Structure, magnetism and magnetic induction heating of NixCo(1-x)Fe2O4 nanoparticles

Highlights: • Ni_xCo_(1-x)Fe₂O₄ were obtained by coprecipitation with particle size of 4–108 nm. • Structural, and magnetic properties were correlated with AC heating efficiency. • AC induced heating efficiency increased with Ni content. • Tailoring the Ni content and particle size allows controlling the SAR value. NiFe₂O₄ and CoFe₂O₄ nanoparticles present different efficiencies as magnetic heating agents due to their respectively soft and hard magnetic properties. The substitution of Ni by Co to form mixed Ni_xCo_(1-x)Fe₂O₄ nanoparticles is employed, along with thermal treatment, to tailor their heating efficiency by modification of their structural and magnetic properties. For that purpose, nanostructured Ni_xCo_(1-x)Fe₂O₄ (with x = 0.00, 0.25, 0.50, 0.75 and 1.00) were synthesized by coprecipitation and annealed at 400, 700 and 1000 °C. The substitution of Ni by Co allows obtaining particles with progressively lower saturation magnetization and magnetic anisotropy. Heat treatment allows modifying the saturation magnetization and coercivity differently, by enhancing hyperfine magnetic fields and increasing the particle size beyond the transition from monodomain to multidomain behaviour, respectively. Samples annealed at 700 °C exhibit the largest coercivity and anisotropy values, whilst magnetization values increase progressively with the annealing temperature. The AC induced heating efficiency of Ni_xCo_(1-x)Fe₂O₄ increases with Ni content due to increased minor hysteresis losses. For a given time of exposure and concentration, selection of the nanoparticle size and Ni content allows to obtain different efficiencies, which can be selected according to the heating applications involved.