Synthesis of hard magnetic α-MnBi phase by high pressure torsion and field assisted annealing

Abstract: Bulk composites using powder blends of Mn and Bi with equal atomic ratios are consolidated and severely deformed by high-pressure torsion (HPT). Subsequent annealing treatments lead to the formation of the ferromagnetic and rare-earth free hard magnetic {\alpha}-MnBi phase. The initial phase formation is studied by in-situ high-energy XRD annealing experiments showing a beneficial influence of HPT-deformation on the amount of {\alpha}-MnBi. The nucleation sites are strongly increased, thus the volumetric {\alpha}-MnBi phase content exceeds 50 vol.%. A lower HPT-pressure of 2 GPa is found to be preferred over 5 GPa to obtain high {\alpha}-MnBi contents. Annealing is done with or without applying an external magnetic field. Additionally, influences of the HPT-induced shear deformation are discussed and correlated with preferred diffusion path ways fostering the formation of anisotropic {\alpha}-MnBi phase.