  The magnetization of the prototypical molecular magnet Mn12-acetate exhibits
a series of sharp steps at low temperatures due to quantum tunneling at
specific resonant values of magnetic field applied along the easy c-axis. An
abrupt reversal of the magnetic moment of such a crystal can also occur as an
avalanche, where the spin reversal proceeds along a "deflagration" front that
travels through the sample at subsonic speed. In this article we review
experimental results that have been obtained for the ignition temperature and
the speed of propagation of magnetic avalanches in molecular nanomagnets. Fits
of the data with the theory of magnetic deflagration yield overall qualitative
agreement. However, numerical discrepancies indicate that our understanding of
these avalanches is incomplete.
