Mössbauer study of the order-disorder andα−γtransitions in FeCo

A Mössbauer study of ${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}$ is used to measure the internal field ($H_{\mathrm{int}}$) at the ${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}$ nucleus, Mössbauer spectral area, and the center shift for a nearly equiatomic FeCo alloy. Measurements were made between 297 and 1300 K with special attention to the region near the order-disorder (OD) transition (1006 K) and $\alpha -\gamma$ transition (1235 K). Near and above the OD transition, $H_{\mathrm{int}}$ changes slowly with temperature compared to the bulk magnetic moment. At the $\alpha -\gamma$ transition, $H_{\mathrm{int}}$ drops to zero abruptly. The Mössbauer spectral area shows a strong temperature dependence near the OD and $\alpha -\gamma$ transitions. In particular, an appreciable decrease in the spectral area is observed at the $\alpha \to \gamma$ transition. The Mössbauer spectral area is analyzed in correlation with the resistivity measurements of Seehra and Silinsky. Considerable deviations from the harmonic behavior near and above the OD transitions are evident. By contrast the center shift does not show any dramatic change near the OD and $\alpha -\gamma$ transitions.