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Magnetic Force Microscopy
Magnetic Force Microscopy
256 Dimension 3100 Manual Rev. D
• Troubleshooting: Section 15.6
• MFM Image Verification: Section 15.6.1
• Saturation in Amplitude Detection: Section 15.6.2
• Optical Interference: Section 15.6.3
15.1 Magnetic Force Microscopy
In MFM, a tapping cantilever equipped with a special tip first scans over the surface of the sample
to obtain topographic information. Using
LiftMode as shown in Figure 15.1a , the tip then raises
just above the sample surface. The surface topography from the initial scan is added to the lift
height to maintain constant separation during the lifted scan. The influence of magnetic force is
measured using the principle of force gradient detection.
15.1.1 Force Gradient Detection
In the absence of magnetic forces, the cantilever has a resonant frequency f
0
. This frequency is
shifted by an amount
∆
f proportional to vertical gradients in the magnetic forces on the tip. The
shifts in resonant frequency tend to be very small, typically in the range 1-50 Hz for cantilevers
having a resonant frequency
f
0
~100 kHz. These frequency shifts can be detected three ways: phase
detection which measures the cantilever’s phase of oscillation relative to the piezo drive;
amplitude detection which tracks variations in oscillation amplitude; and frequency modulation
which directly tracks shifts in resonant frequency. Phase detection and frequency modulation
produce results that are generally superior to amplitude detection .
Figure 15.1a MFM LiftMode Principles
1
2
3
4
5
Magnetic Fields
5. Lifted cantilever reprofiles topography while responding to magnetic influences (second retrace).
4. Lifted cantilever profiles topography while responding to magnetic influences (second trace).
3. Cantilever ascends to Lift scan height.
2. Cantilever retraces surface topography on first retrace.
1. Cantilever traces surface topography on first trace.