Mossbauer studies of ultrafine iron-containing particles on a carbon support

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Abstract
Carbon-supported metallic iron particles with an average diameter of 3.7 nm have been studied in situ by Mossbauer spectroscopy in the temperature range 5-305 K and with external magnetic fields up to 4 T. Depending on the preparation conditions various amounts of amorphous Fe-C particles are also formed. The average magnetic hyperfine fields of the surface atoms of alpha -Fe particles are significantly larger than that of bulk alpha -Fe at 5 K, but decrease much faster with increasing temperature than the bulk hyperfine field. The superparamagnetic blocking temperature for the alpha -Fe particles is about 70 K, but after oxidation it decreases to about 50 K, indicating that the magnetic anisotropy energy constant of the oxidized particles is significantly lower than that of the metallic particles. The temperature dependence of the spectral area is similar in the metallic and the oxidized states. In both cases the vibrational modes are dominated by particle vibrations.