Formation of strong L10-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 L10-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.