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A Magnetic Gas

G. Jo, Y. Lee, J. H. Choi, C. A. Christensen, T. Kim, J. Thywissen, D. E. Pritchard, and W. Ketterle, Observation of itinerant ferromagnetism in a strongly interacting Fermi gas of ultracold atoms, Science 325, 1521 (2009).

For decades, it has been an open question whether it is possible for a gas to show properties similar to a magnet made of iron or nickel.  Iron and nickel are ferromagnetic because they become strongly magnetized below a specific temperature, when unpaired electrons within the material spontaneously align in the same direction.

It is well known that this behavior is caused by quantum mechanics.  The so-called Pauli exclusion principle does not allow two electrons with the same spin orientation to come close to each other.  Therefore, when the electrons interact with each other with repulsive forces, it can be more favorable for them to align their spins because this reduces the repulsive interaction between them, which is strongest at close distance.

All known ferromagnetic materials have a periodic crystal structure, which is known to enhance ferromagnetism.  Until today, it is not known whether gases or liquids without such a structure can become ferromagnetic.

CUA researchers have now addressed this decade-old question by using a gas of ultracold atoms cooled to 150 billionth of 1 Kelvin above absolute zero (-273 degrees C or -459 degrees F).  The Pauli exclusion principle applies to all particles classified as fermions, which includes the proton, electron and neutron, and also composite particles consisting of an odd number of elementary fermions.  Since all fermions have some properties similar to electrons, they can be used to simulate the behavior of electrons in a ferromagnet. In the CUA work, the researchers studied the fermionic atom lithum-6, which consists of three protons, three neutrons and three electrons.

In their experiment, the CUA team trapped a cloud of ultracold lithium atoms in the focus of an infrared laser beam. When they gradually increased the repulsive forces between the atoms (which can be adjusted by varying an external magnetic field), they observed several features indicating that the gas had become ferromagnetic. The cloud first became bigger and then suddenly shrunk. When the atoms were released from the trap, they suddenly expanded faster.

This and other observations agreed with theoretical predictions for a phase transition to a ferromagnetic state.  However, the experiment cannot yet distinguish between a gas which has become ferromagnetic, and a gas with strong ferromagnetic fluctuations.  These questions will be addressed in future work.



Fig.1: When the repulsive forces between fermionic particles increase, the system can undergo a phase transition and become a ferromagnetic.  This is well known for crystalline materials like iron and nickel, but it is still an open question whether it can occur in a gas.  Atomic gases with strong repulsive interactions are experimentally studied at the CUA.



J. H. Choi, C. A. Christensen, G. Jo, W. Ketterle, T. Kim, Y. Lee, D. E. Pritchard, J. Thywissen

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