THz-Driven Spin Dynamics in Orthoferrites with Kramers and Non-Kramers Rare-Earth Ions

We study the THz-driven spin dynamics of the canted antiferromagnet ErFeO_{3} across the spin reorientation temperatures 80-90 K. The amplitudes of the soft antiferromagnetic resonance mode are drastically enhanced by an order of magnitude in the middle of the spin reorientation temperature interval in ErFeO_{3}. Conversely, in the phenomenologically similar magnetic system of TmFeO_{3}, exhibiting the same phase transitions at very similar temperatures, this enhancement is not observed. We attribute this enhancement in ErFeO_{3} to the dynamic interaction of the iron 3d spins with the rare-earth 4f-electronic transitions of the Kramers Er^{3+} ions. We confirm this interpretation by comparing measurements on TmFeO_{3}, where the non-Kramers Tm^{3+} electronic states do not strongly couple to the antiferromagnetic mode. Thus, our results indicate that the character of the microscopic electronic 4f states may drastically alter the ultrafast magnetic response in the materials with otherwise very similar static magnetic properties and phase diagrams.