Magnetic properties of Mn-Al-C permanent magnet alloys

Bulk Mn-Al-C permanent magnets prepared by various techniques

“…19 The tp for the hot-deformed magnet is calculated to be 0.26 and the texture developed is in the plane perpendicular to the deformation direction. In the case of Mn-Al-C magnets prepared by extrusion, Koch et al, 1 Ohtani et al, 4 Sakamoto et al 20 and Bittner et al 19 reported texture formation in the axis parallel to the extrusion direction. Figure 4 shows the XRD patterns of the magnets prepared by the three processes.…”

“…The Mn-Al-(C) magnets with highest H c reported so far was also prepared by hot-extrusion where significant amount of pressure was involved. 4 The SEM secondary images of the hot-compacted, MW-sintered followed by heat-treatment at 600 • C for 10 min and hot-deformed magnets are shown in Fig. 5.…”

“…[1][2][3] The hard-magnetic properties in Mn-Al alloy system originate from the L1 0 τ-phase with high anisotropy field of ∼40 kOe, a relatively high saturation magnetization of ∼96 emu/g and Curie temperature > 300 • C. [1][2][3] The Mn-Al magnet with best properties so far was produced by Matsushita Electrical Industrial company (M r = 6.1 kG, H c = 3.2 kOe, (BH) max ∼7 MGOe) by hot extrusion/swaging. 4 However, the performance improvement as compared to the ferrite magnets was not sufficient to justify the cost of the manufacturing. Furthermore, the invention of RE-based magnets (RECo 5 ) in the 1960's shifted the research interests off from Mn-Al.…”

“…1,5,6 The very recent renewed interest in Mn-Al alloys resulted in a number of investigations that explored various processing techniques. The τ-MnAl phase is metastable, and is usually formed by quenching from the high temperature hcp ε-phase followed by annealing at temperatures between 500-700 • C. 3,4 Prolonged annealing results in decomposition of the τ-MnAl phase to the equilibrium β-Mn and γ 2 -Al 8 Mn 5 phases. 3 Addition of a small amount of C slows down the decomposition of the τ-phase and increases H c .…”

“…3 Addition of a small amount of C slows down the decomposition of the τ-phase and increases H c . 4 Various techniques such as mechanical milling, 7,8 mechanical alloying, 9,10 melt-spinning, 11,12 gas atomization, 13 spark erosion 14 were used to make base material (powder) which then was consolidated to make bulk magnets by spark plasma sintering 15 and equal channel angular extrusion. 16 Unfortunately, all the results reported so far are very inferior compared to the properties reported by Ohtani et al 4 Though the powders exhibited high coercivity (>4.8 kOe), 17 the magnetization was significantly lower and the consolidated bulk magnets showed a further decrease in magnetization and coercivity as well.…”