# Spinel Iron Oxide by the Co-Precipitation Method: Effect of the Reaction Atmosphere
## 1 Introduction  
Spinel ferrite magnetic nanoparticles (MNPs) are attracting increasing interest due to the numerous areas of application that they have potential uses in, including biomedicine [1, 2, 3], magnetic resonance imaging (MRI) contrasting agents [4], drug delivery [5, 6], and wastewater treatment [7]. A characteristic property exploited in all these applications is their superparamagnetism (SPM) [8]. Typically, decreasing particle size until a critical value r\({}_{c}\), induces the transition of the material from a magnetic multi-domainstate (MD) to a magnetic single domain state (SD) [9]. On decreasing the particle size, the energy barrier, \(\Delta\)E (i.e., the energy required for the magnetic moment to flip between two minima) value becomes comparable to or smaller than that of thermal fluctuations (k\({}_{\text{B}}\)T) giving rise then to the so called superparamagnetic (SPM) regime in the system. Among the spinel ferrites, the most commonly used are magnetite (Fe\({}_{3}\)O\({}_{4}\)) and maghemite (\(\gamma\)-Fe\({}_{2}\)O\({}_{3}\)) due to their interesting properties such as high saturation magnetization, low toxicity and chemical stability [10]. However, the morpho-structural features and the magnetic properties of such materials are strongly dependent on the synthesis method used. Accordingly, several synthesis methods have been reported in the literature such as sol-gel [11], thermal decomposition [12, 13], microemulsion [14], and co-precipitation [15, 16, 17, 18]. Among them, co-precipitation is one of the most used methods due to the favorable synthetic conditions (i.e., high reaction yield, safe and inexpensive precursors and solvent). The effect of several synthesis parameters (e.g., type of precipitating agent, stoichiometric ratio, temperature, etc.) on the chemical composition, morphology, size distribution and magnetic properties of MNPs have been investigated [19, 20, 21, 22, 23, 24], but few, and often contradictory works, on the effect of reaction atmosphere have been carried out. It has been reported that a normal reaction atmosphere (i.e., air), leads to higher saturation magnetization (M\({}_{\text{s}}\)) and narrower size distribution [25, 26]. Other studies indicate that a higher M\({}_{\text{s}}\) and a sharper size distribution were obtained using a controlled reaction atmosphere (i.e., inert precipitation medium) [27, 28]. Accordingly, this paper focuses on the effect of reaction atmosphere on the morpho-structural and magnetic properties of spinel iron oxide nanoparticles. To clarify the contradictory data in the literature, a multi-technique approach has been used to investigate the effect of reaction atmosphere on the morpho-structural and magnetic properties of the materials. Spinel iron oxide nanoparticles have been prepared by a co-precipitation approach using air and an inert (i.e., nitrogen) reaction atmosphere. The thorough characterization of materials just shows small differences in morphological properties that can been explained by the influence of the reaction atmosphere on the formation mechanism of the nanoparticles.