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In this paper, the generation scheduling problem for autonomous island systems with significant penetration of non-dispatchable Renewable Energy Source (RES) generation is investigated, proposing a unit commitment-economic dispatch (UC-ED) model which is fully compatible with the provisions of the regulatory framework currently applicable to the Greek non-interconnected island (NII) systems. Two UC-ED model variants are presented and comparatively assessed, in order to illustrate the impact of various system parameters (technical characteristics of the thermal units, reserve requirements, etc.) on the expected operating profile of the NII system. A detailed UC-ED model is first proposed, which takes into account the reserve requirements at different time scales (primary, secondary and tertiary reserves) and respective capabilities of generating units. An alternative, simplified UC-ED model is also presented, formulated on the basis of aggregate spinning reserves, to cater for uncertainties in the actual reserve capabilities of existing thermal units. The developed UC-ED models are applied to an existing NII system to evaluate their performance and results obtained on an annual basis.

Combined wind and pumped-storage virtual power plants, called hybrid power stations (HPS), constitute a realistic and feasible option to achieve high renewable energy source (RES) penetration levels in power systems and particularly in autonomous island grids. Technical issues arising from the integration of HPS in islands have not been sufficiently investigated yet. In this paper, the dynamic behaviour of an island system with an HPS is analysed, the effects of the various HPS operating modes on the transient behaviour and stability of the system are investigated, constraints regarding the operation of the HPS are identified and potential solutions are proposed.

Pumped storage is generally viewed as the most promising technology to increase renewable energy source (RES) penetration levels in power systems and particularly in small autonomous island grids. Combined wind and pumped-storage “virtual power plants”, called hybrid power stations (HPS), constitute a realistic and feasible option to achieve high penetrations, provided that their components are properly sized. In this paper, the optimum sizing is investigated for a pumped storage HPS operating in an island system. The analysis addresses the sizing of the main HPS components (hydro turbines, pumps, wind farm, reservoirs), adopting either the investor’s perspective, where the objective is to maximize the return on the HPS investment, or a system perspective, where the optimization target is the maximization of RES penetration, along with maintaining the lowest possible generation cost in the system. Genetic Algorithms (GAs) are applied for the optimization and a real isolated island power system is used as a study case. The adopted operating policy and pricing principles, which critically affect the optimal sizing of an HPS project, are based on the existing regulatory framework for storage stations in Greek islands.

Pumped storage is generally viewed as the most promising technology to increase renewable energy source penetration levels in power systems and particularly in small autonomous island grids, where technical limitations are imposed by the conventional generating units. The Hybrid Power Station (HPS) of Ikaria Island, Greece, which is currently in the construction stage, will be one of the first wind-hydro-pumped-storage hybrid stations in the world. In this paper, the HPS is described and a suitable operating policy is introduced, both for the HPS and the overall generation system of Ikaria, taking into consideration the existing regulatory framework for HPS in isolated island grids. Based on the proposed policy, the operation of the Ikaria Island system is simulated on an annual basis, in order to quantify the expected benefits from the operation of the HPS.

Sr 2 Fe 1− x Cr x Mo 1− x W x O 6 , x =0, 0.05, 0.10, 0.15, 0.20, and 0.30 compounds have been prepared by the solid state encapsulation technique and their magnetic and magnetotransport properties have been studied. All samples show ferromagnetism at room temperature, the Curie temperature increases with x up to x =0.10 ( T C =426 K), remaining at high values even for x =0.30 ( T C =408 K). The resistivity of the low-doped ( x

This study looks at the synthesis of Fe nanoparticles using a borohydride reduction of metal salts in the presence of carboxyl and methoxyl terminated polyethylene glycol (PEG). As the PEG concentration is varied, there is a corresponding increase in the Fe content of samples as determined by Mossbauer spectroscopy. Particle sizes are between 70 and 300 nm as indicated by transmission electron microscopy. As the polymer concentration is increased the overall saturation magnetization of the coated nanoparticles initially increases due to the increased iron content with a maximum of 70 emu/g for nanoparticles with the largest Fe content. The magnetization then begins to decrease as the polymer concentration begins to contribute to a significant volume fraction of the sample. All samples are magnetically soft with high remanence as indicated by SQUID magnetometry measurements.

The magnetic critical behaviour of the manganese perovskite series La0.67−y(Sr,Ba,Ca)0.33+yMn1−xSnxO3 ( x = 0.01 , 0.02, y = 0 , 0.07) is studied by means of dc magnetic measurements. The substitution of Mn by Sn ions in the samples results in a decrease of the transition temperature. The structure can be described by a rhombohedral unit cell (space group R - 3 C ) for the samples where the A-site is occupied by La and Sr or La and Ba ions and an orthorhombic unit cell (space group Pnma ) for the samples where the A-site is occupied by La and Ca ions. An increase of the cell volume is observed as Mn substituted by Sn ions in all samples. Arrott and scaling plots show that the samples, where the A-site is occupied by La and Sr or La and Ba ions, follow the behaviour of a conventional second-order ferromagnetic transition. The values of the critical exponents ( β , γ ) are between those predicted by the three-dimensional Heisenberg model and mean field theory. In contrast, the samples that contain La and Ca ions in the A-site show anomalous behaviour around the Curie point. Static disorder induced by Sn substitutions leads to a rounding of the transition.

The magnetic critical behavior of the manganese perovskite series La0.67−y (Sr, Ba, Ca)0.33+y Mn1−x SnxO3 (x=0.0l, 0.02, y=0, 0.07) is studied by means of de magnetic measurements and 119Sn Mossbauer spectroscopy. The structure can be described by a rhombohedral unit cell (space group R-3C) for the samples where the A-site is occupied by La and Sr or La and Ba ions and orthorhombic unit cell (space group Pnma) for the samples where the A-site is occupied by La and Ca ions. Arrott and scaling plots show that the samples, where the A-site is occupied by La and Sr or La and Ba ions, follow the behavior of a conventional second-order ferromagnetic transition. In contrast, the samples that contain La and Ca ions in the A-site show anomalous behavior around Curie point. Mossbauer measurements show two magnetic phases below T c . One of them exhibits stronger exchange interactions with more rapid electron transfer between Mn3+/Mn4+, compared to the other.

Polycrystalline double perovskites Sr2Fe1−x Cr x Mo1−x W x O6 with x=0, 0.05, 0.10, 0.15, 0.20, and 0.30 have been prepared by sold state reactions. A continuous decrease of the tetragonal unit cell parameters α and c with increasing x values is observed. The highest Curie temperature T c =426 K is recorded for the x=0.10 compound. 57Fe Mossbauer spectroscopy measurements indicate a non-integral electronic configuration of ≈3d5.3 for the Fe ions at the ordered double perovskite structure for x≤0.20, which reaches ≈3d5.4 for x=0.30. Fe-Mo/W anti-site and anti-phase boundary defects are observed in all samples in equal concentrations of around 3% of the total number of Fe ions in their structure.

The mechanism of the thermal decomposition of Fe2(SO4)3 in air has been studied at different temperatures (520-700 °C) using mainly 57Fe Mossbauer spectroscopy. Iron(III) oxides with corundum (), bixbyite (), spinel () and orthorhombic () structures were identified as solid products of this conversion. A significant influence of the heating temperature on the decomposition mechanism and on the phase composition of reaction products was found.

We report on magnetometry, resistivity and Mossbauer spectroscopy measurements on single-phase Fe3BO5 in a wide temperature range from 4.2 to 620 K. Magnetically split Mossbauer spectra were observed below TN = 114 K, although a proportion of the Fe2+ ions in the basal plane (crystallographic site Fe1) remain paramagnetic down to the ferrimagnetic transition at Tw = 74 K. This unusual magnetic behaviour known as `idle-spin behaviour' can be explained by the competitive magnetic interactions with its neighbouring ordered ions and should be related to the topology of the octahedral Fe1 sites in the crystal structure of Fe3BO5. The behaviour change recorded in the dc magnetization and Mossbauer data indicates that the magnetic structure of Fe3BO5 should change at 74 K, where the spin frustration gradually ceases to exist. Electron delocalization in the Fe3-Fe2-Fe3 triads between pairs of Fe2+ and Fe3+ ions was observed at all temperatures between 4.2 and 180 K and coexists with localized ferric and ferrous (Fe1 and Fe4 crystallographic sites) states. Above 180 K the delocalization in the Fe3-Fe2-Fe3 triads includes the rest of the Fe3+ ions and some evidence can be seen in the resistivity data as well.

The effect of Mo and Fe atoms on the crystal structure and magnetic properties of the intermetallic La 2 Co 17− x Mo x ( x =0.5, 1, 1.5, 2), and La 2 Co 16− y Fe y Mo ( y =0, 1, 2, 3, 4, 6) compounds have been studied by X-ray diffractometry, magnetic measurements and Mossbauer spectroscopy. All samples belong to the rhombohedral Th 2 Zn 17 -type structure and their lattice parameters a and c increase both with Mo and Fe content. From the La–Co–Mo samples only the one with x =0.5 presents planar anisotropy, whereas from the La–Co–Fe–Mo samples only the y =1 has uniaxial anisotropy. The magnetization M S and the Curie temperature T C decrease upon Mo substitution, whereas the anisotropy field H A does not change significantly. In the Fe-substituted compounds M S increases, but the Curie temperature increases slightly for 0⩽ y ⩽4 but decreases in y =6. The low temperature M–T curve shows that the samples La 2 Co 16.5 Mo 0.5, and La 2 Co 10 Fe 6 Mo present a spin reorientation transitions at 70 and 260 K, respectively. Mossbauer samples were obtained for all Fe-containing samples in the temperature range 20–300 K. Above 260 K a jump in the values of the hyperfine fields and quadrupole splitting parameters is observed which can be associated to the spin reorientation.

The four new intermetallic aluminum silicides RE4Fe2+xAl7-xSi8 (RE = Ce, Pr, Nd, Sm) crystallize from the reaction of Si, Fe, and RE (or rare earth oxides) in molten Al at 850 °C. All compounds share the same structure type as determined by single-crystal X-ray diffraction analysis. They form in the space group Cmmm (No. 65) with cell constants of a = 10.909(2) A, b = 16.265(3) A, c = 4.0804(8) A, R1 = 0.0196, and wR2 = 0.0486 for the Sm analogue. The crystal structure is a complex three-dimensional network comprised of repeating layers containing Al, Si, and Fe connected by atoms between the layers. The RE3+ ions are then located within tunnels of the three-dimensional network, running parallel to the c axis with a coordination number of 14. Magnetic susceptibility measurements indicate that the rare earth ions are in a 3+ oxidation state, whereas the Fe atoms are in an effective diamagnetic state. Electronic band structure calculations, carried out on the hypothetical analogue Y4Fe2Al7Si8, predict metal...

Pr 3 (Fe,Ti) 29 hydrocarbide has been synthesised by gas–solid reaction. The hydrocarbide shows a lattice expansion of 5.1%. The Curie temperature, T C , (119°C for the parent alloy) was increased to 373°C. The room temperature saturation magnetisation, M S , of the hydrocarbide is 138.1 Am 2 kg −1 and the average hyperfine field at room temperature is 25.1 T. The magnetic anisotropy of the hydrocarbide changes from an easy cone with an angle of 34° (parent alloy) to a narrower cone with an angle of 26°. The anisotropy field, H A at room temperature is 5.5 T.

Polycrystalline Fe2BO4 was prepared by solid state reactions and its electronic and magnetic properties were investigated by Mossbauer spectroscopy and magnetization measurements. The Mossbauer spectra of Fe2BO4 below 270 K indicate the presence of Fe2+ and Fe3+ sites in the structure, in a ratio 1 : 1. Above this temperature electron delocalization sets in between the divalent and trivalent iron ions and Fe2.5+ states are observed. The temperature dependence of the Mossbauer spectra and magnetization measurements clearly show the onset of magnetic order below 155 K.

Polycrystalline Fe2 BO4 prepared by solid-state reaction was investigated over a wide temperature range using Mossbauer spectroscopy and magnetization and resistivity measurements. The Mossbauer data below 270 K reveal four iron sites (two Fe2+ and two Fe3+ ) in a 1:1:1:1 ratio. Above this temperature, electron delocalization sets in between the divalent-trivalent iron ions and up to 400 K both delocalized and localized iron states are observed. The magnetization and Mossbauer data indicate that this oxoborate mixed-valence system is an L-type ferrimagnet below Tc = 155 K, with the iron atoms in each sublattice having different point symmetries and different magnetic interactions. The resistivity data show the first-order nature of the charge-ordering transition (Tco = 316 K).

The proximity effect on the Curie temperature of exchange-coupled melt-spun nanocomposite Sm2(Fe,Co)15Cr2C2/(Fe,Co) magnets consisting of a fine mixture of hard and soft phases has been examined using thermomagnetic measurements, x-ray diffraction (XRD), Mossbauer and transmission electron microscopic studies. A significant enhancement in the Curie temperature of the 2:17 phase is observed in these magnets (27 °C in Sm2Fe15Cr2C2 and 102 °C in Sm2Fe11Co4Cr2C2) as compared to their patent cast alloys. XRD and Mossbauer analysis confirm that both the cast alloys and the nanocomposite magnets are comprised of the same phases [rhombohedral 2:17 and bcc Fe(Co)]. The unit cell volume of the 2:17 phase is slightly larger in ribbons and it is found to contribute to about 15% of the total Curie temperature enhancement. Microstructural studies revealed that the grain size is reduced from 10 to 20 μm in the cast alloy to 20–60 nm in the melt-spun magnets. The enhancement of the Curie temperature is believed to arise ...

Nonstoichiometric single phase ${\mathrm{Fe}}_{1\ensuremath{-}x}\mathrm{O}$ films with ferrimagnetic properties, large saturation magnetization ${(M}_{s}),$ and low-temperature coercivity ${(H}_{c})$ have been prepared and studied. A model on the relation between ${M}_{s}$ and the lattice parameter (a) is proposed. The model is based on the existence of 16:5 spinel-type defect clusters, and predicts a linear dependence between ${M}_{s}$ and a. The experimental data of ${M}_{s}$ versus a is a straight line passing through the point ${(M}_{s},a)$ of bulk ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}.$ This fact suggests that ${\mathrm{Fe}}_{1\ensuremath{-}x}\mathrm{O}$ is the link between the two phases ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ and FeO, which have seemingly different physical nature and properties, into one class of materials.

Anomalous ferromagnetic-like behavior, with significant magnetization and very large coercivities at low temperatures was observed in α-Fe 2 O 3 /Ag multilayers. Mossbauer studies showed the presence of two magnetic components, bulk-like and surface-like with substantially lower hyperfine field. It was found that the observed magnetic moment per particle can be accounted for by the contribution of only about 5% of the total number of surface Fe 3+ ions. This result can be explained if the magnetic moments on the surface are predominantly randomly oriented resembling a spin glass state. The negligible quadrupole interaction, observed in the Mossbauer spectra at low temperature, strongly supports the last interpretation. Large shifts in the hysteresis loops of field-cooled samples indicate strong exchange coupling between the antiferromagnetic core and the net magnetic moments on the surface.

A ferromagnetic-type behavior has been found in sputtered FeO and α-Fe2O3 thin films at low temperatures. This observation is in contrast to the bulk antiferromagnetic properties of FeO and α-Fe2O3. At room temperature the coercivity of the films is negligible, but below 150 K it increases rapidly and at 10 K it is larger than 3 kOe for both kinds of iron oxide films. The saturation magnetization of the films is large (with a maximum of 270 emu/cc in FeO and 140 emu/cc in α-Fe2O3 samples) and thickness dependent. Mossbauer spectra of α-Fe2O3 films could be analyzed by two magnetic components, one very close to bulk α-Fe2O3 and the other one identified as surface α-Fe2O3. Mossbauer spectra of FeO films showed that the samples were off-stoichiometric, with about 10% Fe deficiency. Clusters of tetrahedrally coordinated Fe3+ ions were observed in an environment similar to that in Fe3O4. The hysteresis behavior in FexO films can be explained by the defect clusters of Fe3+ ions, and by uncompensated spins in th...

The hysteresis behavior of Fe χ (SiO 2 ) 1- χ granular films was studied as a function of preparation conditions. The films were prepared by dc and rf magnetron sputtering over the composition range χ v = 0.1–0.9 volume fraction of Fe. The Tandem deposition method and deposition from a composite target both with and without titanium sublimation, were used to prepare the films. A typical granular structure was observed, with grain size in the range 2–20 rim. X-ray diffraction and selected area diffraction showed an α-Fe (bcc) type structure. Magnetic properties showed that by varying the deposition method and some of the sputtering parameters (sputtering rate, argon flow, film thickness and substrate temperature), it is possible to switch from a relativeIy magnetically hard sample ( H c ∼ 700 Oe) to a soft sample ( H c ∼ 20 Oe). A dramatic increase in H c has been observed at cryogenic temperatures. This result, along with the Mossbauer data, suggests a shell/core granule morphology with an Fe and/or (Fe-Si) core surrounded by Fe-Si-O.

Iron oxide and cobalt oxide thin films were prepared by reactive DC magnetron sputtering in a mixture of Ar and O/sub 2/ gases. The stoichiometry of the oxide films was studied as a function of the amount of O/sub 2/ during deposition. Sputtering conditions, including dc power and oxygen-to-argon ratio, were found which allowed pure FeO, Fe/sub 3/O/sub 4/, /spl alpha/-Fe/sub 2/O/sub 3/, CoO, and Co/sub 3/O/sub 4/ films to be fabricated. The microstructure of the films, and especially the grain shape and size distribution, were quite different for oxide films with different stoichiometry. The crystallographic axes of Fe-O films were randomly oriented in contrast to Co-O films in which a strong texture was found. A strong ferromagnetic-like behavior was observed in FeO and /spl alpha/-Fe/sub 2/O/sub 3/ films, in contrast to the well-established antiferromagnetic behavior in bulk. The reason for this anomalous behavior is attributed to clusters of defects in FeO films and uncompensated surface spins in /spl alpha/-Fe/sub 2/O/sub 3/ films. A large shift (3800 Oe) was observed in the low temperature hysteresis loop of a field cooled sample consisting of 50% Co and 50% CoO. A similar but less pronounced effect was observed in FeO and /spl alpha/-Fe/sub 2/O/sub 3/ films. This result is believed to be caused by a strong exchange coupling between the ferromagnetic and antiferromagnetic constituents in these composite films.

The structural and magnetic properties of fine Fe B particles with a size of about 50 nm and B concentrations of 5, 14, and 17 at.% were studied in as-prepared samples and after annealing in vacuum and in H 2 atmosphere. The samples annealed in vacuum were found to be dominated by α-Fe in the high B concentrations. Oxide components appear in the particles with low B content. Boron is found to prevent oxidation of the as-made samples and promote the formation of mixed valence oxides in the crystallized samples.

SmFe and SmFe(Nb or Zr) alloys, with stoichiometry RE:TM = 2:17 were prepared in ribbon form, using the melt spinning technique. The samples then were annealed and nitrated. X-ray diffraction and Mossbauer spectroscopy were used to study the samples. The presence of Nb(Zr) atoms stabilize the TbCu 7 structure, while the samples without substitutions crystallize in the Th 2 Zn 17 -type structure. The hyperfine parameters for the corresponding phases are deduced from analysis of Mossbauer spectra in the temperature range 15–300 K. At 300 K the mean hyperfine fields are 241 and 219 kOe for the Nb and Zr phases, respectively. The Curie temperature of Sm 2 Fe 17 is higher than that of the intermetallic compound Sm 2 Fe 17 prepared by the arc melting technique, and the Curie temperatures of the TbCu 7 -type phases are in the range 200–230°C. Remarkable enhancements in the coercive fields of the 1:7 nitride products are observed.

57 Fe Mossbauer measurements have been performed on the lamellar iron (II) ethylphosphonate Fe(II)(C 2 H 5 PO 3 )·H 2 O. Mossbauer parameters have been studied as a function of temperature in the paramagnetic state (above 28 K). In the temperature range 150–200 K this compound has been found to undergo a phase transition resulting in a discontinuity in the quadrupole splitting ΔE. This unusual behavior was ascribed to a structural change and not to a transition from an ordered to a quasi-liquid state of the water molecule and/or CH 2 CH 3 moieties. Results of a preliminary infrared study supports this hypothesis.

The structural and magnetic properties of Fe–Pt nanoparticles with Fe 2+ /Pt 4+ molar ratio of 1, 1.25, 1.5, 2 were studied in the as-prepared and annealed samples. The powders were produced by chemical reduction. The magnetization and coercivity of as-made particles were relatively low. After annealing, M S increases and H C decreases with increasing Fe 2+ /Pt 4+ molar ratio. The maximum values of M S and H C obtained were about 75 emu/g and 20 kOe, respectively.

Large differences have been observed in the coercivity values of Fe SiO 2 and Fe BN granular films with the coercivities being much lower in the BN matrix. We have used Mossbauer spectroscopy in an attempt to understand the differences in the magnetic hysteresis behavior. The Mossbauer data suggest a core/shell morphology for the Fe SiO 2 films with an Fe(Fe-Si) core surrounded by an Fe-Si-O shell whose surface consists of Fe2+-Si-O grains. In Fe BN the data indicate the presence of amorphous Fe-B particles in a BN matrix. This hypothesized particle morphology can explain the high coercivity values in SiO2 films and the low Hc in BN because of the low anisotropy of the amorphous phase.

Pumped storage is generally viewed as the most promising technology to increase renewable energy source (RES) penetration levels in power systems and particularly in small autonomous island grids, where technical limitations are imposed by the conventional generating units. In this chapter, an operating policy is proposed for hybrid wind-hydro power stations (HPS) in island grids, in order to increase wind penetration levels, while at the same time minimizing the impact on the conventional generation system and ensuring the viability of the HPS investment. The proposed operating strategy is applied to different autonomous island systems using a dedicated logistic model, in order to evaluate the effect on the overall operation and economics of the island systems and to assess the feasibility of HPS investments. In addition, the real case study of the HPS in Ikaria island, Greece, which is currently in the construction stage and will be one of the first wind-hydro-pumped-storage hybrid stations in the world, is examined and the expected benefits from its operation according to the proposed policy are presented. The material presented in this chapter is based on publications Papaefthimiou et al. (IET Renew Power Gener 3:293–307, 2009, IEEE Trans Sustainable Eng 1:163–172, 2010) available in IET-Renewable Power Generation and IEEE Transaction on Sustainable Energy.

NIGMS NIH HHS (GM-41482) Desulfovibrio gigas ferredoxin II (FdII) is a small protein (α4 subunit structure as isolated; M(r) ≃ 6400 per subunit; 6 cysteine residues) containing one Fe3S4 cluster per α-subunit. The x-ray structure of FdII has revealed a disulfide bridge formed by Cys-18 and Cys-42 approximately 13 Å away from the center of the cluster; moreover, the x-ray structure indicates that Cys-11 forms a disulfide bridge with a methanethiol. In the oxidized state, FdII(ox), the 1H NMR spectra, exhibit four low-field contact-shifted resonances at 29, 24, 18, and 15.5 ppm whereas the reduced state, FdII(R) (S = 2), yields two features at +18.5 and -11 ppm. In the course of studying the redox behavior of FdII, we have discovered a stable intermediate, FdII(int), that yields 1H resonances at 24, 21.5, 21, and 14 ppm. This intermediate appears in the potential range where the cluster (E'0 ≃ -130 mV) is reduced from the [Fe3S4]1+ to the [Fe3S4]0 state. FdII(int) is observed during reductive titrations with dithionite or hydrogen/hydrogenase or after partial oxidation of FdII(R) by 2,6- dichlorophenolindophenol or air. Our studies show that a total of three electrons per α-subunit are transferred to FdII. Our experiments demonstrate the absence of a methanethiol-Cys-11 linkage in our preparations, and we propose that two of the three electrons are used for the reduction of the disulfide bridge. Mossbauer (and EPR) studies show that the Fe3S4 cluster of FdII(int) is at the same oxidation level as FdII(ox), but indicate some changes in the exchange couplings among the three ferric sites. Our data suggest that the differences in the NMR and Mossbauer spectra of FdII(ox) and FdII(int) result from conformational changes attending the breaking or formation of the disulfide bridge. The present study suggests that experiments be undertaken to explore an in vivo redox function for the disulfide bridge. publishersversion published

Pillared structures with an interlayer opening of approximately 0.3 nm were obtained after successive heat treatments of the PILC precursor in reducing and oxidizing conditions. This precursor was prepared by reacting a Na+-montmorillonite with an intercalant containing Al and Fe oxo-hydroxides (Al/Fe = 1). Powder X-ray diffraction, elemental analysis, Fe-57 Mossbauer spectroscopy, catalytic activity measurements and surface area data were used to characterize the samples. On the basis of Mossbauer spectra taken at temperatures between 4.2 and 300 K, it is deduced that oxidizing steps produce Al substituted maghemite which converts into Al substituted magnetite upon reducing heat treatment. Firing the precursor in oxidizing atmosphere forms pillars of few nm in diameter. However, heating under reducing conditions yields pillars of smaller diameter. This later behaviour is maintained even after reheating the material in oxidizing atmosphere. From the temperature dependence of Mossbauer spectra it is deduced that the diameter of the Fe oxide particles in the pillars is smaller than 10 nm. Clays and Clay Minerals

The crystallization behavior and magnetic properties have been investigated in melt-spun Pr7Tb1Fe84.5Nb1.5B6 samples. A c-axis texture of 2:14:1 phase was observed in ribbons spun below 14 m/s. Samples spun above 40 m/s show an amorphous phase, which crystallizes into a TbCu7 structure before it transforms into the 2:14:1 phase. At higher temperatures, (Pr,Tb)1+δFe4B4 phase precipitates from the matrix. Optimum magnetic properties are found with coercivity Hc=8.6 kOe and (BH)max=17.3 MG Oe.

Exchange interactions and charge transfer in the Fe2+Fe3+ pair of the mixed valence [Fe2S2(dimethylmethanebisbenzimidazolate)2]3− trianion have been studied by analysis of Mossbauer spectra in the temperature range of 1.5–180 K and in applied fields of 10 mT, 0.35 T, and 6.2 T. The low‐temperature spectra reveal a ground state with total spin St=1/2 and hyperfine parameters intermediate between values for a Fe2+Fe3+ localized mixed‐valence pair and a fully delocalized system where the two iron atoms are equivalent. A consistent set of hyperfine parameters has been derived by fitting the spectra with a stochastic relaxation model taking into account spin relaxation in the St=1/2 state and electron hopping between the iron ions. An interpretation of the values of the hyperfine parameters has been given by solving a spin Hamiltonian, which includes antiferromagnetic and double exchange in an asymmetric Fe2+Fe3+ pair and which allows partial electron delocalization. Using the value a2=0.8 for the delocalizati...

The magnetic anisotropy fields of nitrogenated NdFe 10 Mo 2 N x compound are found to be 20 and 8.6 T at 10 and 300 K respectively. Magnetically hard NdFe 10 Mo 2 N x with a coercivity of 8.8 kOe at room temperature and 33.7 kOe at 10 K is obtained by nitrogenating melt-spun NdFe 10 Mo 2 ribbons. The coercivity depends strongly on the nitrogenation temperature and time with the maximum values obtained for nitrogenation at 570 °C for 4 h. The high coercivity is related to the purity of the ThMn 12 phase and the uniformity of nitrogenation.

NdFe 10 Mo 2 N 0.5 samples were prepared by heating fine NdFe 10 Mo 2 powders in N 2 gas. The nitrided material maintains the ThMn 12 -type structure but with a unit cell volume about 3% larger than in the original compound. Neutron diffraction experiments showed that the nitrogen atoms occupy the interstitial 2b sites and the Mo atoms the 8i sites consistent with the Mossbauer data. The Curie temperature is greatly enhanced by about 150 K. The magnetocrystalline anisotropy changed strongly from plane anisotropy in NdFe 10 Mo 2 to uniaxial anisotropy in NdFe 10 Mo 2 N 0.5 at room temperatures. The changes in the local crystallographic and electronic structure can explain the strong uniaxial anisotropy of the nitrided material.

Iron nitride powders, with composition Fe4N, have been prepared from Nd-containing precursors using pure ammonia (NH3) gas. The precursors were a mixture of iron oxide and neodymium hydroxide made by coprecipitation of metal salts (FeCl3) and Nd(NO3)3·6H2O) with sodium hydroxide (NaOH) in aqueous solution. The presence of Nd3+ ions in the system promoted the formation of Fe4N. The as-made sample has a saturation magnetization of 150 emu/g (200 emu/g if only the mass of the iron nitride is included). The high magnetization is due to the protection of the particle surface by the neodymium oxide.

We have prepared a new series of R-Fe-B alloys with R=Nd, Pr, and Dy having the Y/sub 3/Fe/sub 62/B/sub 14/ structure using the melt-spinning technique. This metastable phase when annealed at higher temperatures transforms to the R/sub 2/Fe/sub 14/B/sub 1/+/spl alpha/-Fe phase. We have studied the crystallization behavior of these alloys. /sup 57/Fe Mossbauer studies have shown that the as-spun samples were amorphous and the ribbons annealed at 600-620/spl deg/C contain the metastable Y/sub 3/Fe/sub 62/B/sub 14/ type structure with /spl alpha/-Fe. The analysis of the Mossbauer spectra of the R/sub 3/Fe/sub 62/B/sub 14/ phases has yielded average hyperfine fields of 229 kG at 295 K and 315 kG at 17 K.

The magnetic anisotropy fields of nitrogenated NdFe 10 Mo 2 N x compound are found to be 20 and 8.6 T at 10 and 300 K respectively. Magnetically hard NdFe 10 Mo 2 N x with a coercivity of 8.8 kOe at room temperature and 33.7 kOe at 10 K is obtained by nitrogenating melt-spun NdFe 10 Mo 2 ribbons. The coercivity depends strongly on the nitrogenation temperature and time with the maximum values obtained for nitrogenation at 570 °C for 4 h. The high coercivity is related to the purity of the ThMn 12 phase and the uniformity of nitrogenation.

The synthesis, characterization, and reactions of the new «cubane»-type clusters (ET 4 N) 3 [Fe 3 S 4 L 3 M(CO) 3 ], M=MO, L=EtS - , BzS - , p-OMePhS - , PhO - , Cl - ; M=W, L=p-OMePhS - , are described in detail. These clusters are obtained in a good yield as (ET 4 N) + salts, by the reaction of the [Fe 3 S 4 L 4 ] 3- linear trimer with M(CO) 3 (MeCn) 3 , M=Mo, W

Ferrimagnetic properties, large saturation magnetization (Ms) and low temperature coercivity (Hc) was observed in nonstoichiometric Fe1−xO films. This unusual behavior was attributed to the existence of spinel type defect clusters coherently embedded in the FeO matrix. A model, which explains the Fe1−xO properties and predicts a linear dependence between Ms and the Fe1−xO lattice parameter a with the Fe3O4 data falling on the same line, is proposed. The experimental Ms vs a curve is in excellent agreement with the theoretical prediction, which indicates the existence of a fundamental relation between FeO, Fe1−xO, and Fe3O4.

A series of bimetallic complexes, [M II M' II BPMP5O 2 CR) 2 ]X 2 where BPMP is the anion of 2,6-bis[[bis(2-pyridylmethyl)amino]methyl]-4-methylphenol, has been synthesized to serve as models for the diferrous forms of iron-oxo centers in proteins. Complex 1 (M=M'=Fe, R=C 2 H 5 , X=BPh 4 , solvate=0.8 CH 2 Cl 2 ) has been characterized by X-ray diffraction methods as having a (μ-phenoxo)bis(μ-carboxylato)diiron core. 1 crystallizes in the triclinic space group P1

We report Mossbauer studies and magnetic measurements of Bi2Sr4Fe3O12+x. From the magnetic measurements and the Mossbauer spectra we conclude that the Bi2Sr4Fe3O12+x is antiferromagnetic with TN=40 K and θ=-258 K. The analysis of the Mossbauer spectra indicates that Fe exists in two states of valence, with relative abundance Fe 3+ Fe 4+ =0.16 . In addition, Fe3+ is in a high spin state.

The stoichiometry and magnetic properties of reactively sputtered Fe-O/Ag multilayers were studied as a function of the Fe-O layer thickness. The thinnest Fe-O (50 A) layers were highly disordered. However, when the thickness was increased to more than 100 A, well-crystallized films comprised of a mixture of FeO and Fe 3 O 4 were formed. The relative amount of FeO and Fe 3 O 4 was thickness-dependent. Very thick (more than 3500 A) films were essentially single-phase Fe 3 O 4 .

Polycrystalline double perovskites Sr2Fe1−x Cr x Mo1−x W x O6 with x=0, 0.05, 0.10, 0.15, 0.20, and 0.30 have been prepared by sold state reactions. A continuous decrease of the tetragonal unit cell parameters α and c with increasing x values is observed. The highest Curie temperature T c =426 K is recorded for the x=0.10 compound. 57Fe Mossbauer spectroscopy measurements indicate a non-integral electronic configuration of ≈3d5.3 for the Fe ions at the ordered double perovskite structure for x≤0.20, which reaches ≈3d5.4 for x=0.30. Fe-Mo/W anti-site and anti-phase boundary defects are observed in all samples in equal concentrations of around 3% of the total number of Fe ions in their structure.

The hysteresis behavior of Fex(SiO2)1−x granular films was studied as a function of preparation conditions. The films were prepared by dc and rf magnetron sputtering over the composition range of xv=0.1–0.9, where xv is the volume fraction of Fe. The tandem and composite target deposition methods, both with and without titanium presputtering, were used to prepare the films. Typical granular structure was observed in these films, with grain size in the range of 2–20 nm. X-ray diffraction and selected area diffraction showed a bcc α-Fe-type structure. Magnetic measurements showed that by varying the deposition method and some of the sputtering parameters including sputtering rate, argon flow, film thickness, and substrate temperature, it is possible to switch from a relatively magnetically hard sample (Hc∼650 Oe) to a soft sample (Hc∼20 Oe). For the former sample, a dramatic increase in Hc has been observed at cryogenic temperatures. This result, along with the Mossbauer data, suggest a shell/core granule m...

The structural and magnetic properties of two Fe‐Co alloy particles with composition Fe100−xCox(x=45,65) were studied using x‐ray diffraction, x‐ray photoelectron spectroscopy, transmission electron microscopy, superconducting quantum interference device magnetometry, and Mossbauer spectroscopy. The particles were nearly spherical in shape with an average particle size around 350 A. Particles formed long chains and showed a core/shell particle morphology. X‐ray diffraction and Mossbauer studies showed the presence of bcc α‐Fe‐Co and CoFe2O4 phases with the former as the majority phase in Fe55Co45 and the latter the majority phase in Fe35Co65. The room‐temperature coercivities of both of these samples were much higher than those in Fe particles with values exceeding 2.2 kOe. The Fe35Co65 sample showed a drastic temperature dependence of coercivity from 1.5 kOe at 300 K to 15 kOe at 10 K. The structural and magnetic data suggest a core/shell morphology with the surface oxide layer having a very important im...

A new series of rare‐earth iron intermetallic carbide compounds has been prepared by the reaction of powders with methane gas. The alloys prior to the methane heat treatment had the 2:17‐type structure, R2(Fe,Si)17 (R=Y, Ce, Pr, Nd, and Sm). After heat treatment in temperatures up to 873 K the alloys absorb carbon and retain their original structure with a volume expansion over the carbon‐free unit cell. For higher heat treatment temperatures (973–1073 K), a phase transformation occurs leading to the BaCd11‐type structure. These compounds have a lower Curie temperature than those with the Th2Zn17‐type structure, in the range of 453–495 K. Rietveld analysis of neutron diffraction data on NdFe10SiCx with the BaCd11‐type structure are reported. In addition, the average hyperfine parameters determined by Mossbauer spectroscopy are reported for the Nd compounds.