Disputation: Structural Studies of Mn-X (X=Al, Bi): Permanent Magnetic Materials without Rare Earth Metals

How to generate and use electricity in a more efficient way is a major challenge for humankind. In this context, permanent magnets play an important role within a very broad range of electric power applications. The strongest magnets used today are mainly based on alloys that contain rare-earth metals, which are neither economical nor sustainable. The search for new alternative alloys with satisfactory magnetic properties is the major motivation for the investigations summarized in this thesis. Interesting candidates for alternative rare-earth free alloys were selected with τ-MnAl as the basis.

Theoretical studies suggest that such alloys may show good magnetic properties after chemical modifications to optimize them. Another compound with promising magnetic properties is MnBi, included in this study. MnAl-Z (Z= C, B, Ga as doping elements) and MnBi compounds were synthesized through carefully devised high-temperature methods, followed by various milling and annealing steps. The structural phase analysis of the samples was based on X-ray and neutron diffraction.
A systematic microstructural investigation was also performed for selected samples. The phase transitions of MnAl and MnBi during heating and cooling at different rates were studied by in situ X-ray diffraction from a synchrotron source. The magnetic properties were characterized by various methods. By strict control of experimental parameters, the metastable τ-MnAl was found to be directly obtainable using a "drop synthesis” process.

A cooling rate of 10 K/min yielded an almost pure ferromagnetic τ-MnAl phase. A microstructural characterization of similarly synthesized MnAlC samples revealed the presence of phase segregation, a Mn-rich region and an Al-rich grain boundary phase. A cryomilling process was employed which decreased the particle size of the MnAl-C sample. Neutron diffraction data disclosed accompanying amorphous features, related to changes in Mn and Al atom occupancies during the milling process. A flash heating procedure regenerated the structural ordering between Mn and Al in the structure, where the initial magnetic properties were recovered. The MnBi compound was synthesized by a self-flux method in order to isolate single crystals. As for τ-MnAl, in situ diffraction studies were applied for following phase transitions and the magnetic properties were studied.