Abstract
An intriguing pathway for the realization of metallic, rare-earth-free magnets is described. Creation of a so-called “exchange-bias” permanent magnet is contemplated that mimics the microstructure of Alnico magnets, comprised of FeCo-based precipitates that are separated by a metallic antiferromagnetic phase that replaces non-magnetic NiAl-rich phase of alnico. In this manner, the existing shape anisotropy is augmented by exchange-bias anisotropy through interphase coupling, providing enhanced coercivities. As a proof of concept, nanocomposite alloys have been fabricated from rapidly solidified Fe34Co33Mn33 ribbons that phase separate into an antiferromagnetic Mn-rich γ-phase and ferromagnetic α-FeCo nanoprecipitates upon post-solidification processing. A progressive enhancement of coercivity, remanence, and exchange bias at T < TN in the ferromagnetic-antiferromagnetic nanocomposite is noted upon annealing, with maximum properties realized at Tann ≈ 673 K for 30 min. Optimization of the microstructure of these Fe-Co-Mn-based alloys is anticipated to be accompanied by improved magnetic performance that could contribute to the development of next-generation permanent magnets.
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