On the contrast in eddy current microscopy using atomic force microscopes

A magnetic probe of an atomic force microscope oscillating above a sample i nduces eddy currents within conducting materials. The resulting electrodyna mic interaction between sample and probe influences the probe vibrations so that local variations in conductivity of a sample might be imaged with an atomic force microscope by phase shifts and/or amplitude changes of the pro be vibrations. A quantitative evaluation of this effect allows one to judge whether the local conductivity can be measured. This paper presents the ca lculation of attainable contrast in phase and in amplitude images by measur ing the high-frequency flexural vibrations of a rectangular beam cantilever . For the sensor tip dimensions of available cantilevers, the dipole approx imation turned out not to be valid but the monopole approximation yields a reasonable estimation of the contrast. Calculations based on the monopole a pproximation show that the amplitude and the phase changes of the cantileve r vibration ensuing from samples with conductivity changes of 20 MS m(-1) a re below the thermal noise level. Copyright (C) 1999 John Wiley & Sons, Ltd .