  Understanding the influence of dipolar interactions in magnetic hyperthermia
(MH) experiments is of crucial importance for a fine optimization of
nanoparticle (NP) heating power. In this study, we use a kinetic Monte-Carlo
algorithm to calculate hysteresis loops, so both time and temperature are
correctly taken into account. It is demonstrated that this algorithm correctly
reproduces the high-frequency hysteresis loop of both superparamagnetic NPs and
ferromagnetic ones without any ad-hoc parameters. The algorithm is easily
parallelizable so calculation on several processors decreases considerably
calculation time. The specific absorption rate (SAR) of NPs dispersed inside
spherical lysosomes is studied as a function of several key parameters: volume
concentration, applied magnetic field, lysosome size, NP diameter and
anisotropy. The influence of these parameters is illustrated and
comprehensively explained. In summary, the effect of magnetic interactions is
to increase the coercive field, saturation field and hysteresis area of major
loops. However, for small amplitude magnetic field such as the ones used in MH,
the heating power as function of concentration can increase, decrease or
display a bell shape, depending of the relationship between the applied
magnetic field and the coercive/saturation fields of the NPs. The hysteresis
area is found to be well correlated to the parallel or antiparallel nature of
the dipolar field acting on each NP. It is shown that the heating power
increases or decreases sharply in the vicinity of the lysosome membrane. The
amplitude of variation reaches more than one order of magnitude in certain
conditions. Finally, implications of these various findings are discussed in
the framework of MH optimization. It is concluded that feedbacks on specific
points from biology experiments are required for further advance on the
optimization of NPs for MH.
