  The energy released in a magnetic material by reversing spins as they relax
toward equilibrium can lead to a dynamical instability that ignites
self-sustained rapid relaxation along a deflagration front that propagates at a
constant subsonic speed. Using a trigger heat pulse and transverse and
longitudinal magnetic fields, we investigate and control the crossover between
thermally driven magnetic relaxation and magnetic deflagration in single
crystals of Mn$_{12}$-acetate.
