Magnetic properties of nanosized iron oxide particles precipitated in alginate hydrogels

Nanoparticles of γ-Fe2O3 (size 2–3nm) were precipitated in alginate hydrogels by cross-linking sodium alginate with Fe ions in a methanol-water solution. The zero-field-cooled and field-cooled magnetization measurements between 5 and 350K and the hysteresis in the M vs H relation below the blocking temperature indicate superparamagnetic behavior. The temperature dependence of the coercive field is not consistent with the T1∕2 behavior predicted by Neel and Brown for the noninteracting particles. The average diameter of the nanoparticles determined from the magnetic data is consistently larger than the corresponding particle size determined by x-ray diffraction, perhaps due to interparticle magnetic interactions.