Structural studies of the phase separation of amorphous Fe[sub [ital x]]Ge[sub 100[minus][ital x]] alloys

Small-angle x-ray scattering and x-ray-absorption near-edge spectroscopy (XANES) experiments have been performed on amorphous Fe[sub [ital x]]Ge[sub 100[minus][ital x]] alloys over the composition range 0[le][ital x][le]72. The observed small-angle x-ray-scattering patterns were compared both with those calculated for a model assuming segregation of the alloys into particular phases and with scattering patterns calculated for voids in a homogeneous matrix. The x-ray-absorption near-edge-structure data were used to test for phase separation. No large-scale phase separation is observed in the semiconductor-metal transition region (15--25 at. % Fe), but fine-scale, kinetically limited phase separation or other types of composition fluctuations cannot be ruled out. The results also indicate that phase separation occurs for alloys with 37[le][ital x][le]72, with data consistent with separation into amorphous FeGe[sub 2] and Fe[sub 3]Ge. Thus, ferromagnetic moment formation occurs in the phase-separated region, with the transition composition (40--43 at. % Fe) probably linked to [ital a]-Fe[sub 3]Ge percolation, as hypothesized by Janot for the related Fe[sub [ital x]]Sn[sub 100[minus][ital x]] system. This phase separation explains the Mossbauer observation of magnetic'' and nonmagnetic'' Fe atoms in these alloys.