Role of the fuel stoichiometry and post-treatment temperature on the spinel inversion and magnetic properties of NiFe2O4 nanoparticles produced by solution combustion synthesis

Soft-magnetic nickel ferrite (NiFe2O4) nanoparticles were synthesized via solution combustion synthesis (SCS). The influence of the fuel concentration and the post-treatment temperature on the structural and magnetic properties of the samples were studied. The samples produced in a fuel-short regime presented the purest NiFe2O4 nanoparticles, with an average diameter of 32 nm. Spinel inversion degree, composition, crystallite size, and surface area were evaluated regarding their influence on the magnetic behavior of the nanoparticles. Interestingly, the magnetic properties of pure NiFe2O4 samples remained unaffected even after heat-treated above 900 °C. This result is promising for industrial applications that require magnetic stability over temperature changes. NiFe2O4 presented a critical size of 40 nm for single-domain to multi-domain transition. Below 40 nm, the discharge of Ni and α-Fe2O3 from the NiFe2O4 structure is the main factor influencing the magnetism. Pure NiFe2O4 samples treated at different temperatures presented a similar spinel inversion and consequently similar coercivities, approximately 170 Oe