  We obtain a fundamental instability of the magnetization-switching fronts in
super-paramagnetic and ferromagnetic materials such as crystals of nanomagnets,
ferromagnetic nanowires, and systems of quantum dots with large spin. We
develop the instability theory for both linear and nonlinear stages. By using
numerical simulations we investigate the instability properties focusing on
spin avalanches in crystals of nanomagnets. The instability distorts
spontaneously the fronts and leads to a complex multidimensional front
dynamics. We show that the instability has a universal physical nature, with a
deep relationship to a wide variety of physical systems, such as the
Darrieus-Landau instability of deflagration fronts in combustion, inertial
confinement fusion and thermonuclear su- pernovae, and the instability of
doping fronts in organic semiconductors.
