By Peter Weiss
Using a sliver of material shaped like a diving board or cantilever, researchers have detected a force that’s a billionth the strength of a typical chemical bond. At 800 zeptonewtons, it’s the most sensitive measurement of mechanical force ever.
This exquisite sensitivity may finally make it possible to mechanically detect a single electron’s magnetism, say Dan Rugar and John Mamin of IBM Almaden Research Center in San Jose, Calif.
Electrons and most nuclei act like tiny bar magnets because of a quantum mechanical property called spin. Sensing an electron’s spin would be a coup for Rugar and his coworkers. They’ve long been developing a type of magnetic resonance imaging (MRI) microscope for revealing identities and locations in solids of individual atoms–whose magnetism is much subtler than an electron’s (SN: 6/11/94, p. 374). Since the early 1980s, scientists have used other types of microscopes to observe atoms by nonmagnetic means–but just on samples’ surfaces. “If you can do 3-D imaging, that opens up a new vista,” Rugar says.
The new microscope is designed to sense a magnetic force exerted when the spin of the particle repeatedly flips upside down in response to radio signals. To be detected, a spin must oscillate this way for a tenth of a second or so.
Estimates of the magnetic force exerted by a lone electron range from 1,000 to 10,000 zeptonewtons. Three years ago, Rugar’s team built an instrument sensitive enough to detect forces as small as 7,000 zeptonewtons.
However, even the 4-kelvin temperature of that instrument still allowed excessive thermal vibrations in the cantilever. Those tremors cloaked the oscillations caused by an electron’s magnetic force. The scientists operate their new microscope at a mere 40 millikelvins. To further reduce heating, they also use an extremely dim laser for measuring cantilever deflection.
By shaking the cantilever with a piezoelectric crystal, which changes shape when stimulated by electricity, the researchers found they could observe vibrations that flex the cantilever’s end about 0.6 angstrom. That’s less than the diameter of an atom and corresponds to the record-breaking value of 800 zeptonewtons.
