New permanent magnets; manganese compounds

The exponential growth of maximum energy product that prevailed in the 20th century has stalled, leaving a market dominated by two permanent magnet materials, Nd2Fe14B and Ba(Sr)Fe12O19, for which the maximum theoretical energy products differ by an order of magnitude (515 kJ m−3 and 45 kJ m−3, respectively). Rather than seeking to improve on optimized Nd–Fe–B, it is suggested that some research efforts should be devoted to developing appropriately priced alternatives with energy products in the range 100–300 kJ m−3. The prospects for Mn-based hard magnetic materials are discussed, based on known Mn-based compounds with the tetragonal L10 or D022 structure or the hexagonal B81 structure. (paper)