Anomalous field sweep rate dependence of the tunnel relaxation in single-molecule magnet Mn4-Bet

We present a low-temperature magnetometry study of the behavior of the quantum tunneling of the magnetization (QTM) in a Mn4 single-molecule magnet (SMM) as a function of the rate at which a longitudinal magnetic field is swept across a QTM resonance. An initial decrease of the tunnel relaxation rate with increasing field sweeping rate is eventually replaced by an anomalous monotonic increase for moderately high sweep rates.