Butterfly Hysteresis Loop and Dissipative Spin Reversal in theS=1/2,V15Molecular Complex

Time resolved magnetization measurements have been performed on a spin 1/2 molecular complex, so-called ${\mathrm{V}}_{15}$. Despite the absence of a barrier, magnetic hysteresis is observed over a time scale of several seconds. A detailed analysis in terms of a dissipative two-level model is given, in which fluctuations and splittings are of the same energy. Spin-phonon coupling leads to long relaxation times and to a particular “butterfly” hysteresis loop.