G. Marchal

Ion Beam Mixing Effects Induced in the Latent Tracks of Swift Heavy Ions in a Fe/Si Multilayer

By following the experimental results recently published about electronic-energy-deposition-induced effects in metallic materials, a mixing effect is observed in an Fe/Si multilayer irradiated by 650 MeV uranium ions. Mossbauer spectroscopy shows that, after a fluence as low as 1013 cm-2, an Fe 4.5 nm/Si 3.5 nm multilayer has been made almost homogeneous by ion mixing. On electron micrographs, at very low fluence, latent tracks are observed where the magnetic properties are drastically modified from the previous crystalline ferromagnetic state.

Structural and magnetic properties in FexSi1−x amorphous alloys

FexSi1−x amorphous alloys were obtained by the vapor quenching technique. Magnetization measurements and Mossbauer spectroscopy data showed there is a critical composition (xc=0.4) for the appearance of magnetic order which starts with the formation of giant moments; then, an increasing homogeneous behaviour is initiated by percolation of these giant moments. Diffraction studies gave evidences that the magnetic transformation occurs without major structural changes. As a first approximation, Si and Fe atoms can be considered as randomly distributed in a dense random packing of hard spheres.

High electronic excitations and ion beam mixing effects in high energy ion irradiated Fe/Si multilayers

Mossbauer spectroscopy (57Fe) shows evidence for mixing effects induced by electronic energy deposition in nanoscale Fe/Si multilayers irradiated with swift heavy ions. A decrease in the mixing efficiency with electronic stopping power is reported; a threshold is found, under which iron environment modifications no longer occur. The kinetics of Fe–Si phase formation after irradiation suggests the existence of three regimes: (i) for high excitation levels, a magnetic amorphous phase is formed directly in the wake of the incoming ion and an almost complete mixing is reached at low fluence (1013 U/cm2); (ii) for low excitation levels, a paramagnetic Si-rich amorphous phase is favored at the interface while crystalline iron subsists at high fluences; (iii) for intermediate excitation levels, saturation effects are observed and the formation rate of both magnetic and paramagnetic phases points to direct mixing in the ion wake but with a reduced track length in comparison to U irradiation. The measured interfac...

Epitaxial growth of (110) DyFe2, TbFe2 and Dy0.7Tb0.3Fe2 thin films by molecular beam epitaxy

We present the first epitaxial growth of some (110) rare earth-Fe2 (DyFe2, TbFe2 and Dy0.7Tb0.3Fe2 known as terfenol-D) thin films on (110) Nb(11−20) sappire by molecular beam epitaxy. The epitaxy is initiated by the deposition of a thin layer of iron on niobium. The structures are investigated by RHEED and X-ray scattering. Depending on the thermal treatment of the iron thin layer, the films of RE-Fe2, epitaxially grown on it, are either single crystals or present twins related by a 110° rotation about the surface normal. The growth of epitaxial thin films of these compounds is of interest because of the magnetic and magnetostrictive properties these materials may exhibit.

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.

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.

Structure and short-range order of vapour-deposited Si1−xSnx amorphous alloys

Abstract Amorphous Si1−x Snx alloy films have been prepared by the simultaneous evaporation of Si and Sn in high vacuum onto various substrates held at liquid nitrogen temperature. The structure of the alloys was investigated using scanning high-energy electron diffraction, density measurements and Mossbauer spectroscopy. For x<0·5, Sn atoms are shown to be substituted for Si, and selectively surrounded by Si atoms in almost perfect tetrahedral units, in a random continuous network. This trend of the alloys towards order can be traced to the size difference between Si (d = 2·35 A) and Sn (d = 2·80 A).

Thermal stability of titanium hydride thin films

The thermal stability of titanium hydride thin films prepared by reactive evaporation is reported. The release of hydrogen is monitored by the effusion method. The gas evolution spectrum shows two peaks corresponding to the hydrogen effusion from the titanium hydride phase and to the crystallographic transformation from fcc to hcp structure, respectively.

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.