Terahertz Excitation of Exchange Mode in a Cavity Formed by Crystal Interfaces

The interaction of terahertz (THz) radiation with high-frequency spin resonances in complex magnetic materials is central for modern ultrafast magnonics. Here we demonstrate strong variations of the excitation efficiency of the sub-THz exchange magnon in a single crystal gadolinium iron garnet (Gd,Bi)3Fe5O12. An enhancement of the exchange magnon amplitude is observed when its frequency matches an eigenmode of the cavity created by the sample interfaces. Moreover, this enhancement is accompanied by a 5-fold decrease in effective damping of the exchange mode. The THz-exchange magnon interaction in the cavity is analyzed within the developed Landau-Lifshitz-Gilbert formalism for three coupled magnetization sublattices and cavity-enhanced THz field. This work presents a novel approach for the THz excitation of magnetization dynamics in ferrimagnets and outlines promising pathways for the controlled optimization of light-magnetization coupling in single crystals.