Anomalous field sweep rate dependence of the tunnel relaxation in single-molecule magnet Mn-4-Bet

We present a low-temperature magnetometry study of the behavior of the quantum tunneling of the magnetization (QTM) in a Mn-4 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. This behavior is distinct from the drastic reversal of the total sample magnetization caused by thermal avalanches ( hot process), which is commonly observed in other SMMs when increasing the sweep rate, usually due to poor thermal conduction or inadequate thermal anchoring of the sample. We associate the observations to a ( cold ) mechanism by which different collections of molecules within the crystal relax collectively without affecting the rest of the molecules. (C) 2013 Elsevier Ltd. All rights reserved