Control of the size of cobalt ferrite magnetic fluid

Cobalt and iron(II) dodecyl sulfate, Co(DS)(2) and Fe(DS)(2), are used to make nanosize magnetic particles. The size of the particles is controlled by the surfactant concentration. The average size of the particles, determined by transmission electron microscopy and from simulation of Langevin curves, varies from 2 to 5 nm, with 30-35% polydispersity in the size distribution. The magnetic studies are performed on particles having 2, 3, and 5 nm diameter, respectively. These particles are characterized by a superparamagnetic behavior. The saturation magnetization decreases with the particle size and is explained in terms of an increase in the noncollinear structure at the interface. For 5 nm particles, a cubic magnetocrystalline anisotropy is observed. On decreasing the size of the particles, the surface anisotropy strongly increases.