Magnetic properties of γ-Fe2O3 nanoparticles incorporated in a polystyrene resin matrix

gamma-Fe2O3 magnetic nanoparticles ranging in average diameter from 3 to 10 nm were synthesized into a polystyrene resin matrix by an ion-exchange method and characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), Mossbauer spectroscopy, and SQUID magnetometry. The average particle size as determined from XRD and TEM was found to be strongly dependent upon the initial Fe valence state of the starting chloride salt(s) and on the number of steps that the salt introduction and ion-exchange process were repeated. Regardless of the initial Fe valence state and processing conditions, Mossbauer spectroscopy confirmed that the Fe in the resulting nanoparticles existed only as Fe(III) ions and that gamma-Fe2O3 was the only phase present. The values of the saturation magnetization at 5 K were found to be dependent upon the processing conditions and ranged from 203 to 333 emu/cm(3), which are significantly smaller than the bulk value (408 emu/cm(3)) for gamma-Fe2O3. As expected, the nanoparticles exhibited superparamagnetic behavior with the magnetic moments becoming frozen with decreasing temperature as evidenced by the appearance of a six-line splitting in the Mossbauer spectra, a bifurcation in the zero-field-cooled (ZFC) and field-cooled (FC) magnetizations, and an opening in the M-V-vs-H hysteresis curves. The values of magnetic anisotropy constant (1.2-2.1x10(6) ergs/cm(3)) determined from the differences between the ZFC and FC magnetizations were found to be higher than the bulk value (1.1x10(5) ergs/cm(3)) for gamma-Fe2O3, and are probably due to surface 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 in the superparamagnetic regime with a Langevin function indicate fairly broad distributions of particle sizes with the particle sizes being comparable to those deduced from XRD and TEM measurements. The smaller saturated magnetization values found for these nanoparticles combined with the non-zero slope of the high-field magnetization data suggests that these nanoparticles have a non-negligible surface layer of noncollinear spins surrounding a ferrimagnetically ordered gamma-Fe2O3 core.