  Can a gas of spin-up and spin-down fermions become ferromagnetic due to
repulsive interactions? This question which has not yet found a definitive
theoretical answer was addressed in an experiment with an ultracold
two-component Fermi gas. The observation of non-monotonic behavior of lifetime,
kinetic energy, and size for increasing repulsive interactions provides strong
evidence for a phase transition to a ferromagnetic state. It implies that
itinerant ferromagnetism of delocalized fermions is possible without lattice
and band structure and validates the most basic model for ferromagnetism
introduced by Stoner.
