Local structural change under antiferro- and ferromagnetic transition in FeRh alloy

We investigate the local structural change under antiferro- and ferromagnetic transition for annealed Fe50.4Rh49.6 alloy. Both Fe and Rh K-edge EXAFS are analysed up to second nearest neighbours. For first nearest neighbour Fe-Rh (or Rh-Fe), the temperature dependence of the interatomic distance and Debye-Waller factor is well understandable and reproduced by other structural techniques. On the other hand, that for the second nearest Fe-Fe is quite mysterious; the interatomic distance is longer than the value deduced from the corresponding change of the first nearest Fe-Rh but the Debye-Waller factor for Fe-Fe decreases as temperature under the transition. In ferromagnetic phase, the coherent motion of Fe atoms is expected to be prominent. We propose the model that the transition progresses by three steps with temperature, T1, T2, and T3.