Structural studies of the phase separation of amorphous FexGe100-x alloys.

Small-angle x-ray scattering and x-ray-absorption near-edge spectroscopy (XANES) experiments have been performed on amorphous ${\mathrm{Fe}}_{\mathit{x}}$${\mathrm{Ge}}_{100\mathrm{\ensuremath{-}}\mathit{x}}$ alloys over the composition range 0\ensuremath{\le}x\ensuremath{\le}72. The observed small-angle x-ray-scattering patterns were compared both with those calculated for a model assuming segregation of the alloys into particular phases and with scattering patterns calculated for voids in a homogeneous matrix. The x-ray-absorption near-edge-structure data were used to test for phase separation. No large-scale phase separation is observed in the semiconductor-metal transition region (15\char21{}25 at. % Fe), but fine-scale, kinetically limited phase separation or other types of composition fluctuations cannot be ruled out. The results also indicate that phase separation occurs for alloys with 37\ensuremath{\le}x\ensuremath{\le}72, with data consistent with separation into amorphous ${\mathrm{FeGe}}_{2}$ and ${\mathrm{Fe}}_{3}$Ge. Thus, ferromagnetic moment formation occurs in the phase-separated region, with the transition composition (40\char21{}43 at. % Fe) probably linked to a-${\mathrm{Fe}}_{3}$Ge percolation, as hypothesized by Janot for the related ${\mathrm{Fe}}_{\mathit{x}}$${\mathrm{Sn}}_{100\mathrm{\ensuremath{-}}\mathit{x}}$ system. This phase separation explains the Mossbauer observation of ``magnetic'' and ``nonmagnetic'' Fe atoms in these alloys.