tailieunhanh - Highly anisotropic MnBi magnets

The paper presents an approach to loading and compacting of MnBi powders in the 18 kOe magnetic field oriented perpendicular to the pressing direction where MnBi grains can be freely rotated and oriented parallel to the field direction. Based on the energy minimization of the assembly of magnetized grains, the compacting pressure was chosen to optimize two parameters, the mass density and the coercivity iHc of magnets. The prepared MnBi bulk magnet had g/cm3 , Mr /Ms , and (BH)max reached MGOe. | Journal of Science and Technology 54 (1A) (2016) 58-65 HIGHLY ANISOTROPIC MnBi MAGNETS Nguyen Van Vuong*, Nguyen Xuan Truong Institute of Materials Science, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet, Cau Giay, Hanoi * Email: vuongnv@ Received: 30 August 2015; Accepted for publication: 28 October 2015 ABSTRACT The rare-earth-free MnBi magnetic material is promising for high-temperature (150 200 C) application of permanent magnets because of its large magnetocrystalline energy and especially the positive thermal coefficient of coercivity (dHc/dT > 0). Because of the moderate value of the spontaneous magnetization Ms 74 emu/g, the anisotropy of MnBi bulk magnets should be investigated to enhance the remanence Mr. With large ratio Mr/Ms and appropriate microstructure, the squareness of MnBi magnets should have high value leading the remanent coercivity bHc close to the intrinsic coercivity iHc, thus enhancing the energy product (BH)max. The paper presents an approach to loading and compacting of MnBi powders in the 18 kOe magnetic field oriented perpendicular to the pressing direction where MnBi grains can be freely rotated and oriented parallel to the field direction. Based on the energy minimization of the assembly of magnetized grains, the compacting pressure was chosen to optimize two parameters, the mass density and the coercivity iHc of magnets. The prepared MnBi bulk magnet had g/cm3, Mr/Ms , and (BH)max reached MGOe. o Keywords: rare-earth-free MnBi magnets, anisotropy, anisotropic compaction. 1. INTRODUCTION Large magnetocrystalline energy of K MJm-3 and positive thermal coefficient of coercivity (dHc/dT > 0) make the rare-earth-free MnBi magnetic material appealing for producing low cost magnets used in the applications at an elevated high temperature in the range of 150 200 oC [1 - 5]. With the maximum value kG of the spontaneous magnetization Ms and the perfect microstructure, the energy product