Magnetization reversal of nanostructured Co/Pt multilayer dots and films studied with magnetic force microscopy and MOKE

Large scale periodic arrays of magnetic nanostructures consisting of e.g. Co/Pt multilayer dots with perpendicular magnetic anisotropy are possible candidates for future high-density magnetic storage media. We have fabricated periodic dot-arrays by optical interference lithography with Ar⁺ ion lasers operating at wavelengths of 457 nm and 244 nm, respectively. Periodicities range from 125 nm to 1100 nm with dot diameters between 70 nm and 740 nm covering a total area of up to 20 cm² with a maximum dot density of about 4.1×10¹⁰ dots/in². The global magnetic properties of these Co/Pt dot-arrays and of corresponding homogeneous films are investigated by the magneto-optical Kerr effect (MOKE) in polar geometry. Results of the magnetization reversal are compared to magnetic force microscopy (MFM) investigations in external magnetic fields of up to H=1 kOe perpendicular to the sample. A simple MFM-data analysis is described which allows to determine hysteresis loops from a series of MFM images obtained in various external magnetic fields. This then allows to measure the coercivity H_c of a given sample on small length scales. A comparison with MOKE hysteresis loops shows good agreement, although the unknown hysteretic behavior of the tip cannot be separated. The MFM-image analysis described therefore stands as a semi-quantitative method, which is however useful to measure relative changes of the coercivity on small length scales