Engineering magnetic phase transitions and bandgap tunability in Co-doped NiO nanoparticles for spintronic and optoelectronic applications

This study presents a comprehensive investigation of the structural, optical, and magnetic properties of cobalt-doped nickel oxide (Ni_1-xCo_xO, 0.0 ≤ x ≤ 0.08) nanoparticles synthesized via a citrate-assisted sol-gel method. X-ray diffraction (XRD) combined with Rietveld refinement reveals a doping-induced phase evolution from a mixed-phase system (NiO and metallic Ni) to a single-phase cubic NiO structure at x = 0.08. Raman and FTIR analyses confirm the incorporation of Co into the NiO lattice, along with strain-induced structural distortions and lattice disorder. Magnetic measurements demonstrate a distinct transition from ferromagnetic (FM) behaviour at x = 0.04 to antiferromagnetic (AFM) ordering at x = 0.08, attributed to Co-mediated exchange interactions and strain-induced modulation of spin correlations. Optical absorption spectra show bandgap tuning in the range of 2.9–3.5 eV, driven by defect-induced states at low Co content and quantum confinement at higher doping levels. These results highlight the crucial role of Co doping in simultaneously tailoring the structural, magnetic, and optical properties of NiO nanoparticles, establishing their potential for multifunctional applications in spintronic, optoelectronic, and photovoltaic devices.