# Magnetic Vortex and Hyperthermia Suppression in Multigrain Iron Oxide Nanorings
## 3 Results and Discussion  
The phase purity and crystallinity of the nanorings were examined by XRD. Figure 1 shows the representative XRD spectra of \(\alpha\)-Fe\({}_{2}\)O\({}_{3}\) after 5 h of reaction and reduced Fe\({}_{3}\)O\({}_{4}\) nanorings. Both the as-synthesized and annealed samples display sharp peaks, which can be indexed to \(\alpha\)-Fe\({}_{2}\)O\({}_{3}\) and Fe\({}_{3}\)O\({}_{4}\), respectively. The sharp XRD peaks, evidence of high crystallinity, provide an advantage to the Fe\({}_{3}\)O\({}_{4}\) nanorings, as this ensures improved magnetic property, and thus better inductive heating efficiency. The full width at half maxima (FWHM) of all the diffraction peaks of Fe\({}_{3}\)O\({}_{4}\) nanorings (5 h of reaction) are relatively large, suggesting a small crystallite size of the Fe\({}_{3}\)O\({}_{4}\). The average crystallite size of Fe\({}_{3}\)O\({}_{4}\) (5 h of reaction) as calculated using the Debye-Scherrer formula is 15 nm. To further study the microstructure and morphology of Fe\({}_{3}\)O\({}_{4}\) nanorings, TEM imaging of the annealed samples was performed. We have shown earlier that the morphology and size of the \(\alpha\)-Fe\({}_{2}\)O\({}_{3}\) nanoparticles does not change with annealing [28]. TEM images in Figure 2 showed that the annealed Fe\({}_{3}\)O\({}_{4}\) nanoparticles obtained after hydrothermal reaction at 220 \({}^{\circ}\)C exhibit ring morphology and their  
Figure 1: X-ray diffraction (XRD) pattern of as-synthesized (\(\alpha\)-Fe\({}_{2}\)O\({}_{3}\)) and reduced (Fe\({}_{3}\)O\({}_{4}\)) nanorings after 5 h of hydrothermal reaction. The lower patterns with blue and black squares are the JCPDS data for bulk \(\alpha\)-Fe\({}_{2}\)O\({}_{3}\) and Fe\({}_{3}\)O\({}_{4}\), respectively [21; 27; 28]. size is relatively polydispersed. The annealed sample obtained after 5 h of hydrothermal reaction (Figure 2a,b) yielded nanorings with an average outer diameter of \(\sim\)110 nm and an inner to outer diameter ratio of \(\sim\)0.55. As can be seen from the TEM image in the inset, the Fe\({}_{3}\)O\({}_{4}\) nanorings seem to consist of many semispherical Fe\({}_{3}\)O\({}_{4}\) nanoparticles of around 20 nm, which are self-assembled to form a multigrain nanoring. This supports the observation of broad XRD peaks on the Fe\({}_{3}\)O\({}_{4}\) nanoring, which had a crystallite size of 15 nm. To understand the effect of hydrothermal reaction time on the evaluation of nanoring morphology and dimensions, we varied the reaction time while keeping other reaction parameters unperturbed. TEM images of the annealed multigrain Fe\({}_{3}\)O\({}_{4}\) nanorings obtained after 8 and 12 h of hydrothermal reaction (Figure 2c,d) yielded particles with average outer diameter of \(\sim\)110 and 105 nm, respectively, and an inner to outer diameter ratio of \(\sim\)0.5. The average outer diameter and an inner to outer diameter ratio of all the nanorings can be seen in Table 1. The time-dependent studies of hydrothermal reaction suggest that the nanoring morphology and dimensions remain almost the same from 3 to 12 h of reaction. TEM images of the reduced Fe\({}_{3}\)O\({}_{4}\) nanorings after 12 h of hydrothermal reaction (Figure 2d) indicated that prolonging the reaction time causes agglomeration and breaking of the multigrain nanorings. This was also observed in the case of nanotubes, where we showed breakdown of nanotubes after 24 h of reaction [28]. Magnetic hysteresis loops measured at 300 K in multigrain Fe\({}_{3}\)O\({}_{4}\) nanorings after different hydrothermal reaction times of 3, 5, 8, and 12 h are shown in Figure 3a. The saturation magnetization (\(M_{\rm S}\)) and coercive field (\(H_{\rm C}\)) values at 300 K of the Fe\({}_{3}\)O\({}_{4}\) nanorings of different hydrothermal reaction times are summarized in Table 1. In Figure 3a, all the multigrain Fe\({}_{3}\)O\({}_{4}\) nanoring samples show similar behavior at 300 K. Despite having small crystallite sizes, the presence of notable \(H_{\rm C}\) in all