Polarization analysis of low-energy excitations in single-domain Cr

The longitudinal and transverse spin fluctuations in single-domain Cr have been investigated in the transverse spin-density-wave phase using constant energy scans with 4≤E≤25 meV and polarization analysis. The transverse excitations show well-defined peaks centered at the incommensurate satellite positions (1±δ0 0) and a broad peak at the commensurate (1 0 0) position. The longitudinal fluctuations exhibit a much more complicated structure due to the additional appearance of magnetic excitations below 9 meV (Fincher-Burke modes). The fluctuations with longitudinal polarization show a pronounced renormalization between 136 and 230 K raising the possibility that the Fincher-Burke modes are responsible for the phase transition at T_N. Above E≈18 meV, the longitudinal fluctuations decrease with increasing E at T = 230 K in contrast to the transverse fluctuations. Our results indicate that unpolarized beam data is not really useful for making comparisons with existing theories. There is an urgent need to extend polarization analysis to higher neutron energies.