Force Imaging
Force Plots–An Analogy
Rev. B
MultiMode SPM Instruction Manual
177
11.1 Force Plots–An Analogy
A force plot is an observation of tip-sample interactions which yields information regarding the
sample and tip. By way of analogy, suppose a materials researcher must determine how powerful
two different types of magnets are. One magnet is made of iron, the other is a stronger, so-called
“rare earth” magnet.
A simple way of measuring each magnet’s power would be to determine its pull upon a steel plate.
For example, the researcher could hang each magnet from a simple spring scale, “zero” the scale,
then lower the magnet toward a heavy steel plate. At regular distances from the plate, the amount of
pull shown on the scale is recorded. At some unique height above the plate, each magnet is attracted
strongly enough to attach itself to the plate. A plot of height, H, versus magnetic pull gives a
comparative index of each magnet’s power. Similarly, after attachment the researcher could pull
each magnet away from the plate and measure the pulling force at regular intervals until the magnet
breaks free. The pull-off point of each magnet gives an additional index of its holding power.
1
A
representation of this setup during a lowering cycle of one magnet is illustrated in
.
Figure 11.1a
Comparative Index of Pulling Forces
The pulling force is measured at 1cm height intervals while the scale and magnet lower and lift in a
controlled cycle.
depicts a plot of this experiment using two magnets. The plot depicts
each magnet’s attraction as it approaches the plate, and its tenacity when pulled off the plate.
Assuming both magnets are the same size, this reveals information about each magnet’s power.
1. For the sake of simplicity, forces are represented here using a common scale in kilogram units; however, force is properly
measured in Newtons (1 kg = 9.8 N).
H = 11 cm
F = 0 kg
H = 10 cm
F =.08 kg
H = 9 cm
F =.12 kg
H = 8 cm
F =.30 kg
H = 7 cm
F = 1.0 kg
Steel plate
(0 N)
(0.8 N)
(1.2 N)
(2.9 N)
(9.8 N)
2
3
4
5
6
7
8
9
10
11
H
