Nanoscale Imaging

The magnetic resonance force microscope (MRFM) uses an ultrathin silicon cantilever (yellow) with a nanometer size magnetic tip (blue) to detect the magnetic signal from an individual electron buried below the surface of the sample. Because the electron has a quantum mechanical property called “spin,” it acts like a tiny bar magnet and can either attract or repel the magnetic tip. The interaction between the spin and the tip is localized to the bowl-shaped region in the sample called the “resonant slice,” which moves as the cantilever vibrates. With the aid of a high-frequency magnetic field generated by a coil (right, background), the orientation of the electron (green arrow) flips as the resonant slice passes through. The magnetic force between the electron and magnetic tip alternates between attraction and repulsion every time the electron flips its orientation, causing the cantilever frequency to change slightly. A laser beam (left) is used to measure precisely the variations in cantilever vibration frequency. [ JPG with label ] [ Animation ]
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Raffi Budakian, John Mamin and Dan Rugar (left to right) are three of the four members of the IBM Research team who developed and used this Magnetic Resonance Force Microscope to detect the magnetic signal from a single electron. Benjamin Chui is not shown.
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The ability to detect the magnetism from an individual electron is based on the exquisite sensitivity of an ultrathin silicon microcantilever like this one. Shaped like a miniature diving board, this cantilever is 85 micrometers long (roughly the diameter of a human hair) and the shaft is only 100 nanometers thick (some 1,000 times thinner than a human hair). Because the cantilever is so thin, it is extremely flexible and can detect ultrasmall forces in the attonewton range. In the experiments performed at IBM's Almaden Research Center, a tiny magnetic particle is attached to the cantilever tip so it can be used in a magnetic resonance force microscope (MRFM). The cantilever is vibrated at its natural frequency (5,000 cycles per second), and interaction with an electron spin changes the cantilever's vibration frequency by about 1 part per million. (The scale bar represents about 10 micrometers.)
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This magnetic tip (shown colorized in blue) affixed to a magnetic resonance force microscope (MRFM) cantilever is the key element in IBM's detection of the magnetism from a single electron.

Only 150 nanometers thick, this small bit of samarium-cobalt magnetic alloy generates an intense, localized magnetic field. The interaction of this magnetic field with the electron spin causes the cantilever to be either attracted or repelled by the electron spin, depending on the orientation of the electron.

The thickened region at the end the silicon cantilever (on which the magnetic tip sits) is designed to add mass at the end of the cantilever, thus reducing high-frequency tip vibrations that can disturb the spin during measurement. This thickened region measures about 10 micrometers long, 5 micrometers wide and 1 micrometer thick. (The scale bar represents about 1 micrometer.)
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