Ultrathin MgO Coating of Superparamagnetic Magnetite Nanoparticles by Combined Coprecipitation and Sol–Gel Synthesis

Superparamagnetic magnetite nanoparticles coated with an ultrathin (∼1 nm) magnesium oxide (MgO) layer have been synthesized by combining coprecipitation and sol–gel methods. A thorough chemical and structural characterization has been carried out by means of high resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), X-ray energy dispersive spectroscopy (EDS), dynamic light scattering (DLS), and thermogravimetric analysis (TGA). Aberration corrected HRTEM experiments with subangstrom spatial resolution have allowed us to distinguish the ultrathin MgO shell that grows epitaxially on the magnetic cores. The capability of the MgO shell to protect the magnetic nuclei from oxidation up to 600 °C has been demonstrated. The magnetic properties of the material have been studied before and after the coating procedure. The superparamagnetism of the magnetite nuclei at room temperature is preserved even after calcination. The possibility of obtaining particles of controlled size coated with an isolating layer of nanometric thickness and high thermal stability makes the combination of the two synthesis methods used in this work a starting procedure to obtain nanometric powders suitable for technological applications, such as high-frequency electronics. These particles, after the appropriated functionalization, are also potential candidates to be used in biomedical applications.