Investigation of the static and dynamic magnetic properties of CoFe2O4 nanoparticles

We have investigated the magnetic behavior of cobalt ferrite nanoparticles with a mean diameter of 7.2 nm. AC susceptibility of colloidal cobalt ferrite nanoparticles was measured as a function of temperature T from 2 to 300K under zero external DC field for frequencies ranging from f = 10 to 10,000 Hz. A prominent peak appears in both chi' and chi '' as a function of T. The peak temperature T-2 of chi '' depends on f following the Vogel-Fulcher law. The particles show superparamagnetic behavior at room temperature, with transition to a blocked state at T-B(m)similar to 94K in ZFC and 119K in AC susceptibility measurements, respectively, which depends on the applied field. The saturation magnetization and the coercivity measured at 4.2K are 27.3 emu/g and 14.7 kOe, respectively. The particle size distribution was determined by fitting a magnetization curve obtained at 295K assuming a log-normal size distribution. The interparticle interactions are found to influence the energy barriers yielding an enhancement of the estimated magnetic anisotropy, K = 6 x 10(6) erg/cm(3). Mossbauer spectra obtained at higher temperatures show a gradual collapse of the magnetic hyperfine splitting typical for superparamagnetic relaxation. At 4.2 K, the Mossbauer spectrum was fitted with two magnetic subspectra with internal fields H-int of 490, 470 and 515 kOe, corresponding to Fe3+ ions in A and B sites. (C) 2008 Elsevier B.V. All rights reserved.