Characterization of low-energy magnetic excitations in chromium

The low-energy excitations of Cr, i.e., the Fincher-Burke (FB) modes, have been investigated in the transversely polarized spin-density-wave phase by inelastic neutron scattering using a single-Q_± crystal with a propagation vector Q_± parallel to [001]. The constant-momentum-transfer scans show that the energy spectra consist of two components, namely dispersive FB modes and an almost energy-independent cross section. Most remarkably, we find that the spectrum of the FB modes exhibits one peak at 140 K near Q=(0,0,0.98) and two peaks near Q=(0,0,1.02), respectively. This is surprising because Cr crystallizes in a centrosymmetric bcc structure. The asymmetry of those energy spectra decreases with increasing temperature. In addition, the observed magnetic peak intensity is independent of Q, suggesting a transfer of spectral weight between the upper and lower FB modes. The energy-independent cross section is localized only between the incommensurate peaks and develops rapidly with increasing temperature.