Formation of strong L 1 0 -FePd/α-Fe nanocomposite magnets by visualizing efficient exchange coupling.

Conceptual nanocomposite magnets (NCMs) composed of exchange-coupled hard/soft magnetic phases have been expected to show excellent magnetic performance based on simultaneous high coercivity ( H c ) and high saturation magnetization ( M s ). In our previous works, however, the H c was considerably lower than its theoretical value ( H a ), which prevented us from improving the performance of NCMs. Here, we show that the H c of isolated particulate L 1 0 -FePd/α-Fe NCMs is dominated by their phase segregation into core/shell-like structures versus Janus-like structures. Using first-order reversal curve (FORC) analysis, we clearly distinguished a microscopically undetectable difference in the phase-segregation structure in the NCMs, finding both efficient and inefficient exchange coupling. The nanostructurally controlled NCMs dominated by core/shell-like structure with efficient exchange coupling showed the largest energy product (( BH ) max = 17.5 MGOe) in the Fe-Pd system and the highest H c / H a value (26.5%) among all NCM powders.