Biomedical Engineering Reference
In-Depth Information
These include the use of thin- fi lm-coated MFM probes with a wide variety of
magnetic and mechanical properties (see Section 15.6.2), the use of split photo-
diodes to detect cantilever defl ection [5-7], and the use of dynamic mode AFM to
enhance the force sensitivity of the MFM technique (see Section 15.4.2). As a
result, in recent years a spatial resolution of 20-30 nm has been achieved with
MFM, making it an attractive technique for the characterization of MNPs. With
further advances in the fi eld, it is envisaged that MFM may achieve its theoretical
limit of resolution (5- 10 nm).
MFM has found numerous applications in fundamental research and in the data
storage industry. MFM serves as a useful tool for disk-failure analysis, and to
analyze magneto-optical media and corresponding read/write processes, which
usually require a micron-scale spatial resolution. More recent, nonstandard appli-
cations of MFM include magnetic dissipation imaging [8] to investigate magnetiza-
tion dynamics through studying the energy transfer between the cantilever and
the magnetic sample, and low-temperature measurements to investigate magnetic
vortices [9] or local variations in the magnetic penetration length in superconduc-
tors and colossal magnetoresistance materials [10]. The application of MFM to
characterize nanoscale magnetic domains has not yet been completely explored,
mainly because special considerations need to be given to MFM experimental
conditions, sample preparation and probe characterization to enable the qualitative
and quantitative detection of MNPs.
15.3
Comparison of MFM to Other Techniques
Despite the emerging interest of the life sciences community in the use of MNPs,
there is a clear absence of a detection technique for their localization and charac-
terization at the single-particle level. The techniques currently being used to detect
MNPs can broadly be classifi ed as invasive or noninvasive. The limitation in reso-
lution and/or sensitivity of these techniques as compared to MFM is discussed
here.
15.3.1
Invasive Imaging
Superconducting quantum interference device (SQUID) magnetometry is conven-
tionally used to characterize the bulk magnetic properties of materials, including
samples of MNPs with masses of several micrograms. In the recently developed
scanning SQUID microscopy technique, the pick-up coil is scanned over the
surface of the sample where it picks up the stray magnetic fi eld of the magnetic
moments in the sample [11]. As a result, the spatial resolution of the method is
defi ned by the dimensions of this coil. The highest demonstrated resolution to
date is approximately 10
m which is insuffi cient for studying systems of MNPs
[12]. Similarly, the scanning Hall probe microscope (SHPM) suffers from low
μ
