The role of processing parameters on the magnetic properties of directionally solidified Bi/MnBi composites

Eutectic and off-eutectic Bi/MnBi composites have been plane-front directionally solidified at various growth velocities and cooling rates. This results in elongated, aligned MnBi rods dispersed in a Bi terminal solid solution. Magnetization as a function of temperature (4.2, 77 and 295K) and applied field (up to 230 kOe) has been used to evaluate the influence of processing parameters on the magnetic properties. A previously reported magnetic phase (which we call the high coercivity or HC phase) occurs in directionally solidified samples. Magnetization measured parallel to the solidification direction indicates that the HC phase is paramagnetic at room temperature and orders below 295K. The behavior perpendicular to the solidification direction is more complicated and suggests that the parallel direction is the easy axis of magnetization. The volume of the HC phase increases with the cooling rate during solidification, and this phase converts to the expected LTP phase during isothermal annealing. A surface defect model is suggested to explain the coercive field observed for annealed samples containing only the LTP phase and a ferrimagnetic defect-type crystal structure is suggested for the HC phase.