Nanoscaled magnetic materials with tunable properties

Résumé 0

In this thesis, severe plastic deformation by high-pressure torsion is used to produce magnetic materials with tunable properties. Magnetic tunability is ensured by using combinations of materials, which exhibit large miscibility gaps in the thermodynamical equilibrium and consist therefore of one diamagnetic element and at least one ferromagnetic element. Upon processing a considerable grain refinement and intermixing is observed for certain compositions, whereas a microstructural steady state is aimed for each composition investigated. For an understanding of the structure-property relationship in severe plastically deformed materials, a large focus of this study is on the correlation of magnetic and microstructural properties. A spin-glass state, arising in as-deformed Fe-Cu specimens, clearly demonstrates the formation of a supersaturated solid solution upon high-pressure torsion. Tiny, residual Fe-rich particles give rise to superparamagnetic behavior. The amount of the residual Fe-particles is controlled by the composition, maintaining a switching between superparamagnetism to a spin-glass state. A remarkable tunability in the magnetic properties was found by annealing treatments already at relatively low temperatures, enabling a change between superparamagnetic to single-domain and even multi-domain behavior. The replacement of Cu with Ag leads to bulk ferromagnetic properties. For the Ag-based alloys, different deformation behaviors were observed, which are traced back to differences in the mechanical properties of the alloying elements. For the Co-Cu system single-phase solid solutions with ferromagnetic properties develop, whereat large Co-contents exhibit soft magnetic properties. The magnetic moment and the coercivity in the Co-Cu system is found to be controlled by the Co-to-Cu ratio. Furthermore the coercivity is tuned by subsequent thermal treatments, whereby these alloys have been found to exhibit a remarkable thermal stability regarding the microstructure. A further improvement in terms of soft magnetic properties was achieved by substituting small amounts of Co with Fe. For larger Fe-contents, the magnetic behavior changes and a considerable large coercivity is observed.