Tuning the optical band gap and magnetization of oleic acid coated CoFe2O4 NPs synthesized by facile hydrothermal route

•A facile hydrothermal synthesis of oleic acid coated CoFe2O4 was achieved at various temperatures.•Structural and EDX analyses confirm the phase and composition in final products.•A good energy gap tuning was noticed with the particle size.•An enhancement in Ms was noticed from 44.64 (OCF-60) to 76.66 (emu/g) (OCF-180)•A strong correlation between spin-spin and spin-phonon coupling was observed. The high-quality oleic acid coated cobalt ferrite (OCF) nanopowders were prepared by hydrothermal route at 60 °C, 120 °C and 180 °C temperatures. The average crystallite sizes of 5.2 ± 0.07 nm, 13.3 ± 0.19 nm and 18.9 ± 0.23 nm were evaluated for OCF-60, OCF-120 and OCF-180. The SEM images of OCF exhibits agglomerated nanoparticles of roughly spherical shape. Optical energy gap was estimated and found to be enhanced from 1.8 to 2.3 eV with decreasing particle size. The area ratio Av2 of phonon modes occurred at 447 and 600–613 cm−1 are correspond to octahedral and tetrahedral sites, respectively. The enhancement in saturation magnetization MS was observed from 44.64 to 76.66 (emu/g) owing to decrease in area ratio Av2. Value of spin-spin correlation function was found to be decreased from 1.60 to 0.93 with increase in MS/size. A strong correlation between magnetic and optical properties with particle size was observed.