  Heating magnetic nanoparticles with high frequency magnetic fields is a topic
of interest for biological applications (magnetic hyperthermia) as well as for
heterogeneous catalysis. This study shows why FeC NPs of similar structures and
static magnetic properties display radically different heating power (SAR from
0 to 2 kW.g-1). By combining results from Transmission Electron Microscopy
(TEM), Dynamic Light Scattering (DLS) and static and time-dependent
high-frequency magnetic measurements, we propose a model describing the heating
mechanism in FeC nanoparticles. Using, for the first time, time-dependent
high-frequency hysteresis loop measurements, it is shown that in the samples
displaying the larger heating powers, the hysteresis is strongly time
dependent. More precisely, the hysteresis area increases by a factor 10 on a
timescale of a few tens of seconds. This effect is directly related to the
ability of the nanoparticles to form chains under magnetic excitation, which
depends on the presence or not of strong dipolar couplings. These differences
are due to different ligand concentrations on the surface of the particles. As
a result, this study allows the design of a scalable synthesis of nanomaterials
displaying a controllable and reproducible SAR.
