Suppressing antiferromagnetic coupling in rare-earth free ferromagnetic MnBi-Cu permanent magnet

Abstract

Rare-earth free, ferromagnetic MnBi shows a positive temperature coefficient of coercivity from room temperature to 400 K and energy product (BH)max of 17.7 MGOe at 300 K. However, MnBi undergoes a first-order structural phase transformation from a ferromagnetic low-temperature phase (LTP) to a paramagnetic high-temperature phase at 613 K below the Curie temperature (Tc) of 716 K. The transformation is attributed to Mn diffusion into the interstitial site of LTP MnBi unit cell. Interstitial Mn antiferromagnetically couples with the Mn at lattice 2a site, lowering the magnetization. Cu-occupied bipyramidal sites are investigated as a possible means to suppress Mn diffusion into the bipyramidal sites using first-principles calculations based on the density functional theory. Saturation magnetization, magnetocrystalline anisotropy constant (K), and Tc of (Mn0.5Bi0.5)100−xCux (x = 0–33) are reported. The magnetocrystalline anisotropy changes to the out-of-plane direction (x = 13) from the in-plane direction (x = 0.0). Tc decreases gradually to 578 K at x = 33 from 716 K at x = 0.0. The calculations show a slightly lower (BH)max of 15.6 MGOe while it is expected that Cu-occupied interstitial sites will significantly suppress Mn diffusion and raise the temperature of the phase transformation.

Structural ordering at magnetic seeds with twins at boundaries of a core–shell alloy Mn60Bi40 and tailored magnetic properties

“…Rare-earth (R) free alloys, such as MnX, where X=Al, Bi, Sb, or Sn [1][2][3][4][5][6][7][8][9][10][11][12][13], have great potentials of developing exotic properties for using in place of conventional rare-earth alloys, usually R 2 Fe 14 B (R=Nd, Sm, Pr, etc), in magnet technology and allied devices. It is not merely because of the rare-earths are rather expensive, but that also suffer from a global scarcity of the resources, and involve sophisticated processing at a mass-scale production for domestic uses.…”

“…So, it favors them usable over a wide range of temperatures, especially, near room temperatures, safely at500 K [3][4][5][6][7]. They have superior erosion resistance to protect stable properties at uses in ambient air [8][9][10][11][12][13][14][15]. In this view, such alloys in general are preferably eco-friendly for uses as small magnets, servo motors, automobiles, recording media, spintronics, MEMS (micro-electro-mechanical systems), microelectronics, biomedical tools, and other possible devices [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15].…”

“…They have superior erosion resistance to protect stable properties at uses in ambient air [8][9][10][11][12][13][14][15]. In this view, such alloys in general are preferably eco-friendly for uses as small magnets, servo motors, automobiles, recording media, spintronics, MEMS (micro-electro-mechanical systems), microelectronics, biomedical tools, and other possible devices [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15]. Among the various MX alloy series, a X=Bi based alloy is widely studied at near stoichiometry, Mn/Bi ratio∼1, and is made at a mass scale using traditional methods (i) sintering Mn and Bi metals in a fine powder at a certain temperature [8,16] and (ii) melting-casting a predetermined Mn-Bi metal mixture as ingots or ribbons [6][7][8][9][10][11][12].…”

“…Among the various MX alloy series, a X=Bi based alloy is widely studied at near stoichiometry, Mn/Bi ratio∼1, and is made at a mass scale using traditional methods (i) sintering Mn and Bi metals in a fine powder at a certain temperature [8,16] and (ii) melting-casting a predetermined Mn-Bi metal mixture as ingots or ribbons [6][7][8][9][10][11][12]. Grinding, annealing, and ball milling are often used to refine the alloy microstructure of small crystallites of optimized magnetic and other engineering properties [8][9][10][11][12][13][14][15][16]. Bi and Mn metals when mix up and react together form a Mn-Bi alloy of it contains a ferromagnetic (FM) α-MnBi phase (crystallites) of emerging magnetic properties at spins pinned down at single magnetic domains (SMDs) at the surfaces.…”