Magnetic and structural characterization of silver-iron oxide nanoparticles obtained by the microemulsion technique

In the present paper we report the magnetic characterization of silver-iron oxide nanocomposite obtained by the chemical microemulsion method. TEM images and X-ray diffractograms show that the nanocomposite consists of Ag nanoparticles of ～ 7 nm surrounded by a quasiamorphous matrix. The ZFC–FC curves and Mössbauer spectra obtained at different temperatures show a typical evolution of a system composed of weakly interacting nanoparticles with a blocking temperature (T_b) of ～50 K. The analysis of the magnetic data reveals that the matrix is formed by γ-Fe₂O₃ phase with a structural range order of ～2 nm.     

Magnetite nanowires in MCM-41 type mesoporous silica templates

MCM-41 mesoporous material was chosen as a template of very small Fe₃O₄ particles. The results of structural and magnetic studies of magnetite nanowires are reported. The average length of these nanowires is about 70 nm and their diameter is 3 nm. Magnetite polycrystalline nanowires were characterized by means of X-ray diffraction and ⁵⁷Fe Mössbauer spectroscopy (MS). Almost 80% of the particles exist in a superparamagnetic state at room temperature. Mössbauer measurements also provided evidence that the composites displayed a distribution of magnetic particles by size. As a result, strong changes of superparamagnetic and magnetic relative contributions along with temperature were observed.     

Iron environment in calcium-soda-phosphate glasses and vitroceramics

The structure of calcium-soda-phosphate glasses and vitroceramics with relatively high iron content was investigated by X-ray diffraction, electron paramagnetic resonance (EPR) and Mössbauer spectroscopy. The X-ray diffraction analysis proves the vitreous state of the as prepared samples and the development of crystalline phases in the annealed samples. The ferric ions disposed in sites that give rise to the absorption line with g_ef ≈ 4.3 in the EPR spectra of vitreous samples are not more evidenced in the spectra of the annealed samples, from which only of a symmetric and narrowed line with g_ef ≈ 2 is recorded. The room temperature ⁵⁷Fe Mössbauer spectra both of glass and vitroceramic samples consist of two quadrupole doublets characteristic for octahedral sites of Fe²⁺ and Fe³⁺ ions. The isomer shift for glass samples decreases and for vitroceramic samples increases with the iron oxide content.     

Mössbauer study of multiphase iron oxide composites

A representative nanocomposite made of ferrimagnetic γ-Fe₂O₃ and antiferromagnetic α-Fe₂O₃ nanoparticles with estimated weight fractions of ～90 and ～10%, respectively, and grown in transparent silica xerogels has been characterized by Mössbauer spectroscopy with respect to the temperature. The changes of the hyperfine parameters of the different subspectral components have been compared with other macroscopic magnetic and structural measurements. This comparison has allowed us to identify two different superparamagnetic transitions, located at ～50 K and at ～250 K, as well as the phases involved in each transition.     

Spin disorder in Fe-doped manganites

Ln_0.7M_0.3MnO₃ compounds are well-known ferromagnets mediated by a double exchange mechanism. As Mn atoms are substituted by Fe in the ratio Mn_1−xFe_x the magnetic structure dramatically changes, because the ferromagnetic double exchange chain is broken. At low Fe concentrations all compounds are magnetically ordered. For intermediate values ferro (FM), antiferro (AF) and paramagnetic (PM) phases co-exist in a large temperature range, and at x ≈ 0.2 spin or cluster-glass behavior is found. Magnetization, Mössbauer, polarized and low angle neutron scattering as well as muon spin relaxation experiments have been performed on 0 ⩽ x ⩽ 0.30 compounds showing the transit from long range ferromagnetism to spin glass. Co-existence of FM and AF clusters of different size has been found for all doped compounds.     

Structural properties of Cr2O3–Fe2O3–P2O5 glasses,Part I

The structural properties of xCr₂O₃–(40 − x)Fe₂O₃–60P₂O₅, 0 ⩽ x ⩽ 10 (mol%) glasses have been investigated by Raman and Mössbauer spectroscopies, X-ray diffraction (XRD) and differential scanning calorimetry (DSC). The Raman spectra show that the addition of up to 5.3 mol% Cr₂O₃ does not produce any changes in the glass structure, which consists predominantly of pyrophosphate, Q¹, units. This is in accordance with O/P ≈ 3.5 for these glasses. The increase in glass density and T_g that occurs with increasing Cr₂O₃ suggests the strengthening of glass network. The Mössbauer spectra indicate that the Fe²⁺/Fe_tot ratio increases from 0.13 to 0.28 with increasing Cr₂O₃ content up to 5.3 mol%, which can be related to an increase in the melting temperature from 1423 to 1473 K. After annealing, the 10Cr₂O₃–30Fe₂O₃–60P₂O₅ (mol%) sample was partially crystallized and contained crystalline β-CrPO₄ and Fe₃(P₂O₇)₂. The SEM and AFM micrographs of the partially crystallized sample revealed randomly distributed crystals embedded in a homogeneous glass matrix. EDS analysis indicated that the glass matrix was rich in Fe₂O₃ (39.6 mol%) and P₂O₅ (54.9 mol%), but contained only 5.5 mol% of Cr₂O₃. These results suggest that the maximum solubility of chromium in these iron phosphate melts is 5.5 mol% Cr₂O₃.     

Structure and properties of nanocrystalline Cu70Fe18Co12 obtained by mechanical alloying

Cu₇₀Fe₁₈Co₁₂ alloy is prepared by mechanical alloying of pure Cu, Fe, Co powder using a high energy ball mill, with increasing milling time ranging from 4 to 8, 24, 36 and 54 h. The variation of the morphology and the elemental distribution were measured at these different stages on various grains of the alloy using a scanning electron microscope with a dispersive energy analyzer. Atomic clusters of iron were observed on some grains after 8 h of milling, confirming the non-homogenisation of the powder at this stage. Beyond 12 h, the homogenisation is ensured over a volume of one cube micron. Microstructural changes during the mechanical alloying have been studied by X-ray diffractometry (XRD) and Mössbauer spectroscopy. X-ray diffraction measurements confirm the dissolution of iron and cobalt phases in the FCC matrix of copper after 24 h of milling with increase of the structural parameter. This same dissolution was also measured by Mössbauer spectroscopy, confirming that after 4 h of milling the CuFe phase begins to form and iron dissolution is incomplete with partial amount of alpha Fe phase surviving after 36 h of milling.     

Effect of OH-content on thermal and chemical properties of SnOP2O5 glasses

Glasses with 55–60 mol% SnO and 40–45 mol% P₂O₅ have shown extremely large differences in the chemical and thermal properties depending on the temperature at which they were melted. Glasses prepared at low melting temperature, 450–550 °C, had low T_g, 150–200 °C, and low chemical stability. Glasses prepared at high melting temperature, 800–1200 °C, had much higher T_g, 250–300 °C, and much higher chemical stability. No significant differences were found by ¹¹⁹Sn Mössbauer and ³¹P Nuclear Magnetic Resonance spectroscopy. Large differences in the OH-content could be detected as the reason by infrared absorption spectroscopy, thermal analyses, and ¹H Nuclear Magnetic Resonance spectroscopy. In samples with low T_g, a broad OH – vibration band around 3000 nm with an absorption intensity >20 cm^?1, bands at 2140 nm with intensity ～5 cm^?1, at 2038 nm with intensity ～2.7 cm^?1, and at 1564 nm with intensity ～0.4 cm^?1 were measured. These samples have shown a mass loss of 3–4 wt% by thermal gravimetric analyses under argon in the temperature range 400–1000 °C. No mass loss and only one broad OH-band with a maximum at 3150 nm and low absorption intensity <4 cm^?1 could be detected in samples melted at high temperature, 1000–1200 °C, which have much higher T_g, ～300 °C, and much higher chemical stability.     

Influence of thermal treatment upon the diffusion characteristics of Mo in Fe76Mo8Cu1B15 alloy

Mo tracer diffusion in Fe₇₆Mo₈Cu₁B₁₅ alloy was studied using serial sectioning method in the temperature range 548–648 K. Sputtering by Ar⁺ ions was applied as a sectioning technique. The measurement was carried out with the ribbon-like samples in as quenched state (q) and in two states after pre-anneals 683 K/1 h (a1) and 743 K/1 h (a2). The volume fraction of crystalline phase was checked by the Mössbauer spectroscopy. Measured concentration profiles consist of two distinct branches suggesting that two diffusion paths are operating. Diffusion coefficients D_v calculated from the near-surface branch were attributed to Mo volume diffusion in amorphous phase. Their values obtained for (a1) are lower than those obtained for (q) which is due to relaxation of free volume. The higher values, on the other hand, obtained for (a2) are influenced by faster short-circuit diffusion, which was attributed to diffusion in interfacial amorphous phase. Short-circuit diffusivity, P, of Mo was evaluated from concentration tails using both LeClaire’s analysis and Divinski’s theory derived for transition type-A/B regime.     

On the coordination environment of Fe- and Pb-rich solidified industrial waste: An X-ray absorption and Mössbauer study

The oxidation state and coordination environment of Fe and Pb atoms in a series of homogeneous vitrified Pb- and Fe-rich industrial waste glasses is investigated by means of X-ray absorption fine structure (XAFS) and ⁵⁷Fe Mössbauer spectroscopies. The waste content in the studied samples varies between 10 and 60 wt.%. The Mössbauer analysis reveals that even though all the glasses contain both Fe³⁺ and Fe²⁺ ions, the concentration of Fe²⁺ decreases with increasing waste content. The XAFS results demonstrate that the structural role of Fe depends on the waste content and the Fe³⁺ ion occupies increasingly tetrahedral at the expense of octahedral sites as the waste content increases. On the other hand, the Pb²⁺ coordination environment remains unaffected by the waste content. In order to determine the percentage of FeO₆ and FeO₄ polyhedra, we propose a mixed model for the analysis of the Fe-K edge extended-XAFS (EXAFS) spectra according to which X% of the Fe atoms occupy tetrahedral sites while the rest (1 − X)% constitute octahedra. The EXAFS results disclose that when the waste content increases from 10 to 40 wt.%, the percentage of FeO₆ octahedra decreases from 55 to 13 wt.%. When the waste content exceeds 50 wt.%, Fe is predominantly a glass former. The importance of this finding relies with the fact that stabilized products can be produced using a higher amount of Fe-containing toxic waste and a smaller amount of vitrifying agents.     

Influence of annealing on the high frequency magnetotransport properties of melt-spun Fe31Co31Nb8B30 alloys

The influence of furnace annealing and Joule heating on microstructure and high frequency magnetotransport properties of Fe₃₁Co₃₁Nb₈B₃₀ alloy has been studied using differential scanning calorimetery, hysteresis and Mössbauer spectroscopy measurements. Annealing reduces disorder in the specimens via structural relaxation improving soft magnetic properties. In as-cast specimens the spins preferentially remain within ribbon plane whereas annealing above 400 °C they become random. Magnetoimpedance shows a progressive deterioration of the magnetoimpedance response upon furnace annealing as well as Joule heating of the samples.     

Time resolved thermal lens measurements of the thermo-optical properties of Nd2O3-doped low silica calcium aluminosilicate glasses down to 4.3 K

In this work, the thermal lens spectrometry was applied to measure the thermo-optical properties of Nd₂O₃-doped low silica calcium aluminosilicate glasses as a function of temperature, between 4.3 and 300 K. The thermal relaxation calorimetry was used to determine the specific heat, c_p. The results showed a decrease of the thermal diffusivity of about one order of magnitude from 4.3 K up to 300 K, with a T^?1 dependence in the interval between 20 and 70 K and a T^?0.35 between 4.3 and 20 K. The fluorescence quantum efficiencies of the doped samples were calculated down to 50 K, showing a variation of the order of 12% and 25% for the samples with 0.6 and 1.04 mol% of Nd₂O₃, respectively. In addition, the temperature corresponding to the maximum in c_p/T³, the so-called boson peak, was observed at about 17 K for the undoped sample and at lower temperatures for the doped glasses. In conclusion, our results showed the ability of the time resolved thermal lens to determine the thermo-optical properties of glasses at temperatures lower than 300 K, bringing new possibilities for experiments in a wide range of optical materials.     

Molecular dynamics in glass-forming poly(phenyl methyl siloxane) as investigated by broadband thermal,dielectric and neutron spectroscopy

The molecular dynamics of glass-forming poly(methyl phenyl siloxane) (PMPS) is studied by thermal (10⁻³–5 × 10² Hz), dielectric (10⁻³–10⁹ Hz) and neutron (5 × 10⁸–10¹² Hz) spectroscopy. Because of the broad frequency range of 15 orders of magnitude the study provides a precise determination of glassy dynamics in a wide temperature range using different probes. The relaxation rates extracted from the different methods agree quantitatively in both their absolute values and in their temperature dependencies. A detailed analysis of the temperature dependence of the relaxation rate f_p by a derivative technique shows that the α-relaxation of PMPS has to be characterized by a high and a low temperature branch separated by a crossover temperature T_B = 250 K. In both temperature ranges the temperature dependence of f_p has to be described by Vogel/Fulcher/Tammann laws with different Vogel temperatures. Also the analysis of the dielectric strength in its temperature dependence gives a crossover behavior from a low to a high temperature region with a similar value of T_B. T_B can be interpreted as onset of cooperative fluctuations and the formation of dynamical heterogeneities. The dependence of the relaxation rate on the scattering vector Q extracted from neutron scattering obeys a power law τ ∼ Q^−Slope, where the power Slope varies between Slope = 2 and Slope = 3.5 with increasing temperature. This anomalous dependence of the relaxation time on the momentum transfer is discussed in terms of dynamic heterogeneities in the underlying motional processes even at temperatures above T_B. Besides the segmental dynamics the fast Methyl group rotation is considered as well. The relaxation rates of this process have an activated temperature dependence with an activation energy of 8.3 kJ/mol. The data were discussed in the framework of the threefold jump model were the incoherent elastic scattering from ‘fixed’ atoms which are frozen on the time scale of the Methyl group rotation was taken into account.     

Phase evolution and magnetic properties of Co/α-Fe2O3 powder mixtures with different molar ratios treated by mechanical alloying

In this work, we report the phase formation and magnetic properties of Co–hematite powder mixtures with two different molar ratios: Co/α-Fe₂O₃: 1/0.7 and 1/1.3 subjected to high-energy mechanical milling using metallic cobalt and hematite powder as the initial raw material in ambient air atmosphere. The samples were activated with a ball to powder weight ratio (BPR) of 10 and the milled powders were collected after 0, 1, 5, 15, 25 and 30 h. Various characterization techniques such as XRD, HRTEM, VSM and Mössbauer spectroscopy were utilized to study the prepared samples. For the samples with Co/α-Fe₂O₃: 1/0.7 milled for 1 and 5 h the formation of cobalt ferrite was confirmed. However, this was not the case for the samples milled above 5 h for whose both Mössbauer and XRD results confirmed the phase decomposition taken place for the previously formed cobalt ferrite phase. Further, the formation of superparamagnetic nanoclusters of iron oxide, a wustite-like Fe_1?XCo_XO phase and the existence of small amounts of metallic Fe/Fe_1?XCo_X phase/s were also detected for these samples. The presence of the latter phase is not believed to be solely related to contamination from the steel vial/balls used. A mechanochemical-reduction process is assumed to be also possibly responsible for the formation of the observed reduced phases. For the powder mixture with Co/α-Fe₂O₃: 1/1.3, however, increased formation of cobalt ferrite phase was observed by increasing the milling time. The highest maximum magnetization (53 e.m.u/g) and coercive field (500 Oe) was obtained for the sample milled for 25 h among various samples of this series of powder mixture. The lower magnetization obtained for this sample compared to that of the bulk is attributed to the size effect. Furthermore, the structural–magnetic properties relationship of the various powders prepared is discussed in detail.     

Thermal behavior of substituted FeCo-based metallic glasses

Three compositions of metallic glasses, Fe₅₈Co₂₄Nb₃Ta₁Mo₁B₁₃, Fe_61.5Co_20.5Nb₃Ta₁Mo₁B₁₃ and Fe₆₆Co₁₈Si₁B₁₅ were prepared by rapid quenching from the melt and subsequently annealed for 1 hour at 450, 650 and 750 °C. The samples were analyzed by X-ray diffraction (XRD) and Mössbauer spectroscopy. All Mössbauer spectra were fitted with a six-line pattern corresponding to the crystalline α-(FeCo) phase and a hyperfine magnetic field distribution representing the amorphous component. The Mössbauer spectra of annealed Fe₆₆Co₁₈Si₁B₁₅ revealed the presence of a secondary crystalline phase, namely (FeCo)₃(BSi). Moreover, the last Mössbauer spectrum in the set was fitted with an additional sextet, corresponding to the appearance of hematite in the system. It is inferred that the addition of Nb, Ta and Mo considerably delays the onset of bulk crystallization and iron oxidation in these systems. The XRD patterns are in qualitative agreement with the Mössbauer results and are consistent with a surface layer of hematite nanoparticles in the system. The activation energy of 4.34 eV for oxidation was estimated from the Mössbauer results.     

Electronic states of SnO-ZnO–P2O5 glasses and photoluminescence properties

SnO–ZnO–P₂O₅ glasses with 30 and 40 mol% P₂O₅ were prepared by a melting process in an air atmosphere. The glass transition temperature, refractive index, and photoluminescence of the glasses were investigated. The electronic states of Sn(II) and Sn(IV) were determined by Mössbauer spectroscopy. The PO₄ units were investigated by Raman spectroscopy. The glass transition temperature was lower than 450 °C, and decreased as the Sn concentrations increased, so that the minimum was about 250 °C. The refractive index increased as the Sn concentration increased. The emission spectra of the glasses peaked at around 2.0–3.0 eV and depended on the glass compositions.     

Exchange interactions in hydrogen-induced amorphous YFe2

The magnetic properties of hydrogen-induced amorphous (HIA) YFe₂H_x (x = 1.8, 3.0 and 3.4) alloys have been investigated by means of ⁵⁷Fe Mössbauer spectroscopy and atomic pair distribution analysis. The exchange integral (J) estimated from the temperature dependence of the average ⁵⁷Fe hyperfine field shows a tendency to decrease with decreasing the average Fe–Fe interatomic separation. Moreover, an abrupt drop of J is evident at an average Fe–Fe interatomic separation of about 0.25 nm, which is the empirical threshold where the magnetic interaction between Fe moments changes from ferromagnetic to antiferromagnetic. The absence of a spin-glass state in HIA YFe₂(H) is well understood by the enhancement of J through the volume expansion induced by the absorption of hydrogen.     

Electrical properties of annealed a-SiC:H thin films

In the present work the dependence of electrical properties of a-SiC:H thin films on annealing temperature, T_a, has been extensively studied. From the measurements of dark dc electrical conductivity, σ_D, in the high temperature range (from 283 up to 493 K), was found that the conductivity activation energy, E_a, is invariant for T_a ⩽ 673 K and equal to 0.64 eV, whereas for T_a from 673 up to 873 K, E_a increases at about 0.2 eV reaching to a maximum value 0.85 eV at T_a = 873 K, suggesting the optimum material quality. This behavior of E_a as a function of T_a is mainly attributed to relaxation of the strain in the amorphous network, which is possibly combined with weak hydrogen emission for temperatures up to 873 K. For further increase of T_a (>873 K) the phenomenon of hydrogen emission, causes rapid decrease of E_a down to 0.24 eV at T_a = 998 K, deteriorating the material quality. These results are also supported by the measurements of dark dc electrical conductivity in the low temperature range (from 133 up to 283 K), where the dependence of the density of gap states at the Fermi level, N(E_F), on annealing temperature presents the minimum value at T_a = 873 K. The Meyer–Nelder rule was found to hold for the a-SiC:H thin films for annealing temperatures up to 873 K. Finally, the dependence of dark dc electrical conductivity at room temperature, σ_DRT, on T_a showed to reflect directly the dependence of E_a on T_a.     

Photo-crystallization in a-Se imaging targets: Raman studies of competing effects

Photo-induced crystallization of a-Se is investigated by Raman spectroscopy as a function of temperature (250–340 K) and exposure time in thin-film structures used as targets in high-gain avalanche rushing photoconductor (HARP) video cameras. The Stokes-to-Antistokes ratio is monitored to obtain the local temperature T_loc at the laser spot; fluxes (632 nm) of 17 and 10 W/cm² are used. We find a rich temperature behavior that reflects the competition of changes in viscosity and strain, and defines four distinct regimes. No photo-crystallization is seen for T_loc below 260 K, nor in a 15 K range around T_g ～ 310 K. For T_loc in the regime 260–302 K the initial rate of crystal growth after onset of photo-crystallization is temperature independent, whereas for T_loc > 318 K the growth rate is thermally enhanced. Our results are in qualitative accord with a theory by Stephens treating the effects of local strain on the secondary growth of crystalline nuclei in a-Se. We conclude that the observed growth rate between 260 and 302 K is driven by local strain, and that relaxation of this strain near T_g suppresses crystal growth until thermally assisted processes accelerate the photo-crystallization at higher temperatures.     

Strontium environment transition in tin silicate glasses by neutron and X-ray diffraction

The effect of Sr modifier atoms on the structure of stannosilicate glasses of composition (Sr0)_x(SnO)_0.5−x(SiO₂)_0.5, with 0 ⩽ x ⩽ 0.15, has been studied using Mössbauer spectroscopy and neutron and X-ray diffraction. The tin is mostly in the Sn²⁺ state. The Sr–O bond length undergoes a step decrease from (2.640 ± 0.005) Å to (2.585 ± 0.005) Å as x increases from 0.10 to 0.15, indicating a decrease in co-ordination number from 8 to 7. A Sn–Sn distance of 3.507 ± 0.005 Å is revealed by a first-order difference calculation from the x = 0 sample. This is too short to be consistent with significant edge sharing of [SnO₃] trigonal pyramids.