Chemical control of superparamagnetic properties of magnesium and cobalt spinel ferrite nanoparticles through atomic level magnetic couplings

A correlation between the electron spin-orbital angular momentum coupling and the superparamagnetic properties has been established in MgFe2O4 and CoFe2O4 spinel ferrite nanoparticles. The contribution to the magnetic anisotropy from the Fe3+ lattice sites is almost the same in both nanocrystallites as neutron diffraction studies have shown a similar cation distribution in these two types of spinel ferrite nanoparticles. Due to the strong magnetic couplings from Co2+ lattice sites, the blocking temperature of CoFe2O4 nanoparticles is at least 150 deg higher than the same sized MgFe2O4 nanoparticles. Mossbauer spectroscopy studies demonstrate that the magnetic anisotropy of CoFe2O4 nanoparticles is higher than that of the same size MgFe2O4 nanoparticles. These studies indicate that the superparamagnetic properties of nanoparticles can be controlled through chemically adjusting the magnetic anisotropy energy.