High Exchange Bias in Fe3−δO4@CoO Core Shell Nanoparticles Synthesized by a One-Pot Seed-Mediated Growth Method

Abstract

Core–shell nanoparticles (NPs), which consist in a ferrimagnetic (FIM)/antiferromagnetic (AFM) interface and result in exchange bias coupling, became recently of primary importance in the field of magnetic nanoparticles. The enhancement of some applications such as hyperthermia or magnetic storage media based on the miniaturization of devices is correlated to the size reduction of NPs, which results in the decrease of the magnetocrystalline anisotropy and of the blocking temperature. We present here the synthesis of Fe3−δO4@CoO core–shell NPs by a one-pot seed-mediated growth process based on the thermal decomposition of metal complexes at high temperature. A 2 nm thick CoO shell was grown homogeneously from the starting Fe3−δO4 core surface. The Fe3−δO4@CoO core–shell NP structure has been deeply investigated by performing XRD and advanced techniques based on TEM such as EELS and EFTEM. The high quality of the core–shell interface resulted in the large exchange bias coupling at 5 K (HE ≈ 4.1 kOe) between the FIM and the AFM components. In comparison to starting Fe3−δO4 NPs, the dramatic enhancement of the magnetic properties such as a high coercive field (at 5 K, HC ≈ 15 kOe) were measured. Furthermore, the core–shell structure resulted in the enhancement of the magnetocrystalline anisotropy and the increase of the blocking temperature to 293 K.