Biomedical Engineering Reference
In-Depth Information
romagnetic particles (NdFeB) attached to commercial cantilevers [43] offer the
strongest sensitivity, as these particles have both a high magnetization and high
coercivity values. The magnetic fi eld emanating from the particle on the magnetic
force microscope probe can therefore be very strong (
400 G m
− 1
). These tips have
an advantage in imaging paramagnetic particles, as they can have a high force
sensitivity of the detection cantilever and can maximize the magnetic force between
the tip and the sample. However, the physical dimensions of these probes are
generally several hundreds of nanometers, which limits their spatial resolution.
FIB-milled probes are generally diffi cult to replicate identically, and/or to mass
produce.
>
15.6.4
CNT Probes
Carbon nanocone ( CNC ) [40] tips (Figure 15.3 c) or carbon nanotube ( CNT ) [41]
probes (Figure 15.3d) offer a higher resolution because the magnetic material on
the CNT probe (Figure 15.3d) is contained within a narrow cylinder, in contrast
to standard pyramidal probes [44]. The smaller volume of magnetic material that
contributes to the signal results in a higher spatial resolution (
20 nm). The
sharper tip radius of the CNT also allows a better topographic scan, which yields
a more accurate height profi le for the tip to follow. CNT probes are less invasive
than commercially available probes, because a smaller amount of magnetic mate-
rial is brought into close proximity with the sample. Finally, the magnetic material
is distributed on the probe along an easy-to-model straight cylinder, a feature
which may help in more quantitative MFM studies. For studying MFM in biologi-
cal samples which usually have low coercivity, CNT MFM probes offer a useful
alternative.
<
15.7
Probe Calibration
Although the physical dimensions of the probe can be ascertained using scanning
electron microscopy (SEM), the characterization and calibration of its magnetic
properties requires special considerations. These include the tip hysteresis loop
and coercivity, as well as the magnetization distribution of the tip. In particular
for the latter, one does not know how this differs from tip to tip, and how much
magnetic material of the tip-coating is used in the imaging process, even when
considering the simplest possible homogeneous magnetization distribution of the
tip. Sophisticated techniques such as electron holography [45] or vibrating canti-
lever magnetometry [44] have been used to characterize the magnetic force micro-
scope tips through both theoretical and analytical models. Use of the magnetic
force microscope as a microscopic magnetometer to calibrate the tips remains
the most popular method. A quick method of verifying (qualitatively) the tips is
to image magnetic tapes. These samples usually have a longitudinal magnetic
