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Microscope Sizes Up Zeptonewtons of Force
15 March 2001 7:00 pm
SEATTLE--A new microscope is so exquisitely sensitive that scientific terms almost fail to describe it. At the American Physical Society meeting here 12 March, a physicist described an instrument that can sense a mind-bogglingly small force: 820 zeptonewtons. A zeptonewton, mind you, is a unit of force so small that scientists haven't had any use for it; it's about the gravitational pull between you and an ant 10 kilometers away. With such precision, scientists might be able to create three-dimensional pictures of atoms and electrons--a dream that is not possible with current microscopes.
The team's technique is a refinement of magnetic resonance force microscopy (MRFM), in which a tiny magnet is suspended at the end of a fine cantilever. As scientists move the magnet close to a clump of atoms, it interacts with the atomic and electronic spins--the quantum-mechanical properties of subatomic particles that are related to a material's magnetism. The magnet, nudged by the spins, transmits that force to the cantilever, which swings up and down. By measuring the cantilever's motion, scientists can create an image of the clump.
Unfortunately, nature stands in the way of perfect measurements. Individual atoms within the cantilever itself jiggle due to thermal motion, limiting the precision of the measurement; it is like trying to weigh something on a scale that is constantly bumping up and down. So John Mamin and his colleagues at IBM in Almaden, California, set out to reduce the thermal motion by cooling the cantilever down to a fraction of a degree above absolute zero. The result is an instrument so sensitive that if the cantilever senses something for about a second, it can feel a force of about 820 zeptonewtons, or about 10-18 newtons. This is less than the force needed to sense the spin of a single electron.
The feat has won the "unbounded admiration" of physicist John Sidles of the University of Washington, Seattle. "They are, at present, way ahead of any other MRFM group," he adds. However, Mamin cautions that an electron must be "well behaved" and hold still long enough for the cantilever to register its spin. But that time scale will be reduced as the cantilevers get more sensitive, he predicts, and scientists will be able to make three-dimensional pictures with atomic-scale resolution. Says Mamin: "This is the dream."