Nanocrystalline Co30Fe70 alloy synthesized by alkalide reduction

Abstract

Co₃₀Fe₇₀ nanocrystallites have been synthesized by alkalide co-reduction of Co²⁺ and Fe³⁺. As produced, the material consists of highly pyrophoric 4.0 nm nanocrystals with a narrow (<12%) size distribution, that are superparamagnetic with an average blocking temperature <T_B> of ∼20 K prior to removal of the by-products in which they are embedded. Removal of the by-products allows magnetic interactions between the particles with a consequent increase in the transition temperature to greater than 300 K. Annealing at temperatures higher than 300 °C under vacuum resulted in agglomeration and particle growth; annealing above 500 °C resulted in a change in the shape of the particles to largely rectangular prisms with little further growth above 700 °C. M_s increased with increasing annealing temperature reaching values as large as ∼89% of the bulk value (245 emu/g) after annealing at 700 °C while retaining low M_r and H_c. While rapid exposure of the Co₃₀Fe₇₀ nanocrystallites to air resulted in spontaneous combustion, gradual, slow exposure over a period of one week resulted in the formation of a thin surface coating of CoFe₂O₄, or air stable “core-shell” alloy–oxide nanocrystallites.