Instruction manual

Magnetic Force (MFM) Imaging
Magnetic Force Imaging Theory
226 MultiMode SPM Instruction Manual Rev. B
13.1 Magnetic Force Imaging Theory
MFM imaging utilizes the Interleave and LiftMode procedures discussed in Chapter 12; users are
advised to read appropriate sections prior to attempting MFM imaging. Best results will be
obtained with Digital Instruments Quadrex Extender or Basic Extender Modules. These hardware
units allow phase detection and frequency modulation for optimal MFM imaging. Instructions for
software installation for both extender modules can be found at the end of this chapter; complete
instructions for hardware and software installation are included when you purchase your extender
module.
In MFM, a tapping cantilever equipped with a special tip is first scanned over the surface of the
sample in TappingMode to obtain topographic information. Using LiftMode as shown in Figure
13.1a, the tip is then raised just above the sample surface. 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. 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 1-50Hz for cantilevers having a resonant
frequency f
0
~100kHz. 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 13.1a MFM LiftMode principles
All standard MultiModes are capable of MFM imaging using amplitude detection techniques. By
adding a Basic Extender or Quadrex Extender to a NanoScope IIIa controller (see Figure 13.1b), or
by using a NanoScope IV controller, a MultiMode may also be used for frequency modulation or
phase detection, giving improved results. Amplitude detection has largely been superseded by
frequency modulation and phase detection. A more extensive discussion of force gradient detection
and MFM imaging is given in the reprint Magnetic Force Microscopy: Recent Advances and
Applications. Contact Veeco to obtain a copy.
Note: In the instructions below, steps specific to phase and amplitude imaging are
described independently. Use the icons in the margin to locate steps specific to
either frequency modulation and phase detection, or amplitude detection.
1
2
3
4
5
to magnetic influences on second trace and retrace.
Lifted cantilever profiles topography while responding
Cantilever ascends to Lift scan Height.
Cantilever traces surface topography on first trace and retrace.
1 & 2
3
4 & 5