Microstructure and magnetic properties of Co-Ni thin film sputtered in nitrogen atmosphere

It was already reported that the coercivity of the Co-Ni alloy thin film deposited at Ar plus N 2 sputtering atmosphere increases with the increase of the N 2 partial pressure after heat treatment in vacuum [1] [2] [3]. The change of crystalline grain size, crystalline structure and magnetic domain patterns of the film for the varied N 2 partial pressure were investigated by transmission electron microscopy technique. It was found that after 340°C heat treatment, crystalline structure transforms to hcp single phase and grains are completely separated when the Co-15 Ni (at%) films were sputtered at N 2 partial pressure greater than 75% in total pressure of 8m Torr. Foucau-lt mode and defocused mode imaging of magnetic domain patterns showed the decrease of domain width in accordance with the increase of N 2 partial pressure. The coercivity increase with N 2 partial pressure seems to be closely related to the formation of Co-Ni hcp structure which has large uniaxial magnetic anisotropy constants and to the separation of the grains and possibly to the crystalline defects in the grains.