Magnons in antiferromagnetic bcc Cr and Cr₂O₃ from time-dependent density functional theory

We apply time-dependent density functional theory to calculate the transverse magnetic susceptibility of bcc Cr and Cr₂O₃, which constitute prototypical examples of antiferromagnets with itinerant and localized magnetic moments, respectively. The exchange-correlation kernel is rescaled in order to enforce the Goldstone condition and the magnon dispersion relations are extracted based on a symmetry analysis relying on the generalized Onsager relation. Doing so, our calculations yield the characteristic linear magnon dispersion of antiferromagnets in the long-wavelength limit. In the case of Cr₂O₃, we find that the adiabatic local density approximation yields a good qualitative agreement with the measured dispersion, but overestimates the magnon velocity and bandwidth by a factor of 2. Including a Hubbard correction improves the magnon velocity, but at the expense of the overall qualitative agreement with the experimental magnon dispersion. For bcc Cr we find a sharp acoustic magnon mode at low energies with a velocity in agreement with previously reported values. At higher energies, the acoustic magnon mode becomes subject to strong Landau damping and rapidly vanishes once it enters the Stoner continuum. In addition to the acoustic magnon mode, we also observe an additional collective mode along the Γ→R direction with an energy of ∼1 eV, which is located inside the Stoner continuum, but appears to elude the effect of Landau damping.