Fabrication of nanometer-sized ferromagnetic probe

Abstract

A sharp needlelike metal probe can be used in a scanning tunneling microscopy (STM) for surface analysis. The probe requires a small-apex tip to maintain a tunneling current from the sample surface. These conditions give STM an atomic resolution. Such a sharp probe also can be applied in the scanning-electron microscope (SEM) for the electron-beam source from electric-field-induced tunneling-electrons. This field-induced current comprises a high brightness and coherent e-beam which gives the SEM a better resolution. Regardless of the different applications, the sharpness of the metal tip dominates the performance of these applications. Tungsten is a common material that can be etched electrochemically to an ultrasharp needlelike probe for these applications. Soft magnetic materials such as iron and nickel also can be fabricated into a similar shape but it is difficult to generate an ultrasharp probe from a simple electrochemical-etch process due to less anisotropic etching. We report a method to fabricate iron and nickel probes within 40nm of the tip’s radius. The iron and nickel wires were first electrochemically etched at the interface of an air bubble in the electrolyte. Because very high surface tension of the micron sized bubble gives a higher anisotropic etching to the wire which in results in the wire etched into needle like sharp probe. By additional sharpen process, the small air bubble acts as a sharpener, the tip was then “sharpened” into nanometer size by periodic stripping the apex of the tip surface.