Thermally Induced Solid-State Syntheses of γ-Fe2O3 Nanoparticles and Their Transformation to α-Fe2O3 via ε-Fe2O3

The thermally induced solid-state syntheses of -Fe₂O₃ nanoparticles from iron-bearing materials (FeSO₄, Fe₂(C₂O₄)₃ and almandine garnet) are described. Magnetic properties, particles size and the mechanism of the structural change of -Fe₂O₃ nanoparticles have been investigated using ⁵⁷Fe Mössbauer spectroscopy, X-ray powder diffraction (XRD) and atomic force microscopy (AFM). -Fe₂O₃ nanoparticles are transformed into hematite via -Fe₂O₃ as the intermediate.     

Synthesis and application of the uniform particle size of nano-γ-Fe2O3: dispersed nanoparticles of γ-Fe2O3 for green synthesis of aminophosphonates

China started nanotechnology research from 1990s, and has paid much attention to nanoeducation since then. Scientific discoveries from nanoresearch have been introduced timely into high school textbooks, and more specific courses such as Nanomaterials and Nanofabrication are being taught at both college and graduate levels. In this paper, nanotechnology courses at the Graduate University of Chinese Academy of Sciences, Beijing University, and Tsinghua University are chosen to be overviewed, which could be a good indicator of Chinese nanoeducation because of high-quality science and engineering education in these schools. Research institutes also contribute in popularizing nanoscience knowledge through activities such as public open days. These combined efforts substantially promote the public acceptance and understanding of nanoscience and technology in China.     

A Mössbauer and ESR study of LiNbO3-Fe2O3 for low Fe2O3 concentrations

Samples of the system LiNbO₃-Fe₂O₃ prepared by water quenching and by the double-roller quenching method in the range up to 24 mol% Fe₂O₃ were investigated by Mössbauer and ESR spectroscopy. In the water quenched samples up to 11 mol% Fe₂O₃ only the Fe³⁺ and the Fe²⁺ valence states could be detected. The Fe²⁺ concentration decreased with increasing Fe₂O₃ content. Above 11 mol% Fe₂O₃ magnetically split Mössbauer spectra indicated the presence of Fe₂O₃ clusters. The isomer shift values of Fe³⁺ as a function of Fe₂O₃ concentration showed jumps at 6 and 11 mol% Fe₂O₃, whereas no significant changes could be detected in the quadrupole splitting values. The ESR data already exhibited the existence of isolated Fe³⁺ ions and of clusters with Fe-Fe distances less than 8 Å for the lowest Fe₂O₃ concentration. The cluster signal intensity increased with increasing Fe₂O₃ content. The roller quenched samples showed increased Fe²⁺ concentration as compared to the water quenched samples, which suggests that slow quenching results in iron oxidation and cluster formation. For low Fe₂O₃ concentrations a valence state change Fe³⁺Fe²⁺ can easily be obtained by heat treatments in various atmospheres, whereas for higher Fe₂O₃ contents (9.8 mol%) precipitations of-Fe (in reducing atmosphere) and Fe₂O₃ (in air) could be observed in addition to the valence state changes of a remaining part of dissolved Fe ions. On the basis of the obtained results a model was suggested for the unusual behaviour of the lattice parameters observed in LiNbO₃-Fe₂O₃.     

Investigation of the parametric excitation and relaxation of phonons in antiferromagnetic α-Fe2O3

Parametric excitation of magnetoelastic waves was investigated in the easy-plane antiferromagnet α-Fe₂O₃ by parallel and perpendicular microwave pumping over a wide range of frequencies, magnetic fields, and temperatures, and the parametric resonance thresholds were measured. The frequencies of the natural magnetoelastic vibrations of the sample were investigated as a function of the magnetic field and temperature. The results of the measurements were used to calculate the parameters of the magnetoelastic wave spectrum and the rate of relaxation of the excited quasi-phonons. Possible mechanisms for quasi-phonon damping were analyzed.     

Spectroscopy of Reflected Electron Energy Losses of γ-Fe2O3

Physics of the Solid State - The spectra of energy losses of reflected electrons, which are recorded with the energy of primary electrons in the range of 200–3000 eV, are investigated. From...     

Surface structure of γ-Fe2O3(111)

The surface structure of γ-Fe₂O₃(111) has been investigated with a range of surface techniques. Two different surface structures were discovered depending upon surface preparation techniques. Sputtering followed by annealing in vacuum produced a reduced surface characterised by a (2 × 2) LEED pattern, whereas sputtering followed by annealing in 1 × 10^? 6 mbar oxygen produced a surface characterised by a (√3 × √3)-R30° LEED pattern. The latter appears to be a very low conductivity surface, whereas the former has the band gap expected for maghemite (~ 2.0 eV). We propose that the reduced surface is a magnetite-like layer, whereas the oxidised surface is an Fe₂O₃-like layer.     

Lowering of magnetic field intensity at the surfaces of α-Fe2O3 and FeBO3 single crystals

Depth-selective conversion electron Mössbauer spectroscopy was used to study magnetic properties of the thin surface layers of the α-Fe₂O₃ and FeBO₃ single crystals. An analysis of the experimental spectra indicates that the magnetic properties of the layers at a depth of more than ～100 nm from the surface are similar to the properties of crystal bulk, and the corresponding spectra consist of narrow lines. The lines gradually broaden as the crystal surface is approached. The spectra of the ～10-nm-thick surface layers consist of broad lines, indicating a wide distribution δ=2.1 T of the effective magnetic field about its mean value of 32.2(4) T. The experimental spectra were used to determine the effective magnetic fields (H _eff) for the iron ions situated in the surface layers of thickness ～100 nm. The effective fields in these layers were found to gradually decrease at room temperature (291 K) as the crystal surface was approached. The H _eff values in the 2.4(9)-nm-thick surface layer of the α-Fe₂O₃ crystal and 4.9(9)-nm layer of FeBO₃ are 0.7(2) and 1.2(3)%, respectively, smaller than for the nuclei of the ions in the bulk of these crystals.     

High-pressure characteristics of α-Fe2O3 using DFT+U

We have calculated the structural, magnetic and electronic properties of corundum-type α-Fe₂O₃ from first principles using density-functional theory (DFT) and the DFT?+?U method to account for correlation effects in this material. Although the correct magnetic ground state is obtained by pure DFT, the magnetic moments and the band gap are too small, and the predicted structural phase transition coupled with a transition from the insulating high-spin to a metallic low-spin phase at a pressure of 14?GPa is not observed experimentally. We find that considering the Coulomb interaction directly by including a Hubbard-like term U in the density functional greatly improves the results with respect to band gap and magnetic moments. The phase transition is shifted to higher pressures with increasing values of U and disappears for U?>?3?eV. The best overall agreement of structural, magnetic and electronic properties with experimental data is obtained for U?=?4?eV.     

Mössbauer investigation of non-aggregated γ-Fe2O3 particles

Investigation by Mössbauer spectroscopy of non-aggregated nanometric -Fe₂O₃ particles dispersed in polymer is reported. Magnetic interactions between the particles were controlled by varying the particle concentration in the polymer. The results show that over the investigated range, the interactions make the relaxation time shorter. Infield experiments show spin canting which increases with decreasing particle size.     

Magnetic behaviour of γ-Fe2O3 nanoparticles by mössbauer spectroscopy and magnetic measurements

-Fe₂O₃ nanoparticles with varying state of dispersion in a polymer have been investigated by Mössbauer spectroscopy, static magnetic measurements at low applied field, and alternative susceptibility measurements over a large range of frequencies (2×10^–2–10⁴ Hz). The dynamical behaviour was characterized through the variation of the blocking temperature with the characteristic time of the measurement. The Mössbauer blocking temperature was determined according to a procedure described. For quasi-isolated particles an Arrhenius law is demonstrated. Effects of interparticle interactions in concentrated and aggregated systems are satisfactorily explained by the previous model. Dependence of the superparamagnetic susceptibility on the experimental conditions interpreted using the Lorentz or Onsager fields is mentioned.     

Synthesis and characterization of hollow ��-Fe2O3 sub-micron spheres prepared by sol�Cgel

In this work we report the preparation of magnetic hematite hollow sub-micron spheres (??-Fe₂O₃) by colloidal suspensions of ferric nitrate nine-hydrate (Fe(NO₃)₃·9H₂O) particles in citric acid solution by following the sol?Cgel method. After the gel formation, the samples were annealed at different temperatures in an oxidizing atmosphere. Annealing at 180°C resulted in an amorphous phase, without iron oxide formation. Annealing at 250°C resulted in coexisting phases of hematite, maghemite and magnetite, whereas at 400°C, only hematite and maghemite were found. Pure hematite hollow sub-micron spheres with porous shells were formed after annealing at 600°C. The characterization was performed by X-ray diffraction (XRD), Mössbauer spectroscopy (MS) and scanning electron microscopy (SEM).     

Effect of interparticle interactions on the dynamical properties of γ-Fe2O3 nanoparticles

The dynamical properties of γ-Fe₂O₃ nanoparticles dispersed in a polymer have been investigated by means of AC susceptibility measurements in a large frequency range (10<ν<10⁴ Hz) and Mössbauer spectroscopy measurements. The frequency dependence of the blocking temperature, T_B, has been analysed for a series of samples with the same volume distribution and interparticle interactions of different strengths. An increase of the effective energy barrier with increasing interaction strength has been observed unambiguously, in agreement with our model, based on a statistical calculation of the interaction energy for a disordered assembly of particles with volume distribution and easy axes in random directions.     

Specific Features of the Local Structure and Transport Properties of ZrO2–Sc2O3–Y2O3 and ZrO2–Sc2O3–Yb2O3 Crystals

Optics and Spectroscopy - The specific features of the local structure of ZrO2–Sc2O3–Y2O3 and ZrO2–Sc2O3–Yb2O3 crystals are revealed by optical spectroscopy using the Eu3+...     

The modulated magnetic structure of α-Fe2O3: Ga weak ferromagnet

The domain structure of a diluted weak α-Fe₂O₃: Ga ferromagnet is considered. Magnetic fields of specific amplitudes and orientations applied in the basal plane of the crystal are found to produce nonuniform magnetic states due to a periodic deviation of the antiferromagnetic vector from the easy axis of crystallographic anisotropy. A phase diagram of the modulated magnetic state with the triad axis is constructed, and a dependence of the spatial period of modulation on an external magnetic field is studied. A phenomenological model and the nature of the magnetic superstructure discovered are discussed.     

Effect of contacts between γ-Fe2O3 nanoparticles on their phase-transition temperature

The effect of mechanical contacts between γ-Fe₂O₃ particles on the temperature of the γ-α structural transition in them is established by magnetic studies and differential thermal analysis (DTA). The sample in which γ-Fe₂O₃ particles had no mechanical contacts with one another remained in the ferromagnetic state up to T _C = 630°C and had two exothermal DTA peaks. The first peak almost coincided with the Curie temperature, while the second peak attributed to the γ → α structural transition corresponded to 760°C. The magnetic transition for particles with a larger number of contacts was shadowed by the γ → α structural transition with a temperature lowered to 550°C.     

Magnetic properties of γ-Fe2O3 nanoparticles encapsulated in surface-treated polymer spheres

Mössbauer and magnetic characterization of polymer-dispersed γ-Fe₂O₃ nanoparticles treated under different chemical processes are reported in this work. X-ray powder diffraction analysis provides a mean particle size of D ~ 8.0 nm. Whereas Mössbauer spectroscopy data suggest the presence of only Fe^3?+? ions, magnetization measurements indicate the occurrence of a freezing phenomenon in agreement with the thermal evolution of Mössbauer spectra. A core–shell model was used to determine a magnetically disordered layer (shell) of d ~ 1.0 nm covering a region of collinear magnetic moments (core). The chemical treatments with H₂O₂ and Na₂S₂O₈ modify notoriously the magnetic response of the polymer-dispersed nanoparticles.     

Neutron topographic investigation of the Morin transition in flux-grown crystals of α-Fe2O3

The Morin transaction is investigated by neutron diffraction topography on high crystalline flux-grown single crystals of α-Fe₂O₃. They display a widely spread transition, which is complete for some of the crystals, partial for others and even “reentrant” in one case. Topographys indicate that the transition takes place through a succession of a abrupt transitions of regions of the samples, and give images of these regions. We discuss the possible relationship between these regios and the crystal perfection.     

γ-Fe2O3 nanoparticles prepared by laser ablation of a tiny wire

A new method of preparing nanoparticles by pulsed laser ablation of a tiny wire is reported. A Nd:YAG pulsed laser with a wavelength of 1064 nm was used to ablate a 0.5-mm-diameter iron wire in a sealed chamber in a flowing mixed gas of N₂, O₂, and air to generate -Fe₂O₃ nanoparticles. In the meantime, a bulk Fe sample was ablated in the same chamber with the same laser processing parameters in order to compare the effect of the bulk sizes on the production rates and the sizes of the nanoparticles. The experimental results demonstrated that the production rate of nanoparticles prepared by laser ablation of tiny wires was about eight times that of laser ablation of bulk targets with the same composition, while the sizes of the nanoparticles were basically the same. With a higher power density and/or smaller diameters of the metal wires, it is possible to obtain smaller sizes of the nanoparticles with higher production rates. PACS 81.07.Wx