Temperature dependence of the magnetic-moment distribution around impurities in iron

We have made neutron diffuse-scattering measurements of the magnetic-moment distribution around impurities in Fe as a function of temperature in an attempt to obtain information regarding the range of the exchange interactions. The measurements were made on Fe-based alloys containing 2-3 at.% of Si, Ge, Ti, V, Mn, Co, and Ni at temperatures ranging from 300 to 800 K. The FeTi, FeV, FeCo, and FeNi cross sections show very little temperature dependence while the FeSi, FeGe, and FeMn show pronounced thermal effects. This observation can be explained by a nearest-neighbor molecular-field model by assuming that the impurity-host to host-host exchange ratio is near unity for Ti, V, Co, and Ni impurities and small for Mn impurities, an assumption that is supported by $T_c-\mathrm{vs}-c$ data. Furthermore, the observed temperature dependences for FeSi, FeGe, and FeMn are described reasonably well by this model provided that the low-temperature moment distributions are included in the calculation. We conclude that the magnetic moment of an Fe atom depends on its local chemical environment and on its local magnetic environment. We attribute the latter to nearest-neighbor exchange interactions.