Low-temperature phase MnBi compound: A potential candidate for rare-earth free permanent magnets

The low-temperature phase (LTP) MnBi is one of the few rare-earth free compounds that exhibit a large magnetocrystalline anisotropy energy in the order of 106 J/m3. A large coercive field (μ0Hcj) above 1 T can be obtained readily by reducing the crystallite size (D) through mechanical grinding (MG). The room-temperature Hcj values follow a phenomenological expression μ0Hcj = μ0Ha(δ/D)n where the anisotropy field (μ0Ha) is ∼4 T, the Bloch wall width (δ) is 7 nm and the exponent (n) is about 0.7 in our study. The grain refinement upon MG is accompanied by suppression of the spin reorientation transition temperature (TSR) from 110 K to below 50 K. The coercive field starts to exhibit positive temperature dependence approximately 50 K above TSR and the room-temperature magnetic hardening induced by MG could partially be brought about by the lowered onset of this positive temperature dependence. The suppression of TSR by MG is likely to be induced by the surface anisotropy with which the 2nd order crystal field term is enhanced. One of the shortcomings of LTP-MnBi is its poor phase stability under the ambient atmosphere. The spontaneous magnetization decreases considerably after room-temperature aging for 1 week. This is due to oxidation of Mn which leads to decomposition of the MnBi phase. Hence, the surface passivity needs to be established before this material is considered for a permanent magnet in practical uses. Another shortcoming is the limited spontaneous magnetization. The theoretical upper limit of the maximum energy product in LTP-MnBi remains only a quarter of that in Nd2Fe14B. Nevertheless, owing to the unique positive temperature dependence of the first-order anisotropy constant (K1), the hardness parameter (κ) of LTP-MnBi is enhanced above room temperature; κ reaches as large as 2.8 at 580 K. This makes LTP-MnBi a possible candidate for the hard phase in rare-earth free nanocomposite magnets.