Fe Crystals Research Articles

Modelling of the evolution of crack of nanoscale in iron

Metal owns the ability of self-healing to some extent, and the ability of the internal crack healing is most desirable for improving the reliability of metal. A molecular dynamics simulation has been further developed to investigate the evolution of a nanoscale crack in body centred cubic Fe crystal under the conditions of heating or compressive pressure. When system temperature drops, the evolution of the crack that was at elevated temperature has been studied for the first time. N-body potential according to the embedded atom method has been adopted. The original nanoscale crack is expressed by removing some atoms in the centre of the cell, and the minimum vertical distance between the atoms on the top and bottom crack surfaces has been defined as Dm for assessing the process of crack evolution. The results show that a crack healing process can be accelerated significantly with an increase of temperature. When the system temperature decreases, Dm of the crack that was in healing process does not change significantly but fluctuates in a narrow range. This means that the crack healing is the result of Fe atoms diffusing into the crack area but not the thermal stress incurred in the simulation cell at elevated temperature. The pre-compressive pressure under the condition of both biaxial and uniaxial loadings can help promote the crack healing significantly and results in more uniform distribution of defects after healing.

High spin polarization at the Fe/C60 interface in the Fe-doped C60 film

A process of tunneling conduction and the spin-dependent resistivity change (so-called tunneling magnetoresistance effect) in the Fe-doped C60 film with a granular structure is investigated for the current-into-plane device. Cooperative tunneling (cotunneling) through several Fe nanoparticles is suggested to be operative at temperatures lower than 20K. By considering the effect of cotunneling on the magnetoresistance ratio, it is successfully shown that the spin polarization of tunneling electrons generated at the Fe/C60 interface is much higher than that in Fe crystal at low temperature in a similar fashion to that at the Co/C60 interface in the Co-doped C60 films. A strong temperature dependence of spin polarization is observed, suggesting a possible influence by the thermally induced disorders ascribed to the Fe atoms bonded with C60 in the C60Fe compound.

X-shape two-dimensional photovoltaic dark spatial solitons with fully incoherent white light

We report on the first successful experimental observation of X-shape two-dimensional photovoltaic dark spatial solitons in an anisotropic LiNbO3: Fe crystal with fully incoherent white light beams under odd or even initial conditions. This kind of solitons results from the photovoltaic self-defocusing nonlinearity, which depends on the orientation of the principal axes of the crystal.

Photorefractive Properties Varied With Li Composition in $\hbox{LiNbO}_{3}$:Fe Crystals

LiNbO3 : Fe crystals with different Li compositions were prepared in this paper. Both holographic storage and photo-induced light scattering experiments were carried out on these crystals. Our results showed that, with the increase of the Li composition in LiNbO3 : Fe, the response time of the material was shortened largely, while the photoinduced light scattering in the crystals was suppressed significantly. These improvements of the photorefractive properties of LiNbO3 : Fe were attributed to the change of polaronic system in the material. In addition, the rapid change of the photo-induced light scattering with the Li composition was found between 48.9 and 49.3 mol%. It was explained by the threshold effect of photo-induced light scattering with respect to the photovoltaic field.

Modulation instability of bright photovoltaic solitons on a partially incoherent background

We present the observation of incoherent anti-dark photovoltaic solitons in LiNbO3:Fe crystal. This new class of soliton states involves bright photovoltaic solitons on a background beam meeting κ > 1, where κ is the ratio of background illumination photovoltaic constant to that of soliton beam. For κ < 1, dark photovoltaic solitons are generated. Furthermore, this novel type solitons are investigated experimentally by injecting coherent light and partially coherent background of infinite extent. In case of spatial coherence of the background lower than the threshold of incoherent modulation instability, these results indicate that bright photovoltaic solitons can propagate in a stable fashion.

Peculiarities of photonic lattices recorded by Bessel beam technique in LiNbO3:Fe crystal

The aim of this work is the investigation of the peculiarities of recording of the refractive photonic lattices by Bessel beam technique in photorefractive Fe doped LiNbO3 crystal. Optical C-axis of the crystal was oriented along the crystal surfaces. The Gaussian laser beam at 532nm wavelength with 17.7mW is transformed into the Bessel beam by an axicon. The intensity pattern of non-diffracting Bessel beam is imparted into the photorefractive medium, being irradiated, via electro-optic effect, thus creating radially modulated refractive index photonic lattices. The annular refractive photonic lattice created inside the LN:Fe crystal has 10 μm period in radial direction. The recorded photonic lattice has been tested by probe beam. The direct observation of recorded lattice by phase microscope was also performed. The azimuthal dependence of created lattice was observed. The qualitative explanation is given.

Multiple-hologram recording with one-beam encoding

This research proposes a method that uses a single object beam to record multiple images in a photorefractive crystal medium without having to use any reference wave. The object beam in this study is modulated using a lenticular lens array sheet to produce a set of sub-object beams. These beams are then angularly separated on the recording plane and their scattered waves overlapped in an iron-doped photorefractive LiNbO₃ crystal. This single-exposure, multiple-holographic-recording method is simple and proven successful via the experiments that recorded four holograms in a 30 x 1 mm³ LiNbO₃:Fe crystal with single exposure.

Factors that influence the modulation instability in self-defocusing photorefractive crystal

We report on the experimental observation of the modulation instability in self-defocusing photorefractive LiNbO3: Fe crystal. Experimental results show that the modulation instability is related to the illumination time; and it is also related to the angle that between the direction of c-axis and the direction of the threadlike beam that illuminate the crystal. And the modulation instability is also related to the shape of the incident light.

Molecular Dynamics Simulation of Induced Solidification Process of Pure Liquid Fe by Al&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt; Nanoparticles

The molecular dynamics(MD) method has been used to simulate the solidification process of pure liquid Fe induced by a series of Al2O3 nanoparticles of different radii at three temperatures(1750,1730 and 1710 K).The structural evolution of Al2O3 nanoparticles and the effect of these particles on the solidification process of pure Fe have been analyzed.It was found that during the solidification process,the inner structure of the Al2O3 nanoparticles remained crystalline and structural deformation only occurred in surface atoms.The CTIM-2 method showed that the solidified Fe atoms were mainly faced-centered cubic(fcc) and hexagonal closed-packed(hcp) atoms.In addition,the temperature at which solidification took place was influenced by the size of the Al2O3 nanoparticles.The orientations of the resulting Fe crystals were influenced by the extent of the drift of the Al2O3 nanoparticles.

Molecular Dynamics Simulation of Porous Layer-enhanced Dislocation Emission and Crack Propagation in Iron Crystal

The internal stress induced by a porous layer or passive layer can assist the applied stress to promote dislocation emission and crack propagation, e.g. when the pipeline steel is buried in the soil containing water, resulting in stress corrosion cracking (SCC). Molecular dynamics (MD) simulation is performed to study the process of dislocation emission and crack propagation in a slab of Fe crystal with and without a porous layer on the surface of the crack. The results show that when there is a porous layer on the surface of the crack, the tensile stress induced by the porous layer can superimpose on the external applied stress and then assist the applied stress to initiate crack tip dislocation emission under lowered stress intensity KI, or stress. To respond to the corrosion accelerated dislocation emission and motion, the crack begins to propagate under lowered stress intensity KI resulting in SCC.

Optical fabrication of three-dimensional photonic lattices in LiNbO 3:Fe crystals with a single amplitude mask

We fabricate three-dimensional nonlinear photonic lattices in iron-doped lithium niobate photorefractive crystal for the first time by a single amplitude mask. The experimental setup of this method is very simple and flexible. The period of the lattices can be dominated easily. We analyze the three-dimensional photonic lattices by plane wave guiding and far field diffraction pattern imaging. The induced photonic lattices can exist stably for a long time in the photorefractive crystal.

Effect of iron impurities on the photoluminescence and photoconductivity of ZnSe crystals in the visible spectral region

The photoconductivity and photoluminescence of ZnSe:Fe crystals in the visible spectra region are studied. The scheme of optical transitions within Fe2+ impurity centers is established. It is shown that the high-temperature photoconductivity of ZnSe:Fe crystals is controlled by optical transitions of electrons from the 5 E(F) ground state to the higher levels of excited states of Fe2+ ions, with subsequent thermal activation of the electrons to the conduction band. Efficient excitation of intracenter luminescence of ZnSe:Fe crystals is attained with light corresponding to the region of intrinsic absorption in Fe2+ ions.

Optically induced three-dimensional nonlinear photonic lattices in LiNbO 3:Fe crystal

We fabricate three-dimensional nonlinear photonic lattices in self-defocusing iron-doped lithium niobate photorefractive crystal for the first time by an improved Michelson interferometer device. The experimental setup is simple and inexpensive. The period of the induced photonic lattices can be dominated easily. We analyze the three-dimensional nonlinear photonic lattices by plane wave guiding and far field diffraction pattern imaging. The induced nonlinear photonic lattices can exist stably for a long time in the photorefractive crystal.

Light-induced waveguide by a finite self-trapped vortex beam in a photorefractivemedium

We report the formation of a 2D waveguide induced by a self-trapped vortexbeam in a photorefractive–photovoltaic medium. Demonstrations are performedby monitoring the self-trapping dynamic of an annular vortex beam in aLiNbO3:Fe crystal. Control of the experimental observations is consistent with the underlying physicsrevealed by a numerical model.

Microstructure and Magnetic Properties of Fe and Fe-alloy Thin Films Epitaxially Grown on MgO(100) Substrates

Fe, Fe50Co50, and Fe80Ni20 (at. %) single-crystal films with the (100)bcc plane parallel to the substrate surface were prepared on MgO(100) single-crystals heated at 300 °C by ultra high vacuum molecular beam epitaxy. The film growth mechanism, the film structure, and the magnetic properties were investigated. In-situ reflection high energy electron diffraction and X-ray diffraction analyses indicate that the strains in the films are very small though there are fairly large mismatches of −3.7∼−4.3% at the film/substrate interface. Cross-sectional high-resolution transmission electron microscopy shows that misfit dislocations are introduced in the film at the interface. Dislocations are also observed in the film up to around 10∼20 nm distance from the interface. The presence of such dislocation relieves the strain caused by the lattice mismatch. The in-plane magnetization properties of these films reflect the magnetocrystalline anisotropies of respective bulk Fe, Fe50Co50, and Fe80Ni20 crystals.

Effect of Hydroxyapatite Matrix on the Growth of Carbon Nanotubes via a CVD System

This paper reports the fabrication of carbon nanotubes (CNTs) /hydroxyapatite (HA) composites via in-situ process in a chemical vapor deposition (CVD) system and the effect of HA matrix on the growth of CNTs. When the HA matrix is fabricated by a chemical coprecipitation method, deficient HA (D-HA) crystals in nano size have been observed in these composites. There are about 20% weight percent of multi-walled CNTs with a mean diameter of 40~60 nm. When the matrix used as catalyst for CNTs growth without HA in the same way, no CNTs but solid carbon fibers in submicrometer scale have been produced in these composites. Only Fe and MgO crystals can be observed in these composites. When the HA matrix is fabricated by a physical mixing with the presence of D-HA crystals, hollow CNTs with the diameter of 180~210 nm are also produced in these composites. Fe and MgO besides P2O5, D-HA and C crystals can be observed in the composites.

Optimal recording wavelength for maximum diffraction efficiency of thermal fixing in LiNbO_3:Fe

Optimal recording wavelength for maximum diffraction efficiency of thermal fixing in LiNbO(3):Fe crystal is investigated. Holographic gratings are recorded using three typical recording wavelengths including 488, 514, and 633 nm. Optimal switching from recording to thermal fixing is taken into consideration. The fixed holograms are developed by an original recording setup. Diffraction efficiencies of recording and thermal fixing are measured by a two-wave coupling technique. The theoretical and experimental results are analyzed and compared. With a blue beam, the nonvolatile hologram with maximum fixing efficiency is achieved. This work can obtain high persistent diffraction of the nonvolatile holographic storage in LiNbO(3):Fe crystals.

Structure and magnetic properties of Fe epitaxial thin films prepared by UHV rf magnetron sputtering on GaAs single-crystal substrates

Fe thin films were prepared on GaAs single-crystal substrates of (100) B3 , (110) B3 , and (111) B3 orientations by ultra high vacuum rf magnetron sputtering. The effects of substrate orientation and substrate temperature on the film growth, the structure, and the magnetic properties were investigated. On GaAs(100) B3 substrates, Fe(100) bcc single-crystal films are obtained at 300 °C, whereas Fe films consisting of bcc(100) and bcc(221) crystals epitaxially grow at room temperature (RT). Fe(110) bcc and Fe(111) bcc single-crystal films are respectively obtained on GaAs(110) B3 and GaAs(111) B3 substrates at RT–300 °C. The in-plane lattice spacings of these Fe epitaxial films are 0–9% larger than the out-of-plane lattice spacings due to accommodation of lattice mismatch between the films and the substrates. The film strain is decreased by employing an elevated substrate temperature of 300 °C. The in-plane magnetization properties are reflecting the magnetocrystalline anisotropy of bulk bcc–Fe crystal.

Effect of light polarization on holographic recording in glassy azocompounds and chalcogenides

Abstract Light polarization effects on a holographic grating recording in a glassy chalcogenide a-As40S15Se45 film has been experimentally studied and compared with previously studied glassy molecular azobenzene film 8a at 633, using s − s,p − p, CE-1 and CE-2 circular-elliptic recording-beam polarizations (differing by light electric field rotation directions). The azocompound exhibited much higher self-diffraction efficiency (SDE) and diffraction efficiency whereas chalcogenide was more sensitive. Their recording efficiency polarization dependences also were different. SDE up to 45% was achieved in 8a with p − p and up to 2.6% in a-As40S15Se45 with CE-2 polarized recording beams. The polarization changes in the diffraction process were studied as well in these and other materials (11, 16, 19 and a-As2S3 film, LiTaO3:Fe crystal). It was found that light polarization changes in the process of diffraction from gratings recorded vectorially by s−p polarizations depended on chemical composition, wavelength, and exposure time. Vector gratings with SDE up to 25% were recorded in 8a, rotating a linear polarization by 90°. No light polarization changes were found in azobenzene 19 and chalcogenide films and in LiTaO3:Fe crystal, thus showing a vector recording of scalar holograms. The recording mechanisms in azocompounds and chalcogenides are discussed and compared.

Graphene Growth by a Metal-Catalyzed Solid-State Transformation of Amorphous Carbon

Single and few-layer graphene is grown by a solid-state transformation of amorphous carbon on a catalytically active metal. The process is carried out and monitored in situ in an electron microscope. It is observed that an amorphous carbon film is taken up by Fe, Co, or Ni crystals at temperatures above 600 °C. The nucleation and growth of graphene layers on the metal surfaces happen after the amorphous carbon film has been dissolved. It is shown that the transformation of the energetically less favorable amorphous carbon to the more favorable phase of graphene occurs by diffusion of carbon atoms through the catalytically active metal.