We studied the preparation of nanometer-sized magnetic dots on MgO tunnel barriers. The samples were formed by depositing Fe thin layers, which exhibited a 3D Volmer-Weber growth on MgO (001) substrates. The current-voltage characteristics were then measured using a scanning tunneling microscope. By comparing different preparation techniques such as the oxidation of a metallic Mg layer and direct deposition of MgO by electron beam evaporation, we found that direct deposition was the best way to achieve flat MgO surfaces and assemblies of dots with uniform heights. The growth of the Fe dots seems to be mainly governed by the density of surface defects of the buffer, which act as nucleation sites. The density of these defects appears to be modified by the insertion of an iron electrode prior to the deposition of the MgO layer.
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