Muon hyperfine fields in Fe(Al) alloys

The hyperfine field on the muon, $B_{\mathrm{hf}}$, at interstitial sites in dilute Fe(Al) alloys has been measured for four different concentrations of Al and as a function of temperature by the muon-spin-rotation method. The magnitude of $B_{\mathrm{hf}}$, which is negative, decreases at rates ranging from (0.09±0.03)% per at.% Al at 200 K to an asymptotic limit of 0.35±0.03 far above 440 K. This behavior shows that sites near the Al impurity are weakly repulsive to the muon, with an interaction potential of 13±3 meV. In order to fit the temperature dependence of the hyperfine field, it is necessary to hypothesize the existence of a small concentration of unidentified defects, possibly dislocations, which are attractive to the muon. Although the Al impurity acts as a nonmagnetic hole in the Fe lattice, the observed decrease in $B_{\mathrm{hf}}$ is only 35% of the decrease in the bulk magnetization. We conclude that $B_{\mathrm{hf}}$ is determined mainly by the enhanced screening of conduction electrons in Fe and Fe(Al). Since the influence of the Al impurity on the neighboring Fe moments is very small, most of the change in $B_{\mathrm{hf}}$ is therefore attributed to the increase in conduction-electron polarization at the Al impurity.