Critical slowing-down of phonon dynamics in a cobalt(II) single-molecule magnet

Single-molecule magnets (SMMs) are promising candidates for molecule-scale spin qubits and high-density information storage. The lattice vibration inside SMMs can modulate the crystal electric field of the central metal ions, allowing direct transitions between different spin states. Such spin-phonon coupling is a major source of decoherence or unwanted magnetic relaxation, yet can be tailored for spin manipulation. The coupling is thus of central importance for SMMs’ spintronics applications. Here, we investigate the critical behavior of magnetic relaxation inside an eight-coordinate, molecule-magnet complex, cobalt(II)-12-crown-4, through terahertz (THz) time-domain spectroscopy (TDTS) and THz electron paramagnetic resonance (EPR). The temperature-dependent study shows a prominent critical slowing-down of a 0.8 THz phonon oscillation lifetime across the Curie point, indicating strong spin-phonon coupling for this phonon mode. The same critical behavior is not shown in other absorption peaks, e.g., at 0.5 THz. Our study pinpoints the energy scale of spin-phonon coupling in this system, laying the groundwork for ultrafast switching of spin states inside single-molecule magnets.