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.
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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
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
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
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
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
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TL;DR: In this paper, the electrical resistance was measured in applied magnetic fields up to 5 T over the temperature range of 55 −155 K and the temperature dependence of resistivity showed a semiconductor behavior without discontinuity at Verwey point, while the magnetoresistance (MR) exhibited an extremum in its temperature dependence and correspondingly the magnetization manifested a discontinuous decrease, with temperature decreasing through VerWEy point.
11 citations
01 Jan 1992
TL;DR: In this paper, the formation of Mo particles by sputtering in Ar, at high pressures, was studied and the size, shape, and agglomeration of the particles is a strong function of the Ar pressure.
Abstract: Studies were performed of the formation of Mo particles by sputtering in Ar, at high pressures. The size, shape, and agglomeration of the particles is a strong function of the Ar pressure. At the Ar threshold pressure, 150 mTorr, where particle formation in the vapor is first observed the particle size distribution is bimodal. Both the small and large particles were cubes. The large cubes are a self-arrangement of the small cubes. The voltage threshold for sputtering is approximately independent of pressure for pressures less than 220 mTorr. The sputtering rate decreased with increasing pressure above the threshold. There are two competing effects on particle formation. The decreased sputtering rate with increasing pressure makes particle formation more difficult. The decreased mean free path facilitates particle formation.
3 citations