Effect of Co cations and oxygen vacancy on optical and magnetic properties of SrTi_1−xCo_xO₃ nanoparticles prepared by the hydrothermal method

SrTi_1−xCo_xO₃ (x = 0, 0.05, 0.10 and 0.15) nanoparticles were synthesized by the hydrothermal method. Optical and magnetic properties based on Co cations and oxygen vacancy were studied in annealed samples. Phase and structure of all samples studied by X-ray diffraction results reveal a crystal of perovskite type with the formation of impurity phases of SrCO₃ and Co₃O₄ in as-prepared and annealed samples of x = 0.15. Images of annealed samples obtained by scanning electron microscope and transmission electron microscope reveal cubic-like nanoparticles with the increase of average particle size from 103.85 ± 1.15 to 113.14 ± 2.27 nm due to the increase of Co content. In addition, the determined optical band gaps of annealed samples from the obtained ultraviolet–visible spectra are found to decrease from 3.148 to 2.417 eV. X-ray photoelectron spectroscopy results of annealed samples with x = 0.05 and 0.10 indicate the increase of oxygen vacancy (V_o) with increasing Co content and the existence of only Co²⁺ cation in a sample of x = 0.05 and both of Co²⁺ and Co³⁺ cations in a sample of x = 0.10. XANES results of annealed samples confirm the oxidation state 2 + and 3 + of Co cations. Magnetization (M) measurements using vibrating sample magnetometer reveal paramagnetic behavior in as-prepared samples and ferromagnetic behavior in annealed samples with saturation magnetization increases from 0.48 to 3.13 emu/g, owing to the increase of Co content. F-center exchange mechanism due to Co²⁺–V_o–Co²⁺ and Co³⁺–V_o–Co³⁺ couplings are proposed for the ferromagnetism. Temperature dependence of magnetization for annealed SrTi_0.90Co_0.10O₃ sample measured in zero field-cooling and field-cooling modes indicate a Curie temperature above 280 K.