Superparamagnetic cobalt ferrite nanocrystals synthesized by alkalide reduction

CoFe2O4 nanocrystallites have been synthesized by alkalide reduction of CO2+ and Fe3+ to form nanoscale CoFe2, followed by oxidation with aerated water at room-temperature resulting in the nanocrystalline ferrite. As produced, the material consists of 2-4 nm nanocrystals that are superparamagnetic with an average blocking temperature of similar to350 K. Annealing at 100 degreesC in air results in a decrease in the blocking temperature to similar to250 K, with no detectable nanocrystallite growth. The dramatic change in the magnetic properties upon annealing is probably due to removal of crystal defects, namely oxygen vacancies. Further annealing to temperatures as high as 400 degreesC results in little change in the nanocrystallite size or the magnetic properties. Annealing at 500 degreesC results in the onset of significant growth in the nanocrystallite size, reaching 30 nm for material annealed at 1000 degreesC. The saturation magnetization, remanence, and squareness ratio, measured at 300 K, increase smoothly with increasing annealing temperature above 500 degreesC reaching 30 nm, 75 emu/g (94% of bulk value), 28 emu/g, and 0.37 respectively, for material that had been annealed at 1000 degreesC. The unannealed material has the largest coereivity observed in this study, 5.13 kOe at 5 K falling to 116 Oe at 300 K. The coercivity at 300 K declines dramatically to 0.9 Oe upon annealing at 100 degreesC, rising sharply to 67 Oe for material annealed at 500 degreesC, falling to 44 Oe for material annealed at 600degreesC, and then steadily growing with increasing annealing temperature to 59 Oe for material annealed at 1000degreesC. The anomalous increase in coercivity observed for samples annealed at 500 degreesC appears to be due to an increase in the average crystallite aspect ratio, which declines upon annealing at higher temperature.