Coupling Mechanism of Cobalt Ferrite in Matrices - Polymer Solutions and Multiferroics Composites

This thesis focuses on the coupling mechanism of nanostructured cobalt ferrite (CFO) in different hybrid materials and their response to external stimuli, such as temperature, magnetic or electric fields. In the first part of this thesis, ferrofluidic materials consisting of viscous solutions and embedded magnetic nanoparticles are studied offering a wide range of possible applications. For that purpose, the influence of nanoparticles on mechanical and rheological properties of solutions, as well as phase transitions, are investigated by standard magnetometry and AC susceptometry (ACS). To cover a wide range of frequencies, our ACS setup is combined with the one at the University of Cologne. The in-field alignment of magnetic nanoparticles and their particle-matrix interaction are investigated by nanoparticle magnetization dynamics. Brownian motion is identified and interpreted from ACS spectra, whereby the particle size in solutions is extracted. In the second part of this thesis, the coupling mechanism of CFO to piezoelectric materials in multiferroic composites is investigated. Since single-phase multiferroic materials operating at room temperature are rare in nature, multi-phase composites are studied at the nanoscale. In recent years, the topic of multiferroics was revitalized by combinations of nanostructured ferroic materials. Here, a detailed analysis of the magnetoelectric (ME) effect between magnetostrictive and piezoelectric constituents is given. By pulsed laser deposition (PLD) CFO is grown in combination with piezoelectric materials, like barium titanate or bismuth ferrite, in various connectivity schemes: CFO as thin films, as nanopillars and as particles in a piezoelectric matrix. In the last step, CFO is replaced by Terfenol-D to enhance the ME coupling coefficient. The second part of this thesis starts with the preparation technique and synthesis of the multiferroic composites by PLD. The main focus lies on structural and magnetic characterization and the indirect observation of the magnetoelectric coupling. The influence of different material compositions and the preparation technique are studied to obtain a detailed overview of the physical phenomena that govern the strength of the ME effect. An element-specific characterization is done by X-ray absorption spectroscopy. For that, electric-field dependent X-ray magnetic circular dichroism is recorded at the Fe and Co L3,2 absorption edges in the surface-sensitive total electron yield mode. X-ray linear dichroism is investigated at the Fe and Co L3,2 and Bi N5,4 absorption edges at different magnetic fields. In addition, the magnetic field dependent piezoresponse is measured by piezoresponse force microscopy or by magnetic force microscopy to investigate the magnetoelectric effect. In both parts of this thesis, dealing with polymeric hybrid materials and multiferroic composites, the characterization of the constituent itself is given, e.g. structural properties by X-ray diffraction. Then, the coupling mechanism itself is investigated by magnetometry, ACS, microscopy or X-ray absorption spectroscopy.