The correlation between structure and magnetic properties of Fe particles with core-shell structure (with the ratio of their linear dimensions of around 11:2 nm) has been investigated. In the as-prepared particles the surface layer, composed of small clusters of iron oxide, causes suppression of the exchange interactions and strong pinning of the core moments. Annealing leads to reconstruction of the surface layer and formation of big islands of good quality stoichiometric magnetite with sharp Verwey transition at 122 K. The exchange couplings between Fe particles across ${\mathrm{Fe}}_3{\mathrm{O}}_4$ interfaces are enhanced leading to long-range ferromagnetic correlations, reduction of the local anisotropy, and magnetic softening.

• Received 24 July 2003

DOI:https://doi.org/10.1103/PhysRevB.70.054412