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Journal ArticleDOI

Properties of nanocomposites of α-Fe and Fe3O4

TL;DR: In this article, a reduction treatment in hydrogen at a temperature of 683 K for a duration from 5 to 40 min is described, where the incorporation of hydrogen atoms during the reduction process appears to break down the precursor oxide particles.
About: This article is published in Journal of Magnetism and Magnetic Materials.The article was published on 2002-04-01. It has received 21 citations till now. The article focuses on the topics: Superparamagnetism & Magnetization.
Citations
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Journal ArticleDOI
TL;DR: The phenomenology of exchange bias and related effects in nanostructures is reviewed in this paper, where the main applications of exchange biased nanostructure are summarized and the implications of the nanometer dimensions on some of the existing exchange bias theories are briefly discussed.

1,721 citations

Journal ArticleDOI
TL;DR: In this article, the fabrication process and magnetic properties of three types of system consisting of ferromagnetic (FM) particles embedded in an antiferromagnetic matrix are discussed, and the preparation techniques are ball milling, H2 partial reduction of oxides and nanoparticle gas condensation.
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.

80 citations

Journal ArticleDOI
TL;DR: Ferromagnetic resonance (FMR) and ac conductivity measurements have been applied to study a polymer composite containing as filler a binary mixture of magnetite (Fe3O4) and cementite nanoparticles (30-50nm) dispersed in a diamagnetic carbon matrix, which was synthesized by the carburization of nanocrystalline iron.
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...

63 citations

Journal ArticleDOI
TL;DR: In this article, the authors used a theoretical formalism based on a distribution of diameters or volumes of the nanoparticles, and interpreted each single-domain nanoparticle as a core-shell system with magnetocrystalline anisotropy on the core and surface anisotropic on the shell.
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.

55 citations

Journal ArticleDOI
TL;DR: In this article, multiwalled carbon nanotubes (MWCNTs) were synthesized over natural magnetic rock as an efficient catalyst in the absence or presence of water vapor and the produced magnetic nanocomposites were employed for mercury(II) adsorption.

47 citations

References
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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.
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.

2,202 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.
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.

1,566 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.
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.

1,465 citations

Journal ArticleDOI
TL;DR: In this paper, a model of the magnetization within these particles consisting of ferrimagnetically aligned core spins and a spin-glass-like surface layer is proposed, and the qualitative features of this model are reproduced by a numerical calculation of the spin distribution.
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.

1,407 citations