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.