IMPROVEMENT OF MAGNETIC PROPERTIES OF MnBi POWDERS PREPARED BY LOW-ENERGY BALL MILLING

Abstract

Recently MnBi magnetic material attracts a large attention due to its potential for high-temperature permanent magnetic applications. Although its sponatenous magnetization is moderate, Ms ~ 8.2 kG74 emu/g but its large coercivity, iHc > 10 kOe, which results in the theoretical value of energy product (BH)max ~16.8 MGOe. The MnBi single phase is difficult to be prepared by using conventional techniques, such as the arc-melting, melt-spinning and sintering because of the big difference between the melting temperatures of Bi (544 K) and Mn (1519 K). Furthermore, the magnetic properties of magnets are strongly dependent on processing. The heat treatment of arc-melted alloys, the ball milling of annealed alloys, and the bulk magnets fabrication were found to have large effects on (BH)max of MnBi magnets. In this work, we report the effects of decomposition of MnBi LTPlow temperature phase (LTP) into Bi and Mn during low-energy ball milling (LEBM) carried out in xylene and silicon oil protection solvent environments and its influence on the magnetic properties of green MnBi as-milled powders. In both solvents, by LEBM for 120 - 150 min, MnBi arc-melted and annealed alloys were ground into fine particles of 0.5 – 5 µm to increase iHc up to 5 kOe. By LEBM for 120 min, the viscous silicon oil constrained the decomposition of MnBi (LTP) keeping Ms around 56 emu/g instead of 42 emu/g of in-xylene LEBM powders.