“Magnets are all around us – holding postcards on the refrigerator, pointing to magnetic north on a compass, and in speakers and headphones – yet some mysteries remain,” says Joseph H. Thywissen, a professor of physics at the University of Toronto and a visiting member of the Massachusetts Institute of Technology-based team leading the research. “We have perhaps found the simplest situation in which permanent magnetism can exist."
The scientists observed the behaviour in a gas of lithium atoms trapped in the focus of an infrared laser beam. The gas was cooled to 150 nK, less than a millionth of a degree above absolute zero, which is at -273 C. When repulsive forces between the atoms were gradually increased, several features indicated that the gas had become ferromagnetic. The cloud first became bigger and then suddenly shrunk, and when the atoms were released from the trap, they suddenly expanded faster. These observations were reported in the 18 Sept 2009 issue of Science, in a paper titled “Itinerant Ferromagnetism in a Fermi Gas of Ultracold Atoms”.
This and other observations agreed with theoretical predictions for a transition to a ferromagnetic state. Ferromagnetic materials are those that, below a specific temperature, become magnetized even in the absence of a strong magnetic field. In common magnets, such as iron and nickel that consist of a repeating crystal structure, ferromagnetism occurs when unpaired electrons within the material spontaneously align in the same direction.
“Magnetism only occurs in a strongly interacting regime, where calculations – even using today’s fastest computers – are difficult,” says Thywissen. “Since naturally occurring gases do not have strong enough interactions to address the question, we turned to ultra-cold gases for answers.”
If confirmed, these results may enter textbooks on magnetism, showing that a gas of fermions does not need a crystalline structure to exhibit magnetic properties. “The evidence is pretty strong, but it is not yet a slam dunk,” says MIT physics professor and co-principal investigator David E. Pritchard. “We were not able to observe regions where the atoms all point in the same direction. They started to form molecules and may not have had enough time to align themselves.”
Thywissen's interest in the topic of ultra-cold ferromagnetism originated in theoretical work at Toronto led by Professor Arun Paramekanti in the physics department, along with graduate student Lindsay LeBlanc. "We assumed that ferromagnetism did exist for a gas, and then asked what its properties would be," explains LeBlanc. "Surprisingly, we found there were simple energetic signatures of ferromagnetism – that were eventually observed at MIT."
At MIT, the team was led by principal investigator Wolfgang Ketterle, and included graduate students Gyu-Boong Jo, Ye-Ryoung Lee and Caleb A. Christensen, post-doctoral associate Jae-Hoon Choi, and undergraduate student Tony H. Kim. Thywissen is affiliated with the University of Toronto’s Centre of Quantum Information and Quantum Control, and is a Senior Fellow at Massey College.
Canadian funding agencies include the National Science and Engineering Research Council (NSERC) and the Canadian Institute for Advanced Research (CIfAR). US funding included the National Science Foundation, the Office of Naval Research, through a Multidisciplinary University Research Initiative (MURI) program, and by the Army Research Office with funds from the Defense Advanced Research Projects Agency (DARPA) Optical Lattice Emulator (OLE) program.
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