(471a) Influence of Precursor Aging Time On the Magnetic Properties of Cobalt Ferrite Nanoparticles

Cobalt ferrite nanoparticles are of interest because of their high coercivity response and magnetic anisotropy constant, which make them attractive for the design of magnetic storage devices and sensors based on Brownian relaxation mechanism. These nanoparticles can be synthesized with a narrow size distribution by the thermal decomposition of an oleate precursor in a high boiling point solvent. We studied the influence of aging time of the iron-cobalt oleate precursor on the magnetic properties of cobalt ferrite nanoparticles. To this end, an oleate was prepared from Fe⁺³ and Co⁺² salts (Fe/Co=2.25) using sodium oleate and a mixture of ethanol/water/hexane. Afterwards, the oleate was aged at room temperature. Nanoparticles were synthesized using 1-octadecene as solvent and the iron-cobalt oleate with different aging times. The sizes of the synthesized nanoparticles were determined by Transmission Electron Microscope (TEM). X-ray diffraction was used to determine the crystal structure of the nanoparticles. Inductively coupled plasma optical emission spectroscopy (ICP-OES) was used to determine the incorporation of cobalt and iron in the ferrite crystalline structure of the synthesized nanoparticles. The saturation magnetization of the synthesized nanoparticles was determined by measuring the response of the equilibrium magnetization under the application of a DC magnetic field. An increase from 0.5 Am²kg^-1 to 125 Am²kg^-1 was with increasing aging time. The coercivity of cobalt ferrite nanoparticles was recorded at 2, 300, and 400 K. Nanoparticles at 2 K displayed a coercivity increase from Am^-1 to Am^-1 with increasing aging time. These samples also exhibited a coercivity increase from to Am-1 at 300 K. No coercivity was observed for any sample at 400 K. AC susceptibility measurements were recorded at 273 K, from which we determined a Brownian relaxation mechanism for the synthesized nanoparticles. In the case of nanoparticles prepared with the longest aging time the response was as expected for nanoparticles with a Brownian relaxation mechanism.