New approach to modify the magnetic properties of ultrathin Co layers by introducing a special structure of coverage layer have been recently discovered and reported . The double-wedge samples with the following structure: (i) monocrystalline sapphire substrate covered with Mo and subsequently with Au(111) buffer layer (ii) Co wedge with the thickness d increasing from 0 to 2 nm along "x" axis; (iii) a wedge of coverage metal A (A=Au,Ag,Mo,Cr) with the thickness h increasing from 0 to few nm along "y" axis; and (iv) a cap layer of metal B (B=Au,Ag), were grown by MBE technique. They were studied by polar Kerr effect in two experimental setups: (i) millimagnetometer with red laser beam, which locally probes the sample area of millimeter range size; and (ii) micromagnetometer based on an optical microscope. A strong influence of the overlayer designed geometry on both magnetic anisotropy and coercive field has been found. Assuming the exponential dependence exp(-h/h*) of the spatial distribution of both parameters on silver thickness h, one can calculate h* equal to about only 1 nm for both parameters. The size of new "chemical" domain wall constrained by the silver overlayer is theoretically analyzed, showing the possibility of wall width tuning in a wide range by changing the gradient of h. Strong modification of magnetic domain structure is observed in two regions of the sample: (i) high d, where spin reorientation transition from perpendicular to in-plane direction occurs in Co layer and (ii) low d, where magnetization remanence becomes negligible. In these two regions the fractal dimension of the domain wall, as well as the density of nucleation centers, show large increase in comparison to the in-between located region.
This work was supported by the Polish State Committee for Scientific Research (Grant No. 4 T11B 006 24) and partly by the EU "QMDS" project, contract No. HPRN-CT-2000-00134.  M.Kisielewski et al. Phys. Rev. Lett. 89, 8 (2002) 87203