Magnons in antiferromagnetic bcc-Cr and Cr$_2$O$_3$ from time-dependent density functional theory

TL;DR

This study uses time-dependent density functional theory to analyze magnon behavior in antiferromagnetic bcc-Cr and Cr₂O₃, revealing dispersion relations, damping effects, and the impact of different approximations on magnetic excitations.

Contribution

The paper introduces a method to accurately compute magnon dispersion in antiferromagnets using TDDFT with a rescaled exchange-correlation kernel, highlighting the effects of different approximations.

Findings

Cr₂O₃ shows good qualitative agreement with experiments but overestimates magnon velocity.

bcc-Cr exhibits a sharp low-energy magnon mode with velocity matching previous data.

An additional high-energy collective mode is observed inside the Stoner continuum, resistant to Landau damping.

Abstract

We apply time-dependent density functional theory to calculate the transverse magnetic susceptibility of bcc-Cr and Cr$_2$O$_3$, 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 Cr$_2$O$_3$, 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 two. 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 $\Gamma\rightarrow\mathrm{R}$ direction with an energy of $\sim$ 1 eV, which is located inside the Stoner continuum, but appears to elude the effect of Landau damping.