Spin waves are the collective excitations of the magnetization in the GHz-THz range in magnetic systems. They can be used both as promising information carriers for future devices or as a versatile probe for static and dynamic properties of magnetic systems, making them very interesting objects. However, their local investigation at a nanometer scale remains mostly unexplored because of experimental lacking. The goal of my project is to develop a completely new type of tip-enhanced scanning microscope sensitive to the local magnetization dynamics, which allows the probing and imaging of spin waves in thin films in the GHz-subTHz range with a nanometer resolution. The development of this table-top experiment is based on the combination and the synchronization of a tip-enhanced atomic force microscope (AFM) and a Brillouin light spectroscope (BLS). This new approach will gain new insight in the field of magnetic systems by probing local spin waves dynamics. In particular, it will give the possibility to aboard new subjects of studies inaccessible in laboratory without that setup such as the organization of spin waves in nano-objects (patterned films, magnetic textures), the imaging of the antiferromagnetic order or the local coupling between magnetic and structural orders in complex systems. Finally, this technique is compatible with a wide sample environment allowing, in the long term, the local study of the spin waves response to external stimuli. The development of this unique, innovative and ambitious setup will give me the opportunity to start my own experimental research and attract students and collaborators at a local, national and international level.

SPECTRON is an interdisciplinary project that proposes the development of a new microplasma source of atomic nitrogen. This will be manufactured based on a micro-hollow cathode discharge (MHCD) configuration which consumes very low power and achieves huge power densities, thus enhancing the possibility to improve the dissociation of molecular nitrogen. The targeted application is the deposition on large substrates (cm scale) of hexagonal boron nitride (h-BN), a strategic material in photonics, optoelectronics, etc. The originality of the project is the first implementation of a ns pulsed high voltage with a stiff rise time (10-20 ns) which can even ignite multiple microdischarges by increasing the microplasma volume. This power supply is considered for the first time to drive MHCD. Our ambition is to achieve a strongly non-equilibrium plasma with stiff gradients of electron and reactive species densities, thus improving the plasma reactivity and decreasing the average deposition temperature. The new MHCD will be characterized in terms of stable plasma operation using different gas mixtures (N2/Ar and N2/He), materials (dielectric and electrodes), and structural geometry. The plasma physics and dynamics with emphasis on N-atom kinetics will be thoroughly investigated by means of advanced optical diagnostics such as fast imaging, space-time resolved emission spectroscopy, and ultrafast TALIF coupled to streak camera. The results obtained will be used to validate discharge kinetic and CFD models developed by the group. To perform the deposition of quality h-BN films, different precursors will be investigated, and the film quality will be assessed experimentally and correlated to the discharge properties. The consortium consists of young researchers and engineers with very complementary skills in plasma physics and applications, electrical engineering, advanced optical diagnostics, and material synthesis, making it possible to overcome the challenges of the project.

A general continuum modeling framework to describe the kinetics of reconstructive martensitic phase transformation (MT) coupled with crystal plasticity (CP) at the scale of dislocations is still lacking. In this project, we propose to use the geometrically nonlinear elasticity theory as a single unified framework to model reconstructive MT and CP together. The theory is capable of distinguishing the behavior of different crystal symmetries and dealing with nucleation, and propagation of martensitic variants and their interaction with dislocations without ad-hoc assumptions. Nonlinear elasticity theory can be used to model crystal plasticity and martensitic phase transformations if the global invariance of the elastic energy in the space of finite strain tensors is taken into account. In this approach, continuum elasticity takes the form of Landau theory with an infinite number of equivalent energy wells whose configuration is controlled by the symmetry group GL(n, Z). To regularize such a highly degenerate model we use lattice-based discretization which brings a finite cut off length representing a Ginzburg- like characteristic superatomic scale. The model is mesoscopic in nature, in that it is formulated in terms of mesoscopic quantities such as stresses and strains, and at the same time fully incorporates the underlying symmetry of the crystal lattice. Our model shows that crystal plasticity together with phase transitions naturally arises from nonlinear elasticity if the symmetry of the crystal lattice is properly accounted for. It correctly describes plastic slip and displacive martensitic transformations at the atomic scale and long-range interactions between dislocations and different phases; the dislocations cores and phase boundaries are regularized and blurred on the scale of the unit cell. In order to perform quantitative simulations, we will calculate the strain energy by making use of the Cauchy-Born hypothesis, which is capable of bridging information from the atomistic scale to macro-scale and it consists of coupling of the continuum with molecular theories. More precisely, we will deform a homogeneous lattice formed by atoms interacting via an atomistic potential in order to obtain the homogenous strain energy density in the undeformed configuration as a sum of the interactions of the atoms for a given macroscopic deformation gradient. We will apply the model to study the microstructural evolution during reconstructive martensitic transformations observed in materials such as titanium, zirconium and their alloys. They are of substantial interest for several applications in the nuclear, aeronautic and bio-medical fields.

The DESYNIB project aims to develop and optimize an innovative Micro Hollow Cathode Discharge (MHCD) deposition reactor on large surfaces of hexagonal boron nitride (h-BN), in nitrogen atmosphere. This innovative project, driven by a consortium of young researchers having a solid and complementary expertise in plasma processes and material sciences, will open a new exploratory research axis in the Laboratory of Material Sciences and Processes (CNRS LSPM, UPR 3407). h-BN is a strategic material for strong added value applications, such as photonics and electronics. The scientific community still lacks an efficient growth method to deposit homogeneous thin epitaxial h-BN films on large surfaces (we aim to deposit on 5 cm substrates in this project). MHCDs allow high electron densities and therefore high dissociation degree of the precursors to be reached which is particularly suited for nitride deposition given the high bond energy of molecular nitrogen (9.5 eV). Since these micro discharges are not in thermodynamic equilibrium, it is also possible to considerably reduce the deposition temperature compared to conventional processes. This project will be organized in three work packages: (i) fundamental study of the MHCD in reactive gases, (ii) construction and study of a matrix reactor and (iii) h-BN deposition with a matrix reactor. It will involve theoretical and experimental research to understand the fundamental mechanisms governing the deposition process and to optimize the reactor. This latter will be composed of two chambers. A “plasma source stage” will provide an effective source of atoms thanks to the MHCD array while the created discharge will expand into a “deposition stage” where the boron precursor will be injected and the substrate located. The reactor construction involves thoughtful engineering work including manufacturing of the MHCD array and designing the generator used to supply the discharge. The plasma source will be characterized to determine the relevant parameters of the process, such as densities, fluxes and temperatures of reactive species under different operating conditions. State-of-the art diagnostics will be used, based on optical (absorption and emission spectroscopy, fluorescence) and electrical measurements. A volume-averaged model (0D) that allows scaling laws to be obtained and the exploration of a large parameter space, will be developed to study the physics involved in one-hole MHCD. This will allow the reactive species concentrations to be calculated as a function of the external parameters (pressure, power, flow rate). This 0D model will be usefully completed by a hybrid model which will allow the 2D modelling of the plasma expansion in the deposition chamber. This expansion will be studied both in the case of one hole and multi-holes, to take into account possible interactions between adjacent holes. Thorough comparison of experiments and model results will be made in order to improve the sizing of the plasma reactor. The deposition of h-BN will be optimized by varying the key process parameters such as pressure, plasma source-substrate distance, surface temperature, and boron concentration, and through a detailed study of the boron-precursor injection system. The h-BN films will be characterized in the LSPM laboratory by X-ray Diffraction and Raman Spectroscopy to evaluate the phase purity and quality, and also by scanning electron microscopy for the surface morphology observation. Collaborations with experts, already contacted, in Transmission Electron Microscopy (University Paris 7 and University of Zaragoza in Spain) and in confocal microscopy (University Montpellier 2) will allow the quality of the films to be benchmarked. In terms of valorisation, a French industrial company (ANNEALSYS) has already manifested its will to implement such a plasma source into its existing Chemical Vapor Deposition reactors, after the consortium has obtained a proof of concept.

This project aims implement a complete modeling of the hydrogen blistering phenomenon in metals, accounting for every process stages: vacancy diffusion & clustering; bubble formation; bubbles growth, cracking. This model will be applied to two model materials for mechanical and fusion communities: iron and tungsten.