FORCE DETECTION WITH ATTO-NEWTON PRECISION has been achieved by an IBM-Stanford team of physicists led by Daniel Rugar (408-927-2027) at IBM. This work, reported at last week's APS meeting in Kansas City, was carried out with a magnetic resonance force microscope (MRFM), a device that combines nuclear magnetic resonance technology with probe microscope technology. The goal here is nothing less than the ability to make 3-dimensional, non-destructive, in-situ atomic-resolution images of atoms, molecules, defects in solids, dopants in semiconductors, and binding sites in viruses. In the IBM-Stanford setup, a thin silicon cantilever, 230 microns long but only 60 nm thick, is poised above a tiny sample. A magnetic particle mounted on the cantilever interacts (under the additional influence of fields from an RF coil) with tiny volumes of magnetic atoms in the sample. Under just the right circumstances the particle on the cantilever (like a diver on a diving board) will begin to resonantly oscillate; the cantilever's movement shifts a laser interference pattern viewed through an optical fiber. In this way Rugar can measure tiny magnetic interaction with a resolution of 7 x 10^-18 Newtons, the most sensitive force measurement ever made with a probe microscope. (An attoNewton is to the weight of a feather as the weight of the feather is to that of the Hoover Dam.) With further refinement the MRFM process will detect single spins; Rugar hopes to map the spins of electrons in dispersed defect sites in silica. Another speaker at the APS meeting, John Sidles (206-543-3690) of the University of Washington, emphasized possible medical and biological research applications. The first person to suggest the MRFM approach, Sidles observed that the structure of some proteins can be worked out with nearly atomic resolution by crystallizing a sample and then interpreting the pattern of x rays diffracted by the crystal. Other notable proteins, however, do not lend themselves to this process. Moreover, ordinary force microscopes cannot image interesting molecules residing in clefts of biological structures. MRFM, by contrast, will supply 3D images of the hardest-to-get-at molecules, providing information for medical and drug-design research. A Los Alamos-Caltech group, represented by Chris Hammel of Los Alamos (505-665-0759) is applying MRFM to the study of multi-layer electronic devices. Eventually his probe will be able to map buried structures, such as defects in circuit elements. He uses a stiffer, faster-oscillating tip than the IBM version; this does not necessarily improve the force sensitivity but would greatly speed up data acquisition. (Some MRFM figures are available at Physics News Graphics.)

SINGLE MAGNETIC ATOMS DISRUPT SUPERCONDUCTIVITY on an atomic scale. As part of their microscopic study of magnetism, Ali Yazdani and his colleagues at IBM Almaden deposit single manganese (Mn) and gadolinium (Gd) atoms, each of which exerts magnetic forces, onto a niobium metal, which is a superconductor at low temperatures. By measuring the tunneling current that flows from the surface to the probe of a scanning tunneling microscope, the researchers detected the loss of superconductivity in the vicinity of the isolated magnetic atoms. This represents the first time a local loss of the superconducting state at the atomic scale has been detected. The researchers theorize that the atoms break up nearby electron pairs which constitute supercurrents. (Talk at the APS meeting; also Science, 14 March 1997; IBM Press Release, 21 March 1997.)