Unexpected Magnetic Domain Behavior in LTP-MnBi

Journal Article · Mon Jul 01 00:00:00 EDT 2013 · IEEE Transactions on Magnetics

DOI:https://doi.org/10.1109/TMAG.2013.2247983· OSTI ID:1211424

Nguyen, PK; Jin, S; Berkowitz, AE

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 approximate to 10(7) 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, similar to 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 degrees 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 c of some significant fraction of 2K/M-S approximate to 30 kOe would seem reasonable in this polycrystalline sample. By examining "Bitter" patterns, we show that the sample is composed of similar to 50 - 100 mu m 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, H-C increased by several orders of magnitude, as expected.

• Sponsoring Organization: USDOE Advanced Research Projects Agency - Energy (ARPA-E)
• DOE Contract Number: 0472-1554
• OSTI ID: 1211424
• Journal Information: IEEE Transactions on Magnetics, Vol. 49, Issue 7; ISSN 0018-9464
• Country of Publication: United States
• Language: English