  Magnetic nanoparticles constitute potential nanomedicine tools based on the
possibility to obtain different responses triggered by safe remote stimulus.
However, such richness can be detrimental if the different performances are not
accurately differentiated (and controlled). An example of this is the
reorientation of magnetic nanoparticles under the influence of AC fields, which
can be exploited for either magneto-mechanical actuation (MMA) at low
frequencies (tens of Hz); or heat release at large ones (MHz range). While it
is clear that Brownian rotation is responsible for MMA, its heating role in the
high-frequency regime is not clear. In this work we aim to shed light on this
issue, which needs to be well understood for applications in magnetic fluid
hyperthermia (MFH) or heat triggered drug release. Using a Brownian dynamics
(BD) simulation technique, we have theoretically investigated the contribution
of Brownian reversal in disk-shape particles (to enhance the viscous
interaction with the environment) over a wide range of frequencies. Our results
predict essentially negligible hysteresis losses both in the high- and
low-frequency domains, with completely different implications: highly efficient
MMA, but negligible MFH performance. Importantly, complementary micromagnetic
simulations indicate that the large magnetic torque assumption of the BD
simulations is supported by hexagonal-shape disks, up to field amplitudes of
the order of 100 Oe. Larger fields would lead to N\'eel reversal which,
noteworthy, predicts significant heating performance. The possibility of
switching between the MMA and MFH response by changing the amplitude of the AC
field, together with their distinct optimal conditions (large magnetic torque
for MMF; large heating for MFH), points to such hexagonal nanodisks as
promising nanomedicine agents with double mechanical and heating
functionalities.
