Chr. Janot

Magnetism in amorphous Fe-Si alloys

Abstract Fe-Si alloys have been obtained by the vapor quenching technique. Resistance measurements, electron microscopy and diffraction results are typical of an amorphous system. The Mossbauer spectra of these amorphous alloys, fitted in terms of a distribution of hyperfine fields, show the existence of a local magnetic order with, however, a proportion of weakly or even no coupled Fe atoms which is temperature and Fe-concentration dependent.

Structural description of iron-silicon amorphous alloys

Abstract Typical Fe x Si1-x amorphous alloys have been studied by electron diffraction and resistivity measurements in a wide concentration range (0 ≤ x ≤ 0·75). A description of the short-range order in these systems is proposed in terms of a dense random packing of hard spheres, as in pure amorphous metals, if x ≥ 0·3. In the small iron concentration range (x ≤ 0·2) the structure becomes progressively similar to an amorphous semiconductor, as pictured by a continuous random network model. Iron and silicon atoms appear to be distributed without chemical correlations.

Point defects in vanadium investigated by Mossbauer spectroscopy and positron annihilation

By means of the diffusion broadening of the Mossbauer resonance and of a new technique of positron lifetime measurements, point defects in the vanadium refractory metal have been investigated up to the melting point. Values of formation and migration enthalpies and entropies have been obtained for both monovacancies and divacancies: H1VF=(2.2+or-0.4) eV, H2VF=(3.7+or-0.8) eV, S1VF=(4.8+or-1.4)kB, S2VF=(11.0+or-1.4)kB, H1VM=(1.2+or-0.3) eV, H2VM=(0.8+or-0.3) eV.

Crystal growth and precipitation in thin films of amorphous Fe–Au alloys

Abstract Metastable alloys of the Fe–Au system have been prepared by the vapour quenching technique. The films so obtained are studied by resistivity measurements and by transmission electron microscopy and diffraction. Alloys containing more than 60 at. % Fe are amorphous at low temperature and a transformation to a metastable b.c.c. solid solution occurs at a temperature ranging from 80 to 250 K when the concentration varies from 80 to 60 at. % Fe. A further annealing at higher temperature leads to the precipitation of a gold-rich f.c.c. phase which grows, in a platelet structure, from linear defects formerly observed in the metastable b.c.c. phase. Kinetic parameters of the crystallization and orientation relationship between the phases during the precipitation are determined.

Magnetic measurements on amorphous FeSi alloys

Magnetisation measurements on FexSi1-x amorphous alloys show the existence of a critical concentration at x=0.4. On the other hand the magnetic behaviour seems to be quite similar to the one observed in crystallised solid solution.

Crystallization of Au x Si1−x amorphous alloys

Abstract Amorphous Au x Si1−x alloys have been prepared by vapour deposition at 77 K over a wide range of composition (0  x  0·80). Crystallization has been studied by electrical resistivity measurements, electron microscopy and diffraction. A metastable crystalline phase μ is formed before reaching the equilibrium dissociation into crystalline gold and silicon. This metastable phase has a composition near Au3Si and may be obtained alone or in a mixture with silicon depending on the amorphous alloy composition. Short-range order in the amorphous material is discussed with respect to the structure of the μ phase. It is suggested that amorphous alloys appear to exist as a single phase only at the Si-rich end of the composition range and near the eutectic composition.

About the magnetic behaviour of FexMe1-x (Me=Si, Ge, Sn) amorphous alloys

Magnetisation measurements have been carried out in FexSn1-x amorphous alloys obtained by atomic deposition. A critical composition, xcr approximately=0.4, for the onset of ferromagnetism has been found along with a rapid increase of the magnetisation when x>xcr. Comparison with data previously obtained for FexSi1-x and FexGe1-x amorphous alloys suggests that the onset of magnetic ordering may result from localised interactions; details of the short-range order and chemical correlations might be quite different for Si, Ge and Sn compounds. Temperature dependence of the magnetisation has shown that there is not a well defined Curie temperature in amorphous FexSn1-x which is consistent with magnetic inhomogeneities.

High field magnetization measurements in FexSi1−x amorphous alloys

Magnetization in Fex Si1−x amorphous alloys was measured at high magnetic field up to 150 k0e. The experimental data show the existence of critical conditions for the occurence of magnetic order whose first stage is the appearance of giant moments in the 0.35 < × < 0.50 composition range. At larger iron concentration (x ≳ 0.5) a ferromagnet behaviour arises from a percolation process.

Electrical transport properties and phase stability of amorphous Cu x Sn1-x alloys: CuxSn1-x amorphous alloys

Abstract Cu x Sn1-x amorphous alloys with composition ranging from x = 0·25 to x = 0·81 have been obtained using a refined vapour-quenching technique. The composition dependence of the crystallization temperature, T cr, of the electrical resistivity, ρ0, and of the temperature coefficient of the resistivity, α, have been measured. For the first time, clear evidence has been obtained for the existence of maxima in T cr(x) and ρ0(x) corresponding to a minimum in α(x) near x = 0·77.

Magnetic phase diagram in Fe x Sn 1-x amorphous alloys

Amorphous Fe x Sn 1-x alloys have been prepared by a vapour deposition technique and their magnetic properties have been investigated. Competition of ferro- and antiferromagnetic interaction results in quite a complex behaviour : x > 0.45 corresponds to alloys with a typical ferromagnetic order and a transition to a paramagnetic state at a Curie temperature T c ; at intermediate composition (0.40 M is a spin-glass-like transition and a second one at T c > T M is from ferro- to paramagnetism; the alloys on the tin side (0.25 M or remain paramagnetic (x < 0.25) whatever the temperature.