Kids, how are the north and south poles of a refrigerator magnet arranged? How are chemists able to “see” the atoms that they work with? In this activity you will discover how to answer these questions and also gain an understanding of a cutting-edge imaging technology.
Please note: All chemicals and experiments can entail an element of risk, and no experiments should be performed without proper adult supervision.
A refrigerator magnet has many north and south poles, not just two as in a bar magnet. The magnetic poles are nearly always arranged in stripes. A thin probe strip cut along one side of the magnet will be deflected up and down when pulled across the back of the magnet perpendicular to the stripes. In this activity the magnetic force between the probe strip and the magnet depends on the distance between the two surfaces and the relative size and alignment of their magnetic fields. By scanning the surface with the probe, an entire surface image can be obtained. This activity is analogous to atomic force microscopy and it offers a view of magnetic force microscopy (MFM) used for larger-scale imaging.
How:
- Obtain a flexible-sheet refrigerator magnet.
- Cut one 5-mm wide strip along the left edge and another along the bottom edge of the magnet.
- Place the unprinted side of one of the magnetic strips against the unprinted side of the magnet. Drag the strip across the back of the magnet in both directions. Repeat with the second strip. The strip that is alternately attracted and repelled (bounces) should be used as the probe strip. Discard the other strip.
- Experiment with the probe strip that you have retained by pulling it across the surface slowly, then quickly; close to the surface, then far away; at various angles, etc. What does the arrangement of magnetic poles appear to be?
Questions:
Does the magnetic force between the probe strip and the back of the magnet depend on the distance between the two surfaces, i. e., can the probe map variations when it is far from the surface? Would the size of the tip of the probe matter? If the poles are made very small, say nanometer scale, could their arrangement still be determined? Atomic-scale images of a surface can be obtained by atomic force microscopy (AFM). To produce an image, a probe is moved relative to the surface and variations in force are recorded for a series of parallel passes. This force measurement, when plotted as a function of position, provides an image of the arrangement of atoms on a surface.
--------------
Kathleen Carrado Gregar, PhD, Argonne National Labs
[email protected]
October 2010
----------------
References:
“A Refrigerator Magnet Analog of Scanning-Probe Microscopy” by Julie K. Lorenz, Joel A. Olson, Dean J. Campbell, George C. Lisensky, and Arthur B. Ellis in Journal of Chemical Education, Vol. 74 No. 9, 1032, September 1997, http://pubs.acs.org/doi/abs/10.1021/ed074p1032A.
