We present a high resolution electrical conductivity imaging technique based on the principles of eddy current and atomic force microscopy (AFM). An electromagnetic coil is used to generate eddy currents in an electrically conducting material. The eddy currents generated in the conducting sample are detected and measured with a magnetic tip attached to a flexible cantilever of an AFM. The eddy current generation and its interaction with the magnetic tip cantilever are theoretically modeled using monopole approximation. The model is used to estimate the eddy current force between the magnetic tip and the electrically conducting sample. The theoretical model is also used to choose a magnetic tip-cantilever system with appropriate magnetic field and spring constant to facilitate the design of a high resolution electrical conductivity imaging system. The force between the tip and the sample due to eddy currents is measured as a function of the separation distance and compared to the model in a single crystal copper. Images of electrical conductivity variations in a polycrystalline dual phase titanium alloy (Ti-6Al-4V) sample are obtained by scanning the magnetic tip-cantilever held at a standoff distance from the sample surface. The contrast in the image is explained based on the electrical conductivity and eddy current force between the magnetic tip and the sample. The spatial resolution of the eddy current imaging system is determined by imaging carbon nanofibers in a polymer matrix. The advantages, limitations, and applications of the technique are discussed.

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Eddy current testing is a widely used nondestructive evaluation technique of materials. This technique offers measurement of electrical conductivity and magnetic permeability of materials. Since the presence of defects significantly changes the electrical conductivity, eddy current testing is widely used to detect defects, cracks in materials. The resolution in eddy current imaging is mainly limited by the diameter of the sensor, which is most often greater than 500 μm. This paper presents development of an eddy current microscopy based on an Atomic Force Microscope (AFM) to image local variations in the electrical conductivity of materials with submicron resolution. The newly developed methodology has been used to characterize materials with varying electrical conductivities. Simultaneously acquired AFM surface topography images and eddy current images of the same region of the samples are presented. The contrast observed in eddy current images is explained based on the local electrical conductivity variations in the materials. The advantages and limitations of the AFM based eddy current imaging to study the electrically conductive materials are discussed.