Structural and Magnetic Properties of CoFe2O4 Nanoparticles in an α-Fe2O3 Matrix

CoFe2O4 nanoparticles (NPs) of ∼5 nm have been synthesized by the polyol method and subsequently used as seeds in a sol-gel process to prepare a ferrimagnetic/antiferromagnetic CoFe2O4@α-Fe2O3 nanocomposite. During the formation of α-Fe2O3, the CoFe2O4 NPs form clusters embedded within the antiferromagnetic matrix. The combination of data collected by SQUID magnetometry and Mössbauer spectrometry, together with the structural data obtained from X-ray powder diffraction and transmission electron microscopy, allows us to get a complete picture of the complex nanocomposite system. We address the difficulties in synthesizing phase-pure α-Fe2O3 through sol-gel methods, emphasizing that obtaining a pure hematite phase free from contamination requires precise optimization of sintering conditions, which are not yet clearly defined in the current literature. The relationship between the growth of nanoparticles and the increase in coercivity is discussed as a result of the controlled chemical synthesis of the composite. Furthermore, a new feature emerges in the form of a magnetic bias, hinting at the superexchange coupling between the ferrimagnetic CoFe2O4 and antiferromagnetic α-Fe2O3 phases at their interface.