Polarized-beam study of the paramagnetic scattering from bcc iron

Abstract

Reliable magnetic scattering cross sections are obtained for bcc Fe at $T=1.02{T}_{c}$ and $1.06{T}_{c}$ using polarized neutrons and polarization analysis. The constant-$q$ measurements cover a $q$ range of 0.1-0.6 ${\mathrm{\AA{}}}^{\ensuremath{-}1}$ with an energy range extending up to 50 meV. These scans consist of broad energy distributions centered at zero energy with no peaks observed at finite-energy transfers, in contrast to the results reported by Lynn. A simple paramagnetic scattering function $S(q,\ensuremath{\omega})\ensuremath{\propto}[\frac{1}{({\ensuremath{\kappa}}_{1}^{2}+{q}^{2})}][\frac{\ensuremath{\Gamma}}{({\ensuremath{\Gamma}}^{2}+{\ensuremath{\omega}}^{2})}]$ is shown to describe the iron cross sections quantitatively, and in absolute units, for the $q$ and energy range specified above. The peaks observed in constant-energy scans are simply energy slices of the paramagnetic scattering function and thus should not be interpreted as spin-wave peaks. We conclude that in the ($q$,$\ensuremath{\omega}$) region covered in our polarized beam studies, neither propagating spin waves nor giant short-range magnetic order exist in Fe above ${T}_{c}$.