Hydrothermal synthesis of Cu-substituted Ni ferrites: Structural, morphological, and magnetic properties

Cu-substituted Ni ferrite systems are produced following a hydrothermal synthetic route carried out at relatively mild conditions (i.e., below 200 ◦C). Different degrees of substitution were investigated, ranging from bare Ni ferrite to a complete Cu-substituted system (by replacing 100% of Ni with Cu). Morphological and structural characterisations point out that the introduction of Cu in the Ni ferrite (below 50% substitution) causes a gradual reduction of the average diameter of the ferrite polyhedral nanoparticles maintaining the Ni ferrite structure. At higher content of Cu, instead, the morphology evolves toward the co-presence of iron oxides (magnetite and hematite) and copper oxide (tenorite), without the formation of bare Cu ferrite. The partially substituted Ni ferrite nanomaterials exhibit a mostly superparamagnetic behaviour in all samples, registering a reduction of the values of magnetisation saturation by increasing the Cu content. Transmission M¨ossbauer spectroscopy performed on these nanomaterials evidenced a gradual lowering of the Fe fraction occupying the octahedral sites and a consequent increment of the Fe fraction in the tetrahedral sites. Quantitative analysis confirms the gradual reduction of the Ni content within the ferrite samples, coupled with the opposite increment of the Cu content, whereas the Fe content is lower in the case of Ni75-Cu25 sample. This particular behaviour can be associated with the influence of Cu substitution in the Ni ferrite structure, with preferential replacement of Fe from the octahedral B-sites toward the tetrahedral A-sites.