  When magnetic nanoparticles (MNPs) are single-domain and magnetically
independent, their magnetic properties and the conditions to optimize their
efficiency in magnetic hyperthermia applications are now well-understood.
However, the influence of magnetic interactions on magnetic hyperthermia
properties is still unclear. Here, we report hyperthermia and high-frequency
hysteresis loop measurements on a model system consisting of MNPs with the same
size but a varying anisotropy, which is an interesting way to tune the relative
strength of magnetic interactions. A clear correlation between the MNP
anisotropy and the squareness of their hysteresis loop in colloidal solution is
observed : the larger the anisotropy, the smaller the squareness. Since low
anisotropy MNPs display a squareness higher than the one of magnetically
independent nanoparticles, magnetic interactions enhance their heating power in
this case. Hysteresis loop calculations of independent and coupled MNPs are
compared to experimental results. It is shown that the observed features are a
natural consequence of the formation of chains and columns of MNPs during
hyperthermia experiments: in these structures, when the MNP magnetocristalline
anisotropy is small enough to be dominated by magnetic interactions, the
hysteresis loop shape tends to be rectangular, which enhance their efficiency.
On the contrary, when MNPs do not form chains and columns, magnetic
interactions reduces the hysteresis loop squareness and the efficiency of MNPs
compared to independent ones. The present work should improve the understanding
and interpretation of magnetic hyperthermia experiments.
