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 10(6)J/m(3). A large coercive field (mu H-o(cj)) above 1 T can be obtained readily by reducing the crystallite size (D) through mechanical grinding (MG). The room-temperature H-cj values follow a phenomenological expression mu H-0(cj) = mu H-o(a)(delta/D)(n) where the anisotropy field (mu H-o(a)) is similar to 4T, the Bloch wall width (delta) 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 (T-SR) from 110 K to below 50 K. The coercive field starts to exhibit positive temperature dependence approximately 50 K above T-SR 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 T-SR 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 (K-1), the hardness parameter (kappa) of LTP-MnBi is enhanced above room temperature; kappa 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. (C) 2014 Elsevier B.V. All rights reserved.