Structural origin of perpendicular magnetic anisotropy in Ni-W thin films

The magnetic properties and microstructure of electrodeposited Ni-W thin films (0-11.7 at% W in composition) were studied. The film structures were divided into three regions: an FCC nanocrystalline phase (0-2 at% W), a transition region from FCC nanocrystalline to amorphous phase (2-7 at% W), and an amorphous phase (> 7 at% W). In the transition region, (4-5at% W) films with perpendicular magnetic anisotropy (PMA) were found. The saturation magnetization, magnetic anisotropy field, perpendicular magnetic anisotropy and perpendicular coercivity for a typical Ni-W film (4.5 at% W) were 420 kA/m, 451 kA/m, 230 kJ/m and 113 kA/m, respectively. The microstructure of Ni-W films with PMA is composed of isolated columnar crystalline grains (27-36 nm) with the FCC phase surrounded by the Ni-W amorphous phase. The appearance of the interface between the magnetic core of Ni crystalline grains and the Ni-W non-magnetic boundary layer seems to be the driving mechanism for the appearance of PMA. The origin of PMA in Ni-W films is mainly attributed to the magnetoelastic anisotropy associated with in-plane internal stress and positive magnetostriction. The secondary source of PMA is believed to be the magnetocrystalline anisotropy of <1 1 1 > columnar grains and its shape magnetic anisotropy. It is concluded that Ni-W electrodeposited films (4-5 at% W) may be applicable for perpendicular magnetic recording media. (C) 2001 Elsevier Science B.V. All rights reserved.