Mechanism for broadened linewidth in antiferromagnetic resonance

The linewidth of antiferromagnetic resonance (AFMR) is notably broader than that of ferromagnetic resonance (FMR), even when both systems share the same intrinsic Gilbert damping parameter. We found that this phenomenon is primarily attributed to an additional phase delay in the two magnetic sublattices of antiferromagnets caused by Gilbert damping. This phase delay introduces a dampinglike component to the originally dissipationless exchange torque, thereby enhancing the damping and broadening the linewidth. Additionally, two alternative explanations are presented: The transverse component of the exchange coupling decreases the AFMR frequency, thus increasing the damping rate, and the antiferromagnetic eigenmode demonstrates properties of a two-mode squeezed state, intrinsically linked to enhanced damping. These findings introduce a different AFM eigenstate under Gilbert damping, impacting magnetic dissipation in antiferromagnets and providing insights into the experimentally observed broadened AFMR linewidths.