Superparamagnetic Cobalt Ferrite Nanocrystals Synthesized by Alkalide Reduction

Abstract

CoFe 2 O 4 nanocrystallites have been synthesized by alkalide reduction of Co 2+ and Fe 3+ to form nanoscale CoFe 2 , 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 ∼350 K. Annealing at 100 °C in air results in a decrease in the blocking temperature to ∼250 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 °C results in little change in the nanocrystallite size or the magnetic properties. Annealing at 500 °C results in the onset of significant growth in the nanocrystallite size, reaching 30 nm for material annealed at 1000 °C. The saturation magnetization, remanence, and squareness ratio, measured at 300 K, increase smoothly with increasing annealing temperature above 500 °C 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 °C. The unannealed material has the largest coercivity 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 °C, rising sharply to 67 Oe for material annealed at 500 °C, falling to 44 Oe for material annealed at 600 °C, and then steadily growing with increasing annealing temperature to 59 Oe for material annealed at 1000 °C. The anomalous increase in coercivity observed for samples annealed at 500 °C appears to be due to an increase in the average crystallite aspect ratio, which declines upon annealing at higher temperature.

Synthesis and Characterization of CoFe2O4 Hollow Spheres

“…In the present experiment, the coercivity of the CoFe 2 O 4 hollow sphere is 860 Oe, which is close to that of the bulk cobalt ferrite (980 Oe). [38] In comparison to the monodispersed nanocrystals of similar size, [39] the CoFe 2 O 4 hollow spheres show a relatively high coercivity. This is mainly due to the interaction between the nanoparticles composing the hol-low structure.…”

Structural, dielectric and magnetic properties of nickel substituted cobalt ferrite nanoparticles: Effect of nickel concentration

“…16 The structural, transport and magnetic properties of the CoFe 2 O 4 are strongly dependent on the synthesis method, crystallite size etc. 17 The ferromagnetic behavior of the cobalt ferrite is observed with transition temperature around 790K which crystallize in the Co 1−δ Fe δ O 4 tetrahedral A-site coordination and Co δ Fe 1−δ O 4 octahedral B-site coordination where, δ-represents the degree of inversion and when δ = 1, the inverse spinel structure is formed and when δ = 0, the normal spinel structure is possible which is observed in the mixed spinel structure. The value of the δ also depends on the thermal treatment during the synthesis and the crystallite size.…”

Crystallographic and magnetic identification of secondary phase in orientated Bi5Fe0.5Co0.5Ti3O15 ceramics

“…We can infer from this that the 15 The magnetic properties of both strongly depend on cobalt and iron content, particle size, preparation method and particle shape. [16][17][18] The remanent magnetizations of CoFe 2 O 4 and Co 2 FeO 4 lie between 25 and 60 emu/g and 18-24 emu/g respectively, 19,20 and can reach 120 emu/cm 3 for CoFe 2 O 4 thin films. 21 Depending on the grain size, magnetic Curie temperature ranges of 520 C-557 C and 187 C have been identified for CoFe 2 O 4 and Co 2 FeO 4 , respectively.…”