Transitions in the direction of magnetism in Ni/Cu(001) ultrathin films and the effects of capping layers

Ultrathin films of nickel exhibit an unusual sequence of transitions from in-plane to perpendicular magnetization as a function of film thickness. A sharp transition from in-plane to perpendicular magnetization is found near 7 ML thickness, followed by a gradual transition back to in-plane magnetization beginning at 37 ML. This sequence of transitions cannot be explained by the surface or shape anisotropies, both of which favor in-plane magnetization in the thickness range where perpendicular anisotropy is found. We have measured the thickness dependence of these transitions for nickel film wedges, and films capped by nonmagnetic and magnetic overlayers, to experimentally determine the surface, interface, and magnetoelastic anisotropies. We find that both the surface and interface anisotropy constants are negative (favoring in-plane magnetization), with the magnitude of the surface term being larger than that of the interface. A correlation is found between the critical thickness for misfit dislocation formation in the nickel film and a sharp transition in the coercive field. This transition is used to accurately determine the onset of a thickness dependence in the bulk magnetoelastic energy, which causes the magnetization to rotate back into the film plane. This model gives a complete description of the mechanism for the easy-axis changes at both the 7 and 37 ML thicknesses.