Fronts of spin tunneling in molecular magnets

Dissipative spin-tunneling transitions at biased resonances in molecular magnets such as Mn-12 Ac are controlled by the dipolar field that can bring the system on and off resonance. It is shown that this leads to spin relaxation in form of propagating fronts of tunneling, with the dipolar field adjusting self-consistently to provide a zero bias within the front core. There are two regimes of the front propagation: laminar and non-laminar with discontinuous magnetization and dipolar field. In the laminar regime the speed of the front can exceed that of the magnetic deflagration, if the transverse field is large enough. Fronts of tunneling can be initiated by magnetic field sweep near the end of the crystal.