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)max 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)max product, the potentiality of cobalt-ferrite nanoparticles in current permanent magnet technology.