Biomedical Engineering Reference
In-Depth Information
15.3.3
Magnetic Resonance Force Microscopy
Among the upcoming techniques which may match the capabilities of MFM,
magnetic resonance force microscopy (MRFM) is the most promising
scanned probe technique based on the mechanical detection of magnetic
resonance. MRFM was proposed as a method to overcome the sensitivity
limitations of inductively detected MRI, and to push the spatial resolution of
MRI into the nanometer and, ultimately, the atomic scale. MRFM methods have
steadily improved since the fi rst demonstrations, with signifi cant advances in
both electron spin and nuclear spin detection. Recently, the detection of magnetic
resonance on a single electron spin has been demonstrated [25]. MRFM uses
a magnetic tip and an ultrasensitive cantilever to sense the dipolar magnetic
force between the tip and spins in a sample. In this sense, the MRFM arrangement
is similar to that of MFM, and in fact the MRFM and MFM data are collected
simultaneously in a MRFM experiment. Unlike MFM, where the static magnetic
force is detected, the sensitivity of MRFM is enhanced through dynamic manipula-
tion of the sample magnetic moments at the mechanical resonant frequency
of the MRFM cantilever. In general, due to the detection of a dynamic magnetic
force, MRFM can deliver sensitivity greater than that of MFM. For example, it
has recently been demonstrated that MRFM is capable of two-dimensional
( 2 - D ) nuclear magnetic resonance ( NMR ) imaging with 90 nm spatial resolution
[26]. However, MRFM relies on the capability to manipulate magnetic moments
in the sample, which in turn is heavily dependent on the sample magnetic
properties such as the shape, saturation magnetization, ferromagnetic resonance
linewidth, and so on, which would make interpretation of the obtained
results more diffi cult. Whilst the MRFM technique holds great promise for the
investigation of individual nanoparticles, it is still a work-in progress and requires
sophisticated and specialized equipment and experimental conditions - unlike
the MFM technique, which is available on most commercial atomic force
microscopes.
15.4
Physical Principals of MFM
15.4.1
Static Mode
For a magnetic force microscope cantilever, the force
F
(which is experienced
normal to the cantilever) can result in a static defl ection
x
of the cantilever, in
accordance with Hooke's law, given by:
F x
=−
(15.1)
