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International audience; The elongation process under swift heavy ion irradiation (74 MeV Kr ions) of gold NPs, with a diameter in the range 10–30 nm, and embedded in a silica matrix has been investigated by combining experiment and simulation techniques: three-dimensional thermal spike (3DTS), molecular dynamics (MD) and a phenomenological simulation code specially developed for this study. 3DTS simulations evidence the formation of a track in the host matrix and the melting of the NP after the passage of the impinging ion. MD simulations demonstrate that melted NPs have enough time to expand after each ion impact. Our phenomenological simulation relies on the expansion of the melted NP, which flows in the track in silica with modified (lower) density, followed by its recrystallization upon cooling. Finally, the elongation of the spherical NP into a cylindrical one, with a length proportional to its initial size and a width close to the diameter of the track, is the result of the superposition of the independent effects of each expansion/recrystallization process occurring for each ion impact. In agreement with experiment, the simulation shows the gradual elongation of spherical NPs in the ion-beam direction until their widths saturate in the steady state and reach a value close to the track diameter. Moreover, the simulations indicate that the expansion of the gold NP is incomplete at each ion impact.

International audience; The fragmentation of displacement cascades into subcascades in copper and iron has been investigated through the molecular dynamics technique. A two-point density correlation function has been used to analyze the cascades as a function of the primary knock-on (PKA) energy. This approach is used as a tool for detecting subcascade formation. The fragmentation can already be identified at the end of the ballistic phase. Its resulting evolution in the peak damage state discriminates between unconnected and connected subcascades. The damage zone at the end of the ballistic phase is the precursor of the extended regions that contain the surviving defects. A fractal analysis of the cascade exhibits a dependence on both the stage of the cascade development and the PKA energy. This type of analysis enables the minimum and maximum displacement spike energies together with the subcascade formation threshold energy to be determined.

Understanding ballistic effects induced by ion beam irradiation can be a key point for controlling and predicting the microstructure of irradiated materials. Meanwhile, the ion mixing framework suggests an average description of displacement cascades may be sufficient to estimate the influence of ballistic relocations on the microstructure. In this work, the BCA code MARLOWE was chosen for its ability to account for the crystal structure of irradiated materials. A first set of simulations was performed on pure copper for energies ranging from 0.5 keV to 20 keV. These simulations were validated using molecular dynamics (MD). A second set of simulations on AgCu irradiated by 1 MeV krypton ions was then carried out using MARLOWE only, as such energy is beyond reach for molecular dynamics. MARLOWE simulations are found to be in good agreement with experimental results, which suggests the predictive potential of the method.

An inertial regime of the magnetization dynamics has been predicted. We study numerically the corresponding equation in the frequency domain. It is shown that inertial terms generate a second resonance peak of the magnetic susceptibility, which corresponds to the nutation frequency. The interplay between precession and nutation is investigated.

We investigate in details the inertial dynamics of a uniform magnetization in the ferromagnetic resonance (FMR) context. Analytical predictions and numerical simulations of the complete equations within the Inertial Landau-Lifshitz-Gilbert (ILLG) model are presented. In addition to the usual precession resonance, the inertial model gives a second resonance peak associated to the nutation dynamics provided that the damping is not too large. The analytical resolution of the equations of motion yields both the precession and nutation angular frequencies. They are function of the inertial dynamics characteristic time $\tau$, the dimensionless damping $\alpha$ and the static magnetic field $H$. A scaling function with respect to $\alpha\tau\gamma H$ is found for the nutation angular frequency, also valid for the precession angular frequency when $\alpha\tau\gamma H\gg 1$. Beyond the direct measurement of the nutation resonance peak, we show that the inertial dynamics of the magnetization has measurable effects on both the width and the angular frequency of the precession resonance peak when varying the applied static field. These predictions could be used to experimentally identify the inertial dynamics of the magnetization proposed in the ILLG model., Comment: 10 pages, 9 figures

Les simulations presentees reposent sur deux modeles de cohesion phenomenologiques a n corps. A temperature nulle, l'energie des surfaces (100) et (110) est minimum pour un plan atomique terminal de composition mixte (50% Au), en accord avec les resultats experimentaux. Le minimum d'energie des surfaces (11n) est obtenu lorsque toutes les terrasses (100) ont egalement une composition mixte. La hauteur des marches et la largeur des terrasses sont alors doubles de celles d'une configuration standard de surface. Les profils de composition de la surface (100), determines en fonction de la temperature a l'aide de simulations Monte Carlo, confirment les resultats des calculs statiques. En dessous de la temperature critique de transition ordre-desordre (T c ), le premier plan de surface est fortement enrichi en or (50% Au) tandis que le second est tres appauvri (100% Cu). Au-dessus de T c , la composition d'equilibre des plans de surface converge vers la valeur de volume (25% Au) via un profil oscillant amorti. La portee de ces oscillations decroit quand la temperature augmente, en bon accord avec des experiences de diffusion de rayons X. Ces resultats sont compares aux profils de composition de la surface (111) qui ne presentent pas d'oscillations.

We investigate the dissolution law of metallic nanoparticles (NPs) under sustained irradiation. The system is composed of isolated spherical gold NPs (4–100 nm) embedded in an amorphous silica host matrix. Samples are irradiated at room temperature in the nuclear stopping power regime with 4 MeV Au ions for fluences up to 8 × 1016 cm−2. Experimentally, the dependence of the dissolution kinetics on the irradiation fluence is linear for large NPs (45–100 nm) and exponential for small NPs (4–25 nm). A lattice-based kinetic Monte Carlo (KMC) code, which includes atomic diffusion and ballistic displacement events, is used to simulate the dynamical competition between irradiation effects and thermal healing. The KMC simulations allow for a qualitative description of the NP dissolution in two main stages, in good agreement with the experiment. Moreover, the perfect correlation obtained between the evolution of the simulated flux of ejected atoms and the dissolution rate in two stages implies that there exists an...

We investigate the kinetic evolution of a nanocomposite submitted to sustained irradiation. The study focuses on gold nanoparticles embedded within a silica host matrix. Irradiations with 4 MeV Au ++ ions are performed in the temperature range from 300 K to 1100 K and for fluences up to 8 × 1016 cm−2. The experimental results are combined with kinetic Monte Carlo simulations to fully characterize the kinetic evolution of the nanoparticles under irradiation. In particular, the change of the steady state with the temperature is investigated in details. First, we demonstrate the existence of a direct-coarsening regime at high temperature, e.g., above 900 K, and of a coarsening-resistant regime at low temperature, e.g., below 700 K. Second, we show that the transition between these two regimes takes place in a temperature region, where the nucleation of new nanoparticles is hindered, and the mass is redistributed among the existing nanoparticles. We also use our experimental results to critically analyze the ...

We extend a previous small-angle neutron scattering study of sodium octanoate (NaC8) micelles to the ternary system sodium octanoate/pentanol/water. The use of contrast variation through selective deuteration of individual components together with explicit computation of interference effects, permits us to deduce the location of pentanol (C5OH) in the micelles. Our main conclusion is that, although the micelles grow as (C5OH) is solubilised, there is no concomitant variation in the NaC8 aggregation number. At low alcohol concentrations, the C5OH is located near the NaC8 polar heads, while at higher concentration the -OH groups are distributed throughout the micellar core.

The knowledge of the structural and dynamical properties of the Liquid-Liquid Interface (LLI) may be of great help to have a better understanding of some chemical processes such as solute transfer or interfacial adsorption. The LLI have been less systematically studied than the liquid-vapor interfaces from the experimental point of view1. The situation is different for the theoretical part, since the similarity of the two types of interfaces result in a common description for both of them2. Some recent theoretical studies, dealing specifically with the structure of the LLI, provided interesting results on interfacial thickness and tension3–6. Microscopic theoretical models, based on statistical theories of inhomogeneous fluids, require assumptions which are difficult to control. Moreover they cannot give a detailed description of the interface at the atomic scale. This is the reason why Molecular Dynamics (MD) and Monte Carlo (MC) simulations have been developped to model interfaces. Most of these computations deal with liquid-vapor systems7,8, while only one liquid-liquid system has been investigated using MC technique to study a Benzene-Water interface9.

Sodium octanoate micelles have been studied by light scattering at surfactant concentrations up to 1.8 M. The micellar weight as determined by intensity measurements shows an increase in the aggregation number from 11 at the c. m. c. to 26 at 1.8 M. Dynamic light scattering indicates a “reactive” diffusion which is a purely translational effect due to the chemical exchange between the micelles and free monomers.