# Spinel Iron Oxide by the Co-Precipitation Method: Effect of the Reaction Atmosphere
## 2 Materials and Methods  
Spinel iron oxide nanoparticles were synthesized by the co-precipitation method using an aqueous solution of Fe\({}^{2+}\) and Fe\({}^{3+}\) with the stoichiometric ratio 1:2, by alkalization with 30% ammonia solution. The synthesis was done according to the following operating procedure: at 60 \({}^{\circ}\)C, a mixture of 6.21 mmol FeCl\({}_{3}\).6H\({}_{2}\)O (\(>\)99%, Sigma-Aldrich, Schnelldorf, Germany) and 3.15 mmol FeCl\({}_{2}\).4H\({}_{2}\)O (\(>\)99%, Sigma-Aldrich, Schnelldorf Germany) was dissolved in 100 mL 0.01 M HCl solution (Analysis iso, 37%, [PERSON], Paris, France.) previously purified by alternating vacuum and nitrogen (N\({}_{2}\)) gas cycles three times. The N\({}_{2}\) atmosphere was maintained and the solution was alkalinized with 30% ammonium hydroxide (ACS, 30%, [PERSON], Paris, France). After two hours, the resulting precipitate was washed with distilled water in several cycles assisted by centrifugation, then dried overnight in the oven at 60 \({}^{\circ}\)C. Finally, the obtained powder was labeled as FO\(-\)CA. To verify the effect of reaction atmosphere, a second sample, labeled as FO\(-\)NA, was prepared in air, leaving the other experimental conditions unaltered. It is worth mentioning that these experiments have been repeated more than one time, (see supporting information for further details). The crystalline structure was characterized by X-ray diffraction (XRD) measurements, using a TT 3003 diffractometer ([PERSON], Ahrensburg, Germany) equipped with a secondary graphite monochromator, employing CuK\(\alpha\) radiation (\(\lambda\) = 1.5418 A). Data were collected in the 10\({}^{\circ}\)\(-\)70\({}^{\circ}\) 2\(\vartheta\) range with a step size of 0.04\({}^{\circ}\) and counting 4 sec per step. Transmission electron microscopy (TEM) analysis was carried out using a CM200 microscope (Philips, Amsterdam, The Netherlands.) operating at 200 kV and equipped with a LaB6 filament. The samples in form of powder were dispersed in ethanol using ultrasonication. A drop of suspension was deposited on a commercial TEM grid covered with a carbon thin film and maintained in air until complete evaporation of ethanol. Frequency counts of particles size are fitted with a log-normal function [29] using ImageJ software [30]:  
\[\mathrm{P}=\frac{1}{\mathrm{D}\sigma\sqrt{2\pi}}\exp\left[-\frac{\ln^{2}( \mathrm{D}/\langle\mathrm{D}_{\mathrm{TEM}}\rangle)}{2\sigma^{2}}\right] \tag{1}\]  
where \(\sigma\) is the standard deviation and \(\mathrm{D}_{\mathrm{TEM}}\) is the median of the log-normal distribution. Mossbauer spectra were recorded in transmission geometry using a \(\mathrm{{}^{57}Co/Rh}\)\(\gamma\)-ray source mounted on an electromagnetic driving unit undergoing a triangular velocity form. Experiments were carried out at 12 K using a bath cryostat. The quantity of powder is optimized at about 5 mg of Fe/cm\({}^{2}\). The fitting procedure was done using the home-made unpublished MOSFET software involving magnetic sextets and quadrupolar components with Lorentzian lines. DC magnetization measurements were performed with a superconducting quantum interference device (SQUID) magnetometer (H\({}_{\mathrm{max}}\) = 5.5 T; T = 5-400 K). The sample in solid form was fixed using epoxy resin in polycarbonate capsule to prevent any movement of the powder during the measurement. All the magnetic measurements were normalized by the real mass of the sample. Saturation magnetization value (M\({}_{\mathrm{S}}\)) was determined from the portion at high field of M=(H) curves using as a formula:  
\[\mathrm{M}=\mathrm{M}_{\mathrm{S}}\left(1-\frac{\mathrm{A}}{H}-\frac{\mathrm{B }}{\mathrm{H}^{2}}\right) \tag{2}\]  
Fe K-edge (7112 eV) XAFS measurements were carried out at the XAFS beamline of the ELETTRA synchrotron [31]. X-ray absorption spectra were measured in transmission geometry at room temperature, measuring the X-ray intensities before (I\({}_{\mathrm{o}}\)) and transmitted (I\({}_{\mathrm{t}}\)) through the sample using two gas optimized ionization chambers.