Granular Fe3 − x O4-CoO hetero-nanostructures produced by in situ seed mediated growth in polyol: magnetic properties and chemical stability

We report here the synthesis of granular composites based on intimately mixed ferrimagnetic Fe3 − x O4 and antiferromagnetic CoO nanocrystals using forced hydrolysis in a polyol solvent. Typically, 10 nm sized iron oxide nanoparticles were pre-formed in diethyleneglycol and used as seeds to grow epitaxially cobalt monoxide nanocrystals, in the same reaction mixture. X-ray diffraction, transmission electron microscopy, 57Fe Mossbauer spectrometry and dc-magnetometry confirmed the production of the desired hetero-nanostructures, of great interest in the field of magnetic recording, and evidenced the formation of a kind of cobalt ferrite solid solution between the initially formed Fe3 − x O4 and latterly deposited CoO nanocrystals. The resulting Co and Fe concentration gradient significantly affects the low temperature magnetization reversal of the composite nanostructures compared to the pristine iron oxide magnetite-like powder. Moreover, with time, air exposure leads to an almost total transformation of the CoO phase into Co3O4 and the electronic displacement of a great part of the Fe2+ ions into Fe3+ ones in the whole grain volume, along with noteworthy low temperature magnetization and coercivity decrease. This material evolution makes the studied composites less valuable for most of the magnetic based functional applications and underlines the importance to pay attention to their storage and integration into functional devices before use.