Interplay of Magnetic and Structural Anisotropy in Co$\vert$Ni Multilayer Thin Films

Interfacial perpendicular magnetic anisotropy (PMA) is important for spin transfer devices and has been predicted to overcome dipolar shape anisotropy for [t Co$\vert$2t Ni] multilayers with thicknesses, t, of 4 $\mathring{A}$ or less [1]. layered between Cu, however, thickness dependent PMA (t=1, 2, 3, 4 and 6 $\mathring{A}$) is not sufficiently strong to produce perpendicular magnetization. Anomalous diffraction reveals that the Co and Ni are expansively strained by the Cu within the sample plane. As calculated in reference 2 this trigonal strain would be sufficient to overcome the PMA. Ferromagnetic resonance measurements [3] indicate that the net Lande g-factors are enhanced above bulk for all thicknesses, and increase further with decreasing layer thickness. By applying element-specific x-ray magnetic circular dichroism (XMCD) we have been able to study the Co and Ni individually. Both elements show increasing spin to orbit ratios with decreasing thickness magnetized either in-plane or along sample normal. In all cases the spin to orbit ratio is enhanced along the sample normal compared with the in-plane direction. \newline [1] Phys. Rev. Lett. 68, 682 (1992) \newline [2] Phys. Rev. B 69, 104426 (2004) \newline [3] Eur. Phys. J. B 59, 475 (2007)