Characterization of effects of plane-front solidification and heat treatment on magnetic properties of Bi/MnBi composites

Eutectic Bi/MnBi composites (3.2 volume percent MnBi) have been plane-front directionally solidified using the Bridgman-Stockbarger method. For the growth velocity range investigated, the morphology of the composites consists of an aligned ensemble of MnBi rods with mean rod diameters lying below the theoretical size for single domain behavior. Magnetization as a function of temperature, applied field and angle with respect to the solidification direction was used to characterize the magnetic properties. These measurements have been correlated with particle size distributions and volume fraction data. Previous studies have shown that solidification processing conditions and post-solidification heat treatment significantly affect the magnetic properties due to the presence of a nonequilibrium magnetic phase. The nonequilibrium phase has been isolated by varying the processing parameters and the temperature dependence of its magnetization determined. In addition, the transformation kinetics of the nonequilibrium to the equilibrium MnBi phase was investigated by monitoring the magnetization change during isothermal annealing. The intrinsic coercivity of the equilibrium component was found to depend on whether a given percent of equilibrium phase is produced by directional solidification or post-solidification heat treatment anneal.