Magnetic Force Microscopy MFM
Procedure, Techniques and Applications
What is Magnetic Force Microscopy?
Essentially, a magnetic force microscopy (MFM)
is a variant of an Atomic Force Microscope (AFM) typically used for scanning and
studying surfaces with magnetic properties. The probe (scanning tip) of the
magnetic force microscope has a magnetic coating that allows the device to
measure magnetic fields. This makes MFM ideal for imaging the spatial
distribution of the magnetic field of such objects as magnetic tapes.
Main Parts of a Magnetic Force Microscope
- Piezoscanner (piezoelectric
scanner) - This is
the part of the device on which the sample is mounted. The piezoscanner
provides spatial movement of the sample (x, y, z directions) making it possible
to map the sample surface.
- Atomic force microscope
probe - The
probe (silicon probe) of the MFM is coated with a magnetic material (magnetized
using a strong magnet)
- Cantilever - The cantilever is the
part that holds the tip/probe at one end. The cantilever is not magnetized
Probe (tip) Magnetization Procedure
- Having determined the north
pole of the magnet, bring it very close to the tip (1 to 2 mm)
- Slowly move the magnet pole
away from the tip (horizontally)
This simple procedure is used to magnetize the
tip for MFM.
Working Mechanism (How MFM Works)
For this technique, the sample (with magnetic
properties) is mounted on the piezoscanner that moves on an X, Y, Z axis. With the
specimen resting under the cantilever's tip, magnetic force between the
tip/probe and the sample causes the cantilever to bend and the oscillations
Apart from the magnetic forces between the sample surface and the
tip, Van der Waals forces are also in action in this set-up. These (Van der
Waal forces) are weaker forces that exist between the atoms of the tip and
those of the sample surface. However, they are weaker compared to the magnetic
force depending on the distance between the two.
As the tip moves (up and down)
across the sample surface, it also causes the cantilever to oscillate which in
turn causes changes of the angular deflections of the light beam reflecting off
the surface of the cantilever. These changes are detected by a small displacement
transducer (optical sensor)
* Essentially, a magnetic force microscope is an
atomic force microscope (AFM) with a magnetized tip/probe that is used to
measure the magnetic force exerted on the cantilever tip.
* Rather than magnetic
domains, a magnetic force microscope detects and images the stray fields present
on the surface of the sample.
For a sample with oppositely magnetized domains,
the different magnetic poles in the domain would result in stray fields that
can be detected by the magnetic tip on the sample surface. For instance, in
such a set-up, a MFM tip moving across the sample surface would experience two
different forces on the domain wall caused by the different magnetic poles.
Here, one of the forces would be the repulsive force (from one of the magnetic
poles) and attractive force from the other pole. The different forces would
cause the tip and thus the cantilever to oscillate differently between
different regions of the sample during imaging and ultimately give an image
representing the state of the stray fields on the sample surface.
Given that other forces (such as the Van der
Waal forces) apart from magnetic force acts on the tip, it is necessary that
the probe be maintained at a given distance during imaging. Since this often
proves challenging, dual passages is used (Two-Pass Technique) in magnetic
The Two-Pass technique involves scanning the
sample surface twice. For the first scan, intermittent contact mode is used to
map the sample surface. Intermittent contact mode, also refered to as
true-Non-contact, involves using the tip to get the topography of the sample.
The tip is passed across the sample surface similar to the process
used in atomic force microscopy. Therefore, there is some level of contact for
the initial scanning.
During the second scanning, the cantilever tip
is set at a small distance from the sample in order to scan along the
topography line mapped during the first scan. For this phase, the cantilever
tip only interacts with the magnetic force, which allows for imaging.
technique presents a big advantage given that it minimizes the effects caused
by other non-magnetic forces. For instance, by only following the topography
line obtained from the first scan, effects of Van der Waal forces on the
cantilever tip are minimized so that they do not affect the effects caused by
magnetic force. This ensures that only the effects of the magnetic force are
Discussion (Two Pass Technique)
During the first scan that involves contact, the
cantilever tip scans the surface while the recordings made are used to produce
a three dimension (3D) image of the surface. Here, contact with the sample
surface allows for the device to scan the surface down to the atomic level.
first scan plays an important role in that it helps mark out a trajectory
through which the tip passed through. This helps provides a profile of the
surface that is then stored in the connected computer.
During the second phase
of scanning, the tip moves along the marked trajectory above the surface (no
contact). This ensures that the tip is only affected by other forces (magnetic
and to a lesser extent by Van der Waal) rather than contact force. During the
second phase of scanning, oscillations of the cantilever are largely due to the
effects of the magnetic forces (stray fields) from the sample.
Applications of MFM for Studying Nanoparticles - Nanoparticle Characterization
Nanoparticles are nanoscale particles. This
means that they are particles that range between 1 and 100 nanometers in size.
As such, they are too small to be clearly seen under the light microscope let
alone the naked eye.
With such techniques as magnetic force
microscopes, it has become possible to observe and study a wide range of
nanoparticles. One of the biggest
advantages of this technique (MFM) is that it can be used in a variety of
conditions (ambient conditions). This means that the magnetic force microscope can
be used in varying temperatures, in liquid conditions as well as vacuums. This, therefore, makes it an excellent technique for studying various nanoparticles in
their respective environments.
One instance in which magnetic force microscopy
has been used to observe and study nanoparticles is with regards to
Magnetoferritin molecules. In a recent experiment, researchers used Ferritin
from Pyrococcus furiosus, a species of Archaea. This was preferred because of
its capacity to stabilize the magnetic iron nanoparticles during the imaging
To study these molecules, researchers began with atomic force
microscopy in order to obtain the topography images. Essentially, this involved
mounting the sample and moving the AFM tip across its surface. This allowed the
researchers to observe and study the shape and dimensions of the molecules (Magnetoferritin
molecules) on the protein (Ferritin).
In the Two-Pass technique, this is
typically the first phase of imaging. Using this technique, the researchers
were able to clearly identify the shape of these molecules (dimensions,
height). The second pass (also known as the MGM phase) is then performed along
the topography marked during the first pass. From the second pass, the
researchers were able to identify four Magnetoferritin molecules
Ferritin is a protein that can be found in a
variety of organisms ranging from prokaryotic bacteria to mammals. Being a
protein, it is made up of various molecules including the Magnetoferritin
molecules. In the experiment described above, the AFM was first used to map out
the sample surface and highlight the topography of the molecule thereby giving
a general appearance of the molecule.
By itself, atomic force
microscopy is incapable of identifying the magnetic fields of the molecule.
Here, the second pass (MGM Phase) allows for these areas in the molecule to be
identified by using a magnetized tip. Therefore, the two techniques greatly
contribute to understanding the molecule in detail (dimensions of the molecule
based on AFM and the presence of the magnetic molecules based on MGM).
By using the technique (MGM) it has become
possible for researchers to study the magnetic properties of a wide variety of
biological molecules in various organisms (bacteria to mammals). In particular,
the technique has been shown to have a number of advantages including being
non-destructive when studying biological components and providing high resolution.
Imaging and Characterization of Superparamagnetic Iron Oxide Nanoparticles in Liquid (SPIONS)
(SPIONs) are nanoparticles that are currently
being increasingly used in biological sciences. Due to their size and long
half-life, these nanoparticles have been shown to be particularly suitable for
targeted drug delivery, magnetic labeling as well as for treatment of
hyperthermia among cancer patients among others. These applications make it a
valuable nanoparticle in biological sciences and pharmaceutics and several
other fields. Using magnetic force microscope, it has become possible for
researchers to study and understand these nanoparticles.
Sample Preparation (SPION) for Microscopy
For this experiment, researchers required the
- Iron (II) and Iron (III)
chloride (99 percent)
hydroxide (25 percent solution)
- Tetraethyl orthosilicate
(99 percent Sigma)
- 28 percent Ammonium
- Using co-precipitation, the
nanoparticle (SPIONs) was first coated with Bare and Silicon dioxide (SiO2)
- 2.5 milliliters of the iron
solution mixed with water was then added to the tetramethylammonium
- Hydroxide (0.7 mol/L) while
- After stirring for about 30
minutes at room temperature, neodymium magnet was used to separate the black
particles from the solution and
washed three times using tetramethylammonium hydroxide solution (pH 12) to the
point that the particle could no longer be magnetically separated
- The suspension was then
sonicated for about 10 minutes and about 20 mL of the fluid mixed with 10 mL of tetramethylammonium
hydroxide and 160 mL of ethanol
- This was followed by adding
7 mL of tetraethylorthosilicate while stirring and allowed to react at room
temperature for about 18 hours
- SPIONs (coated with
silicone dioxide) were then removed from the solution (magnetically) washed
using ethanol three times and washed using de-ionized water three more times
- SPIONs were then
deposited on mica substrate before imaging
During imaging, researchers started with AFM
imaging, which provided topography images of the sample. The first step allowed
the researchers to determine the height, size and distribution of the SPIONs.
From the images, it was observed that whereas silicone dioxide coated SPIONs
were broadly distributed with higher mean diameter, bare coated SPIONs did not.
However, both were observed to be non-symmetric with right skewed size
For imaging of SPIONs in liquid conditions, the
nanoparticles were first coated with poly methyl-methacrylate (PMMA). Coating
the SPIONs with poly methyl-methacrylate helps secure them to the substrate
temporarily. This is typically done to prevent the nanoparticles from being separated
from the substrate when they are mixed with water.
* Imaging revealed small SPIONs covered by a
layer of PMMA in liquid.
MFM Hard Disk Data Reconstruction
Although studies are being made to improve the
technology for better data recovery, data reconstruction from a hard disk drive
using magnetic force microscopy involves three major steps.
- Obtaining MFM images from a
portion of the disk
- Image processing
- Readback signal extraction
Imaging of the hard drive first involves
selecting a given portion of the disk (e.g. a square of about 100um by 100um)
to be scanned. Once the desired portion has been selected, it is mounted on the
microscope for scanning.
The process starts with operating the probe in tapping
mode. Compared to other modes, the tip (in tapping mode) comes in contact with
the sample surface only briefly. As such, it does not drag along the surface.
This is the first scan that helps mark a trajectory to be followed.
During the second scan, scanning is achieved at
LiftMode where the tip is set at a certain distance from the sample surface
(surface of the hard disk). Determining the ideal distance between the tip and
the sample is of great significance given that the distance selected has a
direct impact on the quality of data obtained. Whereas long distance will affect
the magnetic force between the tip and the sample surface, a very small
distance may result in the tip coming into contact with the surface, which can
cause artifacts to form.
* Slow scan rates increase the rate of obtaining
higher resolution images
The second scan is based on the magnetic force
between the tip/probe and the magnetic field present on the disk. Once scanning
is complete, the images obtained from the scan can then be used for data
reconstruction in the processing phase.
While magnetic force microscope can be used in
the process of data recovery from a hard disk drive, researchers are working on
improving the technique in order to make it more efficient for data retrieval. Despite
its advantages, some of the limitations of MFM in imaging hard disk drives
Low rate of image
acquisition - As already mentioned, higher resolutions are possible with slow
scan rates. This is a big disadvantage of the technique because the process
may take hours, which can frustrate some people
- The sample has to be
specially prepared - Before the sample is scanned, it has to be specially prepared
in order to locate the intended patterns. Again, this may prove
- Accumulation of numerous 2D
images of the same target are almost impossible when one wants to increase
signal to noise ration
- Image area is highly
- The process may not provide
Advantages of Magnetic Force Microscopy
Capacity to separate
magnetic forces from other types of forces
Magnetic force microscopy is primarily used
to scan the magnetic field of a sample. Once the first scan involving contact
has been done, the second scan (non-contact) is dependent on the magnetic force
between the tip and the sample surface. However, other forces such as the Van
der Waal forces exist between the two.
MFM offers a big advantage here because
it is capable of separating these forces. This can simply be achieved by setting
the tip at the ideal distance from the sample once the first scan has been
completed. This makes it possible to scan the surface of the sample based on
the magnetic field.
Application in detecting
nanoscale magnetic domains
Magnetic force microscopy has been shown to be an important
tool for detecting nanoscale magnetic domains in various fields of study
including biological sciences.
In addition to AFM phase and topography images
obtained using this technique, it is possible to detect nanoscale magnetic
domains of such nanoparticles as proteins (e.g. ferritin). This has been shown
to greatly contribute in cancer treatments and the study of biomarkers etc
It can be used in a variety
Compared to other microscopy technique, magnetic force
microscopy can be used in various environment conditions (varying temperatures,
vacuum etc) and even in liquid conditions. This is a big advantage given that
the technique can be used to study and analyze a range of samples in their
natural environment. For instance, using this technique, it is possible for
researchers to study given proteins or other bio-molecules in vitro for better
In addition to the fact
that the technique can be used in varying conditions, it is also capable of
high resolution in these conditions. This makes it an excellent technique for
getting clear images of nanoparticles in their natural environments.
Magnetic Resonance Force Microscopy (MRFM)
Magnetic resonance force microscopy is a type of
magnetic resonance imaging (MRI) that combines the principles of both MRI and
atomic force microscopy to produce three dimensional images of nano-samples
(and in some cases reaching atomic scales). Like magnetic force microscopy, this
technique is also dependent on weak magnetic forces between the sample and tip
of the microscope.
How Magnetic Resonance Force Microscopy Works
Essentially, MRFM uses the same working
principle as magnetic force microscope. Here, the sample is mounted on the
cantilever so that it is in close proximity to the magnet beneath it. With the
magnetic tip moving near the sample, the nuclear of the atoms or unpaired
electrons (on the sample surface) start to spin as they get attracted.
spins then flip which causes the cantilever to move back and forth.
Oscillations of the cantilever can then me measured (by measuring amplitude of
the oscillations) to scan the sample. For the technique to be more effective,
it requires a very sharp probe/tip and a smaller cantilever.
While this technique is similar to magnetic
force microscopy, there are a number of differences that include:
- The sample is mounted on a
smaller cantilever on MRFM
- A magnet is used in MRFM to
produce a magnetic field from the sample
- The sample in MRFM has to be
mounted on the cantilever
Atomic force microscopy (AFM) is itself a type
of scanning probe microscopy that is typically used to measure various
properties (height, friction etc) of a sample surface.
Once the sample is mounted,
the sharp cantilever tip is moved across the sample making it possible to map
the sample surface. As the tip moves across the sample surface, the forces
between it and the surface cause the tip and thus the cantilever to move
(oscillate). This in turn causes changes (displacements) in the laser beam being
reflected off the surface of cantilever; which are then detected by the photo-detector
and recorded. This makes it possible to scan the contours of the sample and
provide images that represent what the surface looks like.
One of the main benefits of AFM is that it can
be used to also detect other forces like Van der Waal and magnetic forces. By
magnetizing the probe, this technique not only can be used to scan the topography
of the sample surface, but also the magnetic fields.
Here, one of the main
differences between AFM and MFM is that whereas the tip/probe can come in contact
with the sample surface in AFM, it is set at a small distance in MFM so that
there is no contact. Secondly, if the tip is not magnetized, the AFM technique
would not be effective in scanning the magnetic field of the sample surface.
The scanning tunneling microscope uses the same principle as AFM. That is, a sharp tip is moved across the sample surface during scanning. However, this technique works by employing a small voltage to either the sharp tip or the sample (both in some cases) making it possible to image the sample surface to the atomic scale.
Using this technique, it is possible to identify individual atoms on the ample surface based on their size and shape.
Of Interest: Electron Microscopy
See Also: Nanotechnology Main Page
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Gustavo Cordova, Brenda Lee and Zoya Leonenko. Magnetic Force Microscopy for Nanoparticle Characterization. January 2016. DOI: 10.17756/nwj.2016-022
Wolfgang Kuch. Magnetic Imaging. Freie Universit¨at Berlin, Institut f¨ur Experimentalphysik, Arnimallee 14, 14195 Berlin, Germany.
Cordova et al., Magnetic Force Microscopy Characterization of Superparamagnetic Iron Oxide Nanoparticles (SPIONs). Nano Biomed Eng 2014; 6(1):31-39. doi: 10.5101/nbe.v6i1.p31-39.