New anisotropic MnBi permanent magnets by field-annealing of compacted melt-spun alloys modified with Mg and Sb

Journal Article · 13 September 2019 · Journal of Magnetism and Magnetic Materials

DOI: https://doi.org/10.1016/j.jmmm.2019.165860 · OSTI ID:1561557

Gabay, Alexander M. ^[1]; Hadjipanayis, George C. ^[1]; Cui, Jun ^[2]

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1. Univ. of Delaware, Newark, DE (United States)
2. Ames Lab. and Iowa State Univ., Ames, IA (United States)

A technique has been designed for manufacturing rare-earth-free MnBi-based magnets capable of filling the existing "gap" between the ferrite and rare-earth permanent magnets. Whereas the earlier approaches relied on sintering of fine and easily degrading single-crystal MnBi particles, the new method achieves refinement of the key α-MnBi phase through melt-spinning combined with appropriate alloying. Modification of the MnBi alloys with Mg and Sb generates a high-coercivity nanostructure of the metastable β' phase. A subsequent compaction at 150 °C produces fully dense materials while converting the β' phase into the stable α phase. Lastly, a short annealing at 265–300 °C in a magnetic field of 3 T increases the fraction of the α phase to 97–98% and aligns the c axes of the α crystallites. A maximum energy product (BH)_max of 11.5 MGOe and an intrinsic coercivity Hc of 5.6 kOe have been obtained in a magnet with the nominal composition Mn₅₀Bi_46.5Mg₃Sb_0.5. Because the coercivity increases with temperature, the maximum energy product of this magnet is still as high as 8.9 MGOe at 175 °C. Increasing the Sb content to 1.5 at.% increases the H_c to 9.3 kOe, but at the same time inhibits the development of the texture thus decreasing the (BH)_max. The addition of Mg was found to increase the c lattice parameter of the α phase resulting in an unusually large ratio c/a ≈1.432.

• Research Organization: Univ. of Delaware, Newark, DE (United States); Iowa State Univ., Ames, IA (United States)
• Sponsoring Organization: USDOE Office of Energy Efficiency and Renewable Energy (EERE), Transportation Office. Vehicle Technologies Office; USDOE Office of Science (SC), Basic Energy Sciences (BES)
• Grant/Contract Number: FG02-90ER45413; EE0007794
• OSTI ID: 1561557
• Alternate ID(s): OSTI ID: 1562554; OSTI ID: 1864013
• Journal Information: Journal of Magnetism and Magnetic Materials, Vol. 495, Issue C; ISSN 0304-8853
• Publisher: ElsevierCopyright Statement
• Country of Publication: United States
• Language: English