Magnetic properties of Co/Pt multilayers deposited on silicon dot arrays

Arrays of silicon dots down to 200 nm in size have been prepared by standard lithography and etching techniques, and then covered by different (Co-0.5 nm/Pt-1.8 nm) multilayers which exhibit perpendicular magnetic anisotropy. On unpatterned substrates, the coercive field varies between 120 and 400 Oe, depending on the buffer thickness and on the number of layers. Magnetic force microscopy (MFM) images show magnetic domains about 1.2 mum in size. Deposition of these multilayers on patterned silicon substrates shows that the profile of the magnetic dots is the same as the initial Si dot profile even when the amount of magnetic material represents 60% of the dot height. Atomic force microscopy cross sections indicate that a negligible amount of material is deposited on the side walls of the dots. MFM images of arrays with a dot spacing large enough to explore the bottom of the grooves show that the magnetic domains in areas between the dots are of the same size as on the unpatterned area, although their coercive field is increased from 170 to 300 Oe. On the top of the dots, single-domain configurations are observed, and the coercive field of the dots ranges from 1600 to 2400 Oe. This distribution of switching fields, which only weakly depends on the dot size, is mainly related to the detailed shape of the dots (in particular the sharpness of the corners) and not to their magnetostatic interaction. This confers to each individual dot a well-defined coercive field. The magnetization reversal of a given dot weakly influences its first neighbors, which means that the dots are essentially independent of each other for the range of Co and Pt thickness used in this study. In the same way, no significant coupling is mediated by the continuous magnetic layer in the bottom of the grooves. These results are confirmed by micromagnetic calculations of the magnetic dot-dot and dot-groove interactions for the various geometries that we have investigated.