Ultracold gases of fermionic atoms (Fermi gases) offer vast opportunities to explore many-body behavior. With modern experimental tools, researchers can make the interaction between the atoms attractive or repulsive, and they can tune the interaction strength over several orders of magnitude. A long-time goal has been to use this tunability to drive the atoms into a ferromagnetic state, in which repulsive interactions cause the atoms to point their magnetic moments in the same direction. The realization of a cold-atom ferromagnet would provide a unique chance to examine the microscopic mechanisms behind magnetism, and to benchmark theoretical models. However, the atoms in these experiments tend to rapidly combine into weakly bound molecules, which obscures the magnetic behavior. Matteo Zaccanti of the European Laboratory for Non-Linear Spectroscopy and the University of Florence, Italy, and colleagues have now developed a new technique that discriminates the atomic signal from that of the molecules, allowing them to observe the emergence of magnetic interactions between the atoms that are indicative of a ferromagnetic phase [1].

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