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. This paper reports the effect of Ta/Bi double buffer layer on the magnetic properties of Co thin film. By inserting a Bi layer into the Ta/Co interface of Ta/Co/Ru, the coercive field is increased by about 6 times. Moreover, a further increase of coercivity is observed for the Ta/Bi/Co/Ru film after 450 ℃ annealing. X-ray photoelectron spectroscopy (XPS) reveals that the Bi atoms diffuse to film surface through the cobalt layer. Positron annihilation spectroscopy (PAS) indicates that the diffusion of Bi atoms promotes the formation of vacancies, which may increase the density of bulk defects in the film and enhance the pinning strength of the magnetic domains of Co. Besides, high-resolution transmission electron microscopy (HRTEM) results confirm the better crystallization of Co layer with a uniform orientation due to the Bi diffusion, which increases the energy barrier for nucleation and movement of the reversal domain. These two reasons lead to the increase of coercivity simultaneously. This finding provides a feasible avenue for the tunable coercive field by the defect engineering.