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The Best Refrigerator Magnet Ever?

19 March 2010 5:21 pm
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Jian-Ping Wang

Limit breaker? The crystal structure of Fe16N2, which one group of researchers says beats the predicted limit for magnetism in a material.

PORTLAND, OREGON—There are limits to just how magnetic a material can be. Or so researchers thought. A compound of iron and nitrogen is about 18% more magnetic than the most magnetic material currently known, a team of materials scientists claims. If such magnets could be produced commercially, they could, for example, allow electronics manufacturers to equip computer hard drives with smaller "write heads" capable of being crammed with more information. Other researchers are reacting to the announcement with caution, however, as similar claims about the controversial material have fallen through in the past.

A material's magnetism originates with its spinning electrons. Each electron acts like a little magnet with its field aligned with the axis of its spin, and when more electrons spin in one direction than in the opposite direction, the material becomes magnetic. For example, an iron atom has four more electrons spinning one way than the other. In a bulk material, the situation is more complicated, as the electron clouds of individual atoms merge into riverlike bands. Electrons spinning "up" flow in different bands from those spinning "down," and the difference between the numbers of highest-energy electrons in the up bands and the down bands determines the material's magnetism—which is smaller than one might expect from the magnetism of a single atom. Using such band theory, researchers can predict which material should have the largest magnetism: iron cobalt.

However, Jian-Ping Wang, a materials physicist at the University of Minnesota, Twin Cities, and colleagues say that a compound of eight parts iron and one part nitrogen, Fe16N2, exceeds this limit by roughly 18%. The key to the material's extremely high magnetism lies in its complicated crystal structure, Wang reported here yesterday at the March Meeting of the American Physical Society. Probing their samples with x-ray, the researchers determined that within them, each nitrogen atom sits in the center of a cluster of six iron atoms and that a couple more iron atoms sit between neighboring clusters. The electrons flowing between the clusters act much like electrons in ordinary iron. But the electrons in the iron atoms in the clusters tend to get stuck, or "localized," where they are. As a result, Wang says, those atoms contribute more like individual atoms to the overall magnetism, driving it way up.

"If it's right, it's super important," says Eric Fullerton, a physicist at the University of California, San Diego. But he stresses the "if." As Wang himself explained, as early as 1972, others had claimed that Fe16N2 is extraordinarily magnetic. In the 1990s, researchers with the Japanese high-tech company Hitachi reported observations that seemed to bolster those claims. However, the evidence was problematic in several ways, Fullerton says. For example, some of the results depended on tricky estimations of exactly what fraction a sample's volume consisted of Fe16N2, which is metastable and tends to fall apart into other crystal structures. Others have not been able to reproduce the Hitachi results, Fullerton says.

Wang, however, says his team has been honing its techniques for years and can now reliably grow samples of Fe16N2. The researchers have also measured the magnetization with a technique called x-ray magnetic circular dichroism, which compares the material's ability to absorb x-ray light whose polarization twirls to the right or to the left. That measure is less sensitive to volume effects than earlier techniques and directly detects the localized electrons, Wang says. The team has also cranked out detailed "first principles" simulations that show the emergence of the localized electrons and make the whole scenario hang together, Wang says.

"He's been able to control things a lot better than other people," says Alan Edelstein, a physicist at the U.S. Army Research Laboratory in Adelphi, Maryland. Still, he hesitates to say it's a done deal. "I think this will be followed up on. We're going to know if this is right." At the very least, Fe16N2 continues to exert its extraordinary pull on the minds of physicists and material scientists.

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