Glass transition of interacting magnetic nanoparticles
Transition vitreuse de nanoparticules magnétiques en interaction
Résumé
This PhD work is an experimental study of glass transitions in colloidal dispersions of charged and magnetic nanoparticles (ferrofluids).
- The colloidal glass transition, observed at high concentrations, leads to an amorphous solid state out of the thermodynamical equilibrium. The charged particles are then strongly interacting, in a potential where the electrostatic repulsions prevail. Thanks to the original properties of ferrofluids, the complete structural degrees of freedom of the particles are considered. Positional ones are probed using static and dynamical scattering techniques (X-rays and neutrons). Glassy dynamics (non diffusivity, aging and intermittency) are reported at nanometric lengthscales. When a magnetic field is applied, the structure of the dispersion becomes anisotropic, as well as the translational dynamics of the particles and its aging. The rotational dynamics of the nanoparticles are probed using magneto-induced birefringence measurements. The rotational dynamics drastically slow down from a volume fraction threshold which depends on the intensity of the repulsions between particles. Its aging is studied on long time scales. An effective age is introduced to unite aging properties at different concentrations.
- At low temperatures, the frozen dispersion constitute an assembly of disordered giant spins, which present some analogies with atomic spin glasses. Using SQUID magnetometry, we study the orientational dynamics of these superspins. We use a method borrowed from spin glasses to extract a dynamical correlation length between superspins; its size increases during the aging.
- The colloidal glass transition, observed at high concentrations, leads to an amorphous solid state out of the thermodynamical equilibrium. The charged particles are then strongly interacting, in a potential where the electrostatic repulsions prevail. Thanks to the original properties of ferrofluids, the complete structural degrees of freedom of the particles are considered. Positional ones are probed using static and dynamical scattering techniques (X-rays and neutrons). Glassy dynamics (non diffusivity, aging and intermittency) are reported at nanometric lengthscales. When a magnetic field is applied, the structure of the dispersion becomes anisotropic, as well as the translational dynamics of the particles and its aging. The rotational dynamics of the nanoparticles are probed using magneto-induced birefringence measurements. The rotational dynamics drastically slow down from a volume fraction threshold which depends on the intensity of the repulsions between particles. Its aging is studied on long time scales. An effective age is introduced to unite aging properties at different concentrations.
- At low temperatures, the frozen dispersion constitute an assembly of disordered giant spins, which present some analogies with atomic spin glasses. Using SQUID magnetometry, we study the orientational dynamics of these superspins. We use a method borrowed from spin glasses to extract a dynamical correlation length between superspins; its size increases during the aging.
Cette thèse est une étude expérimentale des transitions vitreuses de dispersions de nanoparticules magnétiques chargées (ferrofluides). - La transition vitreuse colloïdale, observée à forte concentration, conduit à un solide amorphe hors de l'équilibre thermodynamique. Les particules chargées sont alors en forte interaction, dans un potentiel dominé par les répulsions électrostatiques. Grâce aux propriétés originales des ferrofluides, nous considérons tous les degrés de liberté structuraux des particules. Ceux de position sont sondés par des mesures statiques et dynamiques de diffusion de rayonnement (rayons X et neutrons). Une dynamique vitreuse (non diffusive, vieillissante et intermittente) est observée à l'échelle nanométrique. En présence d'un champ magnétique, la structure des dispersions devient anisotrope, ainsi que la dynamique de translation des particules et son vieillissement. La dynamique de rotation des nanoparticules est sondée par des mesures de biréfringence magnéto-induite. Celle-ci se gèle à partir d'une fraction volumique qui dépend de l'intensité des répulsions entre particules. Son vieillissement est étudié sur des échelles de temps longues. Un âge effectif est introduit pour unir les propriétés de vieillissement à différentes concentrations. - A basse température, la dispersion gelée constitue un ensemble désordonné de spins géants, qui présente des analogies avec les verres de spins atomiques. En utilisant un magnétomètre SQUID, nous étudions la dynamique d'orientation de ces spins géants. Nous utilisons une méthode empruntée aux verres de spins pour extraire une longueur de corrélation dynamique; sa taille augmente au cours du vieilissement.