FeSe Single Crystals Research Articles

Preparation of FeSi single crystals and bicrystals for diffusion experiments by the electron-beam floating zone technique

An electron-beam floating zone melting technique has been used to grow oriented single crystals and bicrystals of FeSi alloys with 6, 10, 12 and 14 at% Si. Procedures for the preparation of monocrystalline and bicrystalline seeds as well as for the growing of single crystals and bicrystals of these alloys are described. Studies of chemical composition of the bicrystalline specimens with aid of electron probe microanalysis revealed a homogeneous distribution of the main components. The behaviour of FeSi bicrystals in contact with a Zn-rich melt has been investigated. The wetting of grain boundaries by the melt and different types of Zn diffusion along the grain boundaries in several concentration intervals with a sharp transition from high to low diffusivity have been observed.

Preparation and properties of FeSi, α-FeSi2 and β-FeSi2 single crystals

FeSi and α-FeSi2 single crystals were prepared by the Czochralski technique. β-FeSi2 and α-FeSi2 single crystals were prepared by chemical vapour transport. FeSi single crystals 18 cm3 in size and α-FeSi2 single crystals of a few cubic centimetres were obtained. β-FeSi2 decomposes peritectically above 970 °C, therefore only needle-like crystals, each about 10 mm long, were grown from the vapour. The temperature dependence of the resistivity was measured from 4.2 K up to room temperature. α-FeSi2 shows a weak temperature dependence of resistivity. The activation energy of FeSi, calculated from ln ρ = f(1T) is equal to 58.2 meV. the Hall coefficient and resistivity of β-FeSi2 single crystals was measured between 30 and 300 K. The as-grown, not intentionally doped β-FeSi2 crystals exhibit n-type conductivity, while Cr and Al doped crystals show p-type conductivity. The Hall coefficient of n-type samples depends on the magnetic field; therefore, the transport properties of β-FeSi2 were explained taking into account two types of carrier, heavy and light. At low temperature (32 K) heavy electrons show a mobility of μn = 48 cm2 V−1 s−1. An impurity band and an additional deep acceptor level are observed in p-type crystals. The activation energies of the shallow acceptor level and deep acceptor level are equal to 55 meV and 100 meV respectively. The mobility of holes μp in p-type crystals reaches a maximum of 1200 cm2 V−1 s−1 at 67 K. Magnetization and magnetic susceptibility measurements in the temperature range 4–320 K show a small positive value of susceptibility and non-linear magnetization for β-FeSi2 but no ferromagnetic phase was detected. α-FeSi2 crystals show linear magnetization and susceptibility similar to β-FeSi2. Magnetization of FeSi is linear but the susceptibility passes through a minimum at about 150–200 K.

Optical Measurements and Band Calculations of FeSi

Reflectivity measurements of high quality FeSi single crystal have been performed at 6 and 300 K over a very wide spectral region from 0.05 to 30 eV. The optical conductivity is obtained by a Kramers-Kronig transformation, and it is discussed in connection with the recent band calculation in which the relativistic spin-orbit effects are taken into account and with other measurements done on FeSi before. The result of direct photoconductivity measurement in the spectral region from 200 to 800 cm -1 is also presented.

Modelling of environment assisted cracking

A critical evaluation of mechanistically based modelling of environment-assisted cracking has been made. Models based on anodic reaction, namely slip-dissolution, anodic reaction-induced cleavage and surface mobility are evaluated from both a mechanistic and quantitative perspective. The film-rupture slip-dissolution mechanism is increasingly perceived to be relevant only to intergranular cracking though not as an exclusive mechanism for this process. The basic principle of film-induced cleavage has been demonstrated experimentally but the range of applicability of the mechanism remains contentious and requires further work. The concept of the surface mobility model is interesting but the quantitative crack growth model is based on unsatisfactory assumptions. In relation to models of environment-assisted cracking based on the effect of hydrogen atoms it is evident that for the non-hydride forming systems the hydrogen enhanced localised plasticity mechanism is gaining wider acceptance though studies of Fe-Si single crystals suggest that for some systems a cleavage mechanism of crack advance may prevail. Quantitative modelling of crack growth based on anodic reaction processes is reasonably well developed, though with a need to provide information about crack-tip electrochemistry and strain rates. These models are comparatively simple but modelling of cracking due to hydrogen atoms is more complex, for example, because of the diffusion and trapping of hydrogen atoms in the metal. Accordingly, models of crack growth in this case tend to be based on rate-limiting processes and in corrosion fatigue in particular there is a need to integrate the chemistry, mechanics, transport and materials interaction in an improved quantitative framework. Mechanistic modelling has an important role in understanding and predicting environment-assisted cracking but the application to life prediction may be limited to specific systems because of the complexities of multiple nucleation events, crack coalescence, crack-tip shielding, crack branching and complex stresses. The intelligent combination of empirical modelling supported by insight from mechanistic modelling provides the most effective approach to predicting environment-assisted cracking in metal-environment systems.

The effect of mechanical stress on the magnetization curves of Ni- and FeSi-single crystals at strong fields

The spatial distribution of the total energy is used to determine the magnetic behavior of ferromagnetic single crystals. For a single ferromagnetic domain there exists only a homogeneous magnetization the direction of which must correspond to a minimum energy direction. Magnetization curves are calculated by observing the stable positions of the magnetization under the application of strong magnetic fields. These magnetization curves show significant variations under different states of mechanical stress for FeSi and Ni crystals. >

The role of stress state on hydrogen cracking in Fe-Si single crystals

While state-of-stress effects have long been known to play a major role in hydrogen embrittlement, there has been little direct evidence outside of thickness effects on threshold stress intensities. Also, there have been recent mechanisms proposed suggesting that enhanced plasticity effects may be more dominant than enhanced decohesion effects. If true, this would mandate different state of stress effects. The goal here was to settle this issue in at least one material, that being a semi-ductile, iron base single crystal. This was achieved by examining the crack initiation site under various loading and hydrogen interaction conditions in precracked Fe-3wt.%Si single crystals oriented in {001}<010> and {001}<110>. A strong dependency of hydrogen-induced cracking on the state of stress was evidenced by the consistent nature of a mid-section initiation stage and tunneling behaviour during crack propagation. This plane stress vs plane strain effect was quantified by analysing both small scale yielding and discretized dislocation computer solutions of the stress field. It was shown that the former did not provide sufficient stress and hydrogen in the local sense while the latter provided both high stress and hydrogen enrichment to cause decohesion. Finally, an assessment is made as to whether surface observations are helpful in sorting out the micromechanical aspects of cracking. This is addressed by examining surface plasticity with respect to the interior crack profile.

メスバウアー分光によるFe-Si単結晶の内部磁場と磁気双極子効果

メスバウアー分光によるFe-Si単結晶の内部磁場と磁気双極子効果

Spin fluctuations in the temperature-induced paramagnet FeSi.

Extensive polarized-neutron experiments have been performed on an FeSi single crystal in a temperature range between 150 and 600 K, and the first concrete evidence that FeSi exhibits temperature-induced magnetism is presented. The magnetic scattering depends strongly on q around ``ferromagnetic'' reciprocal-lattice points, but the integrated intensity over energy is independent of q. The magnetic moment M(q) deduced from the integrated intensity increases with increasing temperature and is in good agreement with the temperature variation of the magnetic moment calculated from the static susceptibility. The width of the energy spectrum depends strongly on both temperature and q. These results can be interpreted in terms of temperature-induced paramagnetism with a narrow-band-gap model.

The influence of silicon on the non-equilibrium surface segregation of carbon in an FeSi 6.3 at% single crystal

The non-equilibrium surface segregation of carbon to the (110) plane of an Fe-Si single crystal was investigated as a function of temperature using Auger electron spectroscopy. It was found that the segregation rates at high temperatures were lower than expected from an extrapolation of low temperature segregation rates. This discrepancy is explained in terms of an interaction between the carbon and silicon atoms on the surface.

The influence of silicon on the non-equilibrium surface segregation of carbon in an Fe-Si 6.3 at% single crystal

Abstract The non-equilibrium surface segregation of carbon to the (110) plane of an Fe-Si single crystal was investigated as a function of temperature using Auger electron spectroscopy. It was found that the segregation rates at high temperatures were lower than expected from an extrapolation of low temperature segregation rates. This discrepancy is explained in terms of an interaction between the carbon and silicon atoms on the surface.

THREE DIMENSIONAL NEUTRON POLARIZATION ANALYSIS IN THE STUDY OF FERROMAGNETIC DOMAIN STRUCTURES

Neutron depolarization experiments have been carried out in which a polarized neutron beam is presented to a sample in either one of the three possible orthogonal polarization directions. Three dimensional polarization analysis of the transmitted beam yields a (3 x 3) depolarization matrix of the sample under investigation. In studying domain structures (DS) various models can be used to describe the D.S. in a limited number of parameters. From the measured depolarization matrix in general 6 independent domain parameters can be determined as a function of magnetic field, temperature, time, stress or other parameters. Various applications of this technique in the study of static and dynamic magnetization processes will be discussed. In particular attention will be given to studies of the amplitude of 90° zigzag walls in a Fe Si-single crystal /1/ and of the domain structure in amorphous Metglas 2826 /2/. As a result of the last it appears possible to analyse the internal mechanical stress distribution as a function of the depth in the foil.

Neutron diffraction topography of Fe(Si) single crystals

Neutron diffraction topography of Fe(Si) single crystals

Crack propagation and cleavage initiation in Fe-2.6%-Si single crystals under controlled plastic crack tip opening rate in various gaseous environments

Crack propagation and crack initiation measurements were carried out in Fe-2.6%-Si single crystals under a new type of control: The elastic strain rate is controlled before yielding. Beyond the yield point, however, the rate of plastic elongation of the specimens is non-linearily controlled. Since plasticity is exclusively concentrated onto the crack tip. the plastic crack tip opening rate is prescribed with reasonable accuracy in this type of experiment. In b.c.c. FeSi single crystals at temperatures below 100°C the crack tip opening rate determines whether the crack advance is ductile or brittle. The critical crack tip opening rate, at which the fracture mode changes, is strongly temperature dependent and yields an activation energy of 0.68 eV for the initiation of cleavage. This value is close to the activation energy reported in the literature for dislocation glide in FeSi. In a small range of the crack tip opening rate, below the critical rate, stable brittle fracture (‘quasi-brittle fracture’) is found. Evidence is reported that plasticity may be responsible in a yet unknown way for the stable quasi-brittle crack growth.

Interaction of moving {110} 90° walls in Fe-Si single crystals with lattice imperfections

Observations, by means of X-ray topography and stroboscopy combined, of moving domain walls and wall junctions in Fe–Si single crystals are reported. Instabilities, surface effects and interactions of 90° {110} walls with dislocations have been observed.

Determination of the orientation of pieces of zigzag Bloch walls in bulk FeSi-Single crystals by neutron small-angle scattering

Small-angle scattering of neutrons allows the determination of the orientation of Bloch walls in the interior of bulk single crystals. The zigzag angle ψ = 28° of the 90°-Bloch wall and its field dependence are measured. We also observe walls or wall pieces with ψ = 0°. With 180°-walls we measure zigzag angles of ψ ≅ 30°.

Thermally activated motion of a 180° Bloch wall in a Fe-Si single crystal

The activated motion of a 180° Bloch wall is investigated in a frame-shaped Fe-Si single crystal. Two kinds of behaviour can be observed. In one of them, the irreversible motion of the wall takes place in a small scale by localized, independent jumps. When a constant magnetic field is applied, the jumps can be thermally activated and the induction increases. The temperature dependence of the change of induction is consistent with a spectrum of energy barriers and can be interpreted by the activation model of Street and Woolley. The order of magnitude of the energy barriers is 1000 K. The magnetic after-effect recorded on a small-amplitude hysteresis loop is proportional to the irreversible susceptibility. The wall motion can also take place by large, collective jumps. The after-effect then exhibits an exponential time dependence which reflects a single energy barrier (∼ 7000 K). When the sample is cycled, the losses per cycle are frequency dependent ; a logarithmic deviation from the linear law is observed at low frequencies, showing that thermal fluctuation assists wall motion. A good agreement is found with experiments previously reported by Sasaki on commercial Fe-Si.

X-ray topography and dynamics: Description of two experiments performed at LURE DCI

Two examples of in situ dynamic experiments using synchrotron white beam X-ray topography are described. The first is concerned with the plastic deformation of FeSi single crystals and the second with the recrystallisation of aluminium polycrystalline samples. Only partial but representative results are presented. The main advantages and shortcomings of the technique are discussed.

Magnetostatic couplings between 180° Bloch walls

We studied Fe-Si single crystals containing small numbers of 180° Bloch walls. The virgin curves and hysteresis cycles reveal strong couplings between wall displacements. We assume that wall bending between pinning points creates interaction fields. A simple model accounts for the Rayleigh law followed by two interacting walls.

Domain structure of FeSi single crystals with Néel orientation

The equilibrium main and closure domain structure at room temperature is fully analysed as function of field. The temperature dependence of the equilibrium domain width is found reversible above 500°C, below it depends strongly on the prior treatment. Structures with completely “anchored” or “movable” Bloch walls can be realized reproducibly.

Pulsed NMR in 〈110〉 FeSi single crystals

The spin-echo spectrum of 〈110〉 FeSi single crystals has been studied. The behaviour of the echo amplitude in dependence of a static magnetic field parallel to the crystal axis is correlated to the magnetic domain structure on the (100) planes. The magnetic field dependence of the echo amplitude is different for different angles between the direction of the high frequency field and the (100) plane.