An oscillating current flowing through a coil produces an oscillating magnetic field. When an electrically conducting material like a metal is brought close to the coil, the oscillating magnetic field produces eddy currents in the metallic material. The strength of the eddy current depends on the electrical conductivity of the material, the distance between the coil and the material, and the frequency of the excitation of the coil. The eddy currents in the electrical conductor produce a magnetic field opposing the magnetic field generated by the coil. The electrical impedance of the coil, placed in close proximity to a metal, is altered due to the eddy currents in the metal. Measurement of the change in the impedance is a method to determine the electrical conductivity of the metal.The presence of a defect or crack under the electromagnetic coil dramatically changes the electrical impedance compared with the same material without a defect. Measurement of the difference between the two impedances has become the basis of the development of eddy current nondestructive evaluation (NDE) of electrically conductive materials. It is possible to produce an image of the local electrical conductivity and magnetic property variations by mapping the impedance data acquired from scans of a conductive sample.
To improve the spatial resolution beyond the coil diameter, modifications to the atomic force microscope (AFM) were developed in the last decade. The magnetic force microscope (MFM) was used to generate and detect eddy currents. A magnetic tip cantilever of a MFM equipped with a piezoelectric element was brought close to an electrical conductor and was vibrated. A vibrating magnetic tip generates eddy currents in the material under the tip. The magnetic field of the eddy currents in the material opposes the motion of the magnetic tip causing damping of the cantilever. Changes in the amplitude of the motion of the cantilever are measured and mapped to obtain an eddy current image of the sample.
While eddy current imaging can be performed with low macroscopic resolution with electromagnetic coils, extremely high resolution in the tens of nanometers range has been achieved with modified AFM. This leaves a significantly large gap of spatial resolution of hundreds of nanometers to hundreds of micrometers. In this work, a new probe design based on the MFM is presented, and is shown to be a promising method to bridge this gap.
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