Complex correlations between microstructure and magnetic behavior in hard nanomagnets : A study of the structural and magnetic properties of SrFe12O19 nanocrystallites

Sammanfattning: Magnetic materials at the nanoscale are of huge scientific and technological interest, as theirmagnetic properties display remarkably different behavior compared to bulk-size. Among na-nostructured materials, M-type hexaferrite SrFe12O19 (SFO) nanoparticles (NPs) has becomeextremely appealing as promising candidate material for the renewal of permanent magnets ap-plications. The conventional method for preparing hexaferrites is high temperature solid statereactions, which produce large crystallites, therefore other approaches are needed for a more ac-curate investigation. These include bottom-up methods such as co-precipitation, hydrothermalsynthesis, and sol–gel auto-combustion. In particular, the latter one was thoroughly studied inthe last 2 years, since it is a novel method, with a unique combination of the chemical sol–gelprocess and the combustion process, whose advantage is the production of nanocrystalline pow-ders. Here our aim is to improve the potentiality of SFO in the application for rare-earth-freepermanent magnets.To begin, the synthesis conditions were optimized to decrease the anneal-ing temperature, without any deterioration in the magnetic performance. The investigation ofthe relationship between the structural/morphological features of the nanocrystallites and the re-sulting magnetic properties was carried out by X-ray powder diffraction (XRPD), transmissionelectron microscopy (TEM), and various characterizations of the magnetic properties by dif-ferent magnetometers. Specifically, a size reduction of such particles yielded the formation ofsingle-domain state below a critical value, which results in an increase of magnetic anisotropyand thus coercivity. However, many factors, e.g., size, shape and interphase conditions, affectthe energy barrier distribution, and deserve careful consideration, as consequently influence themagnetization reversal mode.