  In a dirty metal, electron-electron interactions in the spin-triplet channel
lead to singular corrections to a variety of physical quantities. We show that
these singularities herald the emergence of ferromagnetism. We calculate the
effective action for the magnetic moment of weakly-interacting electrons in a
dirty metal and show that a state with finite ferromagnetic moment minimizes
this effective action. The saddle-point approximation is exact in an
appropriate large-N limit. We discuss the physics of the ferromagnetic state
with particular regard to thermal fluctuations and localization effects.
