Dynamics of colloidal systems of magnetic nanoparticles under influence of magnetic fields investigated by XPCS

This thesis investigates structural properties and the underlying microscopic dynamicsof suspensions of $\alpha$-FeOOH goethite platelets in water under the influence of magneticfields. Goethite particles show unusual physical properties and a rich phase diagram,which makes their suspensions an object of high interest for research in the area of”smart nanoparticles”. Five nanoparticle concentrations were chosen such that differentliquid crystal phases could be studied. The suspensions of platelets of these chosenconcentrations were exposed to magnetic fields of varying strength. Small angle X-rayscattering and X-ray photon correlation spectroscopy data were taken and evaluated.The appearing phases and phase transitions were studied as a function of concentrationand applied magnetic field. For this purpose, order parameters, ellipticity, radial andazimuthal peak positions and widths of scattering features were investigated to clarifythe structural properties in detail. For the analysis of the underlying dynamics, therelaxation rates and the shape of measured time correlation functions were evaluated.The results show that with increasing magnetic field a partial realignment of the plateletsoccurs. This realignment is connected to the magnetic properties of the particles. Thedynamics of the corresponding phases revealed a dependence on the concentration ofnanoparticles in the suspension. At a concentration ofc= 20 vol%the transition from thenematic to the anti-nematic phase traverses a mixed state. The nematic and anti-nematicphases show ballistic motion and very similar properties, even though a realignment ofthe particles from an orientation with the long axis parallel to the applied magnetic fieldin the nematic phase to an orientation with the long axis perpendicular to the magneticfield in the anti-nematic phase occurs. The mixed state of20 vol%-suspension exhibits adiffusive motion of the particles and different characteristics. A significant differencewas observed forc= 16 vol%, where the original nematic state has a transition into theanti-nematic phase via an intermediate state of the mixed phase, too. In contrast to thebehaviour atc= 20 vol%the underlying motion atc= 16 vol%shows ballistic dynamicsonly for the nematic state. At this concentration the mixed state and the anti-nematicphase show both diffusive motions. The formation of different phases and underlyingmicroscopic dynamics are explained through the establishment of qualitative modelbased on dipole-dipole interactions. The results obtained in this thesis describe thestructural properties of goethite suspensions and try to our knowledge for first time tolink the microscopic dynamics of the platelets in the suspensions to their structural andunusual physical properties