Effect of nitrogen incorporation on the ordering transformation of CoPt in CoPt/TiN bilayer films

Highlights: • Nitrogen incorporation effects on CoPt ordering transformation are investigated. • Superabundant vacancies inside the CoPt is proposed. • Ordering parameter and perpendicular coercivity are improved by incorporating nitrogen. • Reduction in ordering temperature is also observed. - Abstract: The effect of nitrogen incorporation on the L1₀ ordering transformation of CoPt thin films deposited by dc magnetron sputtering on MgO substrate with/without TiN seed layers has been investigated. Without TiN seed layer, CoPt film is subjected to a tensile stress, and the perpendicular magnetic anisotropy (PMA) decreases with N₂ gas flow ratio. This is due to the degradation of tensile stress, which is proven to be beneficial to the ordering transformation of L1₀ CoPt. On the contrary, when sputtered on TiN seed layer, CoPt film is subjected to a compressive stress and the PMA increases with N₂ gas flow ratio. The increase of PMA is due to the increase in equilibrium vacancy concentration, which also promotes the diffusion of Co and Pt atoms during annealing. On quartz substrate, the maximum perpendicular coercivity of 11 kOe was obtained at the CoPt/TiN bilayer films with 50% partial N₂ gas flow ratio after post annealing at 700 °C. By partially flowing nitrogen during the deposition, a 200 °C reduction of ordering temperature was also obtained.