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An Attractive Prospect

Researchers get gas to act like a magnet
December 21, 2009

For decades, scientists have wondered whether a gas or liquid can become ferromagnetic–remaining strongly magnetized even in the absence of a magnetic field, as iron and nickel do. Now, for the first time, MIT physicists have observed ferromagnetic behavior in an atomic gas.

MAGNETIC GAS Graduate student Gyu-Boong Jo optimizes the laser beam position on the mirror of the optical setup that produced an ultracold gas of lithium atoms.

The MIT team–led by David Pritchard and Wolfgang Ketterle, physics professors and principal investigators at the Research Laboratory of Electronics–observed the behavior in a gas of lithium atoms cooled to near absolute zero: 150 billionths of a kelvin (-273 °C or -460 °F). If confirmed, the results will prove that a gas of elementary particles known as fermions (the class that includes electrons and protons) can be ferro­magnetic even though it lacks the crystalline structure of common magnets.

In magnets that consist of a repeating crystal structure, such as iron and nickel, ferromagnetism occurs when unpaired electrons spontaneously align in the same direction. The alignment breaks down above a certain temperature, but because that temperature is usually very high–768 °C in the case of iron–magnetism is virtually permanent in these materials. In Fermi gases (which are found in neutron stars, for example), atoms can sometimes act as little magnets that align the way electrons do in crystalline materials. But this phenomenon can occur only under certain circumstances, such as at very low temperatures.

In their experiment, reported in the journal Science, the MIT researchers worked with the Fermi gas lithium-6, trapping a cloud of ultracold lithium atoms in the focus of an infrared laser beam. When they used a magnetic field to gradually increase the repulsive forces between the atoms and observed the results by means of laser illumination, they detected several behaviors indicating that the gas had become ferromagnetic. The cloud first became bigger and then suddenly shrank. When the atoms were released from the trap, the cloud expanded rapidly.

This and other observations agreed with theoretical predictions that if atoms in such materials were confined at very low temperatures, they would spontaneously align to lower their kinetic energy, inducing a ferro­magnetic state. The researchers didn’t detect such alignment directly. But the atoms “started to form molecules and may not have had enough time to develop regions of aligned atoms large enough for us to see,” says ­Pritchard. “The evidence is pretty strong, but it is not yet a slam dunk.” Christophe ­Salomon, research director at France’s National Center for Scientific Research, says the findings offer convincing preliminary evidence that Fermi gases display the type of ferromagnetism found in solid crystalline materials.

The MIT research is part of a program studying novel magnetic materials–which have important applications in data storage, nanotechnology, and medical diagnostics–and the interplay between magnetism and superconductivity. The work is a continuation of earlier research on Bose-Einstein condensates, a form of matter in which particles condense and act as one big wave. ­Ketterle shared the 2001 Nobel Prize in physics for discovering this long-sought form of matter. “We still use the same refrigerator that we used to study Bose-­Einstein condensates,” says Ketterle. “But the science is very different. Ten years ago, I would have never thought that I would study magnetism today.”

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