Magnetic properties of ferromagnetic thin films under external mechanical stresses. Study of spin waves by in-situ Brillouin light scattering. – SpinStress

Development of thin films and nanostructures is a major scientific and technological aim regarding microelectronic, optical and magnetic devices. The mechanical properties of these materials must be studied with attention as they determine the time stability of the structure itself. In an other way, nanostructured materials can reach exceptional mechanical properties, as for example, high thoughness and remarkable tribological performances for multilayered systems.
Several studies have been made concerning the development of characterization techniques of thin films in the elastic domain by in situ tensile experience inside a x-ray diffractometer. The elastic domain corresponds to reversible deformation; so it is easy to understand that the behaviour in this domain and its limits are required parameters for nanostructured thin films fabrication and functionality. Even if the elastic behaviour of materials seems to be relatively simple, our experience in this field has shown that the intrinsic microstructure of thin films (nanometric grains size, particular crystallographic textures and columnar grains along growth direction) induces very heterogeneous elastic behaviour. Thus, discrepancies occur between experience and theoretical models usually applied to bulk materials. Understanding and quantifying these phenomena in view of the structure of the films is an important step for studying transition from elasticity to plasticity.
In the case of magnetostrictive materials, such as Ni, when the film is elastically strained, its magnetic anisotropy can change. This effect is called inverse magnetostriction. In the most of cases, thin magnetic layers are strained during their fabrication (due to thermic and epitaxial stresses). These strains are directly related to structural parameters (crystallographic texture, thickness) that can play a role too, it is thus difficult to know with accuracy the only strain contribution to the magnetic anisotropy in thin films. So, the way proposed in this project is to develop mechanical tests coupled in situ with Brillouin Light Scattering. This new experiment will allow studying spin waves of stressed thin films, and thus increase the possibilities of existing static methods such as hysteresis cycles.
For each films series, we will separately measure elastic strains by in-situ XRD and magnetic properties by in-situ BLS (under magnetic field), for a constant known and controlled external stress state. This new way will permits to link structural, elastic and magnetic properties of thin films.