J. Parellada

Theory of conversion electron Mössbauer spectroscopy (CEMS)

Abstract A theory of conversion electron Mossbauer spectroscopy (CEMS) including second order effects, i.e. secondary electron emission, and detection coincidence corrections has been derived. The theory is applicable to surface films containing any number of distinct layers. The partial spectra are given in terms of analytical functions of the Mossbauer parameters and the physical characteristics of each layer in the sample. Numerical results are compared to available experimental data.

A simple Monte Carlo calculation of kilovolt electron transport

A simple detailed Monte Carlo procedure, based on the continuous slowing down approximation together with the Wentzel model of the atom, is shown to give results in good agreement with experiment and with more involved Monte Carlo calculations. Energy straggling is reproduced by means of the Lenz inelastic cross-section and the distribution of energy losses found in the classical theory of binary collisions.

A simple model for electron scattering: inelastic collisions

A simple scattering model-providing analytical expressions of electron inverse mean free path, stopping power and straggling parameter-is proposed. Such a model can be used in Monte Carlo simulations of electron transport problems. Cross-sections are derived from a schematic Bethe surface based on the single-mode approximation for the free electron gas. Fairly good agreement with experiment results and with other theoretical calculations is found for electron energies in the range between 50 eV and 50 keV for free-electron-like materials. An easy empirical correction to the conduction band cross-section allows one to use the model for transition and noble metals.

Weight functions for integral conversion electron mössbauer spectroscopy (ICEMS)

Abstract Weight functions for ICEMS, computed from Monte Carlo simulation of electron transport, are tested for scaling properties. It is shown that energy scaling holds with high accuracy for a wide range of materials and electron energies. Mass scaling from iron to other materials is shown to be adequate except for low atomic number materials.

Ion beam effects FeNi bilayers

Abstract Ion beam mixing induced by 100 keV Ar+ irradiation in FeNi bilayers has been studied. Conversion electron Mossbauer spectroscopy has been used for structural analysis of the irradiated specimens. An FeNi Invar alloy formed in the mixed region, among other phases, exhibits deviation of the average iron hyperfine field from the Slater-Pauling curve. Two other distinct phases have been identified as solid solutions of nickel in b.c.c. iron and iron in f.c.c. nickel.

Irradiation effects at the Ni-Fe interface

Abstract Layered thin film samples of 100 nm 56 Fe/3 nm 57 Fe/43 nm Ni (coating) or 100 nm 56 Fe/3 nm 57 Fe/63 nm Ni (coating) have been irradiated with 100 keV Ar + or 200 keV Kr 2+ ions, respectively, at doses up to 6 × 10 16 ions/cm 2 . In order to study the effects of irradiation at the Ni/Fe interface, the samples have been analyzed before and after ion bombardment by means of Rutherford backscattering spectrometry, conversion electron Mossbauer spectroscopy and nuclear reaction analysis. For the first time the resonant 58 Ni(p, γ) 59 Cu reaction at E p = 1424 keV has been used to obtain Ni concentration depth profiles at a Ni/Fe interfac Three distinct Fe-Ni phases have been identified by their hyperfine field in the ion beam mixed region after irradiation: a solid solution of Ni in bcc α-Fe, an fcc Fe-Ni alloy (possibly either Ni 3 Fe or Fe 0.64 Ni 0.36 invar alloy), and a nonmagnetic Fe-rich fcc Fe (γ-Fe) phase. The mixing efficiency for fcc Fe-Ni phase formation and for Ni migration into the Fe-rich region as a function of dose has been determined quantitatively indicating that Kr 2+ is more effective than Ar + , as expected and that saturation occurs at higher Ar + doses.

Geometrical effects on line shape and background in experimental Mössbauer spectra

The finite size of the Mössbauer absorber is taken into account to relieve geometrical conditions. An analytical expression reproducing quite accurately the actual shape of the resonant lines is derived. Deviations from the assumptions made have a small effect on line shape. The expression can easily be included in standard Mössbauer fitting routines to obtain accurate values of physical parameters even with bad geometrical conditions.

A simple model for electron scattering: elastic cross sections

A simple method to compute approximate cross sections for the elastic scattering of electrons by atoms in the intermediate energy range ( approximately=500 eV-50 keV) is proposed. The method starts from the static field approximation, the atomic potential being derived from self-consistent procedures. Calculation results are in good agreement with experimental ones for free atoms. For atoms in solids, the solid structure is taken into account by using Wigner-Seitz boundary conditions in the self-consistent calculation. This method is suitable to be used in Monte Carlo simulation of electron transport.

Mixing effect of Fe/Ni multilayers of overall Fe65Ni35 composition

Abstract UHV evaporated Fe 65 Ni 35 multilayers were irradiated with 200 keV Kr ions at doses in the range 1 × 10 15 –5 × 10 16 ions/cm 2 in order to follow the mixing process as a function of the fluence. The samples were analysed by XRD (glancing angle), RBS and CEMS techniques. Both XRD patterns and CEM spectra show that a partial bcc to fcc phase transformation occurs even at the lowest irradiation dose and goes on when the irradiation dose is increased. A remarkable enhancement of the fcc lattice parameter, due to interstitial inclusions of Kr atoms occurs at 4 × 10 16 Kr 2+ /cm 2 dose. At this dose the analysis of the CEM spectrum exhibits an important component, attributed to the Invar phase, and a vanishing central nonmagnetic component, due to fcc iron rich alloy.

Effect of the Wigner-Seitz boundary conditions on internal conversion coefficients

Solid state effects are taken into account in an internal conversion coefficients computation by using Wigner-Seitz boundary conditions. Both the bound and free electron wave functions are calculated from an atomic Dirac-Hartree-Fock-Slater self consistent potential. These internal conversion coefficients are compared with those obtained from the usual free atom boundary conditions.