## 3 Results and Discussion
There was some variation in iron oxide concentration from one sample to another. For instance, well-dispersed SLP was measured at 0.67 wt % (Figure 4A). In this case, an increase of temperature of 0.8 \({}^{\circ}\)C was achieved within 40 s.  
SLP measurement for aggregates containing 17% iron oxide nanoparticles (and 83% cerium oxide) was conducted with an MNP concentration of 0.22 wt % (Figure 4C). An increase of 0.5 \({}^{\circ}\)C was obtained after the application of the magnetic field for 60 s.  
For aggregates made of 100% iron oxide nanoparticles, the SLP measurements were achieved at 0.24 wt % (Figure 4D). For this sample, the increase of temperature up to 0.4 \({}^{\circ}\)C was only obtained for 120 s of excitation. As attended, the temperature of cerium oxide nanoparticles did not increase while alternative magnetic field is on (Figure 4B). Figure 3: (**a**) TEM image of an aggregate of Fe\({}_{2}\)O\({}_{3}\) and CeO\({}_{2}\) NPs; (**b**) Electron energy loss spectroscopy (EELS) analysis of Fe (red) and Ce (green) in the aggregates; (**c**) separated EELS map of the Fe (red) content; (**d**) separated EELS map of the Ce (green) content. By changing the ratio of PAA-Fe\({}_{2}\)O\({}_{3}\) and PAA-CeO\({}_{2}\) particles in the salted mixture from 100 (arbitrary value corresponding to aggregated iron oxide nanoparticles only, that corresponds really to infinity) to 0.17 in order to tune the proportion of both particles into the aggregates, it was possible to tune the mean distance between iron oxide nanoparticles inside the aggregates. The value of 0.01 was arbitrary attributed to well dispersed iron oxide nanoparticles to gain into clarity on the log scale of Figure 5 (this corresponds to a mixing value of 0). SLP value of well dispersed IONPs was found to be 21.27 W/g at 7 mT and 335 kHz (red line in Figure 5). When iron oxide nanoparticles only are aggregated, they exhibit a decreased value of the SLP down to 7 W/g at the same magnetic field and frequency. As aggregation induces a decrease of SLP, this result was almost achieved [4, 23, 24]. By tuning the iron oxide to cerium oxide ratio, we showed that the SLP values remained low for \(r=1.5\) (SLP value of 6.88 W/g). However, when the \(r\) was decreased to 0.5, an increase of the SLP value was measured up to 11.7 W/g. This increase was confirmed when \(r=0.16\) (SLP value of 15.5 W/g). These results are explained by the fact that the iron oxide nanoparticles were \"diluted\" into the aggregates when \(r\) was decreased, thus increasing the mean distance between magnetic nanoparticles. This dilution induced a decrease of dipolar interaction between nanoparticles, so that they could recover their ability to heat up their surrounding medium. The values of well dispersed nanoparticles were not reached for aggregates for two hypothetical reasons. Figure 4: Temperature increase and Box-Lucas fit (red line) versus time for well dispersed MNPs (**A**), cerium oxide nanoparticles (**B**), and aggregates containing 17% (**C**) and 100% (**D**) of iron oxide nanoparticles, respectively. Magnetic field was applied after 60 s recording. First, particles in aggregates were too concentrated to avoid completely dipolar interactions. In a previous study, we established that iron oxide particles in similar aggregates were concentrated at a volume fraction of 20% [17]. For \(r=0.16\), this would correspond to a volume fraction around 3% (that correspond to 15% by weight), which is still a high concentration for the determination of SLP values for iron oxide nanoparticles [25]. As a comparison, SLP values of well dispersed iron oxide nanoparticles was determined at 0.67 wt % to avoid interparticle dipolar interactions. Second, it could be still possible that the Brownian relaxation of iron oxide nanoparticles during hyperthermia was impeached when magnetic particles were located into the aggregates. However, the limit between Neel and Brownian relaxations for maghemite was found to be around 20 nm [26].