Iron magnetic moments in iron/silica gel nanocomposites
01 May 1990-Journal of Applied Physics (American Institute of Physics)-Vol. 67, Iss: 9, pp 4490-4492
TL;DR: In this paper, it is suggested that many of the iron atoms in the as-cured nanocomposites interact antiferromagnetically, and that the magnitude of the effect increases with the Fe concentration.
Abstract: Homogeneous gelled composites of iron and silica containing 11–40 wt. % Fe have been prepared by low‐temperature polymerization of aqueous solutions of ferric nitrate, tetraethoxysilane, and ethanol (with an HF catalyst). X‐ray diffraction, electron microscopy, Mossbauer effect, and magnetization measurements have been used to show that these bulk materials are paramagnetic composites at room temperature and remain in that state to 10 K. In this condition the Fe is present in nanometer‐sized regions and exists in ionic form (both Fe3+ and Fe2+ ). It possesses a large magnetic moment which decreases linearly from 3.9 μB/ Fe atom to 2.8 μB /Fe atom as the Fe content increased from 11% to 40%. For this composition increase, a negative Curie‐Weiss temperature was found which increased in magnitude linearly from −13 to −46 K. It is suggested that many of the iron atoms in the as‐cured nanocomposites interact antiferromagnetically, and that the magnitude of the effect increases with the Fe concentration. After ...
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TL;DR: A substantial amount of work has been carried out in the area of nanocomposite materials for optical applications as discussed by the authors, which are typically constructed by embedding an optically functional phase into a processable, transparent matrix material.
Abstract: A substantial amount of work has been carried out in the area of nanocomposite materials for optical applications. Composites are typically constructed by embedding an optically functional phase into a processable, transparent matrix material. By doing so, the optical properties can be utilized in more technologically important forms such as films and fibers. This review covers many areas of optical composite research to date. Composites with second- and third-order nonlinearities and laser amplification properties are discussed with examples from the recent literature. Other composites, including transparent magnets, may be made using similar structures. The principles used to construct these composites may have important technological applications soon and are therefore summarized in this review.
942 citations
TL;DR: A new magnetic material with appreciable optical transmission in the visible region at room temperature has been isolated as a gamma-Fe(2)O(3)/polymer nanocomposite.
Abstract: A new magnetic material with appreciable optical transmission in the visible region at room temperature has been isolated as a gamma-Fe(2)O(3)/polymer nanocomposite. The synthesis is carried out in an ion-exchange resin at 60 degrees C. Magnetization and susceptibility data demonstrate loading-dependent saturation moments as high as 46 electromagnetic units per gram and superparamagnetism for lower loadings where particle sizes are less than 100 angstroms. Optical absorption studies show that the small-particle form of gamma-Fe(2)O(3) is considerably more transparent to visible light than the single-crystal form. The difference in absorption ranges from nearly an order of magnitude in the "red" spectral region to a factor of 3 at 5400 angstroms. The magnetization of the nanocomposite is greater by more than an order of magnitude than those of the strongest room-temperature transparent magnets, FeBO(3) and FeF(3).
805 citations
TL;DR: In this paper, a review deals with a variety of nanocomposites such as sol-gel, intercalation, entrapment, electroceramic and structural ceramic types, which exhibit superior properties when compared to the monophasic or microcomposite alternatives.
Abstract: The use of nanocomposites in materials processing can lead to monophasic or multiphasic ceramics, glasses or porous materials, with tailored and improved properties. This review deals with a variety of nanocomposites such as sol–gel, intercalation, entrapment, electroceramic and structural ceramic types, which exhibit superior properties when compared to the monophasic or microcomposite alternatives. The utilization of nanocomposites in materials processing is forecasted to have a major impact in catalytic, sensor, optical, electroceramic and structural ceramic materials.
462 citations
TL;DR: In this paper, an Fe2O3−SiO2 composite was prepared by a gelation method that adopts tetraethoxysilane and iron(III) nitrate as starting materials.
Abstract: An Fe2O3−SiO2 composite was prepared by a gelation method that adopts tetraethoxysilane and iron(III) nitrate as starting materials. The dried gel was treated at increasing temperatures, and the samples were characterized by XRD, TEM, magnetic susceptibility measurements, and EPR and Mossbauer spectroscopies. Nanometer size (3−4 nm) X-ray-amorphous iron(III) oxide particles are observed in the samples treated at low temperature. These particles display superparamagnetic behavior in the Mossbauer spectra and susceptibility measurements, and their magnetic moments indicate antiferromagnetic clustering. The occurrence of two sites for iron ions, one in the bulk and one on the surface of nanoparticles, is suggested by EPR and Mossbauer spectroscopies. Heating of the samples to higher temperatures (T > 700 °C) gives rise to a small increase of the particle size. Simultaneously XRD and TEM exhibit the formation of γ-Fe2O3 crystalline particles, Mossbauer spectra reveal a large change in the magnetization, magne...
249 citations
TL;DR: In this paper, the structure and magnetic properties of a series of Fe2O3−SiO2 nanocomposites (9−33 wt % Fe 2O3), prepared by a sol−gel method and submitted to thermal treatments in the temperature range 300−900 °C, were investigated through XRD, TEM, EPR, and magnetic susceptibility measurements.
Abstract: The structure and the magnetic properties of a series of Fe2O3−SiO2 nanocomposites (9−33 wt % Fe2O3), prepared by a sol−gel method and submitted to thermal treatments in the temperature range 300−900 °C, were investigated through XRD, TEM, EPR, and magnetic susceptibility measurements. Superparamagnetic iron(III) oxide nanoparticles with a narrow size distribution, dispersed over the amorphous silica matrix, are present in all the samples. They are mostly amorphous, antiferromagnetic in the samples treated at low temperatures. At T > 700 °C, a lot of γ-Fe2O3 crystalline ferrimagnetic nanoparticles (4−6 nm) are formed, while a further increase of the temperature results in the γ- to α-Fe2O3 transformation. The variation of iron oxide content affects the abundance of γ-Fe2O3 formation, which reaches the maximum percent values in the more dilute samples. In the more concentrated samples, while the amount of maghemite is still growing, antiferromagnetic α-Fe2O3 begins to form. As a consequence, the saturation...
213 citations
References
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TL;DR: In this article, the effects of different types of catalysts on gelation time, porosity, bulk and apparent density, and volume shrinkage on drying were observed. And the authors proposed mechanisms for catalysis for the catalysts considered.
Abstract: In the sol-gel synthesis of silica, a silicon-containing raw material, a solvent, water, and a catalyst are utilized. By varying the catalyst, dramatic effects on gelation time, porosity, bulk and apparent density, and volume shrinkage on drying were observed. For example, porosities ranging from two to sixty-eight per cent can be obtained for dried and fired gels. Mechanisms for catalysis have been proposed for the catalysts considered.
497 citations
TL;DR: In this article, a series of magnetized and unmagnetized ferromagnetic materials were studied and the magnetic properties of these materials were investigated using a monochromatic beam polarization method, which was found to compare favorably with other methods with respect to polarization value, beam intensity, and ease of obtainment.
Abstract: Neutron diffraction studies are reported on a series of magnetized and unmagnetized ferromagnetic materials. The diffraction patterns for unmagnetized, polycrystalline samples of Fe and Co are found to possess both nuclear and magnetic components with the latter in agreement with the magnetic scattering theory with respect both to intensity of scattering and form factor angular variation. Studies on the magnetic structure of ${\mathrm{Fe}}_{3}$${\mathrm{O}}_{4}$ are shown to strongly support N\'eel's proposed ferrimagnetic structure. Predictions of the theory regarding intensity effects upon sample magnetization are fully confirmed and the Schwinger-Halpern-Johnson formulation of the interaction function between the neutron's magnetic moment and the internal fields in a ferromagnet is substantiated. A pronounced variation of intensity around the Debye ring in the diffraction pattern for a magnetized sample is found. Neutron polarization effects in the Bragg scattered beams from magnetized crystals of Fe and ${\mathrm{Fe}}_{3}$${\mathrm{O}}_{4}$ have been studied and it is shown that very highly polarized beams are obtained for certain reflections. This method of monochromatic beam polarization is found to compare very favorably with other methods with respect to polarization value, beam intensity, and ease of obtainment.
153 citations
TL;DR: The sol-gel process has been extended to the preparation of new diphasic xerogels leading to new hybrid ceramic-metal materials as mentioned in this paper, which can be controlled by both preparation steps: the gelation parameters and subsequent thermal treatment.
129 citations
TL;DR: In this paper, X-ray diffraction data, characterized by the presence of a diffuse scattering peak centered at 20−24 degrees and the absence of any strong Bragg scattering from the iron-containing regions, indicates that these bulk materials are comprised of nanometer-sized regions of iron compounds embedded in a silica gel matrix.
Abstract: Homogeneous gelled composites of iron and silica containing 5–30 wt. % Fe have been prepared by low temperature polymerization of aqueous solutions of ferric nitrate, tetraethoxysilane, and ethanol (with an HF catalyst). X-ray diffraction data, characterized by the presence of a diffuse scattering peak centered at 20≈24 degrees and the absence of any strong Bragg scattering from the iron-containing regions, indicates that these bulk materials are comprised of nanometer-sized regions of iron compounds embedded in a silica gel matrix. Scanning electron microscopy observations show that this matrix is characterized by the presence of many interconnected pores and that the size of these pores is related to the particle size of the Fe-containing regions. The paramagnetic nature of these materials at room temperature, as well as the small size of the iron-containing regions, is indicated by the appearance in many of the samples of only a high intensity central doublet in the 57 Fe M6ssbauer spectra. The Mossbauer effect data demonstrates that the form of the iron can be changed by a subsequent treatment in an atmosphere of ammonia or hydrogen at elevated temperatures: for a 10 wt. % Fe sample treated with ammonia, only a central doublet was observed but with a much larger quadrupole splitting and isomer shift. Both of these subsequently treated materials became superparamagnetic at room temperature. In addition, magnetic susceptibility measurements indicate that the hydrogen treated material becomes a spin glass at low temperatures.
16 citations
TL;DR: In this article, the solgel technique was used to fabricate α-Fe/mullite composite materials with iron concentrations ranging from 20-60 wt.% Fe, which exhibited ferromagnetic hysteresis with saturation magnetization values as high as 115 EMU/g.
Abstract: The sol-gel technique was used to fabricate α-Fe/mullite composite materials with iron concentrations ranging from 20–60 wt.% Fe. Microscopy studies showed the microstructure to consist of particulate α-Fe dispersed uniformly throughout a mullite matrix. Iron particle size, estimated by TEM, ranged from 20–500 nm. The materials exhibited ferromagnetic hysteresis with saturation magnetization Ms values as high as 115 EMU/g.
3 citations