Nanoscale Engineering Toward Bulk Exchange-Spring Permanent Magnetic Fe/MnAl Nanocomposites*

The ability to amplify the performance of exchange-spring nanocomposites through correlation and tailoring of chemical order, microstructural scale and magnetic response is necessary for the development of novel high-performance permanent magnets. To this end, rapid solidification of near-equiatomic MnAl for incorporation into the model exchange-spring Fe/MnAl system has produced alloys containing 20-30\% ferromagnetic $\tau$-MnAl with the high-temperature parent hexagonal $\epsilon$-MnAl phase composing the balance. While typically formed by briefly annealing the quenched $\epsilon$-phase, this work has confirmed direct attainment of nanoscaled $\tau$-MnAl via cryogenic milling. Magnetic and structural data indicate a significantly decreased chemical order accompanied by increased lattice strain with increased milling time. Control of the chemical order and nanostructure of near-equiatomic MnAl will favor metastable configurations to foster robust exchange coupling between $\tau$-MnAl and $\alpha$-Fe to create high performance magnetic nanocomposites.