For the first time, scientists at MIT have observed the ferromagnetic behavior of an atomic gas, addressing a decades-old question of whether it is possible for a gas to exhibit properties similar to those of a magnet in iron or nickel.
The MIT team observed the behavior in gas of lithium atoms cooled to 150 billionths of 1 Kelvin above absolute zero (-273 degrees C or -459 degrees F). The work, reported in the September 18 issue of the journal Science, was led by Wolfgang Ketterle, John D. MacArthur Professor of Physics, and David E. Pritchard, Cecil and Ida Green Professor of Physics. If confirmed, the MIT result could enter magnetism textbooks, showing that a gas of elementary particles called fermions doesn’t need a crystal structure to be ferromagnetic.
For decades, the question of whether it is possible for a gas or a liquid to become ferromagnetic has remained open. Ferromagnetic materials are those which, below a certain temperature, are strongly magnetized even in the absence of a magnetic field. In common magnets such as iron and nickel which consist of a repeating crystal structure, ferromagnetism occurs when unpaired electrons in the material spontaneously align in the same direction.
Electrons, but also neutrons and protons, are elementary particles classified as fermions. Atoms and molecules made up of an odd number of fermion particles are considered as composite fermions. Since all fermions have properties similar to electrons, they can be used to simulate the behavior of electrons in a ferromagnet. In this work, the researchers studied the fermionic atom lithum-6, composed of three protons, three neutrons and three electrons.
Like electrons, these lithium-6 atoms act like small magnets that can line up in the same direction under certain circumstances. In nature, fermionic liquids or gases exist as electron gases, in liquid helium 3, and in neutron stars.
“Not all liquid or gaseous fermion systems in nature have strong enough interactions to become ferromagnetic,” says graduate physics student Gyu-Boong Jo, a member of the research team. “But for lithium atoms, we can use atomic physics tricks to tune the interactions between atoms to an arbitrary strength, just by changing an external magnetic field.”
In their experiment, the MIT team trapped a cloud of ultracold lithium atoms at the focus of an infrared laser beam. As they gradually increased the repulsive forces between the atoms, they observed several features indicating that the gas had become ferromagnetic. The cloud grew at first, then suddenly shrunk. When the atoms were released from the trap, they suddenly expanded faster.
Convincing, but not yet a ‘slam dunk’
This observation and others agree with theoretical predictions of a phase transition to a ferromagnetic state. “The evidence is pretty strong,” says Pritchard, “but it’s not a slam dunk yet. They’ve started forming molecules and may not have had enough time to grow regions of aligned atoms. big enough for us to see.”
Ketterle adds that he and his colleagues have many ideas for studying this new form of matter more closely: “One thing is certain: we have made an important discovery, which will advance our understanding of magnetism.
Christophe Salomon, research director at the French National Center for Scientific Research, says the results provide compelling evidence that fermionic gases exhibit the same type of ferromagnetism found in solid crystalline materials. To fully prove the case, he says, “it would be nice to see direct observation of ferromagnetism – that all spins are parallel.”
The MIT research is part of a program studying new 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 previous research on Bose-Einstein condensates, a form of matter in which particles condense and act as a single large wave. Ketterle was awarded the 2001 Nobel Prize for the discovery and study of this long-sought new 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 never thought I would study magnetism today.”
Ketterle and Pritchard are principal investigators at MIT’s Research Laboratory of Electronics. In addition to Ketterle, Pritchard, and Jo, the MIT team included graduate students Ye-Ryoung Lee and Caleb A. Christensen, postdoctoral associate Jae-Hoon Choi, undergraduate student Tony H. Kim, and Joseph H Thywissen, University Visiting Professor. of Toronto. All MIT researchers are members of the MIT-Harvard Center for Ultracold Atoms.
The MIT research was supported by the National Science Foundation, the Office of Naval Research, through a MURI program, and by the Army Research Office with funds from the DARPA OLE program.