Butterfly Hysteresis Loop and Dissipative Spin Reversal in the S = 1/2, V15 Molecular Complex

Time resolved magnetization measurements have been performed on a spin 1/2 molecular complex, so-called V₁₅. 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.