Mössbauer effect study of charge and spin transfer in Fe-Cr

The influence of temperature and time of annealing on hyperfine fields and isomer shifts has been studied for a range of Fe–Cr alloys containing 1–45 at% Cr. It has been revealed that up to 15at% Cr neither time nor temperature of annealing practically does affect the hyperfine parameters. For more concentrated samples, however, both temperature and time of annealing are important. In particular, the Mössbauer spectrum of Fe–45.5 at% Cr annealed at 700°C for 5 h was a single-line indicating that the sample was paramagnetic. The observed changes of the hyperfine fields and the isomer shifts have been interpreted in terms of a spin and charge trasfer, respectively.Strong linear correlations between the following quantities have been revealed: the hyperfine field H(0,0) and the isomer shift IS(0,0); the average hyperfine field $\text{H}$ and the average isomer shift $\text{IS}$; the average hyperfine field $\text{H}$ and the average number of Cr atoms in the first two coordination spheres, $\text{N}$. It has been calculated from the first two correlations that a) a change of polarization of itinerant s-like electrons of one electron is equivalent to a change of the hyperfine field of 1602 kOe, and b) on average, a unit change of s-like electron polarization is equivalent to 3277 kOe. The two constant are very close to theoretical estimations, which can be found in literature. Correlation between the hyperfine field and the isomer shift led to a conclusion that the substitution of Fe atoms by Cr ones decreases the density of spin-up electrons on average by 0.026 electrons per one Cr atom in a unit cell. These electrons are most likely trapped by Cr atoms, because the hyperfine field at neighbouring Fe nuclei decreases and the density of charge at those nuclei increases at the rate of 0.029 electrons per one Cr atom in a unit cell.Based on the results obtained, a formula describing the dependence of the average hyperfine field on Cr contents and on the heat treatment has been postulated for Fe–Cr alloys.