High temperature stability of maghemite in partially oxidized basalt lava

Mössbauer spectroscopy of basalt lava samples, exhibiting reversible thermal magnetization (J_S-T) curves with Curie temperatures of about 580°C, has revealed considerable amounts of maghemite (γ-Fe₂O₃) in many samples. In view of the expected instability, of maghemite at temperatures above 350°C, this reversibility is rather surprising. Here we report Mössbauer studies on heated lava samples, showing high content of maghemite. The samples were kept at 600°C in oxidizing, reducing, and inactive atmospheres, respectively, for different lengths of time, and then analyzed with Mössbauer spectroscopy at room temperature. The Mössbauer spectra showed that maghemite is stable in the oxidizing atmosphere for at least several hours. In the inactive atmosphere a considerable amount of maghemite still exists after two hours heating. In the reducing atmosphere maghemite had transformed to magnetite after only 30 minutes.     

Superferromagnetic nanostructures

Interactions between magnetic nanoparticles can lead to superferromagnetic ordering, i.e. ordering of their magnetic moments at low temperatures. The use of a simple mean field theory, describing the temperature dependence of the order parameter is discussed. This model is found to give excellent fits to experimental results. In systems of particles with pure dipole interactions, the degree of ordering depends critically on the geometrical configuration of the particles. The application of superferromagnetic nanostructures for magnetic refrigeration is also discussed.     

Interparticle interactions in agglomerates of alpha-Fe2O3 nanoparticles: influence of grinding

We have chemically prepared a sample of antiferromagnetic alpha-Fe2O3 nanoparticles by a gel-sol technique. M?ssbauer spectra of the as-prepared sample showed that superparamagnetic relaxation was suppressed due to strong magnetic interparticle interactions even at room temperature. However, subsequent grinding of the sample by hand in a mortar for some minutes resulted in fast superparamagnetic relaxation of some of the particles. The effect was even more dramatic if the alpha-Fe2O3 powder was ground for a longer time or together with nonmagnetic eta-Al2O3 nanoparticles. Similar effects were found after low-energy ball milling. Thus it is found that the agglomeration of the nanoparticles during preparation under wet conditions results in strong magnetic interparticle interaction, but a relatively gentle mechanical treatment is sufficient to break up the agglomerates, resulting in much weaker interactions. We show that these effects can also be seen when a soil sample containing magnetic nanoparticles is ground.     

Magnetic field dependence of the spin—spin relaxation time of ferric ions in Fe(NO3)3·9H2O and in frozen aqueous solutions

The electronic relaxation of ferric ions in Fe(NO₃)₃·9H₂O and in frozen aqueous solutions has been investigated by use of Mössbauer spectroscopy. The application of an external magnetic field causes a pronounced increase of the spin—spin relaxation time, an effect which is contrary to that normally found in ferric compounds. An explanation is given based on a difference in the ionic state energies of the neighbouring ions induced by the applied magnetic field.     

Selective flow injection analysis of ultra-trace amounts of Cr(VI), preconcentration of it by solvent extraction, and determination by electrothermal atomic absorption spectrometry (ETAAS)

A rapid, robust, sensitive and selective time-based flow injection (FI) on-line solvent extraction system interfaced with electrothermal atomic absorption spectrometry (ETAAS) is described for analyzing ultra-trace amounts of Cr(VI). The sample is initially mixed on-line with isobutyl methyl ketone (IBMK). The Cr(VI) is complexed by reaction with ammonium pyrrolidine dithiocarbamate (APDC), and the non-charged Cr(VI)–PDC chelate formed is extracted into IBMK in a knotted reactor made from PTFE tubing. The organic extractant is separated from the aqueous phase by a gravity phase separator with a small conical cavity and delivered into a collector tube, from which 55 μl organic concentrate is subsequently introduced via an air flow into the graphite tube of the ETAAS instrument. The operations of the FI-system and the ETAAS detector are synchronously coupled. A significant advantage of the approach is that matrix constituents, such as high salt contents, effectively are eliminated. The extraction procedure was optimized by a simplex approach. A central composite design was subsequently employed to verify the estimated operational optimum. An 18-fold enhancement in sensitivity of Cr(VI) was achieved after preconcentration for 99 s at a sample flow rate of 5.5 ml min⁻¹, as compared to direct introduction of 55 μl of sample, yielding a detection limit (3σ) of 3.3 ng l⁻¹. The sampling frequency was 24.2 samples h⁻¹. The proposed method was successfully evaluated by analyzing a NIST Cr(VI)-reference material, synthetic seawater and waste waters, and waste water samples from an incineration plant and a desulphurization plant, respectively.     

Uniform spin wave modes in antiferromagnetic nanoparticles with uncompensated moments

In magnetic nanoparticles the uniform precession (q = 0 spin wave) mode gives the predominant contribution to the magnetic excitations. We have calculated the energy of the uniform mode in antiferromagnetic nanoparticles with uncompensated magnetic moments, using the coherent potential approximation. In the presence of uncompensated moments, an antiferromagnetic nanoparticle must be considered as a kind of a ferrimagnet. Two magnetic anisotropy terms are considered, a planar term confining the spins to the basal plane, and an axial term determining an easy axis in this plane. Excitation energies are calculated for various combinations of these two anisotropy terms, ranging from the simple uniaxial case to the planar case with a strong out-of-plane anisotropy. In the simple uniaxial case, the uncompensated moment has a large influence on the excitation energy, but in the planar case it is much less important. The calculations explain recent neutron scattering measurements on nanoparticles of antiferromagnetic α-Fe₂O₃ and NiO.     

POLYMER SCAFFOLDS BEARING AZOBENZENE-POTENTIAL FOR OPTICAL INFORMATION STORAGE

The fundamental optical storage mechanism of the laser light eddressable azobenzene moiety is briefly introduced.A modular and flexible synthesis design furnishes polyester matrices covalently integrating cyanoazobenzene in regularlyspaced side chains. Thin films of these materials are particularly well suited for holographic storape. Notable figures of meritsof liquid crystalline polyesters are response time to blue-green laser light of the order of nanoseconds, storage capacityexpressed as 5000 lines/mm, and high, permanent (almost nine years) diffraction efficiency of the order of 50% or greater,and erasability, The implications of the main chain nature for polyester morphology and for the permanency of the inducedanisotropy are discussed, The design and methods of preparation of other significantly different polymer scaffolds supportingcyanoazobenzene are elaborated. Oligopeptides always result in amorphous materials, whereas copolymethacrylates anddendritic or hyperbranched polyesters provide some materials that exhibit liquid crystallinity. However, none of these scaffolds affords materials that result in permanent anisotropy when exposed to interfering laser light.     

Effect of particle size on carburization of Fe-Co alloy catalysts

Bimetallic Fe-Co alloy catalysts supported on silica have been studied by use of combined Mössbauer absorption and emisson spectroscopy. Effects of carburization on samples with different particle size are discussed.     

Mössbauer spectroscopy and the iron on Mars

Both the USSR and US have plans of performing rover missions and sample return missions to the planet Mars in the coming decade. There may be possibilities of performingin situ Mössbauer spectroscopy on Mars or on fresh samples returned to a space station.     

Magnetic interaction between ultrafine amorphous Fe1-xCx alloy particles in ferrofluids

Ferrofluids with ultrafine magnetic particles of amorphous Fe_1-xC_x were studied by Mössbauer spectroscopy, transmission electron microscopy and magnetic measurements. Magnetic measurements showed that the initial susceptibility follows a Curie-Weiss law, indicating magnetic interaction between the particles. The influence of the magnetic interaction on the estimation of the particle size, made from the field dependence of the induced hyperfine splitting, is discussed.