Magnetic Properties of γ-Fe2O3 Nanoparticles Precipitated in Alginate Hydrogels

gamma-Fe2O3 magnetic nanoparticles ranging in average diameter from 2 to 4 nm were precipitated within an alginate hydrogel and characterized by X-ray diffraction (XRD), Mossbauer spectroscopy, and SQUID magnetometry. Regardless of the initial Fe valence state of the starting chloride salt, Mossbauer spectroscopy confirmed that gamma-Fe2O3 was the only phase present. As expected, the nanoparticles exhibited superparamagnetic behavior with the magnetic moments becoming frozen with decreasing temperature as evidenced by a bifurcation in the zero-field-cooled (ZFC) and field-cooled (FC) magnetizations and a hysteresis in the M-V-vs.-H curves. The values of effective magnetic anisotropy (similar to 10(6) ergs/cm(3)) determined from the differences between the ZFC and FC magnetizations were found to be an order of magnitude larger than the magneto-crystalline anisotropy for bulk gamma-Fe2O3, and are probably the result of surface and particle size dependent effects. Likewise, the nanoparticle size distributions as deduced from the blocking temperature distribution function f(T-B) based on fits to the difference in the ZFC and FC magnetization curves as well as from fits of the M-V-vs.-H curves with a Langevin function in the superparamagnetic regime indicate fairly broad distributions of particle sizes with the particle sizes being comparable to those deduced from XRD measurements. The smaller saturated magnetization values found for these nanoparticles than the bulk value combined with the non-zero slope of the high-field magnetization data suggests that these nanoparticles have a non-negligible surface layer of non-collinear, spins surrounding a ferrimagnetically ordered gamma-Fe2O3 core.