The giant effect of magnetic ordering on a sound velocity in a σ-Fe₅₅Cr₄₅ alloy

We studied atomic dynamics of $\sigma \text{-}{\text{Fe}}_{100-x}{\text{Cr}}_x$ (x = 45 and 49.5) alloys using nuclear inelastic scattering of synchrotron radiation. For the σ-Fe₅₅Cr₄₅ alloy, the derived reduced iron partial density of phonon states reveals a huge difference in the low-energy region between magnetic and paramagnetic states. The latter implies a ∼36% increase of the sound velocity in the magnetic phase, which testifies to a magnetically induced hardening of the lattice. Estimations of the longitudinal and transverse components of the sound velocity based on theoretically calculated elastic constants for the σ-CoMo alloys imply that both components are affected by magnetism. (letter)