Structural and Magnetic Characterization of Transition Metals-Substituted Cobalt Ferrite Nanocrystals

Cobalt ferrite is a versatile nanomaterial that has been explored due to its wide range of potential applications (technological, biomedical, environmental, among others). It can be synthesized using different methods such as hydrothermal, sol-gel, and coprecipitation. Ferrites at the nanoscale can be optimized by tuning characteristics such as: crystal size, composition, cationic distribution, and shape. The cationic distribution and interaction between them will affect and modify the magnetic properties of ferrites. Accordingly, transition metal (TM) ions were used to substitute Co ions according to TM_xCo_1-xFe₂O₄, where ‘x’ corresponds to the atomic fraction of transition metal. Ni²⁺ and Mn²⁺ were selected as Co substituting species based on the similitude between the ionic radius of Co²⁺(0.82 Å), Ni²⁺ (0.78 Å), and Mn²⁺(0.91 Å). In addition to the ionic radii, the coordination number and magnetic nature of Mn and Ni species were considered. All ferrite samples were prepared by the conventional coprecipitation method. Mn and Ni atomic fractions, ‘x’, between 0.0 and 1.0 were evaluated.<br/>Introducing foreign ions into the spinel structure will cause a change into the superexchange interaction (occurs via intermediate atoms or ions, in this case, oxygen). It is a type of inter-particle interaction that allows the magnetic moments in a solid to interact with each other. This interaction will vary depending on the element causing changes into the magnetic properties (coercivity and saturation magnetization) due to different electronic configuration. X-Ray Diffraction (XRD) and Vibrating Sample Magnetometry (VSM) results confirmed the strong influence of crystal size, ferrite composition, and lattice distortion on the corresponding magnetic properties at the nanoscale. XRD results suggest there was no a remarkable crystal growth by increasing the incorporation of Mn ions. The substitution of Co²⁺ by Mn²⁺ caused an increment in the cubic lattice parameter, ‘a’, attributed to the slightly larger ionic radius of Mn respect to Co. In contrast, the incorporation of Ni ions systematically increased the average crystallite size from 11 nm to 18 nm. A decrease in lattice parameter was observed as the Ni atomic fraction increased. It is attributed to the smaller ionic radius of Ni respect to Co. VSM results show a tailor in coercivity and saturation magnetization. Coercivity values between 14 and 2583 Oe, as well as saturation magnetization values ranging between 10 and 67 emu/g were obtained.