Exchange-coupled fct-FePd/α-Fe nanocomposite magnets converted from Pd/Fe₃O₄ core/shell nanoparticles

We report the controlled synthesis of exchangecoupled face-centered tetragonal (fct) FePd/α-Fe nanocomposite magnets with variable Fe concentration. The composite was converted from Pd/Fe₃O₄ core/shell nanoparticles through a high-temperature annealing process in a reducing atmosphere. The shell thickness of core/shell Pd/Fe₃O₄ nanoparticles could be readily tuned, and subsequently the concentration of Fe in nanocomposite magnets was controlled. Upon annealing reduction, the hard magnetic fct-FePd phase was formed by the interdiffusion between reduced α- Fe and face-centered cubic (fcc) Pd, whereas the excessive α-Fe remained around the fct-FePd grains, realizing exchange coupling between the soft magnetic α-Fe and hard magnetic fct-FePd phases. Magnetic measurements showed variation in the magnetic properties of the nanocomposite magnets with different compositions, indicating distinct exchange coupling at the interfaces. The coercivity of the exchange- coupled nanocomposites could be tuned from 0.7 to 2.8 kOe and the saturation magnetization could be controlled from 93 to 160 emug^-1. This work provides a bottom-up approach using exchange-coupled nanocomposites for engineering advanced permanent magnets with controllable magnetic properties.