Development of rare-earth-free bulk magnets with energy product up to 12 MGOe in field annealed Mn–Bi–Mg–In–Sb alloys

Abstract

A treatment consisting of annealing in a moderately strong, ≤30 kOe, magnetic field was applied to arc-melted ingots and compacted melt-spun alloys with composition Mn50Bi50-x-y-zMgxInySbz. The high degree of texture of the magnetic α phase induced with annealing of the ingot samples (except the Mn–Bi–Sb one) allowed for accurate determination of the phase anisotropy field. Alloying was found to increase the anisotropy field from 45 kOe in the Mn50Bi50 alloy to 65 kOe in the Mn50Bi45Mg3In0.5Sb1.5 alloy. In the compacted melt-spun alloys subjected to field annealing, a higher degree of texture could be achieved after compaction which had involved less heating, when a smaller fraction of the metastable β′ phase had been converted into the α phase; compaction at room temperature allowed for the highest degree of texture in the Mn50Bi45Mg3In0.5Sb1.5 magnet. The addition of indium was found to decrease the solidus temperatures of the alloys, facilitating the development of the α phase during the field annealing and modifying the properties of the resulting magnets. Introduction of indium resulted in a higher coercivity Hc of the Mn–Bi–Mg–In magnets and in a better texture of the Mn–Bi–Mg–Sn–In magnets. The former effect allowed, in particular, for a viable maximum energy product (BH)max of 10 MGOe to be developed in the Mn50Bi46.5Mg3In0.5 magnet through annealing in a relatively low field of 15 kOe, whereas the latter effect allowed for an excellent combination of (BH)max = 11.6 MGOe and Hc = 8.5 kOe in the Mn50Bi45Mg3In0.5Sb1.5 magnet. The highest (BH)max of 12 MGOe was realized in the Mn50Bi46Mg3In0.5Sb0.5 magnet.