Tuning of the magnetic properties of iron oxide core-shell nanoparticles by controlled oxidation.

Superparamagnetic iron oxide core-shell particles with average sizes of 10 and 14 nm were synthesized by thermal decomposition of an iron oleate precursor in a high-boiling solvent. Sodium oleate was used as surfactant to produce cubic particles. The particles show a wustite (FeO) structure in the core and spinel structures of maghemite (γ-Fe2O3) or magnetite (Fe3O4) in the shell. Due to interfacial coupling of the antiferromagnetic core to the ferrimagnetic shell, the particles exhibit a pronounced exchange bias (EB) effect, resulting in a shifted hysteresis loop when measured below the superparamagnetic blocking temperature. The EB behavior was systematically investigated depending on the core/shell ratio, which was achieved by artificial oxidization of the particles. With decreasing core/shell ratio, the superparamagnetic blocking temperature, the coercivity, as well as the EB field decrease. In addition, magnetic memory measurements indicate the presence of a superspin glass state at low temperatures.