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Burn, Magnet, Burn

Fenella Saunders

Mountaineers who tremble at the thought of an avalanche have some company in the physics lab. A kind of avalanche that occurs in magnetic crystals is just as destructive to physicists' experiments as is crashing snow to an unsuspecting skier. In so-called magnetic avalanches, the molecule-sized magnets that make up a magnetic crystal suddenly flip their spins. These uncontrolled events, discovered in the early 1990s, were catastrophic to experiments, but their cause and propagation remained a puzzle. The solution, it turns out, may be of more relevance to fire fighters than to ski patrols.

Magnetic avalancheClick to Enlarge Image

Myriam P. Sarachik and graduate student Yoko Suzuki, at the City College of the City University of New York (CUNY), set up an experiment to track magnetic avalanches. The results led their collaborator, Eugene M. Chudnovsky of CUNY's Lehman College, to recognize an unexpected phenomenon: The magnetic avalanche progressed through the crystal exactly like a flame front moving through a chemically burnable substance, such as a piece of paper. As the group reported in the September 30, 2005, issue of Physical Review Letters, these annoying events may turn out to be a boon for engineers studying deflagration—the movement of a flame through a material.

At the center of this discovery are crystals only about a millimeter long, grown by Sarachik's collaborators at the University of Florida in Gainesville. These crystals are made of manganese-12 acetate, widely used to study quantum tunneling and a contender for use in magnetic memory storage and quantum computing. Using micrometer-sized sensors created at the Weizmann Institute of Science in Israel, Suzuki put together an electronic measurement system fast enough to take readings at 11 points along the length of the crystal during the avalanche. Prior to this, investigators had looked only at what happened to the magnetic field of the entire crystal during an avalanche.

Suzuki submerged the sensor-laden crystals in liquid helium-3 at temperatures below one kelvin—very close to absolute zero. She placed the apparatus in a magnetic field and gradually varied the field strength until an avalanche occurred. She found that there was a threshold energy, a field strength above which an avalanche would abruptly commence. She also found that the avalanches always seemed to start from an edge and progress in a wave, reversing the spins of the crystal's molecular magnets in an abrupt front traveling down the length of the crystal. The speed of the avalanche was constant but depended on the strength of the external magnetic field, varying from one to 15 meters per second.

Chudnovsky saw in the results an analogy to chemical burning. The moving band of flipping molecular magnetic spins in the crystal is equivalent to a flame front progressing through a piece of paper, for instance, and the area behind the front, where the molecular magnet spins had flipped to align with the external field, is like the ash left behind a fire. The material ahead of the magnetic avalanche is analogous to unburned material. "We took formulas for how a flame propagates in a chemical substance, which actually are a hundred years old, and they work perfectly well in describing this phenomenon," Chudnovsky says. "That's how we were convinced that this was a new phenomenon, magnetic deflagration."

The group still doesn't entirely understand what conditions set off the avalanche. "It's a probabilistic event," explains Sarachik. "We sweep the magnetic field and then at some field it goes boom, and you can't predict it." But in the same way that a fire requires energy to sustain itself, the "ignition" of a magnetic avalanche also seems to require a magnetic field of a certain strength; near the threshold energy it peters out partway through the crystal. "It's like a fire that goes out before it finishes its fuel," says Sarachik. However, once there is successful ignition, when the molecular magnets's spins in one area flip over to line up with the external magnetic field, they release energy that helps the next section to flip as well, propagating the process.

Sarachik says scientists who study deflagration might use this set-up as a more controllable way to look at flame fronts. In addition, she notes, the crystals are reusable. In experiments following flame fronts along paper, "you have to use a different sample every time, and one piece of paper may be different from the next. In these crystals, the ‘ash' behind the propagating front has spins pointing in the direction of the field, but there's nothing destructive about that. So you can repeat the experiment over and over, without any doubt whether it's the same sample or not." Sarachik and her colleagues are now planning to see whether there is a magnetic equivalent to detonation as well.

The cognitive leap that Chudnovsky made between magnetism and burning is also a testament to a well-rounded education. "Deflagration is mostly studied by engineers who work on rocket propellants and gasoline engines," he said. "These days I think very few physicists know what deflagration is. But to get my Ph.D. in Russia, I was required to study all subjects in physics. So I was lucky that at some time in my life I studied this material."

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