Magnetic Force Microscopy
TN00031, R1
Preliminary steps
This technical note is an guide to MFM imaging with the EasyScan 2 and
Mobile S AFM systems.
Requirements
Equipment
• EasyScan 2 with extension and dynamic modules, or Mobile S AFM
• Software version 1.5 or higher
• Magnetic Probe: Nanoworld MFMR or Applied Nanotech MAGT
• Strong permanent magnets
Knowledge
• Dynamic mode AFM
Theoretical Background
The aim of Magnetic force microscopy (MFM) is to image a spatial distribution of a magnetic field. Usual samples for MFM can be magnetic tapes,
hard disks and magneto-optical disks.
FT = n ( n FM )
FT
n
Cantilever
FM
Sample
Forces in MFM: Force gradient of a point dipole.
When performing MFM measurements the tip shouldn't come into
mechanical contact with the surface as the influence of the mechanical contact is usually stronger. Therefore during MFM the tip is scanned at a
defined distance above the sample. The magnetic force which is measured
in MFM can be approximated by the force gradient of a point dipole seen
by the magnetic tip (see figure Forces in MFM (p.1)), where n is the unit
vector normal to the cantilever plane and FM the magnetic force When the
AFM is operated in dynamic mode, e. g. when the tip is vibrated in z direction (vertically to the sample) during the scanning, the influence of the
magnetic force in this direction is dominant and thus determines the contrast the MFM image (see figure MFM in Dynamic Mode (p.2)).
magnetic
field lines
FM
Sample
Contrast
MFM in Dynamic Mode: MFM imaging in dynamic mode.
The magnetic filed influences the spring constant of the cantilever probe in
the following way:
k o = k – F T'
where k is the original spring constant. Therefore a force gradient pointing
away from the tip decreases the spring constant and a force gradient pointing towards the tip increases the spring constant. Like in dynamic mode,
this can be related to a decrease in topography for the lower spring constant
and an increase in height for a higher spring constant.he effect of the magnetic force on the amplitude and phase signal are also comparable to the
dynamic mode topography imaging. For the amplitude the change of the
spring constant induces a shift of the resonance peak and the difference in
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the amplitude is thereby related to the magnitude of the magnetic field (see
figure Amplitude and Phase Shift (p.3)). The same shift is also observed in
the phase signal (see figure Amplitude and Phase Shift (p.3)). The advantage
of the phase signal over the amplitude signal is its local linearity and the
thus simpler and linear relation between the force and the phase shift:
Q
dϕ = ---- F T'
k
where Q is the Q-factor of the cantilever probe.
f0'
f0'
f0
shift
phase shift
attenution
frequency
Phase
shift
Amplitude
f0
frequency
bla
Amplitude and Phase Shift: Left: The magnetic tip sample interaction influences the spring
constant of the cantilever probe ant provokes a shift in the resonance frequency. The resonance
shift can be detected in the amplitude signal. Right: The magnetic tip sample interaction
influences the spring constant of the cantilever probe and provokes a shift in the resonance
frequency. This shift is related to the phase shift.
Technical Implementation
The most important change compared to the standard dynamic mode is
that the probe needs to be scanned at a defined distance above the surface.
This has been implemented in the following way:
As shown in figure Implementation (p.4) at every line start (forward and
backward scan) a spectroscopy is performed. Based on that the distance at
which the probe will be scanned is calibrated. The probe is then lifted at the
chosen height and linearly scanned over the sample surface. The feedback
is switched off.
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y
x
linear line scan
displacement
from sample
z
sample
Implementation: Linear scan at a given height over the sample surface.
Caution
Like in spectroscopy, the distance is always seen from the tip, i.e. a negative
value means a displacement upwards as the tip is then moving away (backwards) from he surface. Entering a positive value could crash the tip into
the sample.
Preparation for the Measurement
• Mount the MFM Probe on the AFM.
Usually new MFM cantilevers are not magnetised. Thus we need to magnetise the tip with a strong permanent magnet as show in figure Magnetisation of the Tip (p.5):
• Determine the north-pole of the permanent magnet (for example using a
compass).
• Mark the north-pole, in order to be able to magnetise the tip in a reproducible direction.
• Bring the pole of the magnet to within 1-2 mm distance of the tip.
• Move the magnet away from the tip in a direction perpendicular to the
cantilever surface.
• Put the scan head on the small sample stage.
• Create a new cantilever type according to the specifications of the manufacturer (see Software reference).
• Select cantilever type in the software.
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N
Magnetisation of the Tip:
• Select the phase contrast imaging mode.
• Select the advanced user interface mode.
• Create the chart arrangement in the image window as described in figure
MFM Chart Arrangement (p.5).
Phase (Filter:line fit)
Topography (Filter: raw data)
Amplitude (Filter: raw data)
MFM Chart Arrangement:
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Measurement
In this part you will find a step by step recipe.
Approaching the Surface
• Set the free vibration amplitude to 200mV.
• Clear the check box ‘Auto. start imaging’ in the positioning panel as
shown in figure Auto Start (p.6). A direct start could harm the tip.
Auto Start: Stopping the auto. start scanning in the positioning window.
• Clear the check box ‘Enable Constant Height mode’ as shown in figure
Constant Height Mode (p.7), i.e. that the constant height mode is
switched off. Otherwise the feedback is disabled and the tip could be
harmed.
• Approach the surface.
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TN00031 © 2006 NANOSURF AG
Constant Height Mode: Switching off the constant height mode in the imaging window.
Calibrating the Vibration Amplitude
The calibration value of the vibration amplitude depends on the cantilever
type, because of changes in the length and resonance frequency of the cantilever. As it is important to know the vibration amplitude it needs to be
calibrated. This needs to be done only when a cantilever was changed. Use
the following procedure to calibrate the vibration amplitude:
• Measure a vibration amplitude vs. distance curve in spectroscopy mode
as shown in figure Amplitude Calibration (p.8).
• Determine the slope with the length tool and calculate the m / V calibration value.
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Amplitude Calibration:
• Retract the tip from the surface again.
• Set the free vibration amplitude to a value in order of the magnetic feature
size you intend to measure. An example is given in the chapter Example
Measurement and Hints (p.10).
Caution
Only change the amplitude calibration value when the tip is far away from
the sample. Otherwise you could break the tip.
Starting the Measurement
• Coarse approach close to the sample again.
• Ensure that the check box ‘Auto. start scanning’ is still cleared.
• Approach the surface.
• Start imaging the surface with a small range (1-2µm).This prevents you
from harming the tip.
As the scan at a defined distance above the surface is linear, we need to
correct the tilt of the sample in order to avoid a tip crash.
• Correct the tilt angle for the 0° and 90° scan angle.
Caution
Only scan a few topography lines to correct the tilt. Scanning to long in
contact might harm the magnetic layer and demagnetize the tip.
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TN00031 © 2006 NANOSURF AG
Hint
If you have the scripting interface option installed you can also correct the
tilt with the ‘Correct XY-Slope’ function in the script menu.
• Set the desired scan distance in the ‘Relative Tip-Position’ as shown in
figure Distance from Sample (p.9) at a safe distance (>=500nm) immediately after the tilt correction.
Distance from Sample:
Attention
A negative value means a displacement away from the surface. A positive
value will crash the tip into the sample.
• Activate the constant height mode by checking the box ‘Enable Constant
Height Mode’ shown in figure Constant Height Mode (p.7).
• Decrease the distance manually until you reach the optimum contrast, an
example is given in the chapter Example Measurement and Hints (p.10).
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Example Measurement and Hints
The following examples were measured on a hard disk sample taken from
a defect hard disk. This was a Western Digital WD100BB, 10GB, with
only a single read-write head.
Selecting the Vibration Amplitude
The magnitude of the MFM (phase) signal, the phase noise and the resolution of the MFM signal depend on the vibration amplitude. When the
amplitude is too small, the noise is too high, when the amplitude is too
large, the resolution becomes low, and ultimately the signal will also
become too small. It can be shown that the vibration amplitude should be
of a similar magnitude as the smallest details you wish to see. The accurate
vibration amplitude can be calculated by multiplying the previously measured V/m value from the spectroscopy with the mean feature size. In our
the measurement shown in figure Vibration Amplitude (p.10), the mean
feature size is 80nm, the amplitude calibration gave 5 mV/nm, therefore
the corresponding vibration amplitude is
mV
amplitude = 80nm ⋅ 5 -------- = 400mV .
nm
Vibration Amplitude: Left: This image was recorded with 30nm (150mV) vibration amplitude
and shows a higher resolution and a lower contrast compared to the image on the right. Right:
This image was recorded with 80nm (400mV) vibration amplitude and shows a lower resolution
but a higher contrast compared to the image on the left.
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TN00031 © 2006 NANOSURF AG
Selecting the Relative Tip-Position
In Order to achieve the best contrast, the distance to the sample must be as
small as possible. At a certain point, however the tip will touch the surface
and will be harmed when the contact is to strong. A safe procedure as illustrated in figure Scanning Distance (p.11) is to start from a safe tip to sample
distance (e.g. -500nm) and reduce it step wise until the tip begins to feel
the surface. Immediately increase the distance until the topography features
disappear from the phase signal.
Scanning Distance: Top Left: Magnetic contrast increased when the distance to the sample was
decreased. Top Right: When the surface topography gets visible in the phase scan, you are probably
scanning to near. Bottom Left: Here the topography and some dirt particles are visible in the phase
scan. Also here the distance was to small. Bottom Right: Topography of the HD sample.
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© 2006 NANOSURF AG, SWITZERLAND, TN00031, R1
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