Distance dependence of the phase signal in eddy current microscopy

TL;DR

This study investigates the distance dependence of phase signals in eddy current microscopy with magnetic tips, revealing long-range electromagnetic interactions that enable conductivity measurements at a distance from the surface.

Contribution

The paper demonstrates that the phase signal originates from long-range electromagnetic forces, challenging previous monopole approximation models and enabling conductivity detection from afar.

Findings

Long-range electromagnetic forces cause the phase signal.

Conductivity changes of 4.5×10^7 (Ω·m)^-1 are detectable at a distance.

The phase signal is not primarily due to topography or local forces.

Abstract

Atomic force microscopy using a magnetic tip is a promising tool for investigating conductivity on the nano-scale. By the oscillating magnetic tip eddy currents are induced in the conducting parts of the sample which can be detected in the phase signal of the cantilever. However, the origin of the phase signal is still controversial because theoretical calculations using a monopole appoximation for taking the electromagnetic forces acting on the tip into account yield an effect which is too small by more than two orders of magnitude. In order to determine the origin of the signal we used especially prepared gold nano patterns embedded in a non-conducting polycarbonate matrix and measured the distance dependence of the phase signal. Our data clearly shows that the interacting forces are long ranged and therefore, are likely due to the electromagnetic interaction between the magnetic tip and the conducting parts of the surface. Due to the long range character of the interaction a change in conductivity of $\Delta\sigma=4,5\cdot10^{7} (\Omega$m$)^{-1}$ can be detected far away from the surface without any interference from the topography.