## 3 Results and Discussion  
### Structural and Morphological Analysis  
The phase purity of the Fe\({}_{3}\)O\({}_{4}\)/\(\gamma\)-Fe\({}_{2}\)O\({}_{3}\) core/shell nanoparticle samples was confirmed by XRD analysis. The pattern is presented for \(2\theta\) ranging from 20\({}^{\circ}\) to 70\({}^{\circ}\) along with all the peaks indexed according to the Fe\({}_{3}\)O\({}_{4}\) core and shown in Figure 1a. We did not observe any signature of secondary phases, which confirms the phase purity of our samples. The XRD peak due to (311) plane for both Fe\({}_{3}\)O\({}_{4}\) and \(\gamma\)-Fe\({}_{2}\)O\({}_{3}\) phases were found to overlap with each other whereas the peaks due to (511), (422), (311), (220) and (440) lattice planes corresponding to Fe\({}_{3}\)O\({}_{4}\) core phase were clearly visible. The particles in all samples were observed to be almost spherical in shape. Figure 2 displays the TEM images and the particle size histograms of the three samples. The particle size histograms were obtained from TEM measurements on many particles. Some aggregations can be observed in the TEM images. The average particle sizes calculated from the TEM images for samples S1, S2 and S3 samples were found to be 9 nm, 11 nm and 13 nm, respectively. The particle sizes were also obtained from the XRD patterns by using [PERSON]'s formula; \(D_{\text{P}}=\frac{0.94\lambda}{\beta\cos\theta}\), where \(D_{\text{P}}\) is the average crystallite size, \(\lambda\) is the X-ray wavelength, \(\beta\) is the full width of half maximum (FWHM) of the XRD line, and \(\theta\) is the Brag's angle. The average particle sizes calculated from [PERSON]'s formula were found to be 9 nm, 11 nm and 13 nm, for samples S1, S2 and S3, respectively, which exactly match the average particle sizes obtained from the TEM measurements. The average core and shell dimensions were obtained from several HRTEM images. The core diameters were found to be approximately 8 nm for all the samples. In addition to XRD analysis, the structure and morphology of the nanoparticles were investigated using several microscopy tools such as high resolution transmission electron microscopy (HRTEM), and selected area electron diffraction (SAED). The average shell thicknesses for samples, S1, S2 and S3 were found to be 1, 3 and 5 nm, respectively. Figure 1b displays the HRTEM and SAED images of sample S2. Similar HRTEM images were obtained for the other samples as displayed in Figure 1c,d. In the HRTEM images, the core appeared darker than the shell and thus displaying the core/shell structure of the nanoparticles. The high crystalline nature of the cores of the nanoparticles are clearly observed in Figure 1b. The Fe\({}_{3}\)O\({}_{4}\)/\(\gamma\)-Fe\({}_{2}\)O\({}_{3}\) core/shell structure of the nanoparticles were confirmed from the selected area electron diffraction (SAED) patterns. A representative of the SAED patterns is shown in Figure 1e for sample S2. The ring type pattern shown in Figure 1e ascertains the polycrystalline nature of the sample is also confirmed by this figure. The presence of Fe\({}_{3}\)O\({}_{4}\) and \(\gamma\)-Fe\({}_{2}\)O\({}_{3}\) crystal planes forming the core/shell structure. The highest intensity peaks (311) for both Fe\({}_{3}\)O\({}_{4}\) and \(\gamma\)-Fe\({}_{2}\)O\({}_{3}\) phases were observed for \(d\) = 0.253 nm and 0.252 nm, respectively. This indicates that the (311) plane observed in Figure 1e corresponds to the superimposed planes of both the phases. The (511), (422), (311) and (220) planes of Fe\({}_{3}\)O\({}_{4}\) corresponding to \(d\) = 0.161, 0.171, 0.253 and 0.296 nm, respectively, in addition to the (321) and (221) planes of \(\gamma\)-Fe\({}_{2}\)O\({}_{3}\) corresponding to \(d\) = 0.253 nm and 0.278 nm, respectively, are all present in Figure 1e. This clearly confirms that our core/shell nanoparticles are Fe\({}_{3}\)O\({}_{4}\)/\(\gamma\)-Fe\({}_{2}\)O\({}_{3}\) nanoparticles [37].