# Magnetic Vortex and Hyperthermia Suppression in Multigrain Iron Oxide Nanorings
## 3 Results and Discussion
If this was due to effect of the increased reaction time (i.e., increased grain size, saturation magnetization, etc. ), one would expect to observe a larger SAR value for the 12 h sample, which did not happen in our case. A possible explanation is that as compared to the 8 h sample, the iron oxide nanorings, as we saw in TEM images (Figure 2), tend to degrade and agglomerate at 12 h, which would hinder their magnetic response at low fields, as indicated by the dynamic hysteresis loops in Figure 5. Nevertheless, further study is needed to fully understand this phenomenon, which is beyond the scope of the present paper. In addition, our results demonstrated that the SAR values of the multigrain Fe\({}_{3}\)O\({}_{4}\) nanorings can be appreciably improved when nanorings align with the field direction. From the calorimetric measurements, it was observed that the SAR values of the nanoring samples are relatively small at AC fields lower than the effective magnetic anisotropy. To understand the nature of heating in multigrain Fe\({}_{3}\)O\({}_{4}\) nanorings at low fields, we performed the AC magnetometry measurements at fields \(<\) 400 Oe for the Fe\({}_{3}\)O\({}_{4}\) nanorings (8 h and 12 h). Figure 5a-d show the SAR values and dynamic hysteresis loops in the range of 0-400 Oe at 302 Hz. The heating efficiency and SAR values of the nanoring samples were calculated from the hysteresis loop area (SAR = area \(\times\) frequency) [28]. Figure 5c,d shows that at low fields the hysteresis loop resembles Rayleigh loops, and area of the loops are quite small. As the field was increased, the loop area showed a rapid increase [28]. Figure 4: SAR vs. AC magnetic field plots for all the samples, dispersed in 1 mg/mL of (**a**) water, (**b**) 2% randomly suspended agar solution, and (**c**) pre-aligned to an external dc magnetic field. From Figure 5a-d, it can be seen that the area of the hysteresis loop and the SAR values almost remained unchanged until reaching an AC field of \(\sim\)200 Oe, and showed a steep increase when the AC field was increased above 200 Oe. It can be seen that for the 8 h sample, this increase is steeper and reaches higher SAR values, as was observed in calorimetric measurements. It should be noted that both the calorimetric and AC magnetometry measurements showed very similar SAR values. The AC magnetometry measurement results can be elucidated considering the ratio of the anisotropy field (\(H_{\rm K}\)) to the applied AC field. Depending on the \(H_{\rm K}\) of the nanostructures, the dynamic hysteresis loop shows two regions. At low field levels, when \(H<0.5\,H_{\rm A}\), since the power absorption is mainly caused by viscous losses in the system, a minor loop is obtained. Conversely, when the field \(H>0.5\,H_{\rm A}\), the hysteresis losses dominate, maximum heat power is transferred to the nanostructures, and the hysteresis loop area increases. Below the applied field of 200 Oe, which is \(\sim\)0.5 of \(H_{\rm K}\) of the nanorings (Figure 3b), the hysteresis loop area and the SAR values are negligible, while with the increase in the field, the hysteresis loop area and the SAR values showed a rapid increase. However, since the maximum AC applied field is still lower than \(H_{\rm K}\), the AC loop area is not maximized, and higher fields would be needed to saturate the AC loops [57]. Therefore, AC magnetometry measurements shed light on the low-field heating response of the nanorings that cannot be inferred from the calorimetric measurements.