Inversion of magnon lifetime of ferromagnetic and exchange resonance modes in ferrimagnets

Antiferromagnetically coupled ferrimagnets exhibit both ferromagnetic resonance and exchange resonance modes. The antiferromagnetic exchange resonance mode, characterized by a higher magnon frequency than the ferromagnetic resonance mode, holds promise for fast spintronic applications. However, as higher magnon frequencies are typically associated with shorter magnon lifetimes, the exchange resonance mode is expected to decay more rapidly than the ferromagnetic resonance mode, leading to challenges for long-lived information transfer and coherent dynamics. Here we demonstrate that this inverse relationship between frequency and lifetime can be broken in ferrimagnets with two inequivalent magnetic sublattices. Using time-resolved magneto-optical Kerr effect spectroscopy on CoGd, we observe that the exchange resonance mode exhibits a longer magnon lifetime than the ferromagnetic resonance mode near the angular momentum compensation point. Our theoretical and simulation models reveal that this inversion of magnon lifetime arises from the inequivalence in magnetic damping of the two sublattices. The unique combination of higher frequency and longer lifetime in the exchange resonance mode of ferrimagnets highlights its potential for high-speed and energy-efficient spintronic devices.