Instruction manual

Magnetic Force (MFM) Imaging
MFM Using Interleave Scanning and LiftMode
Rev. B MultiMode SPM Instruction Manual 229
To correctly track the cantilever phase, the Phase offset parameter must be adjusted. This is
automatically done in AutoTune; alternatively, Zero Phase can be selected from the Channel 2
panel. The phase curve should appear as in Figure 13.2a, decreasing with increasing frequency, and
crossing the center line (corresponding to a 90° phase lag) at the peak frequency. The phase curve
then measures the phase lag between the drive voltage and the cantilever response. Again, vertical
gradients in the magnetic force cause a shift ∆f
0
in the resonance frequency. In this case, resonance
shifts give rise to phase shifts ∆φ which then give an image of the magnetic force gradients; see
Figure 13.2b.
Note: The Extender electronic modules give a measure of the phase lag of the
cantilever oscillation relative to the piezo drive. This measurement is
monotonic versus frequency as is the true phase lag in degrees. The Extender
measurement; however, has slightly different nonlinear characteristics vs.
frequency. The measurement technique allows optimal signal-to-noise ratios;
however, absolute values of phase data should be taken as approximate. Users
requiring quantitative measures of force gradient are advised to use frequency
modulation (see Section 13.2.2). Proceed to Step 4 below.
Figure 13.2b Shift in phase at fixed Drive frequency
• Setting a Drive Frequency for Amplitude Detection: Set the Drive frequency to the
left side of the cantilever resonance curve, as shown in Figure 13.2c. This can be done
by using the AutoTune feature to first find the resonance peak, then using Offset under
the Cantilever Tune menu bar to manually move the drive frequency to the side of the
resonance. For maximum sensitivity, set the Drive frequency to the steepest part of the
resonance curve. As the tip oscillates above the sample, a gradient in the magnetic force
will shift the resonance frequency f
0
; (see Figure 13.2d). Tracking the variations in
oscillation amplitude while in LiftMode yields an image of the magnetic force
gradients.
Proceed to Step 4 below.
∆φ
Drive Frequency
90
0
180
Phase (deg)
∆F
0