Biomedical Engineering Reference
In-Depth Information
resonance frequency. If its magnetic moment is low, automatic tuning will be
ineffective and manual tuning will be required. As a result of the involvement of
human judgment in tuning, even with careful manual tuning of a low moment
sample the saturation moment was found to vary by more than
5% over 10 con-
secutive measurements. This limits the use of AGM to low-moment samples. In
addition, the measured moment is very sensitive to sample placement due to the
gradient fi eld, and therefore care must be taken to ensure that the sample and the
calibration standard are identical in mass and size in order to obtain a calibrated
reading. When measuring low-coercivity samples (
±
100 Oe or smaller), it is neces-
sary to reduce the magnitude of the gradient fi eld in order to maintain accuracy.
This, in turn, in reduces the sensitivity of the AGM procedure (compared to VSM)
and also severely limits the suitability of AGM for measuring low coercivities, as
the alternating fi eld cycles through part of the hysteresis loop. Similarly, the gradi-
ent fi eld limits the accuracy of remanence measurements, as the fi eld is cycling
through a minor loop around remanence.
In VSM (Figure 16.5b), the sample is vibrated in the magnetic fi eld using a
vibrator mechanism. The magnetizing fi eld (DC) is provided by an electromagnet
driven by a DC bipolar power supply, while the signal voltage is generated in the
detection coils due to the changing fl ux emanating from the vibrating sample. The
output measurement displays the magnetic moment (
M
) as a function of the fi eld
(
H
). In this stepped-fi eld mode, the magnetic fi eld was changed in selected step
sizes and the magnetic moment measured with the fi eld held stationary. The
advantage of the stepped- fi eld system is that the fi eld between measurement points
can be changed at a very high rate (
∼
2 kOe s
− 1
), and consequently passing through
all the fi eld points does not take very much time. In fact, more time is spent at
each measurement fi eld to measure the signal several times and average the result
(digital signal averaging), which leads to a signifi cant noise reduction if the noise
is uncorrelated. However, the noise fl oor for VSM is 10
− 6
emu, which is two orders
of magnitude higher than that for AGM. In addition to AGM and VSM, SQUID
magnetometry is also frequently used to determine the magnetic properties of
Co-based nanoparticles, and in particular low-temperature studies of magnetic
nanoparticles. However, details of this technique will not be provided at this point.
>
16.2.3
Morphology
Atomic force microscopy is commonly used to study the morphology of materials.
The atomic force microscope consists of a microscale cantilever with a sharp tip
(probe) at its end that is used to scan the specimen surface (see Figure 16.6). The
cantilever is typically silicon or silicon nitride, and has a tip radius of curvature on
the order of nanometers. When the tip is brought into the proximity of a sample
surface, forces between the tip and the sample lead to a defl ection of the cantilever,
according to Hooke's law. Depending on the situation, the forces measured in
AFM include mechanical contact force, van der Waals forces, capillary forces,
chemical bonding, electrostatic forces, magnetic forces (see magnetic force micros-
