J-P Rozet

Observation of dynamical substate mixing of fast ions in solids

Using high-resolution x-ray spectroscopy, we have measured, as a function of target thickness, the relative intensities of the fine-structure components of the Balmer line emitted by fast hydrogen-like krypton ions propagating through thin carbon and copper targets. Our results are in clear disagreement with the predictions of a rate-equation model accounting for collisional l mixing. On the other hand, good agreement is found with a model taking solely into account a wake field-induced Stark mixing of degenerate n,l,j substates. Within this model, the values obtained for the electric field agree well with those deduced from measured total stopping power, which indicates that the effect of core electrons must be considered. Furthermore, off-diagonal density matrix elements of the initial capture process to the n = 3 states are inferred from the experimental intensities. A comparison, for carbon targets, with available (gas-phase) calculations of these matrix elements, reveals important differences.

Modulating the phase transition temperature of giant magnetocaloric thin films by ion irradiation

Magnetic refrigeration based on the magnetocaloric effect at room temperature is one of the most attractive alternative to the current gas compression/expansion method routinely employed. Nevertheless, in giant magnetocaloric materials, optimal refrigeration is restricted to the narrow temperature window of the phase transition (Tc). In this work, we present the possibility of varying this transition temperature into a same giant magnetocaloric material by ion irradiation. We demonstrate that the transition temperature of iron rhodium thin films can be tuned by the bombardment of ions of Ne 5+ with varying fluences up to 10 14 ions cm --2 , leading to optimal refrigeration over a large 270--380 K temperature window. The Tc modification is found to be due to the ion-induced disorder and to the density of new point-like defects. The variation of the phase transition temperature with the number of incident ions opens new perspectives in the conception of devices using giant magnetocaloric materials.

Transport of fast excited ions through thin solids probed by X-ray spectroscopy

Over more than a decade, experimental studies of the production and transport of projectile excited states in thin solid targets have been performed at GANIL for Ar17+ and Kr35+ in the so-called high velocity domain. A range of target thicknesses from single collision condition to equilibrium has been investigated. X-ray spectroscopy techniques have allowed us to determine absolute nl populations of core and Rydberg projectile states, as well as the relative population of fine structure substates (nlj). In parallel, theoretical simulations to treat on the same footing all the competing processes, i.e., collisions, radiative decay and dynamical screening due to the wake field, have been developed. Methods based on either master equations or Monte Carlo approaches have allowed us to reach an unprecedented precision in the description of the ion transport in matter in the perturbative regime. In particular, a first direct measurement of the wake field usually extracted from ion stopping power has been performed.

Improvement of the ETACHA Code towards low velocities and many-electron ions

Knowledge of the detailed evolution of the whole charge state distribution of projectile ions colliding with targets is required in several fields of research such as material science, atomic, plasma and nuclear physics, in particular in regards of the several foreseen large scale facilities. Starting from the previous ETACHA model [1], we present extension of its validity domain towards lower velocities and larger distortions. Moreover, the system of rate equations is now able to take into account ions with up to 60 electrons.

Numerical simulations of purification and final charge state analysis of the slow ion beam for the FISIC project

Synopsis The Fast Ion –Slow Ion Collisions (FISIC) project consists of a crossed-beam arrangement to study ion-ion collisions in the intermediate velocity regime. For the low energy channel, ion trajectory simulations has been used to develop a complete beam line that includes a new Omega type purification system located just before the collision point and a charge state analyzer after interaction.

Low energy Ne ion beam induced-modifications of magnetic properties in MnAs thin films

Investigations of the complex behavior of the magnetization of manganese arsenide thin films due to defects induced by irradiation of slow heavy ions are presented. In addition to the thermal hysteresis suppression already highlighted in Trassinelli et al (2014 Appl. Phys. Lett. 104 081906), we report here on new local magnetic features recorded by a magnetic force microscope at different temperatures close to the characteristic sample phase transition. Complementary measurements of the global magnetization in different conditions (applied magnetic field and temperatures) enable the film characterization to be completed. The obtained results suggest that the ion bombardment produces regions where the local mechanical constraints are significantly different from the average, promoting the local presence of magneto-structural phases far from the equilibrium. These regions could be responsible for the thermal hysteresis suppression previously reported, irradiation-induced defects acting as seeds in the phase transition.

Hints on the origin of the thermal hysteresis suppression in giant magnetocaloric thin films irradiatied with highly charged ions

In a recent experiment we demonstrated the possibility to suppress the thermal hysteresis of the phase transition in giant magnetocaloric MnAs thin film by interaction with slow highly charged ions (Ne 9+ at 90 keV) [1]. This phenomenon has a major impact for possible applications in magnetic refrigeration and thus its reproducibility and robustness are of prime importance. Here we present some new investigations about the origin and the nature of the irradiation-induced defects responsible for the thermal hysteresis suppression. Considering in particular two samples that receive different ion fluences (two order of magnitude of difference), we investigate the reliability of this process. The stability of the irradiation-induced defects with respect to a soft annealing is studied by X-ray diffraction and magnetometry measurements, which provide some new insights on the mechanisms involved.

X-ray spectroscopy as a tool to enlighten the growth of Van der Waals nanoparticles in a supersonic jet

The clustering thermodynamics in a supersonic jet of argon is studied using x-ray emission induced by keV electrons and slow highly charged ions (HCIs). The high sensitivity of the HCI interaction dynamics allows to probe the very first steps of the argon bunch in a supersonic jet.

Impacts of highly charged ions as seeds in a magneto-structural phase transition of magnetocaloric thin films

Investigation on modifications of structural and magnetic properties of magnetocaloric thin films induced by slow highly charged ions bombardment under well-controlled conditions is presented. The ions induce defects/constraints that facilitate nucleation of one phase with respect to the other in the first-order magnetostructural MnAs, with a consequent suppression of thermal hysteresis, but without any significant perturbation on the other structural and magnetic properties.

High-resolution x-ray spectroscopy to probe quantum dynamics in collisions of Ar17+,18+ ions with atoms and solids, towards clusters

We report on studies of projectile excited states produced by electron capture in both low and high velocity regimes, and when highly charged ions (HCIs) collide either with dilute or dense matter. Quantum effects in the interaction dynamics are probed via high-resolution x-ray spectroscopy for Ar17+ at 7 keV u−1 and for Ar18+ at 13.6 MeV u−1 on Ar, N2 or CH4 gas targets and on carbon solid foils. Relevant comparison between those two collision velocity regimes, and between gaseous and solid targets reveal specific features. In particular, the effect of multiple capture process occurring within a single-collision with gaseous target can be compared with the consequence of multistep collisions arising at surfaces and in solid-bulk at low velocity. At high velocity, beside evidence for collective response of the target electrons due to the wake field induced by HCI passing through the solid-bulk, we demonstrate that excitation and ionization collision processes damp the populations of projectile excited states for long ion transit times. The evolution of the np population as a function of n in solid is at variance from the 1/n3 law found in gas, and the disagreement increases with solid target thickness. We have also tackled studies of HCIs in collision with clusters showing that x-ray spectroscopy provides a powerful tool to sign the presence of clusters in a supersonic gas jet.