Recently MnBi magnetic material attracts a large attention due to its potential for hightemperature permanent magnetic applications. Although its sponatenous magnetization is moderate, Ms ~ 74 emu/g but its large coercivity, iHc > 10 kOe, which results in the theoretical value of energy product (BH)max MGOe. | Vietnam Journal of Science and Technology 56 (1A) (2018) 72-78 IMPROVEMENT OF MAGNETIC PROPERTIES OF MnBi POWDERS PREPARED BY LOW-ENERGY BALL MILLING Truong Xuan Nguyen1, *, Chi Kim Thi Hoang2, Khanh Van Nguyen2, Vuong Van Nguyen1 1 Institute of Materials Science, VAST, Hoang Quoc Viet Street, Cau Giay District, Ha Noi 2 Faculty of Physics, Hanoi National University of Education, Xuan Thuy Street, Cau Giay District, Ha Noi *Email: truongnx@ Received: 15 August 2017; Accepted for publication: 5 February 2018 ABSTRACT Recently MnBi magnetic material attracts a large attention due to its potential for hightemperature permanent magnetic applications. Although its sponatenous magnetization is moderate, Ms ~ 74 emu/g but its large coercivity, iHc > 10 kOe, which results in the theoretical value of energy product (BH)max 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 low 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 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 – 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. Keywords: Low-energy ball milling (LEBM),
