Polarization dependence of the magnetic fluctuations in Cr belowTN

We have investigated the magnetic excitations in single-Q, single-domain Cr below $T_N$ using polarization analysis of inelastically scattered neutrons for energy transfers up to 18 meV. The results show that the longitudinal $\mathrm{}\left(L\right)\mathrm{}$ magnetic modes are enhanced below $\simeq 8\mathrm{meV}$ with respect to the transverse $\mathrm{}\left(T\right)\mathrm{}$ modes at low temperature. The latter are independent of the polarization with respect to the ordering vector ${\mathbf{Q}}_{\pm }^{\prime }$ in the transverse spin-density-wave phase, i.e., they are isotropic. The $L$ and $T$ modes are not affected by the spin-flop transition to the longitudinal spin-density-wave phase at $T_{\mathrm{sf}}=121\mathrm{}\mathrm{K}$ indicating that the transition is due to anisotropies and that it is not an intrinsic feature of the spin-density wave. There are indications that the $L$ modes have a less steep dispersion curve and are more damped than the $T$ modes in accordance with the three-band model by Fishman and Liu. With increasing temperature the intensity from the $T$ and $L$ modes increases strongly at small $E$ transfer and their $E$ dependence becomes similar. In particular, the enhancement of the $L$ modes with respect to the $T$ modes vanishes. Significant inelastic scattering is present at the silent peak positions. It surpasses even the scattering at the allowed peak positions at large $E$ transfers. We argue that the silent inelastic scattering is partly responsible for the “commensurate” scattering at the (100) position due to resolution effects and that the major cross section at (100) is due to the longitudinally polarized Fincher-Burke mode at 4.2 meV.