Investigation of synthetic strategies for enhancing the energy product of spinel ferrite nanoparticles

This work was focused on the development of synthesis strategies for enhancing the energy product of spinel ferrite nanoparticles. The final goal was that of finding the best conditions to realize novel materials with improved magnetic properties, which could be used as building blocks for the realization of rare earth free - permanent magnet with higher energy products than commercial ferrites (i.e. strontium ferrite). To this aim, we focused on quasi-zero-dimensional magnetic materials and as possible strategies to enhance their performances we explored the coupling at the nanoscale with magnetic components endowed with complementary properties, and the modification of the intrinsic core properties by solvent mediated post-synthetic treatment. In the first part of this work, hybrid nanocomposite magnets were designed, where the effective exchange-coupling through the interface between hard and soft magnetic constituents or between AFM and a F(i)M phases is exploited to increase the energy product of the material. In particular, metal/ferrite nano-heterostructures, NHSs, were synthesized by an optimized one-pot thermal decomposition method, which allowed us to tune the composition of the final product by tuning the synthetic parameters. The best NHSs obtained by this approach had energy product of 19 kJm-3, higher than that of the commercial hexagonal ferrite. With the aim of developing a system able to display exchange bias at room temperature, iron oxide AFM|FiM core|shell (CS) NPs doped with two divalent cations (Co(II) and Ni(II)) were also synthetized. The co-doping with cobalt and nickel revealed to be a good compromise for obtaining nanocrystals stable to further oxidation processes, and with increased anisotropy, bias and Néel temperature (260 K) than the standard CS iron oxide NPs. The second part of the thesis was devoted to the improvement of the magnetic performances of the prepared CS AFM|F(i)M hybrid NPs and of standard cobalt ferrites NPs, by solvent mediated post-synthesis treatments. In the first case a mild oxidation of the Fe0.95O|Fe3O4, Co0.3Fe0.7O|Co0.8Fe2.2O4 and Ni0.17Co0.21Fe0.62O|Ni0.4Co0.3Fe2.3O4 CS NPs was realized by solvent mediated annealing in the presence of air, with the aim of investigating the role of the nature of the divalent ions on the structure and room temperature magnetic behaviour of the oxidized product. The Ni(II) and Co(II) amount during the oxidation process was found to play a crucial role in the formation of crystalline antiphase boundaries, leading to a mosaic texturing of the spinel structure in the NPs, characterized by enhanced exchange bias and high field magnetization with respect to the standard AFM|F(i)M NPs. The second investigated approach was the solvent mediated annealing treatment of standard cobalt ferrites NPs. This approach has proven to be an excellent strategy for increasing of the 48 % the BHmax value of the material thanks to the reduction of the lattice microstrain generated during the crystallites’ growth. The presented results provide indications on the validity of the proposed approaches. A further investigation is required to obtain materials characterized by exchange effect at room temperatures through a synthesis that can be carried out on a large scale.