Dynamics of anisotropic magnetic fluids

In this work, the particle system consisting of ferrimagnetic barium hexaferrite (BaHF) nanoplatelets suspended in 1-butanol ?is studied. This system has been shown to develop a nematic phase in the field-free state at high particle volume fractions, ?mediated by the intricate balance of attractive and repulsive interactions. In the nematic phase, the molecular order is ?coupled to the magnetic order, due to the magnetic moment fixed to the basal plane of the platelets. This facilitates the ?realization of a ferromagnetic nematic material - a true liquid magnet.?

Due to the magnetic nature of the particles, the study with magnetic characterization techniques is possible. This work ?focuses on the alternating current susceptometry (ACS), where the dynamic magnetic susceptibility spectrum is obtained. In ?this spectrum, the magnetic response of the particles to an oscillating magnetic field is acquired. Within this response, ?concentration-dependent and multifaceted effects are studied, ranging from the relaxation of single particles, to the onset ?of magnetic interparticle interactions at increased particle concentrations, to the self-assembly of particles. The latter can be ?induced by both increased attractive interactions and decreased repulsive interactions. The fixed magnetic moment leads to ?the dominance of the Brownian relaxation mechanism, allowing the determination of the field-dependent Brownian ?relaxation times. The application of theoretical models for various field configurations, and either including or neglecting ?magnetic dipole interactions yields the magnetic properties of the particles, such as magnetic moment and interaction ?parameter. In addition, the rotational diffusion properties are elucidated, which are strongly influenced by the particles ?shapes and their polydispersity.?

Another possible characterization technique is the magnetic actuation of the particles and the subsequent measurement of ?optical properties of the samples in polarized optical microscopy (POM). In the context of this work, a magneto-optical ?measurement setup was therefore developed. This system combines a three-axial coil system and the corresponding ?current sources with a modular POM setup. The system offers various measurement modes, ranging from the generation of ?rotating and oscillating magnetic fields, with superimposed DC offset fields, to the measurement of the sample step ?response and the compensation of ambient static magnetic fields. The optical response can be characterized by rotating ?polarizers and the transmitted intensity can be evaluated either by photodiodes, or by the combination of a long-distance ?microscope and a camera. The capabilities of the system were demonstrated by the example of the highly topical formation ?of stripe-like patterns in suspensions of BaHF nanoplatelets in 1-butanol.?