Synthesis of CoFe 2 O 4 Nanoparticles via the Ferrihydrite Route
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Synthesis of CoFe2O4 Nanoparticles via the Ferrihydrite Route A. Manivannan, A. M. Constantinescu and M. S. Seehra Physics Department, West Virginia University, Morgantown, WV 26506-6315 ABSTRACT Nanoparticles of CoFe2O4 in the size range of 5 nm to 36 nm have been synthesized by first producing two-line Co-doped ferrihydrite and annealing it at temperatures between 573 and 1073 K. Co-doped ferrihydrite is produced by reacting appropriate amounts of FeCl3/CoCl2 solutions with NH4OH at pH = 7. Thermogravimetric measurements provide evidence for the ferrihydrite to CoFe2O4 conversion between 500 and 700 K and x-ray diffraction is used to confirm the transformation to CoFeO4 and determine the particle size. The lattice constant increases initially with the increase in particle size approaching a constant value above 20 nm. The measured Tc equals 788 K for the 36 nm particles. Measurements of magnetization as a function of temperature and magnetic field confirm that the precursor Co-ferrihydrite is a superparamagnet with Tb 40 K, whereas for the 30 nm CoFe2O4, the coercivity Hc 7 kOe at 20 K, decreasing to Hc 800 Oe with Ms 60 emu/g at 300 K. INTRODUCTION In recent years, studies of nanoscale magnetic particles have attracted considerable interest, both from a fundamental point of view, and in view of their applications in magnetic fluids and magnetic recording [1-2]. Bulk CoFe2O4 is a magnetic ferrite with the inverse spinel structure and Curie temperature Tc 793 K. Nanoparticles of CoFe2O4 have been synthesized in thin film form [3], as microemulsions [4-7], and by chemical synthesis [8-10]. According to a recent paper by Grigorova et al [10], the magnetic parameters such as coercivity Hc, remanence Mr and saturation magnetization Ms vary considerably with the preparation technique, perhaps due to the relative distributions of Co2+, Fe2+ and Fe3+ ions on the A (tetrahedral) and B (octahedral) sites of the inverse spinel structure. Consequently, alternative methods of synthesis are desirable. In this work, we report the synthesis of CoFe2O4 nanoparticles in the size range of 5.5 nm to 36 nm using a wet chemical precipitation method, similar to the one we have used for nanoparticles of two-line ferrihydrites (FeOOH ⋅ 4H2O) [11,12]. Thermogravimetric analysis (TGA) shows that at temperatures above about 500 K, the cobalt-ferrihydrite produced converts to nanoscale CoFe2O4, the particle size increasing with the increase in the annealing temperature. We also report some initial magnetic measurements on these nanoparticles. Details of the procedures and experimental results are presented below. EXPERIMENTAL DETAILS To prepare nanoscale CoFe2O4, the water solutions of FeCl3 ⋅ 6H2O and CoCl2 ⋅ 6H2O were mixed in appropriate amounts to yield the atomic ratio of Co:Fe = 1:2. This was followed by the addition of ammonium hydroxide to form a precipitate. This precipitate was filtered and washed until pH = 7 was achieved in the solution. The precipitate was dried GG6.32.1
overnight at 323 K. The x-ray diffraction (XRD) pattern o
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