MnBi-based magnets prepared from melt-spun alloys: Effect of α → β phase transformation during field annealing

Microstructure and magnetic properties of MnBi powders prepared by various ball milling processes

“…The low-temperature phase MnBi (LTP-MnBi) of MnBi alloy displays substantial uniaxial magnetic anisotropy (K 1 =1 * 10 6 J m −3 ) [3], excellent magneto-optical Kerr effect [4], and magnetoresistance effect [5]. The predicted maximum energy product (BH) max of the LTP-MnBi is 18 MGOe, which has prompted extensive research efforts on the purity and performance optimization of LTP-MnBi [6][7][8][9][10][11][12]. However, LTP-MnBi is formed through a solid-state reaction between Mn and Bi elements, and the large difference in their melting points hinders the formation of LTP-MnBi [13].…”

A core–shell magnet Mn70Bi30 grown at seeds in magnetic fields and its impacts on its spin-dynamics, Curie point and other tailored properties

“…An inbuilt shell interlocks surface spins at the FM surfaces a way to facilitating H c at integrated core-shell spins [37,38]. A counterpart M r (remnant) at a duly large H c at a squire B-H loop, M r /M s → 0.9, is a key to devise (BH) max , a maximum 12.8 MGOe reported at doped (y 4 at%) MnBi alloys (y = Sb, In, Mg, Mg) [39,40], or 13 MGOe at ball milled MnBi powders [17], at a practical remedy of a fine nanostructure.…”

“…Although an almost pure α-MnBi phase is formed in duly annealed melt-spin alloy ribbons, it exhibits a rather poor H c unless it is well dispersed as tiny nanocrystals (NCs) in a Bi-rich matrix [2,40]. A routine process used to devise magnets from such alloys still needs to refine them into small NCs, which are compacted, field aligned and sintered in a desired shape [11,39].…”

“…A routine process used to devise magnets from such alloys still needs to refine them into small NCs, which are compacted, field aligned and sintered in a desired shape [11,39]. The NCs rather get degraded lacking to design a large (BH) max beyond 7.3 MGOe or so [39,40]. Annealing in fields is realized a simple recipe to devise α-MnBi aligned in a high grade nanotexture [16,[39][40][41][42][43][44][45].…”

“…The NCs rather get degraded lacking to design a large (BH) max beyond 7.3 MGOe or so [39,40]. Annealing in fields is realized a simple recipe to devise α-MnBi aligned in a high grade nanotexture [16,[39][40][41][42][43][44][45]. Though tuning field aligned FM NCs at anneals was proposed a long ago by Boothby et al in 1958 [46], yet the field annealed MnBi lacks either a high H c [41,45], or a duly large α-MnBi yield [16,42].…”