Itinerant Ferromagnetism with Ultracold Atoms

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Science  18 Sep 2009:
Vol. 325, Issue 5947, pp. 1507-1509
DOI: 10.1126/science.1179767

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Ferromagnets, such as those made of iron or nickel, are called itinerant because the electrons whose spins aligned to create the magnetic state are extended and are the same as the ones responsible for conduction. Ferromagnetism was a mystery for classical physics, and its explanation in terms of spin, exchange interactions, and repulsions between identical particles was a triumph of early quantum mechanics. However, it proved difficult to apply these early models to real ferromagnets in a quantitative way, both because the simple models neglect important features relevant in real materials and because theoretical tools to properly treat the strong correlation problem have only recently been developed. Fortunately, the simple models studied in the early days of quantum mechanics can also be applied to fermions other than electrons. On page 1521 of this issue, Jo et al. (1) provide evidence for an analog of ferromagnetism in an ultracold gas of neutral lithium-6 atoms. When repulsive interactions between these freely moving particles are sufficiently strong, a transition to ferromagnetic ordering is seen.