Magnetic anisotropy and thermal stability study of perpendicular Co/Ni multilayers

The perpendicular magnetic anisotropy (PMA) and thermal stability of Ta (3.0 nm)/Cu (t_Cu)/[Co (0.29 nm)/Ni (0.65 nm)]_N/Ta (3.0 nm) multilayers are investigated with various Cu thicknesses (t_Cu) and Co/Ni periodic numbers (N). A relatively thick Cu seed layer and a suitable Co/Ni repetition number benefit the PMA with larger coercivity and remanence ratio. But the magnetic hysteresis loop for the structure with N in excess of 10 exhibits a bow-tie shape due to the formation of multi-domains, which may cause a significant drop of the remanence ratio. The post-annealing experiments show that the thermal stability of PMA increases with increasing N. In addition to the intermixing of Co and Ni atoms at interfaces, we consider that the diffusion of nonmagnetic atoms into the magnetic layers is also responsible for the PMA decay with increasing annealing temperature.