanoparticle gas condensation. The magnetic properties of the FM/AFM composites are shown to depend strongly on the morphology of the system (e.g., nanoparticle size), the AFM anisotropy and the AFM-FM coupling. For example, all the studied systems exhibit coercivity enhancement below the Neel temperature of the AFM. However, while Co nanoparticles embedded in CoO exhibit loop shifts of thousands of Oe, Fe nanoparticles in Cr2O3 only show a few Oe shifts. An interesting effect evidenced in all systems is the increase of remanence (MR) which, in the case of Co-CoO, ultimately leads to an improvement of the superparamagnetic blocking temperature of the nanoparticles. more

79 citations

Journal ArticleDOI
Abstract: Ferromagnetic resonance (FMR) and ac conductivity have been applied to study a polymer composite containing as filler a binary mixture of magnetite (Fe3O4) and cementite (Fe3C) nanoparticles (30–50nm) dispersed in a diamagnetic carbon matrix, which was synthesized by the carburization of nanocrystalline iron. Ac conductivity measurements showed thermally activated behavior involving a range of activation energies and power law frequency dependence at high frequencies similar to conducting polymer composites randomly filled with metal particles. Ferromagnetic resonance measurements revealed a relatively narrow FMR line at high temperatures indicating the presence of ferromagnetic nanoparticles, where thermal fluctuations and interparticle interactions determine the FMR temperature variation. An abrupt change of the FMR spectra was observed at T<81K (ΔT⩽1K) coinciding with a sharp anomaly resolved in the temperature derivative of the ac conductivity. This behavior is attributed to the Verwey transition of F... more

62 citations

Journal ArticleDOI
Yüksel Köseoğlu1, H. Kavas1, and B. Aktaş2Institutions (2)
Abstract: Superparamagnetic nanoparticles of magnetite (Fe 3 O 4 ) 2 nm in size were produced by a co-precipitation method. Superparamagnetic resonance (SPR) spectra at room temperature show a broad line with a Lande g-factor, g eff ≈ 2. It was observed that, as the temperature decreased to 24 K, the apparent resonance field decreases while the line width considerably increases. We used a theoretical formalism based on a distribution of diameters or volumes of the nanoparticles. The nanoparticles behave as single magnetic domains with random orientations of magnetic moments which are subject to thermal fluctuations. A Landau-Lifshitz line shape function presents adequate results which are in good agreement with the experimental ones. A single set of parameters provides good fits to the spectra recorded at different temperatures. At high T the SPR line shape is governed by the core anisotropy and the thermal fluctuations. By decreasing the temperature, the magnetic susceptibility of shell spins increases. As a result of this, the surface spins produce an effective field on the core leading to a decrease of resonance field, B r . Also, the effective anisotropy increases as the shell spins begin to order. So, the results are interpreted by a simple model, in which each single-domain nanoparticle is considered as a core-shell system, with magnetocrystalline anisotropy on the core and surface anisotropy on the shell. more

52 citations

Journal ArticleDOI
Abstract: Monodisperse nanocrystalline rhombohedral composites of Fe and Fe3O4 magnetic materials have been obtained employing a reduction reaction, in a flowing gas mixture of H2 and N2, of single-crystalline, submicron-sized α-Fe2O3 rhombohedral precursors This synthesis is significant in that we were able to create a nanocomposite with hard and soft magnetic phases juxtaposed within one discrete, anisotropic structure In turn, the precursor hematite rhombohedra of reproducible shape were successfully prepared using a facile, environmentally friendly, large-scale molten-salt reaction Rhombohedra represent a high-surface-area, anisotropic formulation of an industrially important material (iron oxide), which is an active component of gas sensors, photocatalysts, and other types of catalytic materials Moreover, the predictive formation of these materials has been investigated through a systematic variation of experimental parameters Extensive structural characterization of as-prepared samples has been performed more

47 citations

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Journal ArticleDOI
TL;DR: Mise en evidence d'un interaction d'echange intercouche antiferromagnetique et d'une magnetoconductivite en saturation dans les superreseaux Co/Cr et Co/Ru. more

Abstract: We report the discovery of antiferromagnetic interlayer exchange coupling and enhanced saturation magnetoresistance in two new metallic superlattice systems, Co/Cr and Co/Ru. In these systems and in Fe/Cr superlattices both the magnitude of the interlayer magnetic exchange coupling and the saturation magnetoresistance are found to oscillate with the Cr or Ru spacer layer thickness with a period ranging from 12 \AA{} in Co/Ru to \ensuremath{\simeq}18--21 \AA{} in the Fe/Cr and Co/Cr systems. more

2,122 citations

Journal ArticleDOI
TL;DR: Giant magnetoresistance in heterogeneous thin film Cu-Co alloys consisting of ultrafine Co-rich precipitate particles in a Cu-rich matrix is observed, modeled by including spin-dependent scattering at the interfaces between the particles and the matrix, as well as the spin- dependent scattering in the Co- rich particles. more

Abstract: We have observed giant magnetoresistance in heterogeneous thin film Cu-Co alloys consisting of ultrafine Co-rich precipitate particles in a Cu-rich matrix. The magnetoresistance scales inversely with the average particle diameter. This behavior is modeled by including spin-dependent scattering at the interfaces between the particles and the matrix, as well as the spin-dependent scattering in the Co-rich particles. more

1,547 citations

Journal ArticleDOI
TL;DR: The observed isotropic giant magnetoresistance (GMR) in nonmultilayer magnetic systems using granular magnetic solids is shown to occur in magnetically inhomogeneous media containing nonaligned ferromagnetic entities on a microscopic scale. more

Abstract: We have observed isotropic giant magnetoresistance (GMR) in nonmultilayer magnetic systems using granular magnetic solids. We show that GMR occurs in magnetically inhomogeneous media containing nonaligned ferromagnetic entities on a microscopic scale. The GMR is determined by the orientations of the magnetization axes, the density, and the size of the ferromagnetic entities. more

1,434 citations

Journal ArticleDOI
Abstract: Nickel ferrite nanoparticles exhibit anomalous magnetic properties at low temperatures: low magnetization with a large differential susceptibility at high fields, hysteresis loops which are open up to 160 kOe, time-dependent magnetization in 70 kOe applied field, and shifted hysteresis loops after field cooling. We propose a model of the magnetization within these particles consisting of ferrimagnetically aligned core spins and a spin-glass-like surface layer. We find that qualitative features of this model are reproduced by a numerical calculation of the spin distribution. Implications of this model for possible macroscopic quantum tunneling in these materials are discussed. more

1,346 citations

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