Interaction of iron atoms with the Si(100)-2 × 1 surface

Interaction of iron atoms with the Si(100)-2 × 1 surface at room temperature is studied by core-level photoelectron spectroscopy using synchrotron radiation for Fe coverages ranging from a fraction of a monolayer to six monolayers. It is shown that the Fe/Si(100)-2 × 1 interface is chemically active: the Fe-Si solid solution forms early in deposition of iron on silicon. When the Fe coverage reaches four to five monolayers, the state of the system is changed and Fe₃Si silicide arises.     

Silicide formation in bilayer ultrathin iron and cobalt films on silicon

The processes that occur in ultrathin (up to 1 nm) Fe and Co layers during deposition onto the Si(100)2 × 1 surface in various sequences and during annealing of the formed structures to a temperature of 400°C are studied. The elemental and chemical compositions of the films are analyzed by in situ high-resolution X-ray photoelectron spectroscopy using synchrotron radiation, and their magnetic properties are determined using the magnetic linear dichroism effect in the angular distribution of Fe 3p and Co 3p electrons. It is shown that, when iron is first deposited, the formed structure consists of the layers of FeSi, Fe₃Si, Co-Si solid solution, and metallic cobalt with segregated silicon. The structure formed in the alternative case consists of the layers of CoSi, Co-Si solid solution, Co, Fe-Si solid solution, and Fe partly covered by silicon. All layers (apart from FeSi, CoSi) form general magnetic systems characterized by ferromagnetic ordering. Annealing of the structures at temperatures above 130dgC (for the Co/Fe/Si system) and ～200°C (for Fe/Co/Si) leads to the formation of nonmagnetic binary and ternary silicides (Fe _x Co_{1 ? x} Si, Fe _x Co_{2 ? x} Si).     

Influence of the thermal power of a Fe atomic flux on the formation of Cu/Fe nanofilms on a Si(001) substrate

The growth of a Fe sublayer 1.5–14.0 monolayers (MLs) thick and a Cu film (about 5 MLs) on this sublayer is studied at a reduced temperature (1240°C) and an elevated temperature (1400°C) of a Fe source and at a reduced temperature (900°C) of a Cu source. The films are examined by Auger electron spectroscopy, low-energy electron diffraction, and atomic force microscopy. As metal sources, thin Fe and Cu strips on a Ta foil are used. It is shown that a nonequilibrium 2D phase forms in the Fe-on-Si(001) film up to a thickness of 4–5 MLs. This phase appears as closely packed atomically smooth nanoislands. When the thickness of the film exceeds 4–5 MLs, the nonequilibrium Fe phase changes to the bulk (3D) phase of Fe and its silicide Fe _x Si. At Fe source temperatures of 1240 and 1400°C, the nonequilibrium phase consists of Fe with Si segregated on the Fe surface, and a Fe-Si mixture. Copper on the nonequilibrium Fe and Fe-Si phases grows, respectively, as a smooth layer Cu with Si segregated on the top and in the form of Cu-Fe and Cu-Si mixtures. Cu islands growing on the bulk Fe and Fe _x Si phases have smaller and larger sizes, respectively.     

Initial stages of iron silicide formation on the Si(1 0 0)2 × 1 surface

The initial stages of iron silicide growth on the Si(1 0 0)2 × 1 surface during solid-phase synthesis were investigated by photoelectron spectroscopy using synchrotron radiation. The experiments were made on iron films of 1-50 monolayer (ML) thickness in the temperature range from room temperature to 750 °С. Our results support the existence of three stages in the Fe deposition on Si(1 0 0) at room temperature, which include formation of the Fe-Si solid solution, Fe₃Si silicide and an iron film. The critical Fe dose necessary for the solid solution to be transformed to the silicide is found to be 5 ML. The solid-phase reaction was found to depend on the deposited metal dose. At 5 ML, the reaction begins at 60 °С, and the solid-phase synthesis leads to the formation of only metastable silicides (FeSi with the CsCl-type structure, γ-FeSi₂ and α-FeSi₂). A specific feature of this process is Si segregation on the silicide films. At a thickness of 15 ML and more, we observed only stable phases, namely, Fe₃Si, ε-FeSi and β-FeSi₂.     

Formation of iron and iron silicides on silicon and iron surfaces. Role of the deposition rate and volumetric effects

A thin iron film deposited at the rate of 10³ nm/sec on the Si(001) surface and a sandwich structure silicon/iron/Si(111) are studied by Surface Magneto-Optic Kerr Effect, High Resolution Electron Microscopy and X-ray Photoelectron Microscopy methods. The phases present in the structures are identified. Both structures are non-uniform. The ultra-fast-deposited film is magnetically hard (H _c=45 Oe), it contains the silicide Fe₅Si₃. The XPS line shift by +0.55 eV with respect to the pure iron 2p 3/2 level is attributed to Fe₅Si₃. The cross-section image of the sandwich structure shows the presence of enhanced-intermixing channels crossing the Si-rich layer. Iron atoms are the main diffusion species both at the Fe/Si(111) and Si/Fe interfaces. The nature of the volume defect and internal stresses in the transforming iron silicides and their effects on material intermixing and film growth process are discussed.     

Effects of magnetic anisotropy and exchange in Tm2Fe17

Neutron diffraction experiments have been carried out to study the magnetocrystalline anisotropy of two (2b and 2d) Tm sublattices and four (4f, 6g, 12j, and 12k) Fe sublattices in ferrimagnetic compound Tm₂Fe₁₇ (space group P6₃/mmc). We have determined the temperature dependence of the magnitude and orientation of magnetization for each of the thulium and iron sublattices in the range (10?C300) K. A spontaneous rotation (at about 90 K) of the Tm and Fe sublattice magnetizations from the c-axis to the basal plane is accompanied by a drastic change in the magnetization magnitude, signifying a large magnetization anisotropy. Both Tm sublattices exhibit an easy-axis type of the magnetocrystalline anisotropy. The Fe sublattices manifest both the uniaxial and planar anisotropy types. The sublattice formed by Fe atoms at the 4f position reveals the largest planar anisotropy constant. The Fe atoms at the 12j position show a uniaxial anisotropy. We find that the inelastic neutron scattering spectra measured below and above the spin-reorientation transition are remarkably different.     

Effect of Nitrogenation and Hydrogenation on the Magnetic Properties and Structure of the Sm<Subscript>2</Subscript>Fe<Subscript>17</Subscript> Alloy: Analysis of XMCD Data

Changes in the local magnetic and structural properties of Sm₂Fe₁₇ alloys at nitrogenation and hydrogenation of samples have been studied by the X-ray magnetic circular dichroism (XMCD) technique at the Fe K absorption edge and Sm L₃ absorption edge using synchrotron radiation. The results have been discussed in comparison with X-ray diffraction data and macroscopic vibration magnetometry measurements. The observed changes in XMCD spectra indicate a noticeable effect of nitrogenation on the local magnetic properties of sublattices of both iron and samarium, whereas hydrogenation of samples makes a small effect. The mentioned effects have been analyzed and discussed in terms of the effect of nitrogen (N) and hydrogen (H) interstitial atoms on Sm 5d and Fe 4p electronic states. The effect of nitrogenation is larger than the effect of hydrogenation because the volume expansion of the crystal lattice of initial Sm₂Fe₁₇ in the case of nitrogenation is larger than that in the case of hydrogenation. The studied local magnetization curves for samarium and iron sublattices in magnetic fields up to 17 T also indicate a strong increase in the magnetocrystalline anisotropy at nitrogenation.     

Diffusion mechanisms in the Fe_3Si alloys

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Diffusion mechanisms in the Fe3Si alloys

In this paper,the possible reascon for the high thermal vacancy concentration and the low migration barriers for the Fe atorn diffusion in the stoichiometric DO3 structure Fe3Si have been discussed.The high thermal vacancy conenrrarion was attributed to the compression of Fe-Fe atcmic pairs and the tension of Fe-Si atomic pairs in Fe75Si25.The deformations (compression or tensicn)of the atompairs incrcase the interatomic potentials and thus dercrease the enthalpies of vacancy formation.The low minration bamiers for the Fe atom diffusion in Fe75Si25 were related to the symmetric property of the triangualar bamiers.Additionally.it was considered that the Si atoms in Fe3Si could probably migrate via nearest-neighbour jumps without disturbing the long-range order of atomic arrangements,provided that during the diffusion process the residence time on the antistructure sites is very short.     

Size effects and magnetization of (Fe/Si)<Subscript><Emphasis Type="Italic">n</Emphasis></Subscript> multilayer film nanostructures

The temperature dependence of the magnetization of (Fe/Si) _n multilayer films with nanometer layers is investigated. The films are prepared through thermal evaporation under ultrahigh vacuum onto Si(100) and Si(111) single-crystal substrates. It is revealed that the thickness of individual iron layers in (Fe/Si) _n multilayer films affects the magnetization and its temperature dependence. The inference is made that this dependence is associated with the formation of a chemical interface at the Fe-Si boundaries. The characteristics of the chemical interface in the (Fe/Si) _n films are estimated.     

Low temperature thermal annealing-induced α-FeSi2 derived phase in an amorphous Si matrix

Thermal annealing-induced recrystallisation in Fe ion-implanted Si was investigated by transmission electron microscopy. Single crystals of Si(111) were implanted with 120 keV Fe ions to a fluence of 1.0×10¹⁷ cm^-2 at cryogenic temperature. A buried amorphous Fe-Si layer in an amorphous Si matrix was formed in the as-implanted sample. Nanobeam electron diffraction revealed that metastable α-FeSi₂ precipitates embedded in the amorphous Si matrix were formed after annealing at 350 °C for 8 h. The formation of this α-FeSi₂-derived phase was unusual, because it has been observed only in epitaxially grown thin films. Based on the Fe_1-xSi (0<x<0.5) phase with the CsCl structure, which is another metastable phase in the Fe-Si binary system, we discuss the formation process of the metastable α-FeSi₂ in the amorphous matrix. PACS 61.43.Dq; 61.14.Lj; 61.80.Jh     

Quasi-lattice model for the thermodynamic properties and microscopic structure of molten Fe-Si alloy

The quasi-lattice model has been used to study the concentration dependant thermodynamic properties and microscopic structure of Fe-Si alloys in molten state. We have determined the free energy of mixing, excess entropy of mixing, the concentration-concentration structure factor in long wavelength limit [S_CC(0)] and the Warren-Cowley short range order parameter (α₁) of Fe-Si liquid alloy at 1873 K. The observed asymmetry in the properties of mixing of Fe-Si alloy in molten state is successfully explained on the basis of the quasi-lattice model. The analysis suggests that the Fe₂Si complexes are most likely to exist in the liquid state and are strongly interacting in nature. The theoretical analysis suggests that the pairwise interaction energies between the species depend considerably on temperature and the alloy is more ordered towards Fe rich region.     

A Mössbauer spectroscopic study of 3d magnetism of R2Fe14B

Guided by the occupancies and iron magnetic moments μ³, ⁵⁷Fe Mössbauer parameters of Y₂Fe₁₄B at 250K, and in turn for other temperatures, of the sublattices of iron were deduced. Plots of μ(T) in reduced coordinates, through the established correlation between hyperfine field Hn and μ, show that the corresponding state of different iron sites is different and all experimental points fall below Brillouin function. The relation between exchange integral deviation parameter Δ and standard deviation of Fe-Fe interatomic distances S is linear, indicating electrostatic nature of exchange interactions between spins in neighboring atoms. It is inclined to the view that fluctuations of exchange integral is responsible for low T_c of R₂Fe₁₄B.     

Mössbauer study of magnetic order in RBa2Fe3O8

Complete replacement of copper by iron in RBa₂Cu₃O₇ leads to RBa₂Fe₃O₈ (R=Y, rare earth). Mössbauer spectroscopy measurements of⁵⁷Fe and¹⁵¹Eu in RBa₂Fe₃O₈ (R=Y, Eu, Ho, Er) at temperatures 4.2–800 K have been performed. Some of the spectra reveal two inequivalent iron sites, probably corresponding to iron in the Fe(2) site (fivefold oxygen coordination) and in the Fe(1) site (octahedral oxygen coordination). In all compounds the iron moments order antiferromagnetically at the same Néel temperatureT _N720 K. The¹⁵¹Eu Mössbauer spectra of EuBa₂Fe₃O₈ show that the Eu ion is trivalent and exposed to a small exchange field from the iron sublattices.     

Magnetic-dichroism study of iron silicides formed at the Fe/Si(100) interface

The interplay between the phase composition, electronic structure, and magnetic properties of the Fe/Si(100)2×1 interface has been studied at the initial stages of its formation (at Fe doses up to 8 Å). The experiments were carried out in ultra high vacuum by using high-resolution photoelectron spectroscopy with synchrotron radiation. The interface magnetic properties were examined in terms of magnetic linear dichroism in angle-resolved Fe 3p core-level photoemission. It was found that at room temperature a disordered Fe–Si solid solution is formed at the first stage of Fe deposition (≤3.4 Å). In the coverage range of 3.4–4.3 Å the solid solution transforms into Fe₃Si. However, the in-plane ferromagnetic ordering of the silicide occurs only at 6.8 Å Fe that demonstrates the thickness dependence of the magnetic properties of Fe₃Si. The subsequent sample annealing to 150°C transforms Fe₃Si to ε-FeSi, leading to the disappearance of ferromagnetic behavior.     

High field Mössbauer effect study of LaFe12O19

Mössbauer effect measurements on ⁵⁷Fe in LaFe₁₂O₁₉ at room temperature and at 4.2 K in an external magnetic field of 10 T confirm the suggestion of Lotgering[2] that the replacement of Ba²⁺ in BaFe₁₂O₁₉ by La³⁺ is accompanied by a valence change of Fe³⁺ to Fe²⁺ in the 2a-sublattice. The corresponding Fe²⁺-subspectrum has been detected. The increased hyperfine fields of the 12k- and 2b-sites in LaFe₁₂O₁₉ are discussed in terms of the different exchange paths between the iron sublattices.     

Phase analysis in α-Fe after high-dose Si ion implantation by depth-selective conversion-electron Mössbauer spectroscopy (DCEMS)

Si⁺ ions of 50 keV in energy were implanted into α-Fe (95% ⁵⁷Fe) with a nominal dose of 5 × 10¹⁷ cm^?2 at 350°C. The depth distribution of the Fe-Si phases formed by ion implantation after annealing at 300 and 400°C for 1 h was studied quantitatively by depth-selective conversion-electron Mössbauer spectroscopy (DCEMS). Ordered Fe₃Si and ε-FeSi was observed.     

The magnetic and hyperfine properties of iron in silicon carbide

The magnetic and hyperfine properties of iron impurities in 3C- and 6H- silicon-carbide are calculated using the abinitio method of full-potential linear-augmented-plane-waves. The iron atoms are introduced at substitutional carbon, Fe _C , and silicon, Fe _Si , sites as well as at the tetrahedral interstitial sites with four nearest neighbours carbon atoms, Fe _I (C), and four nearest neighbours silicon atoms, Fe _I (Si). The effect of introducing vacancies at the neighbours of these sites is also studied. Fe atoms with complete neighbors substituted at Si or C sites are found to be nonmagnetic, while Fe atoms at interstitial sites are magnetic. Introduction of a vacancy at a neighboring site reverse the picture.     

Point defects in FeAl

Summary Point defects in annealed B2-phase FeAl samples in the range 47–53 at.% Fe were studied using⁵⁷Fe M?ssbauer spectroscopy. Spectra were analyzed using local environment models according to which point defects in atomic shells close to probe atoms induce shifts in the nuclear monopole interaction. For well-annealed samples, better results were obtained assuming only the presence of Fe_Al antisite and V_Fe vacancy defects, and not of Al_Fe antisite defects. Monopole interactions of⁵⁷Fe probes on the Fe and Al sublattices having no defects in the first two shells were about +0.27 and −0.03 mm s⁻¹, respectively, with respect to Fe in alpha-Fe metal. The shifts induced by Fe_Al and V_Fe defects in the first shells of Fe probes on the Fe and Al sublattices were −0.15 and −0.24 mm s⁻¹, respectively, and, in the second shells, +0.06 and +0.011 mm s⁻¹. In addition to structural defects needed to accommodate deviations from stoichiometry, annealed samples were found to contain several percents of Fe_Al and V_Fe defects due to lattice disorder, with greater disorder in Fe-deficient alloys. Paper presented at the ICAME-95, Rimini, 10–16 September 1995.     

CEMS study of silicon implanted iron

Thin layers of iron-rich Fe-Si alloys were formed by silicon implantation into iron at room temperature with different energies (100, 200, and 300 keV) and ion doses (2 × 10¹⁷ to 1×10¹⁸ cm^–2). The produced layers were investigated by⁵⁷Fe conversion electron Mössbauer spectroscopy (CEMS) to identify the phases formed by the ion implantation. Auger electron spectroscopy (AES) was used to measure the concentration depth profiles of the implanted silicon. Depending on the implantation parameters different disordered Fe-Si structures were detected. At low doses only magnetic phases were formed while at high doses a non-magnetic phase with a hitherto unknown structure appeared. Annealing of the samples resulted first in the formation of a D0₃-like short-range order and a slow decrease of the non-magnetic phase, and subsequently in the migration of Si out of the investigated depth range.