The nucleation and growth mechanism of exchange-biased magnetically contrasted highly crystallized and perfectly epitaxied Fe 3 O 4 -CoO nanoaggregates was investigated using local probe analysis, namely 57 Fe Mössbauer and L 2,3 Co XMCD spectroscopies, and advanced magnetometry. The granular nanocomposites were produced by a two-step chemical route. Spinel iron oxide nanoparticles were first synthesised by forced hydrolysis of iron acetate in a polyol and then used as seeds for rock-salt cobalt monoxide nanocrystalline growth. In one case, this crystallochemical step is carried out by separating the seeds, washing and drying them before mixing them in a fresh polyol solution of cobalt precursors. In another case, it is conducted by introducing, directly in the seed reaction solution, the cobalt precursors in the same Fe/Co nominal atomic ratio. It appears that the composite formation proceeds differently in the two cases. It is driven, in the ex situ route, by Co 2+ cation diffusion into the spinel lattice leading to the formation of Co x Fe 3−x O 4 core single crystals around which CoO nanocrystals start growing. Conversely, in the in situ route, it is driven by Co 2+ cation reactivity with unreacted dissolved Fe 3+ , conducted to the coating of the preformed Fe 3 O 4 cores by a CoFe 2 O 4 -like thin interlayer, itself decorated by outer CoO nanocrystals. These reaction schemes are well confirmed by magnetometry experiments. The low-temperature magnetic properties of the produced composites are consistent with the establishment of an exchange-bias feature into hard-
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