is beyond the range of our susceptometer but its contribution
at 111.5 kHz is negligible as shown in figure 3. For samples
dispersed in wax N´eel relaxation will be the only susceptibility
loss mechanism as the particles will not be able to physically
rotate.
The heating properties of the colloids were measured
using a Nanotherics Magnetherm system [15]. The system
consists of a sample coil enclosure, a function generator and
a power amplifier forming a resonant circuit. The system
is monitored by an output from the coil to an oscilloscope.
The samples were placed in a 15ml tube (10mm × 100mm)
which was then placed inside an expanded polystyrene jacket
which acted as an insulator. The ac frequency is varied by
changing the capacitor and coil configuration. This allowed us
to study the heating characteristics of the samples at different
fields and constant frequency. All measurements were made
under identical conditions and concentrations and are therefore
comparable. Figure 4 shows the SAR for all samples as a
function of the applied field. For Isopar M and Isopar V,
C = 2206 J kg−1 K−1 and for the wax C = 2140 J kg−1 K−1.
The mass ratio of particles to liquid φ = 20 mg Fe ml−1
for all samples. This concentration of iron corresponds to
approximately 28 mg of Fe3O4 per ml or a ferrofluid with a
concentration of 30 G. In colloids of these concentrations and
with the dispersion quality that can be seen in theTEMimages,
the effect of dipole–dipole interactions will be small. However,
we cannot exclude the possibility that aggregates or flocculates
of particles would affect the hysteresis. These results together
with those for hysteresis loss are shown in table 2.
Table 2 shows the contribution to the SAR from hysteresis
and susceptibility loss effects for all samples which were
calculated from equations (11) and (7), respectively. From the
calculated data it is clear that the contribution to the SAR from
susceptibility loss is very small except for sample A where it
is 17% of the total. For samples B and C it is insignificant.
Hence for in vivo use with tissue having a minimum viscosity
of 10 cP there will be little or no contribution to the SAR from
susceptibility loss.