Quantum tunneling of the magnetization was reported in 1996 for a crystal of Mn 12 ac molecules with S=10. Quoting Thomas and coworkers, " in an applied magnetic field, the magnetization shows hysteresis loops with a distinct 'staircase' structure: the steps occur at values of the applied field where the energies of different collective spin states of the manganese clusters coincide. At these special values of the field, relaxation from one spin state to another is enhanced above the thermally activated rate by the action of resonant quantum-mechanical tunneling ". Quantum tunneling of the magnetization was reported in ferritin present in horse spleen proteins