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When Push Comes to Shove

Thu, 06/30/2005 - 8:00pm
Bill Weaver, Ph.D.
When Push Comes to Shove

Quantum imaging with magnetic resonance force microscopy



The amount of force required to press the keys of a computer keyboard is around 1 newton (N). That’s about the force provided by a pad of Post-it notes as it mistakenly rests on your spacebar, filling the buffer until your computer emits a barrage of clicks for help. On the high side of things, the world-record holder GE90-115B turbofan jet engine provides over 560,000N of thrust (equivalent to a 64-ton anvil resting on your spacebar). Since they are extremely large on a human scale, the effects of massive forces on hefty objects can be easily observed. It’s a bit more difficult to visualize tiny forces in our mind’s eye since we don’t often see their effects.

Let’s consider the weight of an individual Post-it note from our pad and we have 0.01N. Take some really sharp scissors and cut it into a million equal pieces. We are
 
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. Image reproduced by permission of IBM Research, Almaden Research Center. Unauthorized use not permitted. 
down to 10-8N or 10 nanonewtons (nN). In the mid-1980s, researchers at Stanford University and IBM developed the atomic force microscope (AFM), an instrument capable of measuring this level of force between the sharp tip of an AFM probe and a surface. This non-destructive nudge permits the AFM to sense the distance between the tip and surface-layer atoms while it is scanned across the sample. Modern AFMs capable of creating 3-D images of surfaces having features 0.1nm – 8µm in height determine the force optically by measuring the deflection of a laser beam that is bounced off a short cantilever connecting the tip to the probe body. To achieve high sensitivity, the cantilever is vibrated at its resonance frequency and the surface force affects this frequency resulting in a detectable modulation of the reflected optical beam.

Although the atomic-scale AFM images are breathtaking, it is a surface technique while the Grail of nanotechnology is the ability to image an entire molecule in 3-D. Not wishing to disturb a sample by poking it with a sharp tip, Professors John Sidles and Joseph Garbini at the University of Washington, in collaboration with researchers from Cornell University, the University of Michigan, Army Research Lab and IBM are extending the capabilities of Magnetic Resonance Imaging (MRI) down to the atomic level. Their technique, known as Magnetic Resonance Force Microscopy (MRFM), combines the non-invasive 3-D nature of MRI with the atomic-scale scanning ability of the AFM. In MRFM, the AFM tip is replaced by a tiny permanent magnet and the cantilever is oscillated over the sample in a perpendicular configuration, much like the pendulum of a grandfather clock. A strong static magnetic field causes atomic nuclei to spin parallel or anti-parallel to its field lines as in the MRI technique. However, a second weaker oscillating magnetic field is used to flip the nuclear spins between parallel and antiparallel orientations instead of the radio-frequency light used by MRI. As the cantilever swings above the sample at its resonance frequency, the magnetic field of its tip experiences a force on the order of 10-18N or 1 attonewton (aN) from the flipping nuclei. To visualize this force, we need to slice one of our petite nN Post-its into 10 billion smaller pieces.

In order to maximize the effect of the aN atomic spin force on the delicate cantilever and differentiate it from other molecular and electrostatic forces, the frequency of the oscillating magnetic field is adjusted to match the resonance frequency of the tip, providing a small push on the tip while it is directly overhead, much like a child pushing their friend on a swing set from the middle as they swing by. After this frequency is found, the oscillating magnetic field is turned off, causing the cantilever to miss its push, and turned back on when the cantilever is at the extreme point of its swing, causing the atomic spin force to push in the opposite direction on the next pass. If you have ever experienced the misfortune of pushing against a friend on a swing at the midpoint of their cycle, one or both of you have had the chance to consider your actions as you lay sprawled on the ground.

This detection technique, known as interrupted oscillating cantilever-driven adiabatic reversals (iOSCAR), has been used successfully by the IBM group led by Dr. Dan Rugar to detect the spin of a single unpaired electron in silicon dioxide. This initial success was obtained by averaging the iOSCAR signal over a 13-hour period to overcome the effects of other forces in the sample. Sidles’ group believes it is possible to measure individual nuclear spins in as little as 10 seconds with appropriate advances in the technique over the next five years. If they are successful, MRFM will easily push aside other contenders for the title of the world’s most useful microscope.

Bill Weaver is an assistant professor in the Integrated Science, Business and Technology Program at La Salle University. He may be contacted at sceditor@scimag.com.

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