Unexpected Magnetic Domain Behavior in LTP-MnBi

Low-temperature-phase MnBi (LTP-MnBi) has attracted much interest as a potential rare-earth-free permanent magnet material because of its high uniaxial magnetocrystalline anisotropy at room temperature, K ≈ 10⁷, ergs/cc, and the unusual increase of anisotropy with increasing temperature, with an accompanying increasing coercive force (H_C) with temperature. However, due to the complex Mn-Bi phase diagram, bulk samples of LTP-MnBi with the optimum saturation moment, ~75-76 emu/g have been achieved only with zone-refined single crystals. We have prepared polycrystalline samples of LTP-MnBi by induction melting and annealing at 300°C. The moment in 70 kOe is 73.5 emu/g, but H_C is only 50 Oe. This is quite surprising-the high saturation moment indicates the dominating presence of LTP-MnBi. Therefore, an H_C of some significant fraction of 2K/Ms ≈ 30 kOe would seem reasonable in this polycrystalline sample. By examining “Bitter” patterns, we show that the sample is composed ~ 50-100 μm of crystallites. The randomly oriented crystallites exhibit the variety of magnetic domain structures and orientations expected from the hexagonal-structured MnBi with its strong uniaxial anisotropy. Clearly, the reversal of magnetization in the sample proceeds by the low-field nucleation of reversed magnetization in each crystallite, rather than by a wall-pinning mechanism. When the annealed sample was milled into fine particles, increased by several orders of magnitude, as expected.