Magnetic properties of cobalt-ferrite nanoparticles embedded in polystyrene resin

Samples of maghemite and cobalt-ferrite nanoparticles (sizes, 3–10nm) were prepared by cross-linking sulfonated polystyrene resin with aqueous solutions of (1) FeCl2, (2) 80%FeCl2+20%CoCl2, (3) FeCl3, and (4) 80%FeCl3+20%CoCl2 by volume. Chemical analysis, x-ray powder-diffraction, and Fe57 Mossbauer spectroscopic measurements show that samples 1 and 3 consist of γ-Fe2O3 nanoparticles (sizes, ∼10 and 3nm) and sample 2 and 4 consist of CoxFe3−xO4 nanoparticles (sizes, ∼10 and 4nm). The temperature dependence of the zero-field-cooled and field-cooled magnetizations at low temperatures, together with a magnetic hysteresis in the M versus H data below blocking temperatures, demonstrate superparamagnetic behavior. The introduction of Co in the iron oxide-resin matrix results in an increase in the blocking temperature of nanoparticles.