Nanocrystalline Co@@d30@Fe@@d70@ alloy synthesized by alkalide reduction

Co@@d30@Fe@@d70@ nanocrystallites have been synthesized by alkalide co-reduction of Co@@u2+@ and Fe@@u3+@. 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@dB > of 620 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@ds increased with increasing annealing temperature reaching values as large as 689% of the bulk value (245 emu/g) after annealing at 700 'C while retaining low M@dr and H@dc. While rapid exposure of the Co@@d30@Fe@@d70@ 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 CoFe@d2O@d4, or air stable "core-shell" alloy-oxide nanocrystallites.