A Spin on Magnetic Resonance Imaging

A billionth of a billionth of a newton is a minuscule force by any yardstick. Yet researchers at IBM have just demonstrated that a new microscopy method can measure the 2-attonewton (2 x 10^-18 N) force associated with the magnetic moment (spin) of a single electron [Nature, 430, 329 (2004)].

The ability to detect individual electron spins advances the developing field of magnetic resonance force microscopy (MRFM), which eventually may provide high-resolution images of biomolecules and nanoelectronic components. The force-measurement improvement may also be an enabling step in the development of spin-based quantum-computing methods.

The MRFM concept was proposed just over a decade ago by physicist John A. Sidles of the University of Washington, Seattle. The idea is that combining the advanced capabilities of mag netic resonance imaging (MRI) techniques with the sensitivity and spatial resolution of atomic force microscopy can lead to a powerful imaging tool cap able of revealing the three-dimensional structure of complex molecules with atomic resolution. Additionally, the method could provide NMR-type chemical information on a fine scale.

Daniel Rugar, manager of Nanoscale Studies at IBM's Almaden Research Center in San Jose, Calif., teamed up with Sidles in 1992 to conduct early MRFM experiments and has continued working on the technique since that time. In the current study, Rugar and IBM coworkers Raffi Budakian, H. John Mamin, and Benjamin W. Chui used a specially fabricated silicon cantilever fitted with a 150-nm-wide SmCo magnetic tip to sense electron spins in a sample of vitreous silica. The team treated the sample with gamma rays to produce a low concentration of silicon dangling bonds--unsaturated valencies that are sites of unpaired electrons.

By suspending the cantilever tip roughly 100 nm above the sample while simultaneously applying a high-frequency magnetic field, the group is able to use an interferometer to measure small changes in the cantilever's vibration frequency caused by interactions between an electron spin and the magnetic tip. Magnetic interactions are limited to a small bowl-shaped region, known as the resonant slice, that extends some 100 nm below the surface of the sample. In future imaging applications, a 3-D composite picture of a molecule--a protein, for example--would be generated by scanning the tip, and hence the resonant slice, through the sample.

The demonstration of single-electron-spin detection "is a heartening milestone in realizing the dream of high-resolution MRI," notes P. Chris Hammel in a commentary in the same issue of Nature. Hammel, a physics professor at Ohio State University, Columbus, adds that the achievement of the IBM team will "dramatically alter the horizons for high-resolution imaging."

An additional 1,000-fold increase in magnetic sensitivity is required to make the leap from detecting individual electron spins to detecting single nuclear spins, which will be needed to image molecules, Rugar and coworkers point out. Considering that the latest results represent a sensitivity improvement of a factor of 10 million compared with the experiments of the early 1990s, Rugar asserts that "the remaining required improvement hardly seems out of the question."