Cobalt and magnesium ferrite nanoparticles: preparation using liquid foams as templates and their magnetic characteristics

Abstract

An easy and convenient method for the synthesis of cobalt and magnesium ferrite nanoparticles is demonstrated using liquid foams as templates. The foam is formed from an aqueous mixture of an anionic surfactant and the desired metal ions, where the metal ions are electrostatically entrapped by the surfactant at the thin borders between the foam bubbles and their junctions. The hydrolysis is carried out using alkali resulting in the formation of desired nanoparticles, with the foam playing the role of a template. However, in the formation of ferrites with the formula MFe₂O₄, where the metal ion and iron possess oxidation states of +2 and +3, respectively, forming a foam from a 1:2 mixture of the desired ionic solutions would lead to a foam composition at variance with the original solution mixture because of greater electrostatic binding of ions possessing a greater charge with the surfactant. In our procedure, we circumvent this problem by preparing the foam from a 1:2 mixture of M²⁺ and Fe²⁺ ions and then utilizing the in situ conversion of Fe²⁺ to Fe³⁺ under basic conditions inside the foam matrix to get the desired composition of the metal ions with the required oxidation states. The fact that we could prepare both CoFe₂O₄ and MgFe₂O₄ particles shows the vast scope of this method for making even multicomponent oxides. The magnetic nanoparticles thus obtained exhibit a good crystalline nature and are characterized by superparamagnetic properties. The magnetic features observed for CoFe₂O₄ and MgFe₂O₄ nanoparticles are well in accordance with the expected behaviors, with CoFe₂O₄ particles showing higher blocking temperatures and larger coercivities. These features can easily be explained by the contribution of Co²⁺ sites to the magnetocrystalline anisotropy and the absence of the same from the Mg²⁺ ions.