Hence calculations based on a constant value of K will give an
overestimate of the expected heating. Assuming a Gaussian
distribution of K with a standard deviation of 0.2 times the
mean, a reduction in the calculated value of the specific
absorption rate (SAR) of about 50%would result depending on
the region of the SFD that generated the hysteresis heating. For
each grain size the limitation of hysteresis heating would be at
a value of V associated with the median value of the anisotropy
constant. There is no known technique for the measurement of
the distribution of anisotropy constants in fine particle systems.
Hence at the present time our theory is as accurate as possible.
The calculation of the heat generated by the frictional
drag on the particle (Pstir) is extremely complex. We have
no value for the drag coefficient between a coated nanoparticle
and the liquid. However, the flow of the liquid around such
particle will almost certainly be turbulent and hence none of
the standard equations of hydrodynamics apply. Nonetheless
magnetic stirring of the liquid will lead to a frictional heating
effect. This effect is not only impossible to calculate but also
impossible to control. Stirring may also lead to a lag between
the alignment of the moment and the field which will create
another form of magnetic heating similar to the susceptibility
loss.
The amount of heat generated by magnetic nanoparticles
is normally quantified in terms of the SAR which represents
the power generated per unit mass of magnetic material in
the solution. Depending on the exact procedure followed to
measure the SAR, widely differing results may be obtained.
Furthermore factors such as container shape and sample
volume can lead to different SAR values for the same particles
[11]. SAR is given by