Defect-mediated nucleation of α-iron in Au-Fe alloys

The low-temperature magnetic properties of Au-Fe alloys, such as spin-glass or mictomagnetic behavior, are known to be dependent on thermal history and therefore on the atomic order in the sample. The nature of this order is still controversial, and the discussion includes the presence of short-range order as well as the precipitation of Guinier-Preston (GP) zones. It is frequently neglected in this context that there is a miscibility gap in Au-Fe at low temperatures, where ferromagnetic α-iron should coexist with the solid solution. The conditions for α-iron nucleation have not been studied systematically in the range of compositions, where spin-glass or mictomagnetic behavior appears. We have used a combination of Mössbauer spectroscopy, x-ray diffraction, and small-angle scattering to investigate the nucleation of α-iron in ${\mathrm{Au}}_{0.85}$ ${\mathrm{Fe}}_{0.15}$ and ${\mathrm{Au}}_{0.75}$ ${\mathrm{Fe}}_{0.25}$: (1) This nucleation appeared to be a discontinuous process related to the presence of a high dislocation density. (2) No homogeneous precipitation, like the formation of GP zones, was observed. (3) All experimental results seem to be in agreement with the hypothesis of a (11/20) special-point short-range ordering in the solid solution.