Multiple Material Property Characterization Using Induced Currents and Atomic Force Microscopy

The invention of AFM has enabled the development of eddy current microscopy techniques with better spatial resolution than that of conventional eddy current imaging systems.In magnetic force microscopy (MFM), a magnetic probe is oscillated above a magnetic surface.The oscillating magnetic probe generates eddy currents.This concept was used in the development of an MFM based eddy current microscopy (Hoffmann et al., 1998).This technique was used to image local variations in electrical conductivity of a sample consisting of TiC precipitates in Al 2 O 3 matrix with nanometer scale resolution.However, since the magnetic field of an MFM tip is small, this technique is not suitable to image small variations in conductivity.The sensitivity of this technique was improved by using large magnetic fields from a tip made ofpermanent magnet (Lantz et al., 2001).This resulted in increased sensitivity but reduced the spatial resolution down to hundreds of nanometers.From the above discussion it is evident that it is difficult to achieve both high resolution and high sensitivity to local variations in electrical conductivity using eddy current microscopy by MFM.To improve the sensitivity, a flexible cantilever capable of detecting small variations in the forces can be employed.However, in MFM techniques, a stiffer cantilever, vibrated at its resonant frequency, is used in order to make the cantilever sensitive only to the long-range magnetic forces.But by using a stiffer cantilever, it is difficult to measure small forces generated due to very small variations in the electrical conductivity.The magnetic tips used in MFM have small magnetic field strength.Therefore, the eddy current density that can be induced in the sample material is limited.