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Papers by pierron-bohnes veronique

Journal of Materials Research, 1995

ABSTRACT

Defect and Diffusion Forum, 2008

Chemical ordering kinetics in L1 0 -and B2-ordered AB binary intermetallics was simulated by mean... more Chemical ordering kinetics in L1 0 -and B2-ordered AB binary intermetallics was simulated by means of Monte Carlo (MC) technique implemented with vacancy mechanism of atomic migration. While vacancy concentration is usually much lower than the antisite defect concentration in L1 0 -ordered systems, triple defects are generated in particular B2-ordered systems. The latter definitely affects the chemical ordering process and requires that full thermal vacancy thermodynamics is involved in B2-ordering simulations. The study on L1 0 -ordered binaries was dedicated to FePt thin layers considered as a material for ultra-high-density magnetic storage media. Metastability of the L1 0 c-variant with monoatomic planes parallel to the layer surface and off-plane easy magnetization was revealed. Thermal vacancy formation in B2-ordered binaries was modelled by implementing a mean-field Hamiltonian with a specific formalism of phase equilibria in a latticegas composed of atoms and vacancies. It was demonstrated that for particular pair-interaction energetics, equilibrium concentrations of vacancies and antisites result mutually proportional in well-defined temperature ranges. The MC simulations of B2-ordering kinetics involved the modelled equilibrium vacancy concentration and reproduced the experimentally observed low rate of the process.

Catalysis Today, 2004

The frequencies of the normal modes of vibration of FePd single crystal have been measured using ... more The frequencies of the normal modes of vibration of FePd single crystal have been measured using inelastic neutron scattering. The measurements were performed for the L1 0 ordered structure at 300 and 860 K and for the fcc disordered state at 1020 K. A general lowering of phonon frequencies with increasing temperature is found in FePd, the dependence being the strongest for acoustical transverse [ξ ξ ξ] branches. The migration energies as well as several thermodynamic quantities have been calculated from the densities of states. The contribution of ordering to the migration energy, as experimentally determined in this work, agrees well with the obtained one by Monte Carlo simulations in the L1 0 structure.

Computational Materials Science, 2010

Nano- and meso-scale simulation of chemical ordering kinetics in nano-layered L10-AB binary inter... more Nano- and meso-scale simulation of chemical ordering kinetics in nano-layered L10-AB binary intermetallics was performed. In the nano- (atomistic) scale Monte Carlo (MC) technique with vacancy mechanism of atomic migration implemented with diverse models for the system energetics was used. The meso-scale microstructure evolution was, in turn, simulated by means of a MC procedure applied to a system built of

Computational Materials Science, 2000

The thermodynamic parameters that drive the atomic migration in 2D and 3D binary alloys are studi... more The thermodynamic parameters that drive the atomic migration in 2D and 3D binary alloys are studied using Monte-Carlo simulations. The model is based on a vacancy±atom jump mechanism between nearest neighbour sites, with a constant vacancy concentration, so that only the migration counterpart E M of the total activation energy E A E M E F , where E F is the vacancy formation energy) is determined. The ordering kinetics are well described by exponential like behaviours with one (respectively two) relaxation time in the 2D (respectively 3D) cases. The temperature dependence of the long relaxation time is an Arrhenius law yielding eective migration energies. Slowing down of the relaxation is observed at the Order±Disorder transitions. In both cases, we observe a linear variation of the eective migration energy with the Order±Disorder critical temperature. Ó

Intermetallics, 2009

Monte Carlo simulation of chemical ordering kinetics in nano-layered L1 0 AB binary intermetallic... more Monte Carlo simulation of chemical ordering kinetics in nano-layered L1 0 AB binary intermetallics was performed. The study addressed FePt thin layers considered as a material for ultra-high-density magnetic storage media and revealed metastability of the L1 0 c-variant superstructure with monoatomic planes parallel to the surface and off-plane easy magnetization. The layers, originally perfectly ordered in a c-variant of the L1 0 superstructure, showed homogeneous disordering running in parallel with a spontaneous re-orientation of the monoatomic planes leading to a mosaic microstructure composed of a-and b-L1 0 -variant domains. The domains nucleated heterogeneously on the surface of the layer and grew discontinuously inwards its volume. Finally, the domains relaxed towards an equilibrium microstructure of the system. Two ''atomistic-scale'' processes: (i) homogeneous disordering and (ii) nucleation of the a-and b-L1 0 -variant domains showed characteristic time scales. The same was observed for the domain microstructure relaxation. The discontinuous domain growth showed no definite driving force and proceeded due to thermal fluctuations. The above complex structural evolution has recently been observed experimentally in epitaxially deposited thin films of FePt.

Journal of Alloys and Compounds, 2004

The local atomic arrangement was investigated by neutron diffuse scattering in a FePd single crys... more The local atomic arrangement was investigated by neutron diffuse scattering in a FePd single crystal at 1020 K. The measurements were carried out in the (1 0 0) and (1 1 0) reciprocal planes using a time-of-flight analysis to separate elastic and inelastic contributions. A set of Warren-Cowley short-range order (SRO) parameters (α i ; i = 1, 14) were obtained by fitting the data using the Borie-Sparks formulation. We mainly find a large positive α 2 SRO parameter (attractive interaction between identical atoms). The Krivoglaz-Clapp-Moss approximation, used to estimate the effective pair interaction energies, leads to a strong negative value of the interaction V 2 between second nearest neighbors.

Materials Science and Engineering B-advanced Functional Solid-state Materials, 2006

Ionic irradiation is used to magnetically pattern CoPt alloy films, using an appropriate mask to ... more Ionic irradiation is used to magnetically pattern CoPt alloy films, using an appropriate mask to protect arrays of CoPt dots with a 1 mm 2 size, while the remainder of the film is exposed to ionic irradiation. Before irradiation, we obtain a layer with the Ll 0 tetragonal structure, ordered and with a strong perpendicular magnetic anisotropy energy in the growth direction. After irradiation, the unprotected regions become disordered and magnetically soft, whereas the protected regions remain magnetically hard. Spatially selective irradiation thus provides a spatial distribution of magnetic anisotropy and hence of the magnetization direction, perpendicular to plane in the ordered zones and in-plane in the disordered zones. The interest of ionic irradiation is that the film recovers its initial roughness after elimination of the mask which is promising for the data storage applications.

Catalysis Today, 2006

Patterned CoPt films were fabricated using a combination of e-beam lithography and He+ ion irradi... more Patterned CoPt films were fabricated using a combination of e-beam lithography and He+ ion irradiation to produce a planar array of ordered CoPt squares separated by disordered CoPt areas: (i) by molecular beam epitaxy was deposited a CoPt ordered film which corresponds to a “natural” multilayer: alternating pure cobalt and pure platinum (001) planes. (ii) The film was covered by

Defect and Diffusion Forum, 2001

ABSTRACT

Journal of Materials Research, 1995

ABSTRACT

Defect and Diffusion Forum, 2008

Chemical ordering kinetics in L1 0 -and B2-ordered AB binary intermetallics was simulated by mean... more Chemical ordering kinetics in L1 0 -and B2-ordered AB binary intermetallics was simulated by means of Monte Carlo (MC) technique implemented with vacancy mechanism of atomic migration. While vacancy concentration is usually much lower than the antisite defect concentration in L1 0 -ordered systems, triple defects are generated in particular B2-ordered systems. The latter definitely affects the chemical ordering process and requires that full thermal vacancy thermodynamics is involved in B2-ordering simulations. The study on L1 0 -ordered binaries was dedicated to FePt thin layers considered as a material for ultra-high-density magnetic storage media. Metastability of the L1 0 c-variant with monoatomic planes parallel to the layer surface and off-plane easy magnetization was revealed. Thermal vacancy formation in B2-ordered binaries was modelled by implementing a mean-field Hamiltonian with a specific formalism of phase equilibria in a latticegas composed of atoms and vacancies. It was demonstrated that for particular pair-interaction energetics, equilibrium concentrations of vacancies and antisites result mutually proportional in well-defined temperature ranges. The MC simulations of B2-ordering kinetics involved the modelled equilibrium vacancy concentration and reproduced the experimentally observed low rate of the process.

Catalysis Today, 2004

The frequencies of the normal modes of vibration of FePd single crystal have been measured using ... more The frequencies of the normal modes of vibration of FePd single crystal have been measured using inelastic neutron scattering. The measurements were performed for the L1 0 ordered structure at 300 and 860 K and for the fcc disordered state at 1020 K. A general lowering of phonon frequencies with increasing temperature is found in FePd, the dependence being the strongest for acoustical transverse [ξ ξ ξ] branches. The migration energies as well as several thermodynamic quantities have been calculated from the densities of states. The contribution of ordering to the migration energy, as experimentally determined in this work, agrees well with the obtained one by Monte Carlo simulations in the L1 0 structure.

Computational Materials Science, 2010

Nano- and meso-scale simulation of chemical ordering kinetics in nano-layered L10-AB binary inter... more Nano- and meso-scale simulation of chemical ordering kinetics in nano-layered L10-AB binary intermetallics was performed. In the nano- (atomistic) scale Monte Carlo (MC) technique with vacancy mechanism of atomic migration implemented with diverse models for the system energetics was used. The meso-scale microstructure evolution was, in turn, simulated by means of a MC procedure applied to a system built of

Computational Materials Science, 2000

The thermodynamic parameters that drive the atomic migration in 2D and 3D binary alloys are studi... more The thermodynamic parameters that drive the atomic migration in 2D and 3D binary alloys are studied using Monte-Carlo simulations. The model is based on a vacancy±atom jump mechanism between nearest neighbour sites, with a constant vacancy concentration, so that only the migration counterpart E M of the total activation energy E A E M E F , where E F is the vacancy formation energy) is determined. The ordering kinetics are well described by exponential like behaviours with one (respectively two) relaxation time in the 2D (respectively 3D) cases. The temperature dependence of the long relaxation time is an Arrhenius law yielding eective migration energies. Slowing down of the relaxation is observed at the Order±Disorder transitions. In both cases, we observe a linear variation of the eective migration energy with the Order±Disorder critical temperature. Ó

Intermetallics, 2009

Monte Carlo simulation of chemical ordering kinetics in nano-layered L1 0 AB binary intermetallic... more Monte Carlo simulation of chemical ordering kinetics in nano-layered L1 0 AB binary intermetallics was performed. The study addressed FePt thin layers considered as a material for ultra-high-density magnetic storage media and revealed metastability of the L1 0 c-variant superstructure with monoatomic planes parallel to the surface and off-plane easy magnetization. The layers, originally perfectly ordered in a c-variant of the L1 0 superstructure, showed homogeneous disordering running in parallel with a spontaneous re-orientation of the monoatomic planes leading to a mosaic microstructure composed of a-and b-L1 0 -variant domains. The domains nucleated heterogeneously on the surface of the layer and grew discontinuously inwards its volume. Finally, the domains relaxed towards an equilibrium microstructure of the system. Two ''atomistic-scale'' processes: (i) homogeneous disordering and (ii) nucleation of the a-and b-L1 0 -variant domains showed characteristic time scales. The same was observed for the domain microstructure relaxation. The discontinuous domain growth showed no definite driving force and proceeded due to thermal fluctuations. The above complex structural evolution has recently been observed experimentally in epitaxially deposited thin films of FePt.

Journal of Alloys and Compounds, 2004

The local atomic arrangement was investigated by neutron diffuse scattering in a FePd single crys... more The local atomic arrangement was investigated by neutron diffuse scattering in a FePd single crystal at 1020 K. The measurements were carried out in the (1 0 0) and (1 1 0) reciprocal planes using a time-of-flight analysis to separate elastic and inelastic contributions. A set of Warren-Cowley short-range order (SRO) parameters (α i ; i = 1, 14) were obtained by fitting the data using the Borie-Sparks formulation. We mainly find a large positive α 2 SRO parameter (attractive interaction between identical atoms). The Krivoglaz-Clapp-Moss approximation, used to estimate the effective pair interaction energies, leads to a strong negative value of the interaction V 2 between second nearest neighbors.

Materials Science and Engineering B-advanced Functional Solid-state Materials, 2006

Ionic irradiation is used to magnetically pattern CoPt alloy films, using an appropriate mask to ... more Ionic irradiation is used to magnetically pattern CoPt alloy films, using an appropriate mask to protect arrays of CoPt dots with a 1 mm 2 size, while the remainder of the film is exposed to ionic irradiation. Before irradiation, we obtain a layer with the Ll 0 tetragonal structure, ordered and with a strong perpendicular magnetic anisotropy energy in the growth direction. After irradiation, the unprotected regions become disordered and magnetically soft, whereas the protected regions remain magnetically hard. Spatially selective irradiation thus provides a spatial distribution of magnetic anisotropy and hence of the magnetization direction, perpendicular to plane in the ordered zones and in-plane in the disordered zones. The interest of ionic irradiation is that the film recovers its initial roughness after elimination of the mask which is promising for the data storage applications.

Catalysis Today, 2006

Patterned CoPt films were fabricated using a combination of e-beam lithography and He+ ion irradi... more Patterned CoPt films were fabricated using a combination of e-beam lithography and He+ ion irradiation to produce a planar array of ordered CoPt squares separated by disordered CoPt areas: (i) by molecular beam epitaxy was deposited a CoPt ordered film which corresponds to a “natural” multilayer: alternating pure cobalt and pure platinum (001) planes. (ii) The film was covered by

Defect and Diffusion Forum, 2001

ABSTRACT