Tuning the exchange-coupling effect in raspberry-like γ-Fe2O3@CoO nanoparticles engineered through the single variation of the surfactant concentration in the synthesis process

Abstract

This work describes a method allowing for a controlled synthesis of raspberry-like γ-Fe₂O₃@CoO core–shell nanoparticles with tunable morphology and magnetic properties. The presence and number of CoO satellites, which in the last instance modifies the magnetic functionality of the nanoparticles, is modulated in a simple manner by adjustment of the concentration of the surfactant (oleic acid) used during the synthesis. The crystallinity of both core and satellite materials has been verified by atomic resolution scanning transmission electron microscopy and electron energy-loss spectroscopy (STEM-EELS). A systematic study of the system combining STEM-EELS imaging, Mössbauer spectroscopy, X-ray diffraction and SQUID measurements (5–350 K) has allowed establishing a suitable correlation between chemical, structural, morphological and magnetic properties. Magnetic measurements have proven an effective exchange-coupling that exists between the core (γ-Fe₂O₃) and the satellite (CoO). Of interest, in view of practical applications, is the possibility of varying the resulting bias field by controlling the concentration of satellites in this exchange-coupled ferrimagnetic/antiferromagnetic system: an increase in the oleic acid concentration results in an increased number of CoO satellites, leading to an enhanced exchange bias field accompanied by an increased coercivity and a higher blocking temperature. This simplified synthesis approach opens a new path towards the engineering of novel hybrid nanostructures with functional properties.