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 f rom in-plane to perpendicular magnetization as a function of film thic kness. A sharp transition from in-plane to perpendicular magnetization is found near 7 ML thickness, followed by a gradual transition back t o in-plane magnetization beginning at 37 ML. This sequence of transiti ons cannot be explained by the surface or shape anisotropies, both of which favor in-plane magnetization in the thickness range where perpen dicular anisotropy is found. We have measured the thickness dependence of these transitions for nickel film wedges, and films capped by nonm agnetic and magnetic overlayers, to experimentally determine the surfa ce, interface, and magnetoelastic anisotropies. We find that both the surface and interface anisotropy constants are negative (favoring in-p lane magnetization), with the magnitude of the surface term being larg er than that of the interface. A correlation is found between the crit ical 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 m agnetoelastic energy, which causes the magnetization to rotate back in to the film plane. This model gives a complete description of the mech anism for the easy-axis changes at both the 7 and 37 ML thicknesses.