Structural and\nMagnetic Properties of CoFe<sub>2</sub>O<sub>4</sub> Nanoparticles\nin an α‑Fe<sub>2</sub>O<sub>3</sub> Matrix

CoFe<sub>2</sub>O<sub>4</sub> nanoparticles (NPs) of\n∼5\nnm have been synthesized by the polyol method and subsequently used\nas seeds in a sol–gel process to prepare a ferrimagnetic/antiferromagnetic\nCoFe<sub>2</sub>O<sub>4</sub>@α-Fe<sub>2</sub>O<sub>3</sub> nanocomposite.\nDuring the formation of α-Fe<sub>2</sub>O<sub>3</sub>, the CoFe<sub>2</sub>O<sub>4</sub> NPs form clusters embedded within the antiferromagnetic\nmatrix. The combination of data collected by SQUID magnetometry and\nMössbauer spectrometry, together with the structural data obtained\nfrom X-ray powder diffraction and transmission electron microscopy,\nallows us to get a complete picture of the complex nanocomposite system.\nWe address the difficulties in synthesizing phase-pure α-Fe<sub>2</sub>O<sub>3</sub> through sol–gel methods, emphasizing\nthat obtaining a pure hematite phase free from contamination requires\nprecise optimization of sintering conditions, which are not yet clearly\ndefined in the current literature. The relationship between the growth\nof nanoparticles and the increase in coercivity is discussed as a\nresult of the controlled chemical synthesis of the composite. Furthermore,\na new feature emerges in the form of a magnetic bias, hinting at the\nsuperexchange coupling between the ferrimagnetic CoFe<sub>2</sub>O<sub>4</sub> and antiferromagnetic α-Fe<sub>2</sub>O<sub>3</sub> phases at their interface.