High performance MM–FeCo–B spark plasma sintered magnets with nonmagnetic grain-boundary phase

This work presents results on the effect of mischmetal (MM) content on the magnetic properties, phase formation, microstructure, and intergranular interactions in MM12+xFe78-xCo2B6 (x = 0, 4, 8) magnets. The MM-FeCo-B magnets have been prepared by Spark Plasma Sintering method from powders obtained by grinding precursor nanocrystalline ribbons. A significant increase in coercivity of almost 50%, from 5.4 kOe to 10.2 kOe, has been obtained when the MM content increased from 12 at% to 20 at%, while the remanence decreased with about 30%, from 100.3 to 74.45 emu/g. X-ray diffractograms analysis indicates that for a MM content higher than 12%, in addition to the 2: 14: 1 main phase, secondary phases are formed, the amount of which increases when the MM content increases. Electron microscopy investigations have shown that the secondary phases accumulate at the boundaries of the main phase grains. In the case of the magnet with 20 at% MM content the main phase grains are completely isolated. As a result, the magnets show a weakening of the exchange interaction, explaining thus the increase in coercivity. The best magnetic properties, such as coercivity of about 8.7 kOe and maximum energy product of 11.19 MGOe, have been obtained for the magnet with 16% MM content, which is promising for practical applications in terms of filling the gap between ferrites and Nd-Fe-B magnets.