Cobalt ferrite nanoparticles: Achieving the superparamagnetic limit by chemical reduction.

An unanticipated superparamagnetic response has been observed in cobalt ferrite materials after thermal treatment under inert atmosphere. Cobalt ferrite particles were prepared via normal micelle precipitation that typically yields Co_xFe_3-xO₄ nanoparticles (x=0.6-1.0). While samples thermally treated under oxygen show majority spinel phase formation, annealing in nitrogen gas yields materials consisting of Co-Fe alloy, FeS, and CoFe₂O₄ spinel. After thermal treatment, thermomagnetic studies reveal composition-insensitive, but highly treatment-sensitive, saturation magnetization, coercivity, blocking temperature, and Verwey transition temperature dependence. Extremely high saturation magnetization (159 emu/g) with low coercivity (31 Oe) was observed for one of the treated compositions, which drastically deviates from prototypical cobalt ferrite with large magnetocrystalline anisotropy. We attribute such unique magnetic response to Co-Fe alloy coexisting with FeS and CoFe₂O₄ spinel where the diameter of the metallic phase is below the superparamagnetic limit. While thermal treatment in nitrogen was not anticipated to yield Co-Fe alloy, chemisorbed surfactant molecules (i.e., sodium dodecylsulfate) are postulated to act as reducing agents in the present scenario. [ABSTRACT FROM AUTHOR]