On the exact crystal structure of exchange-biased Fe3O4–CoO nanoaggregates produced by seed-mediated growth in polyol

Abstract

The nucleation and growth mechanism of exchange-biased magnetically contrasted highly crystallized and perfectly epitaxied Fe₃O₄–CoO nanoaggregates was investigated using local probe analysis, namely ⁵⁷Fe 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 Co²⁺ cation diffusion into the spinel lattice leading to the formation of Co_xFe_3−xO₄ core single crystals around which CoO nanocrystals start growing. Conversely, in the in situ route, it is driven by Co²⁺ cation reactivity with unreacted dissolved Fe³⁺, conducted to the coating of the preformed Fe₃O₄ cores by a CoFe₂O₄-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-magnet@antiferromagnet and soft-magnet@hard-magnet@antiferromagnet structures, respectively.