Fabrication of MnBi/Bi composite using dilute master alloy solidification under high magnetic field gradients

By means of the application of a high gradient magnetic field during the solidification process of a dilute Bi-Mn master alloy, bulk MnBi/Bi composite magnets have been prepared. The observation on the microstructure indicates that MnBi grains aggregate at a specified end of the specimen with the local volume fraction of the MnBi phase above 40% and a high degree of orientation. This method avoids other unfavourable phases occurring in the solidified microstructure. The easy magnetization c axis of MnBi grains parallel to the imposed field is attributed to the rotation of magnetized MnBi grains under the effect of a high magnetic field, and the agglomeration of grains at the specified end results from the magnetization force induced by the field gradient. Simultaneously, the mutual action among magnetized grains makes the grain spacing uniform. Uniform orientation and the high degree of crystal ensure the improvement in the magnetic property of composite materials.