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Can a gas of spin-up and spin-down fermions become ferromagnetic because of repulsive interactions? We addressed this question, for which there is not yet a definitive theoretical answer, in an experiment with an ultracold two-component Fermi gas. The observation of nonmonotonic behavior of lifetime, kinetic energy, and size for increasing repulsive interactions provides strong evidence for a phase transition to a ferromagnetic state. Our observations imply that itinerant ferromagnetism of delocalized fermions is possible without lattice and band structure, and our data validate the most basic model for ferromagnetism introduced by Stoner. 

Experimental Procedure

The first step is the production of a spin-polarized Li Fermi gas in the |F = 3/2,mF = 3/2> state by sympathetic cooling with bosonic Na atoms in a magnetic trap. The Li cloud was then loaded into a deep optical dipole
trap with a maxium power of 3W and radial trap frequency of ∼3.0 kHz, followed by an RF transfer into the lowest hyperfine state |F = 1/2,mF = 1/2>. Additional axial confinement was provided by magnetic fields. An equal mixture of |1> and |2> spin states (corresponding to the |F = 1/2,mF = 1/2> and |F = 1/2,mF = −1/2> states at low magnetic field) was prepared by a Landau-Zener RF sweep at a magnetic field of 590 G, followed by 1 s for decoherence and further evaporative cooling at 300 G. Finally, the optical trapping potential was adiabatically reduced over 600 ms, and the field increased back to 590 G. The trap had a depth of 7.1 μK and was nearly cigar shaped with frequencies  300 Hz (radial) and  70 Hz (axial).