# Spinel Iron Oxide by the Co-Precipitation Method: Effect of the Reaction Atmosphere
## 3 Results and Discussions  
### Morpho-Structural Properties  
XRD patterns (Figure 1) of both samples were indexed according to the spinel iron oxide (FO) structure (JCPDS card No.75-449). No extra phase has been detected and an average crystallite size of 9.0(9)nm was obtained for both samples by the Williamson-Hall method [34] (Table 1). To investigate the magnetic structure of the sample Mossbauer spectroscopy under intense magnetic field (8T), applied parallel to the \(\gamma\) -beam has been recorded at low temperature (12 K) (Figure 2). When Mossbauer spectra of ferrites are recorded under an intense magnetic field, the magnetic splitting of the A-site Fe\({}^{3+}\) ions increases and the B-site splitting decreases. Therefore, the applied field makes it possible to separate the A and B patterns and to obtain information about the inversion degree. For a thin sample, where thickness effects are negligible and where the direction of the hyperfine field is at random with respect to the gamma ray direction, the relative area of the Zeeman sextet are in the ratio:  
\[3:2\text{p}:1:1:2\text{p}:3 \tag{3}\]  
where p is dependent on the canting angle \(\vartheta\):  
\[\text{p}=\frac{2\sin^{2}\vartheta}{1+\cos^{2}\vartheta} \tag{4}\]  
By normalizing the total area to 1, the area of lines 2-5 (A\({}_{2,5}\)) is equal to:  
\[\text{A}_{2,5}=\frac{1}{2}\sin^{2}\vartheta \tag{5}\]  
In case of the absence of the canting effect, lines 2-5 disappear [35; 36]. Fitting parameters are reported in Table 2. Spectra of both samples clearly show the inverse spinel structure of the particles. The high oxidation degree of magnetite was confirmed by the low isomer shifts (8) values [37]. Indeed, the mean isomer, which probes the electronic density at the \({}^{57}\)Fe nuclei, that is the valence state, are consistent with the presence of pure Fe\({}^{3+}\) ions for both samples. The measurement at 77 K (see supporting information for further details) confirm this scenario with an isomer shift (0.45 mm/s) is about compatible with the presence of maghemite. X-ray Absorption Near-Edge Structure (XANES) spectroscopy measurements have also been performed on the FO\(-\)CA sample further confirming the presence of mainly maghemite (see supporting information for further details). The second and fifth lines have a non-zero intensity, indicating the presence of a non-collinear (canted) structure for iron magnetic moments with respect to the applied \begin{table}
\begin{tabular}{c c c c} \hline
**Sample** & **D\({}_{\text{XRD}}\) (nm)** & **D\({}_{\text{TEM}}\) (nm)** & **D\({}_{\text{m}}\)(nm)** \\ \hline FO\_CA & 9.0(9) & 9.0(9) & 11(1) \\ \hline FO\_NA & 9.0(9) & 11(1) & 12(1) \\ \hline \end{tabular}
\end{table} 
Table 1: Crystallites size from X-ray Diffraction (DXRD), particle diameter from TEM (D\({}_{\text{TEM}}\)) and magnetic diameter (D\({}_{\text{m}}\)) for FO-CA and FO-NA nanoparticles, uncertainties on the last digit are given in parentheses. Figure 1: XRD patterns of samples synthesized in controlled atmosphere, nitrogen (FO-CA, solid line) and in normal atmosphere, air (FO-NA, dashed line). field. The canting angle(\(\theta\)) of iron spins in tetrahedral and octahedral sites for FO\(-\)CA and FO\(-\)NA is relatively small suggesting the high crystallinity of the particles. Sample morphology has been investigated by transmission electron microscopy. Figure 3a,b show bright field TEM images showing general views of FO\(-\)CA and FO\(-\)NA, respectively. Samples contain aggregates of nanoparticles whose dimensions have been deduced by TEM images and statistically analyzed. A log-normal size distribution was obtained for both FO\(-\)CA and FO\(-\)NA, (see supporting information for further details) with mean size equal, within experimental error, for both samples, (Table 1). To further investigate the crystallographic structure, selected area electron diffraction (SAED) measurements have been performed on FO\(-\)CA and FO\(-\)NA.