Exploring the Magnetic Properties of Cobalt-Ferrite Nanoparticles for the Development of a Rare-Earth-Free Permanent Magnet

We present for the first time an in-depth magnetic characterization of a family of monodisperse cobalt-ferrite nanoparticles (NPs) with average size covering a broad range of particles sizes (from 4 to 60 nm), synthesized by thermal decomposition of metal-organic precursors. Metal precursors, surfactants, and synthetic parameters were settled in order to fine-tune the particle size, which preserves a narrow particle size distribution. The morphology of the family of cobalt-ferrite NPs shows a size-dependent behavior, evolving from sphere to octahedrons for size larger than 20 nm and passing through a cubic habit for intermediate sizes. The evolution of the magnetic properties was studied as a function of the particle size and shape, particularly focusing on those determining the best performance as permanent magnet. Although saturation and remnant magnetization increase monotonously with size, reaching a constant value above 20 nm, the coercive field exhibits a nonmonotonic behavior with two distinct maxima values for low and room temperature, respectively. In addition, we evaluated the (BH)<inf>max</inf> product, the figure of merit of permanent magnets, obtaining the highest value ever reported in the literature for cobalt-ferrite NPs (i.e., 2.1 MGOe (18 kJ/m^-3) for 40 nm NPs). This study allowed us to establish, at least on the basis of the (BH)<inf>max</inf> product, the potentiality of cobalt-ferrite nanoparticles in current permanent magnet technology.