Activated Spin Resonance with THz Attempt Frequency in SmMn₂Ge₂

Relaxation techniques are used commonly to characterize non-equilibrium phenomena such as freezing in spin glass and domain wall motion in ferromagnets. Here we investigate the unusual re-entrant ferromagnetic state in SmMn₂Ge₂ using ac-susceptibility in the frequency range 0.1 Hz – 1 kHz. Surprisingly, we find Debye-like relaxation with an energy barrier E_B ≈ 840 k_B and attempt frequencies in the THz range. We discuss the origin of this resonance and, in particular, the implication for single-spin resonance of Sm spins.