Mossbauer studies on the superparamagnetic behavior of CoFe2O4 with a few nanometers

CoFe2O4 nanoparticles with a cubic spinel structure are prepared by a high-temperature thermal decomposition method. The average particle sizes are 4.6 and 5.7 nm for CoFe2O4 made with two kinds of solvents by TEM. Mossbauer spectra of 4.6 nm particles displayed a superparamagnetic behavior as demonstrated by a single line with zero hyperfine fields, but that of 5.7 nm particles did not at room temperature. It is considered that anisotropy energy was still more superior to thermal energy because of particle size of 5.7 nm CoFe2O4. Furthermore, Mossbauer spectra exhibited the typical spectrum shapes of the CoFe2O4 at 4.2 K. The spectrum at 4.2 K was fitted using two magnetic components of hyperfine fields H-hf = 540.4, 512.6 kOe and isomer shifts delta = 0.40, 0.30 mm/s for 4.6 nm and H-hf = 542.7, 512.8 kOe and delta = 0.41; 0.29 mm/s for 5.7 nm corresponding to Fe3+ ions at site A and site B, respectively. (C) 2006 Elsevier B. V. All rights reserved.