Extraterrestrial magnetite studied by Mössbauer spectroscopy

The meteorite Orgueil is a carbonaceous chondrite of type CI. Carbonaceous chondrites contain Fe(III), Fe(II) and in some cases metallic iron, indicating that they are in a state far from thermodynamic equilibrium. In Orgueil about 40% of the iron is present in magnetite (Fe₃O₄). In this work a sample of magnetite grains extracted from Orgueil has been studied by Mössbauer spectroscopy. It has been found that the magnetic phase contains about 11% of maghemite and that the remaining magnetite has a vacancy concentration smaller than 0.006, corresponding to the formula Fe_2.994O₄.     

Growth of Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> films on the Si(111) surface covered by a thin SiO<Subscript>2</Subscript> layer

Magnetite polycrystalline films are grown by variously oxidizing a Fe film on the Si(111) surface covered by a thin (1.5 nm) SiO₂ layer. It is found that defects in the SiO₂ layer influence silicidation under heating of the Fe film. The high-temperature oxidation of the Fe film results in the formation of both Fe₃O₄ and iron monosilicide. However, the high-temperature deposition of Fe in an oxygen atmosphere leads to the growth of a compositionally uniform Fe₃O₄ film on the SiO₂ surface. It is found that such a synthesis method causes [311] texture to arise in the magnetite film, with the texture axis normal to the surface. The influence of the synthesis method on the magnetic properties of grown Fe₃O₄ films is studied. A high coercive force of Fe₃O₃ films grown by Fe film oxidation is related to their specific morphology and compositional nonuniformity.     

Mössbauer investigations of Fe and Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> magnetic nanoparticles for hyperthermia applications

Magnetic nanoparticles of magnetite Fe₃O₄ and Fe synthesized by physical vapor deposition with a fast highly effective method using a solar energy have been studied. Targets have been prepared from tablets pressed from Fe₃O₄ or Fe powders. Relationships between the structure of nanoparticles and their magnetic properties have been investigated in order to understand principles of the control of the parameters of magnetic nanoparticles. Mössbauer investigations have revealed that the nanoparticles synthesized from tablets of both pure iron and Fe₃O₄ consist of two phases: pure iron and iron oxides (γ-Fe₂O₃ and Fe₃O₄). The high iron oxidability suggests that the synthesized nanoparticles have a core/shell structure, where the core is pure iron and the shell is an oxidized iron layer. Magnetite nanoparticles synthesized at a pressure of 80 Torr have the best parameters for hyperthermia due to their core/shell structure and core-to-shell volume ratio.     

Monitoring by Mössbauer spectroscopy the thermal reduction of hematite into magnetite: the surface effect and charge disproportionality in iron oxide nanoparticles

Nanoparticles of magnetite Fe₃O₄ were synthesized by thermal reduction of hematite α-Fe₂O₃ powder in the presence of high boiling point solvent. The structural transformations and magnetic properties of the obtained nanoparticles were investigated by the ⁵⁷Fe Mössbauer spectroscopy, X-ray diffraction, and magnetic measurements. The content of hematite and magnetite phases was evaluated at each step of the chemical and thermal treatment of the product. An increase of saturation magnetization with the reaction time correlates with an increase of concentration of magnetite in the samples. The electron hoping between Fe^2?+? and Fe^3?+? ions in the octahedral sites of the magnetite nanoparticles and Verwey phase transition were investigated. It was established that not all iron ions in the octahedral sites participated in electron hoping Fe^2?+????Fe^3?+? above the Verwey temperature T _V, and the charge distribution could be expressed as $\big( {{\rm Fe}^{3+}}\big)_{{\rm tet}} \big[ {{\rm Fe}_{1.85}^{2.5+} {\rm Fe}_{0.15}^{3+} }\big]_{{\rm oct}} {\rm O}_4$ .     

利用天然磁铁矿简易绿色合成磁性Fe3C@C纳米材料

采用改进的溶胶凝胶法,以天然磁铁矿为铁源,开发出一种制备过程简单且环境友好且低成本的磁性Fe₃C@C纳米材料制备策略. 其中,柠檬酸作为多元羧酸络合剂,不但可以有效地溶解不同铁源,例如Fe、Fe₃O₄或天然磁铁矿,形成柠檬酸铁盐络合物;还可以在热解过程中作为碳源,形成包裹碳层. 通过控制高温热解过程可以直接形成特殊的核-壳结构形态. Fe₃C@C纳米材料具有超顺磁性特性(38.09 emu/mg).     

Magnetic imaging at the atomic level

Magnetic contrast at the atomic level has been observed for the first time in scanning tunneling microscopy experiments on a magnetite (Fe₃O₄(001)) surface using in-situ prepared ferromagnetic Fe tips. A periodic corrugation with a 12 Å periodicity is clearly observed along the rows of FeB-sites which corresponds to the repeat period of Fe²⁺ and Fe³⁺ along these rows. This periodicity is not observed by using non-magneticW tips although the rows of FeB-sites can be resolved as well. The magnetic contrast observed with Fe tips is attributed to the different spin configurations of the magnetic ions Fe²⁺ and Fe³⁺ in Fe₃O₄.     

The magnetic hyperfine interaction of111Cd in the spinels Fe3O4 and Co3O4

The perturbed angular correlation (PAC) method was applied to study the temperature dependence of the magnetic hyperfine fields in¹¹¹In-doped polycrystalline Fe₃O₄ and Co₃O₄. The critical behaviour near the magnetic phase transitions has been investigated. The changes occuring near the Vervey transition in Fe₃O₄ and the path of the supertransferred magnetic fied in Co₃O₄ are discussed. The results are compared with results obtained from Mössbauer spectroscopy.     

Synthesis and characterization of L10 FePt nanoparticles from Pt–Fe3O4 core-shell nanoparticles

Pt/Fe₃O₄ core-shell nanoparticles have been prepared by a modified polyol method. Pt nanoparticles were first prepared via the reduction of Pt(acac)₂ by polyethylene glycol-200 (PEG-200), and layers of iron oxide were subsequently deposited on the surface of Pt nanoparticles by the thermal decomposition of Fe(acac)₃. The nanoparticles were characterized by XRD and HR-TEM. The as-prepared Pt/Fe₃O₄ nanoparticles have a chemically disordered FCC structure and transformed into chemically ordered fct structure after annealing in reducing atmosphere (4% H₂, 96% Ar) at 700 °C. The ordered fct FePt phase has high magnetic anisotropy with coercivity reaching 7.5 kOe at room temperature and 9.3 kOe at 10 K.     

Mössbauer study of a Fe3O4/PMMA nanocomposite synthesized by sonochemistry

Magnetite nanoparticles of 10 nm average size were synthesized by ultrasonic waves from the chemical reaction and precipitation of ferrous and ferric iron chloride (FeCl₃ · 6H₂O y FeCl₂ · 4H₂O) in a basic medium. The formation and the incorporation of the magnetite in PMMA were followed by XRD and Mössbauer Spectroscopy. These magnetite nanoparticles were subsequently incorporated into the polymer by ultrasonic waves in order to obtain the final sample of 5 % weight Fe₃O₄ into the polymethylmethacrylate (PMMA). Both samples Fe₃O₄ nanoparticles and 5 % Fe₃O₄/PMMA nanocomposite, were studied by Mössbauer spectroscopy in the temperature range of 300 K–77 K. In the case of room temperature, the Mössbauer spectrum of the Fe₃O₄ nanoparticles sample was fitted with two magnetic histograms, one corresponding to the tetrahedral sites (Fe^3?+?) and the other to the octahedral sites (Fe^3?+? and Fe^2?+?), while the 5 % Fe₃O₄/PMMA sample was fitted with two histograms as before and a singlet subspectrum related to a superparamagnetic behavior, caused by the dispersion of the nanoparticles into the polymer. The 77 K Mössabuer spectra for both samples were fitted with five magnetic subspectra similar to the bulk magnetite and for the 5 % Fe₃O₄/PMMA sample it was needed to add also a superparamagnetic singlet. Additionally, a study of the Verwey transition has been done and it was observed a different behavior compared with that of bulk magnetite.     

A study of the new compound YBa4Fe3O11−y

The compound YBa₄Fe₃O _y was synthesized where the Fe atoms completely substituted for the Cu atoms in YBa₄Fe₃O _y . The X-ray phase analysis of the synthesized material showed that the diffraction pattern corresponds to the data for the compound YBa₄Fe₃O _y .⁵⁷Fe Mössbauer measurements of the compound were performed at room temperature after different heat treatments: high-temperature synthesis in an oxygen atmosphere and in air, and low-temperature annealing in oxygen and in vacuum. No magnetic components are observed after any heat treatments. The Fe atoms in the compound have two valence states, Fe³⁺ and Fe⁴⁺. The idealized model of the 1-4-3 Fe structure is discussed.     

Magnetism and thermal induced characteristics of Fe2O3 content bioceramics

Magnetic properties of Li₂O–MnO₂–CaO–P₂O₅–SiO₂ (LMCPS) glasses doped with various amounts of Fe₂O₃ were investigated. There is a dramatic change in the magnetic property of pristine LMCPS after the addition of Fe₂O₃ and crystallized at 850 °C for 4 h. Both the electron paramagnetic resonance and magnetic susceptibility measurements showed that the glass ceramic with 4 at% Fe₂O₃ exhibited the coexistence of superparamagnetism and ferromagnetism at room temperature. When the Fe₂O₃ content was higher than 8 at%, the LMCPS glasses showed ferromagnetism behavior. The complex magnetic behavior is due to the distribution of (Li, Mn)ferrite particle sizes driven by the Fe₂O₃ content. The thermal induced hysteresis loss of the crystallized LMCPS glass ceramics was characterized under an alternating magnetic field. The energy dissipations of the crystallized LMCPS glass ceramics were determined by the concentration and Mn/Fe ratios of Li(Mn, Fe)ferrite phase formed in the glass ceramics.     

Mössbauer studies of stoichiometry of Fe<Subscript>3</Subscript>O<Subscript>4</Subscript>: characterization of nanoparticles for biomedical applications

The iron oxide Fe₃O₄, the mineral magnetite sometimes called ferrosoferric oxide, is notoriousy non-stoichiometric even in bulk form so its formula may be written Fe_3?δO₄. In nanoparticle form, where it has applications in medicine and information technology, it is even more susceptible to oxidation. In this paper we report synthesis and studies of superparamagnetic Fe₃O₄ nanoparticles with controlled diameters of 5.3, 10.6 and 11.9 nm. In room temperature spectra, departures from stoichiometry δ of up to 0.02 were estimated from the relative amounts of Fe ³⁺/ Fe ²⁺ and from their isomer shifts. This cannot be used for very small particles of diameter 10.6 nm and less as they are superparamagnetic at room temperature and do not show hyperfine splitting owing to fast relaxation. Such particles have promise for use in enhancing MRI signals. The magnetic spectrum is restored by the application of a relatively small magnetic field (10 kG). As the temperature is lowered the relaxation slows down and 6-line magnetic hyperfine patterns appear below a blocking temperature T_B. The values of T_B obtained are lower than those of many other researchers reported in the literature, suggesting that our particles are less affected by magnetic interactions between them. At low temperatures all the spectra are similar and closely resemble that of bulk Fe₃O₄ confirming that departures from stoichiometry are small.     

Enhanced atomic-scale contrast on Fe3O4 (100) observed with an Fe STM tip

Clean (100) surfaces of a synthetic single crystal of magnetite (Fe₃O₄) have been prepared in situ using current pulses in a scanning tunneling microscope without subsequent annealing. We have observed atomically resolved terraces with rows of Fe²⁺ and Fe³⁺ ions of the B-sublattice (octahedrally coordinated lattice sites). Along these rows a long-distance corrugation (∼12 ?) has been observed at 300 K using in situ prepared Fe tunneling tips. This corrugation is interpreted as a Wigner localization associated with a Verwey transition above 300 K in the top surface layer. Received: 26 September 2000 / Accepted: 27 October 2000 / Published online: 3 May 2001     

PLD of Fe3O4 thin films: Influence of background gas on surface morphology and magnetic properties

Ablation of Fe₃O₄ targets has been performed using a pulsed UV laser (KrF, λ = 248 nm, 30 ns pulse duration) onto Si(100) substrates, in reactive atmospheres of O₂ and/or Ar, with different oxygen partial pressures. The as-deposited films were characterised by atomic force microscopy (AFM), X-ray diffraction (XRD), conversion electron Mössbauer spectroscopy (CEMS) and extraction magnetometry, in order to optimise the deposition conditions in the low temperature range. The results show that a background mixture of oxygen and argon improves the Fe:O ratio in the films as long as the oxygen partial pressure is maintained in the 10⁻² Pa range. Thin films of almost stoichiometric single phase polycrystalline magnetite, Fe_2.99O₄, have been obtained at 483 K and working pressure of 7.8 × 10⁻² Pa, with a high-field magnetization of ∼490 emu/cm³ and Verwey transition temperature of 112 K, close to the values reported in the literature for bulk magnetite.     

Growth,electronic properties and thermal stability of the Fe/Al2O3 interface

Soft X-ray photoelectron spectroscopy and resonant photoemission have been used to study the growth and electronic properties of Fe ultrathin films deposited on Al₂O₃ substrates. A simultaneous multilayer growth mode has been found for Fe growth at room temperature. For iron coverages below 1 ML, Fe²⁺ species are formed at the Fe/Al₂O₃ interface, followed by the formation of a metallic iron overlayer. The bonding of Fe at very low coverages occurs by charge transfer from Fe to surface oxygen atoms, and neither hybridisation of Fe and Al states nor reduction of the Al₂O₃ substrate are observed. The thermal stability of the interface has been also studied in the range 673–873 K. Annealing produces Fe agglomeration in such a way that some areas of the Al₂O₃ substrate become fully Fe-depleted. In these Fe-depleted areas, Fe²⁺ completely disappears and Al⁰ reduced species are formed. This behaviour would explain the decrease in the magnetoresistance performance of magnetic tunnel junctions after annealing above 573 K.     

Aloe vera plant-extracted solution hydrothermal synthesis and magnetic properties of magnetite (Fe3O4) nanoparticles

Magnetite (Fe₃O₄) nanoparticles have been successfully synthesized by a novel hydrothermal method using ferric acetylacetonate (Fe(C₅H₈O₂)₃) and aloe vera plant-extracted solution. The influences of different reaction temperatures and times on the structure and magnetic properties of the synthesized Fe₃O₄ nanoparticles were investigated. The synthesized nanoparticles are crystalline and have particle sizes of ～6–30 nm, as revealed by transmission electron microscopy (TEM). The results of X-ray diffraction (XRD), High resolution TEM (HRTEM) and selected area electron diffraction (SAED) indicate that the synthesized Fe₃O₄ nanoparticles have the inverse cubic spinel structure without the presence of any other phase impurities. The hysteresis loops of the Fe₃O₄ nanoparticles at room temperature show superparamagnetic behavior and the saturation magnetization of the Fe₃O₄ samples increases with increasing reaction temperature and time.     

铁表面氧化的研究

用XPS,UPS和AES研究铁早期氧化过程,认为在温度不太高和氧分压不太大的条件下,多晶铁片与氧作用生成的化合物是Fe₃O₄而不是FeO和α-Fe₂O₃,也不是三种化合物的分层分布。在600℃以上高温下,氧分压近于1×10^-6Torr下,出现了铁表面上氧的热脱附过程而不是氧化过程。 关键词：     

Oxidation of iron studied by PAC and CEMS

A metallic Fe specimen, implanted with ¹¹¹In, was oxidized and subsequently annealed in a high vacuum for PAC spectroscopy. This treatment gave rise to a huge PAC signal. The magnitude of the hyperfine field was found to be one third of that in metallic Fe. CEMS on an enriched Fe foil given exactly the same treatment has revealed that a maghemite phase (γ-Fe₂O₃) is formed right after the oxidation treatment and a magnetite phase (Fe₃O₄) after the vacuum annealing. ¹¹¹In in the magnetite phase was found to give rise to a PAC signal with large amplitude. PAC spectroscopy in an external magnetic field has revealed that the site of ¹¹¹In is the tetrahedral site of the magnetite with the hyperfine field of +12 T, which is in excellent agreement with those in the ferrites. The present method of oxidation of metallic Fe with nuclear probes in it is quite useful for the study of oxidation processes. Also, it provides us with a simple means to prepare ferrite specimens incorporated with nuclear probes. This revised version was published online in August 2006 with corrections to the Cover Date.     

Magnetic,electronic and structural properties of ZnxFe3−xO4

A series of samples Zn_xFe_3−xO₄ have been prepared by the chemical coprecipitation technique and characterized by X-ray diffraction (XRD), vibrating sample magnetometry (VSM) and X-ray photoelectron spectroscopy (XPS). XRD demonstrates all the samples of Zn_xFe_3−xO₄ have a spinel structure same as Fe₃O₄. The magnetic hysteresis loops of Zn_xFe_3−xO₄ obtained from VSM indicate that the saturation magnetization has a maximum when x is ∼1/3. The chemical states of Fe atoms and Zn atoms in zinc ferrites have been measured using XPS and Auger electron spectroscopy (AES). The Fe 2p core-level XPS spectra and Zn L₃M₄₅M₄₅ Auger peaks have been analyzed and the results have been discussed in correlation with the samples’ magnetic properties. These results suggest most of Zn atoms occupy the tetrahedral sites and a small amount of them occupy the octahedral sites.