Unraveling the mechanism of the one-pot synthesis of exchange coupled Co-based nano-heterostructures with a high energy product.

The development of reproducible protocols to synthesize hard/soft nano-heterostructures (NHSs) with tailored magnetic properties, is a crucial step to define their potential application in a variety of technological areas. Thermal decomposition has proved to be an effective tool to prepare such systems, but it has been scarcely used so far for the synthesis of Co-based metal/ferrite NHSs, despite of their intriguing physical properties. We found out a new approach to prepare this kind of nanomaterial-based on a simple one-pot thermal decomposition reaction of metal-oleates precursors in the high boiling solvent docosane. The obtained NHSs are characterized by the coexistence of Co metal and Co-doped magnetite and are highly stable in air atmosphere, thanks to the passivation of the metal with a very thin oxide layer. The investigation on the influence of the metal precursor composition (a mixed iron-cobalt oleate), of the ligands (oleic acid and sodium oleate) and of the reaction time on the chemical and structural characteristics of the final product, allowed us to rationalize the reaction pathway and to determine the role of each parameter. In particular, the use of sodium oleate is crucial to obtain a metal phase in the NHSs. In such a way, the one-pot approach here proposed allows the fine control of the synthesis, leading to the formation of stable, high performant, metal/ferrite NHSs with tailored magnetic properties. As an instance, the room temperature maximum energy product was increased up to 19 kJm-3 by tuning the Co content in the metal precursor.