Single Crystal Of Magnetite Research Articles

Spin and orbital magnetic moment of reconstructed2×2R45∘magnetite(001)

The surface of a magnetite single crystal with (001) orientation has been prepared by sputtering/annealing cycles providing the $\sqrt{2}\ifmmode\times\else\texttimes\fi{}\sqrt{2}R{45}^{\ensuremath{\circ}}$ reconstruction. The distribution of magnetic domains on the surface has been imaged by x-ray magnetic dichroism in a photoemission microscope. The easy axes are along the surface in-plane $\ensuremath{\langle}110\ensuremath{\rangle}$ directions. The near-surface magnetic moment was determined by applying the sum rules to XMCD spectra obtained with different kinetic energies of the secondary electrons. A reduced total moment of $3.3 {\ensuremath{\mu}}_{B}$ and a ratio of about 0.10 between orbital and spin moment was found, which we attribute to the surface reconstruction.

What is the correct Fe L23 X-ray absorption spectrum of magnetite?

Various groups have reported Fe L23 X-ray absorption spectra (XAS) of magnetite (Fe3O4), each claiming to be that of magnetite, but which contradict each other. Here we report an XAS study of two kinds of magnetite: one is biogenic magnetite nanocrystals extracted from the magnetotactic bacterium Magnetovibrio blakemorei strain MV-1; the other is synthetic, abiogenically produced nano-magnetite. We see significantly different XAS spectra of these two materials. Only when the abiogenic magnetite was reduced under H2 did it give the same spectrum as the biogenic sample. Extensive heating of the biogenic magnetite in air produced spectra similar to that of the abiogenic magnetite. These two spectra are typical of the range of published Fe L23 spectra of magnetite. X-ray diffraction confirmed that the biogenic material is stoichiometric Fe3O4, and showed that the as-received or partly reduced abiogenic material is a non-stoichiometric oxide, intermediate between magnetite and maghemite (γ-Fe2O3). When the membrane which surrounds magnetosome chains was intact, the biotic magnetite single crystals were surprisingly resistant to oxidation. This study clarifies a significant confusion existing in the literature as to the correct Fe L23 X-ray absorption spectra of magnetite and maghemite.

Full vector low‐temperature magnetic measurements of geologic materials

AbstractThe magnetic properties of geologic materials offer insights into an enormous range of important geophysical phenomena ranging from inner core dynamics to paleoclimate. Often it is the low‐temperature behavior (<300 K) of magnetic minerals that provides the most useful and highest sensitivity information for a given problem. Conventional measurements of low‐temperature remanence are typically conducted on instruments that are limited to measuring one single‐axis component of the magnetization vector and are optimized for measurements in strong fields. These instrumental limitations have prevented fully optimized applications and have motivated the development of a low‐temperature probe that can be used for low‐temperature remanence measurements between 17 and 300 K along three orthogonal axes using a standard 2G Enterprises SQuID rock magnetometer. In this contribution, we describe the design and implementation of this instrument and present data from five case studies that demonstrate the probe's considerable potential for future research: a polycrystalline hematite sample, a polycrystalline hematite and magnetite mixture, a single crystal of magnetite, a single crystal of pyrrhotite, and samples of Umkondo Large Igneous Province diabase sills.

Oxidation of Magnetite(100) to Hematite Observed by in Situ Spectroscopy and Microscopy

We study where and how hematite (α-Fe2O3) nucleates and grows during the oxidation of magnetite(100) single crystals. Hematite inclusions grow along ⟨110⟩ directions of the magnetite (Fe3O4), leading to a biaxial array of hematite slabs in an electrically conducting matrix of magnetite. The slab arrays form in both bulk single crystals and thin films of magnetite. Atomic force microscopy reveals that the surface growth of magnetite that accompanies hematite formation is faster adjacent to the hematite slabs. In situ X-ray photoelectron and X-ray absorption spectroscopies at 600 °C in an oxygen environment reveal that the conversion of the Fe2+ in magnetite to Fe3+ in hematite occurs without the formation of the metastable phase maghemite (γ-Fe2O3). We offer an explanation of why Fe3O4(100) oxidizes faster than Fe3O4(111).

The Verwey transition in nanostructured magnetite produced by a combination of chimie douce and spark plasma sintering

Magnetite nanoparticles about 10 nm sized were synthesized by the polyol method. Zero-field-cooled (ZFC)-FC measurements showed a blocking temperature ∼170 K and the absence of the Verwey transition. They were subsequently consolidated by spark plasma sintering at 750 °C for 15 min, leading to a high density (92% of the theoretical density), solid body, with grains in the 150 nm range. X-ray diffraction patterns exhibited a spinel single phase with cell parameters corresponding to the magnetite structure. Magnetic measurements showed a decrease of coercivity from 685 Oe (54.5 kA/m) at 118 K to 90 Oe (7.2 kA/m) at 139 K. ZFC measurements at 25 Oe presented a three-fold magnetization increase as temperature increased; a small transition between 116 and 117.5 K, followed by a larger one from 117.6 to 124 K. The first transition can be associated with a complex crystallographic transition and delocalization of Fe2+-Fe3+, while the second one can be attributed to spin reorientation due to the magnetocrystalline anisotropy constant (K1) change of sign as previously observed only in magnetite single crystals.

Heterogeneous reduction of U 6+ by structural Fe 2+ from theory and experiment

Computational and experimental studies were performed to explore heterogeneous reduction of U 6+ by structural Fe 2+ at magnetite (Fe 3O 4) surfaces. Molecular Fe–Fe–U models representing a uranyl species adsorbed in a biatomic bidentate fashion to an iron surface group were constructed. Various possible charge distributions in this model surface complex were evaluated in terms of their relative stabilities and electron exchange rates using ab initio molecular orbital methods. Freshly-cleaved, single crystals of magnetite with different initial Fe 2+/Fe 3+ ratios were exposed to uranyl-nitrate solution (pH ∼ 4) for 90 h. X-ray photoelectron spectroscopy and electron microscopy indicated the presence of a mixed U 6+/U 5+ precipitate heterogeneously nucleated and grown on stoichiometric magnetite surfaces, but only the presence of sorbed U 6+ and no precipitate on sub-stoichiometric magnetite surfaces. Calculated electron transfer rates indicate that sequential multi-electron uranium reduction is not kinetically limited by conductive electron resupply to the adsorption site. Both theory and experiment point to structural Fe 2+ density, taken as a measure of thermodynamic reducing potential, and sterically accessible uranium coordination environments as key controls on uranium reduction extent and rate. Uranium incorporation in solid phases where its coordination is constrained to the uranate type should widen the stability field of U 5+ relative to U 6+. If uranium cannot acquire 8-fold coordination then reduction may proceed to U 5+ but not necessarily U 4+.

Measurement of z-direction component of electron spins field-emitted from a single-crystal magnetite whisker

A 90° sector type spin rotator was developed for measurement of the z-direction component of a spin polarization, which is parallel to the emitter axis. The rotator enables us to measure all components of electron spins field-emitted from a single crystalline magnetite. In-plane component of spin polarization dominated of field-emitted electrons from single crystalline magnetite whisker, thus it is suggested that the magnetization of the magnetite whisker results from the anisotropy of crystalline structure rather than its shape.

Kinetics of the low-temperature hydrogen reduction of single-crystal magnetite

The hydrogen reduction of plates made from synthetic magnetite single crystals is studied in the temperature range 400–570°C. A mathematical model is developed and applied to describe the kinetics of gaseous reduction of plane finite objects. The following time contributions of individual process stages are distinguished with the least squares method and numerical integration: the nucleation of an iron phase, a chemical interaction of magnetite with hydrogen, and external and internal mass transfer. The diffusion and kinetic reduction parameters are calculated. The activation energy of the chemical stage (87.5 kJ/mol) is determined. An equation for the rate constant of the interaction of magnetite with hydrogen is proposed for temperatures below 570°C. A significant effect of the processes of consolidation and sintering of reduced iron on the internal diffusion parameters is detected in the low-temperature range.

Large magnetorefractive effect in magnetite

We carried out a magneto-optical spectroscopic study of magnetite (Fe3O4) thin films and single crystals at optical wavelengths. We observed a relevant quadratic magneto-optic contribution that can be notably larger than first-order linear magneto-optics at some range of wavelengths in the visible region. These unusual quadratic effects are particularly strong at the Verwey temperature (100–120 K) and decay slowly away from this transition temperature. We attribute this remarkable magneto-optical response to a magnetorefractive effect associated with the field-dependent polaronic conductivity at optical frequencies, which interestingly enough is already noticeable at room temperature.

Fabrication of a spin‐polarized electron emitter with <110>‐oriented magnetite whisker

Abstract<110>‐oriented single crystal magnetite (Fe3O4) whiskers 30 to 300 nm in diameter were synthesized on a stainless steel plate by means of the combustion flame thermal oxidation process. An electron source with this single magnetite whisker was fabricated by using a micro‐sampling instrument in an FIB system. The temperature dependence of the spin polarization of field‐emitted electrons from this electron source was measured by using a Mott electron polarimeter. The spin polarization of this sample was found to be 15% at room temperature. This emitter is an oxidative resistance, and can be handled in the atmosphere. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Fabrication of spin-polarized electron emitter with single ⟨110⟩-oriented magnetite whisker

Magnetite (Fe3O4) is oxidative resistant and has a half metallic property and is expected to have spintronic applications. In this work, ⟨110⟩-oriented single crystal magnetite whiskers, 30–300 nm in diameter, were synthesized on a stainless steel plate by means of the combustion flame thermal oxidation process. An electron source with this single magnetite whisker was fabricated by using a microsampling instrument in a focused ion beam system. The spin polarization of the electron source was found to be 15% at room temperature by using a Mott electron polarimeter.

Characterizing magnetism of individual magnetosomes by X-ray magnetic circular dichroism in a scanning transmission X-ray microscope

Soft X-ray scanning transmission X-ray microscopy (STXM) was used to measure the Fe L 2, 3 X-ray magnetic circular dichroism (XMCD) signal from individual, 30 nm diameter magnetosomes in a magnetotactic bacterium, specifically the marine vibrio strain MV-1. The Fe L 2, 3 spectra recorded with circularly polarized X-rays from an elliptically polarizing undulator were very similar in shape and magnitude to those of fully saturated magnetite (Fe 3O 4). As previously determined by other techniques, our results show that the magnetic moments of individual magnetosomes arranged in linear chains in intact cells of strain MV-1 are all oriented in the same direction. The magnitude of the XMCD signal averaged over a chain of 9 magnetosomes in one cell is similar to that of single crystal magnetite. The spectral shape is slightly different, and indicates that magnetosomes have excess Fe (II), as noted in previous bulk studies of biogenic magnetite. To our knowledge, this is the first measurement of the XMCD of individual magnetosomes in magnetotactic bacteria.

Mid-infrared reflectance spectra and optical constants of six iron oxide/oxyhydroxide phases

We have determined the real and imaginary indices of refraction ( n and k) for six iron oxide/oxyhydroxide phases—magnetite, maghemite, goethite, lepidocrocite, akaganéite, and ferrihydrite. A single crystal of magnetite was used to derive bulk n and k values from 100–2000 cm −1 (5–100 μm). Synthetic nanocrystalline samples of maghemite, goethite, lepidocrocite, akaganéite, and ferrihydrite were pressed into compact pellets used to determine bulk n and k values from 100–1200 cm −1 (8.33–100 μm). All values of n and k (the optical constants) were determined from specular reflectance spectra acquired at 2 cm −1 spectral sampling using classical Lorentz–Lorenz dispersion theory. In this paper, we present the optical constants of all six minerals and the oscillator parameters with which they were modeled. Use of these optical constants could aid in radiative transfer models of terrestrial dust as well as Mars, the Moon, and airless bodies in the Solar System.

Fabrication of a Spin-Polarized Electron Source with a Single Magnetite Whisker

<110>-oriented single crystal magnetite whiskers 30 to 200 nm in diameter were synthesized on a stainless steel plate by means of the combustion flame thermal oxidation process. Microscopic analyses have revealed that the whiskers have some defects such as stacking faults and dislocations and also that manganese is the dominant impurity. A spin-polarized electron source with a single magnetite whisker was fabricated by using a micro-sampling instrument in a focused ion beam (FIB) system. The spin polarization of this sample was measured by using a Mott electron polarimeter and was found to be 15% at room temperature.

Charging and Chemical Reactivity of Gold Nanoparticles and Adatoms on the (111) Surface of Single-Crystal Magnetite: A Scanning Tunneling Microscopy/Spectroscopy Study

We present a scanning tunneling microscopy (STM)/scanning tunneling spectroscopy (STS) study of a model catalyst system consisting of supported gold nanoparticles on a reduced Fe3O4(111) surface in ultrahigh vacuum. Gold forms two electrically distinct nanoparticles on an iron oxide surface upon annealing multilayer Au/Fe3O4(111) at 500 °C for 15 min. I (V) curves taken via STS measurements show that large gold nanoparticles (∼8 nm) exhibit a metallic electronic structure and, thus, are likely neutral. Single gold adatoms appear to be strongly bonded to the oxygen sites of the Fe3O4(111) surface, and tunneling electrons are observed to flow predominantly from the STM tip to the Au adatoms and into the oxygen sites of the surface. The site-specific adsorption of the gold adatoms on oxygen surface atoms and the size-sensitive nature of the electronic structure suggest that Au adatoms are likely positively charged. When this Au/Fe3O4(111) system is dosed with CO at 260 K, adsorption of CO molecules normal to...

Monodisperse mesoporous superparamagnetic single-crystal magnetite nanoparticles for drug delivery

In this contribution, we report a facile, gram-scale, low-cost route to prepare monodisperse superparamagnetic single-crystal magnetite NPs with mesoporous structure (MSSMN) via a very simple solvothermal method. The formation mechanism of MSSMN is also discussed and we think that Ostwald ripening probably plays an important role in this synthesis process. It is also interestingly found that the size and morphology of mesoporous Fe 3O 4 NPs can be easily controlled by changing the amount of NaOH and 1,2-ethylenediamine (ETH). Most importantly, the MSSMN can be used as an effective drug delivery carrier. A typical anticancer drug, doxorubicin (Dox), is used for drug loading, and the release behaviors of Dox in two different pH solutions are studied. The results indicate that the MSSMN has a high drug loading capacity and favorable release property for Dox; thus, it is very promising for the application in drug delivery.

Magnetoresistance in magnetite: Switching of the magnetic easy axis

The influence of the external magnetic field B ( B ≤ 4 T) on resistivity in magnetite single crystal was studied at few temperatures both below and above the Verwey transition temperature T V, and in two 〈1 0 0〉 type cubic directions. We have succeeded to confirm our predictions that the magnetic axis switching affects electronic transport. It was also found that the transverse resistivity ( B ⊥ current direction) exceeds longitudinal one, but only for T > T V, with no obvious systematics below the transition temperature. This may indicate that atomic disorder, inherent and poorly defined in a material with structural domains, is the primary factor that governs transport properties below the transition temperature. Finally, and concomitant with the last statement, only very small magnetic field dependence in magnetite below T V may be inferred from our data.

Electro-precipitation of Fe 3O 4 nanoparticles in ethanol

The preparation of superparamagnetic magnetite (Fe 3O 4) nanoparticles by electro-precipitation in ethanol is proposed. Particle average size can be set from 4.4 to 9 nm with a standard deviation around 20%. Combination of wide-angle X-ray scattering (WAXS), Electron energy loss spectroscopy (EELS) and Mössbauer spectroscopy characterizations clearly identifies the particles as magnetite single-crystals (Fe 3O 4).

One-Pot Template-Free Synthesis of Monodisperse and Single-Crystal Magnetite Hollow Spheres by a Simple Solvothermal Route

Uniform-sized, monodisperse, and single-crystal magnetite hollow spheres with a diameter of 200−300 nm and a shell thickness of ∼50 nm have been successfully synthesized in high yield using a template-free solvothermal route. The reaction duration and the ethylenediamine amount are shown to play important roles in the formation of the magnetite hollow spheres. X-ray diffraction, X-ray photoelectron spectroscopy, Fourier tranform IR, scanning electron microscopy, transmission electron microscopy (TEM), and high-resolution TEM were used to characterize the products. The results show that the prepared single-crystalline hollow spheres are composed of well-aligned magnetite and have a cubic structure. The magnetite hollow spheres show a high saturation magnetization of ca. 68 emu/g, a remnant magnetization of ca. 13 emu/g, and a coercivity of ca. 94 Oe at room temperature. A possible mechanism for the formation of magnetite hollow spherical structures is proposed based on the experimental observations. The prepared magnetite hollow spheres have promising applications in biomedical fields due to their above characteristics.

Fischer–Tropsch Synthesis: Morphology, Phase Transformation and Particle Size Growth of Nano-scale Particles

An unpromoted ultrafine iron nano-particle catalyst was used for Fischer–Tropsch synthesis (FTS) in a CSTR at 270 °C, 175 psig, H2/CO = 0.7, and a syngas space velocity of 3.0 sl/h/g Fe. Prior to FTS, the catalyst was activated in CO for 24 h which converted the initial hematite into a mixture of 85% χ-Fe5C2 and 15% magnetite, as found by Mossbauer measurement. The activated catalyst results in an initial high conversion (ca. 85%) of CO and H2; however the conversions decreased to ca. 10% over about 400 h of synthesis time and after that remained nearly constant up to 600 h. Mossbauer and EELS measurement revealed that the catalyst deactivation was accompanied by gradual in situ re-oxidation of the catalyst from initial nearly pure χ-Fe5C2 phase to pure magnetite after 400 h of synthesis time. Experimental data indicates that the nucleation for carbide/oxide transformation may initiates at the center of the particle by water produced during FTS. Small amount of ɛ′-Fe2.2C phase was detected in some catalyst samples collected after 480 h of FTS which are believed to be generated by syngas during FTS. Particle size distribution (PSD) measurements indicate nano-scale growth of individual catalyst particle. Statistical average diameters were found to increase by a factor of 4 over 600 h of FTS. Large particles with the largest dimension larger than 150 nm were also observed. Chemical compositions of the larger particles were always found to be pure single crystal magnetite as revealed by EELS analysis. Small number of ultrafine carbide particles was identified in the catalyst samples collected during later period of FTS. The results suggest that carbide/oxide transformation and nano-scale growth of particles continues either in succession or at least simultaneously; but definitely not in the reverse order (in that case some larger carbide particles would have observed). EELS-STEM measurement reveals amorphous carbon rim of thickness 3–5 nm around some particles after activation and during FTS. Well ordered graphitic carbon layers on larger single crystal magnetite particles were found by EELS-STEM measurement. However the maximum thickness of the carbon (amorphous or graphitic) rim does not grow above 10 nm suggesting that the growths of particles are not due to carbon deposition.