Structural and magnetic properties of Fex-C1-x nanocomposite thin films

Iron-carbon nanocomposite thin films with iron concentrations ranging from 12 to 74 at. % were grown by ion-beam sputtering codeposition at different substrate temperatures. The microstructure of the films was characterized by transmission electron microscopy, atomic force microscopy, extended x-ray absorption fine structure, and grazing incidence small-angle x-ray scattering. A granular morphology consisting of body-centered-cubic iron-rich nanoparticles (2-5 nm in diameter and 3-8 nm in height with a relatively sharp size distribution) regularly distributed within a more or less graphitic matrix was obtained. Structural properties as well as magnetic ones were found to depend strongly on composition, substrate temperature, and postdeposition treatments (annealing or Ar+ irradiation). The temperature dependence of the susceptibility exhibited a superparamagnetic response with blocking temperature in the range of 13-180 K. The maximum low-temperature (5 K) coercivity value obtained in this study was 850 Oe for the film with the lowest iron content (x = 0.12). The saturation magnetization was found to be reduced compared to the corresponding bulk value for pure alpha-Fe, and increased with increasing substrate temperature. Otherwise, when increasing the iron content, a decrease in coercivity was observed in correlation with an increase in remanent magnetization. (C) 2000 American Institute of Physics. [S0021-8979(00)05407-4].