Fabrication and characterization of small magnetic tunnel junction by atomic force microscope for observation of Coulomb blockade

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Well-defined nanostructures of oxide can be fabricated on metal thin films by applying a negative bias on a conductive atomic force microscopy (AFM) cantilever. With using this technique, planar-type magnetic tunnel junctions have been fabricated. The junction area of this structure can be much smaller than that of a conventional multilayer-type junction. This is advantageous for fabrication of single electron devices exhibiting Coulomb blockade. Ni thin film of 8-20 nm were deposited on SiO2/Si substrates by rf magnetron sputtering. The films were patterned to stripe shapes by photolithography and dry etching. Then nanowires of oxide were fabricated by the AFM nano-oxidation technique, and planar-type magnetic tunnel junctions were successfully obtained.A diode characteristic was observed in current-voltage measurement at the temperature range of 17-300K. It was found that the fabricated nanowires of oxide could be a tunnel barrier. Formally the width of the barrier was thick but the AFM nano-oxidation using tapping mode of the AFM operation realized a thinner width. The barrier height of 1.5 eV and barrier width of 1.7 nm were derived from a calculation using the Simmons formula fitting. There results are promising for ferromagnetic single electron transistors exhibiting Coulomb blockade.