Magnetic and microwave studies of high-spin states of single-molecule magnet Ni ₄

Quantum tunneling of the magnetization in a single-molecule magnet has been studied in experiments that combine microwave spectroscopy (10-50 GHz) with low temperature high sensitivity micro-Hall effect magnetometry (T = 0.4 K). This method enables the monitoring of spin-state populations in the presence of microwave radiation and a direct measure of the energy splitting between low lying high-spin states. We present results that show the level repulsion between such states as a function of magnetic field in the SMM Ni ₄ (S = 4), which clearly indicates the formation of high-spin superposition states. The absorption linewidths provide a lower bound on the transverse relaxation time (τ ₂) or decoherence time of these superposition states of ∼0.5 ns. Studies as a function of microwave power and magnetic field sweep rate suggest that the energy relaxation rate decreases with increasing longitudinal field and energy splitting between states.