Size dependent structural and magnetic properties of FeO–Fe3O4 nanoparticles

Abstract

The magnetic properties of monodisperse FeO–Fe₃O₄ nanoparticles with different mean sizes and volume fractions of FeO synthesized via decomposition of iron oleate were correlated to their crystallographic and phase compositional features by exploiting high resolution transmission electron microscopy, X-ray diffraction, Mössbauer spectroscopy and field and zero field cooled magnetization measurements. A model describing the phase transformation from a pure Fe₃O₄ phase to a mixture of Fe₃O₄, FeO and interfacial FeO–Fe₃O₄ phases as the particle size increases was established. The reduced magnetic moment in FeO–Fe₃O₄ nanoparticles was attributed to the presence of differently oriented Fe₃O₄ crystalline domains in the outer layers and paramagnetic FeO phase. The exchange bias energy, dominating magnetization reversal mechanism and superparamagnetic blocking temperature in FeO–Fe₃O₄ nanoparticles depend strongly on the relative volume fractions of FeO and the interfacial phase.