[Zurück]

M. Atif:
"Development and Investigation of New Magnetoelectric Composites";
Betreuer/in(nen), Begutachter/in(nen): R. Grössinger, W. Linert; Institut für Festkörperphysik, 2011; Rigorosum: 02.08.2011.

New processing techniques to enhance the magnetostrictive property of materials with economical impact are always subject of interest to material science. In order to improve the magnetostrictive properties, comprehensive understanding of each parameter that controls the magnetostrictive strain is necessary. In this work, the influence of different processing and annealing conditions on the magnetic as well as magnetostrictive properties of polycrystalline cobalt ferrite have been explored. These magnetic properties were then correlated to the occupation of cations in the tetrahedral (A) and octahedral (B) sites. After producing the cobalt ferrite by ball milling, the effect of pressing the powder hydrostatically as well as magnetic annealing has been investigated. It has been found that the saturation magnetization increases with hydrostatic pressures due to changes in the cation distribution i.e. transfer of Co2+ ions from B- to A- sites whereas the magnetic anisotropy reduces due to an increase of the Co2+ concentration at the A-sites; consequently the coercivity decreases. However, the hysteresis loops for the magnetically annealed samples indicate a field induced uniaxial anisotropy caused by magnetic annealing. Significant improvements in the magnetostriction and strain derivative have been achieved in all prepared cobalt ferrite samples by magnetic annealing. After magnetic annealing, the maximum magnetostriction increased by 100% whereas the corresponding maximum strain derivative increased by 200%, respectively, measured along the hard axis at room temperature. The results can be explained by the field induced uniaxial anisotropy which alters the domain structure and the magnetization process. Summarizing, the observed variation in the longitudinal magnetostriction with increasing hydrostatic pressure as well as due to magnetic annealing is caused by the induced magnetic anisotropy. It has been observed that the presence of small and uniform grains with a higher degree of inverted spinel structure in the hydrostatic pressed sintered samples is responsible for the enhancement of magnetostrictive strain.
After optimizing the magnetostrictive properties, cobalt ferrite based magnetoelectric composites (with PZT) have been prepared under different processing parameters. Varying the experimental parameters, the main emphasis was on the basic understanding of the magnetoelectric (ME) phenomenon in the ME composites. The target here was to study the effect of production parameters such as composition, microstructure, heat treatment, synthesis procedure etc. All prepared samples have been investigated by microstructural, magnetic, magnetostrictive and magnetoelctric measurements in order to correlate the physical properties with the process parameters in the synthesized composites.

Magnetism. Magnetostriction, Magnetoelectric Effects,Ferroelectrics