Magnetic properties of iron nitride-alumina nanocomposite materials prepared by high-energy ball milling

The structural and magnetic properties of the granular iron nitride-alumina composite materials, (Fe_xN)_0.2(Al ₂O₃)_0.8 and (Fe_xN) _0.6(Al₂O₃)_0.4, fabricated using high-energy ball milling have been determined by using X-ray diffraction, Mössbauer spectroscopy, and magnetization measurements. The Mössbauer spectra, fit with a distribution of hyperfine fields between zero and 40 T, indicate that the weighted average field decreases with increasing milling time. The isomer shift increases with milling time because of a reduced iron 4s-electron density at the grain boundaries. Coercive fields as high as 325 and 110 Oe are obtained for (Fe_xN)_0.2(Al₂O ₃)_0.8 at 5 and 300 K, respectively; the increase in the coercive field upon cooling indicates the presence of superparamagnetic particles. The coercive field increases with milling time because of the reduced particle size. The decrease in the magnetization results from the increase in both the superparamagnetic fraction and the concentration of surface defects with increased milling time.