The Interplay of Crystal‐Field Transitions and Exchange Spin Dynamics in a Ferrimagnet

Rare‐earth iron garnets offer an ideal platform for exploring the interplay of low‐energy excitations and the complex temperature‐dependent magnetization dynamics. In these systems, exchange coupling between rare‐earth and iron sublattices generates high‐frequency collective spin excitations. In addition, the robust spin‐orbit coupling of localized 4 electrons triggers the crystal‐electric‐field (CEF) transitions at THz frequencies. Despite extensive research into the garnet spin dynamics, the interplay between CEF excitations and exchange modes has remained largely unmapped. Using temperature‐dependent THz time‐domain spectroscopy, we demonstrate a hybridization between the Yb‐ion CEF excitation and the Yb‐Fe exchange mode in Gd 3/2 Yb 1/2 BiFe 5 O 12 . This coupling is characterized by a significant redistribution of spectral and temporal weights as the material approaches its magnetization compensation temperature. Notably, the Yb–Fe exchange mode exhibits an anomalous redshift upon cooling – a reversal of the conventional blue shift typically driven by increased exchange coupling. We trace this phenomenon to a modification of Yb–Fe exchange anisotropy, driven by the interplay of the Fe exchange field and Yb CEF excitations. These findings highlight the critical role of CEF‐mediated exchange coupling in shaping low‐energy spin dynamics, positioning rare‐earth garnets as a cornerstone for future THz spintronic technologies.