Structural and magnetic study of the annealing of Fe–Co nanoparticles

Post-synthesis thermal treatments are often used to induce the crystallization of amorphous nanoparticles obtained by wet chemical synthesis in order for them to display the physical properties of the bulk. We have studied the structural and magnetic evolution of FeCo magnetic nanoparticles synthesized via an organometallic route during a soft controlled annealing treatment. Despite very mild conditions, the structural study, carried out using WAXS, EXAFS and Mössbauer spectroscopy, evidences the presence of carbon atoms in the as-synthesized structure and their continuous insertion in the structure upon heating, as they form a rather ordered structure including carbides, before being expelled from the particles as the crystallization of the bcc alloyed phase takes place at higher annealing temperatures. The magnetic properties, recorded along the process, accordingly show highly depleted values as the carbon atoms insert in the structure, and reach the Ms bulk value as the bcc phase is restored.