During the solidification of Bi-Mn alloys from the mushy zone between 262 ℃ and 355 ℃ in a static magnetic field up to 1.0 T, the alignment behavior of the MnBi primary phase in the alloys was studied experimentally. The ferromagnetic MnBi crystals are magnetized and rotate their c-axes parallel to the field. Then, the oriented crystals congregate and grow up under the influence of magnetic interactions so that the alignment structure of the rod-like MnBi phase is produced. It is found that the orientation of MnBi crystals occurs immediately when the heating temperature or magnetic intensity reaches a critical value and beyond the value the orientation factor Γ of the MnBi phase is increased with increasing the heating temperature and the magnetic intensity. The critical temperature for Bi-6%Mn alloy is 275 ℃ in a 1.0 T magnetic field and increases to 290 ℃ when the magnetic field intensity decreases to 0.1 T. When the alloys are hold at 275 ℃, the critical magnetic intensity for Bi-6%Mn alloy is 0.4T and 0.05 T for Bi-3%Mn alloy. The orientation and congregation of the MnBi crystals were analyzed in terms of the magnetic anisotropy of the crystals and their magnetic interactions.
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