Observation of double hysteresis in a MnMn$_6$(CH$_3$H$_5$O$_3$)$_3$ single-molecule magnet

We report on high field and low temperature magnetization measurements of a novel {MnMn$_6$}(CH$_3$H$_5$O$_3$)$_3$ complex. A home-made micro-Hall-probe magnetometer was used to perform the characterization at mK temperatures and fields up to 17\,T. Most 3d-ion based single-molecule magnets, known up to now, have a spin ground state well separated from the first excited state, leading to the formation of giant spin at low temperatures. In contrast to this situation, the ground state (S$=$6) of the present complex can already be exited at moderate magnetic fields. Surprisingly, magnetic hysteresis was observed for both the ground state and the first exited state leading to a double hysteresis in the low temperature magnetization measurements. The blocking temperature was found to be T$_B \approx$1.3\,K. Origin and possible consequences of this unusual behavior will be discussed.