EXAFS study of local order in Ag-Mn spin-glass alloys

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Abstract
In order to correlate changes in magnetic properties in the Ag-Mn spin-glass alloys with changes in short-range order (SRO), EXAFS investigations of the Ag and the Mn K edges have been performed on Ag-Mn quenched alloys. These measurements follow X-ray diffuse scattering experiments on single crystals, which showed that SRO exists in Ag-Mn quenched alloys of composition greater than 6 at.% Mn. A knowledge of the chemical order which exists in these alloys is necessary to understand the variation of the magnetic susceptibility after annealing at moderate temperatures. As usual in EXAFS analysis, a check on the phase shifts has been achieved with elemental metals (Ag and alpha -Mn). The magnitude of the local variation of the lattice parameter about Mn atoms and the value of the Cowley SRO parameter ( alpha 1) was measured for five alloys with Mn concentrations from 6-24 at.%. For each sample the Mn-Ag distance differed by only 2% from the Ag-Ag distance measured at the same temperature in pure silver. Thus only a very weak distortion effect exists in these alloys. The Cowley parameters ( alpha 1) are in good agreement with the results of the analysis of X-ray diffuse scattering experiments. This confirms that the number of first-neighbour Mn atoms to an Mn atom is less than the statistical value.