科研之友
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摘要
Fe-MgO nanocomposites synthesized by the mechanical high-energy transfer technique were characterized by x-ray diffraction (XRD), transmission electron microscopy, Mossbauer spectroscopy and dc magnetization studies. By varying the duration of milling, powder with different grain sizes in the range 17-40 nm was produced. XRD and Mossbauer measurements could not detect the presence of any form of iron oxide in the nanocomposites. Transmission electron micrographs showed a shape transformation from spherical to acicular-like geometry in the samples ball milled for more than 36 h. Dc magnetization measurements show an increase in the coercive field with decrease in average grain size, with a sharp increase at the point of shape transformation. A marked decrease from the expected values of saturation magnetization and magnetic moment per formula unit was observed. This decrease was ascribed to the magnetic dead layer formed at the surface of the nanoparticles, which increased with the increase in milling time. The nanocomposites ball milled for more than 36 h exhibit a two-component Mossbauer spectra due to the presence of two phases: a crystalline phase and an additional phase corresponding to the atoms residing at the grain boundaries between the crystallites. The grain boundaries give rise to a distribution of hyperfine magnetic fields, typical of an amorphous-like compound.
- 出版日期2007-8-7
