Itinerant Ferromagnetism in a Fermi Gas of Ultracold Atoms

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.

For more details, see Science 325 , 1521-1524 (2009).

The power of Feshbach resonances

   Feshbach resonances occur in a cold atomic gas when there is a possible bound molecular state with magnetic moment which differs from the vector sum of the magnetic moments of its constituent atoms.  This allows the energy of the molecular state to be adjusted relative to the energy of the free atoms.  Things become interesting when the energies are nearly equal at magnetic fields which can be practically generated in the presence of ultracold atoms.  Under such circumstances, coupling between the free and bound states can raise or lower the energies of the free atoms (effectively causing repulsion or attraction as the atoms try to move to lower their energy), or can allow the free atoms to bind into a molecule while releasing very little energy.  The repulsion and attraction have led to many interesting studies of strongly interacting many-body systems (see the polar molecule project on this page, or one of many projects on atomic BCS superfluids).

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).