Effect of Mn grinding time on structural, chemical, and magnetic properties of the Manganese Bismuth prepared by sintering in vacuum

Abstract

This work reports the changes in structural, chemical, and magnetic properties of the low-temperature phase manganese bismuth (LTP-MnBi) sintered from mixtures of Bi and Mn ball-milled for different times. The milling time was varied from 1 to 15 hr to produce Mn powder with different particle sizes. The average particle size reduced from ~400 µm (original size) to 35 ± 5 µm and 6 ± 2 µm after 1 and 15 hr milling times, respectively. The LTP-MnBi powder was sintered at 275 °C at vacuum pressure below 5 × 10−7 mbar for 12 hours. By increasing the Mn grinding time, the maximum energy product ((BH)max) of LTP-MnBi decreased from 1.98 ± 0.05 to 1.59 ± 0.07 MGOe, and the saturation magnetization (M s) decreased from 53.42 ± 0.90 to 44.32 ± 0.72 emu/g. The x-ray diffraction patterns indicated the reduction of LTP-MnBi content as a function of the milling time, which is agreed with the decrease in the M s value. This is supported by the x-ray photoelectron results, which also showed the increment of Mn oxides on the surface as a function of Mn milling time. The unexpected decrease in M s, which results in a significant reduction of magnetic performance, might be due to the presence of the oxides preventing diffusion during sintering.