Bulk defects induced coercivity modulation of Co thin film based on a Ta/Bi double buffer layer

•Significant regulation on coercivity of Co film is researched by defect engineering.•Based on a Ta/Bi double buffer layer, coercivity is tuned with the Bi diffusion.•Increment of bulk defect intensity in film enhances pinning strength on domain wall.•Magnetic anisotropy enhancement leads to the coercivity increment simultaneously.