Eddy current microscopy

Abstract

Oscillating ferromagnetic probes, as typically used in magnetic force microscopy (MFM), induce eddy currents within conducting materials. These currents lead to an electrodynamic interaction between the probe and sample. As a consequence, the oscillation of the probe is affected, leading to a contrast in the phase, amplitude, or frequency-shift image. Eddy current imaging is highly sensitive to local variations in the material composition, even involving the analysis of subsurface features. Furthermore, it is possible to image stray fields of ferromagnetic domains and walls with nonmagnetic but conducting probes being oscillated at close proximity to magnetic samples. The latter configuration induces eddy currents within the probe, leading to images with a resolution comparable with that of MFM. In order to understand the contrast, the induced eddy current density and the resulting force between probe and sample are calculated by solving the Helmholtz equation for a dipole oscillating above a conducting surface. Theory and experimental data confirm that eddy current microscopy (ECM) is a new powerful tool providing force microscopy with a certain material-sensitive contrast and additionally permitting an absolutely nondestructive imaging of the softest magnetic materials.The quality of materials in the near-surface region can be examined by using eddy current techniques, which are well known from nondestructive evaluation [1-3]. Inhomogenities such as precipitates or scratches, can significantly change the conductivity of materials and therefore also influence the local eddy current density. Usually, eddy currents are induced by suitable coils and are then detected inductively [2,3]. In most advanced setups, SQUIDs are employed [1,3]. The spatial resolution depends on the size of emitter and receiver and is low if compared with standard microscopy. The invention of scanning tunneling microscopy (STM) in 1982 [4] and the subsequent development of related methods have opened the possibility of analyzing a variety of physical quantities of a sample by applying one uniform principle: the detection of locally varying interaction by raster-scanning a sharp probe in close proximity across the sample surface. Probably the most important breakthrough arose from the possibility of measuring forces on a local scale with atomic force microscopy (AFM) [5]. This involves the investigation of insulating samples with high resolution. Both short-and long-range forces, are accessible. The latter category involves, as a particular kind of probe-sample interaction, magnetostatic forces. If these forces are utilized, magnetic domain configurations can be imaged with sub-100-nm resolution [6,7].In magnetic force microscopy (MFM), a ferromagnetic probe is scanned at some distance across the sample. The magnetostatic force acting on the probe or a related quantity is measured and plotted versus the probe position [7]. In conventional MFM, a dither piezo is used to oscillate the probe. The magnetostatic interaction leads to a change in o...

Distance dependence of the phase signal in eddy current microscopy

“…al. [16,6]. They estimated the effect using typical parameters for ambient conditions and found for a change in conductivity of ∆σ = 10 7 (Ωm) −1 a phase shift of approximately 1…”

“…Obtaining information on the local conductivity of a sample can be achieved by using eddy current microscopy (ECM) [6,7,8]. This method is based on the well-established method of dynamic scanning force microscopy (for a review see e. g. [9,10]).…”

Multiple Material Property Characterization Using Induced Currents and Atomic Force Microscopy

“…For soft magnetic samples, the magnetization of the sample will be affected by the stray magnetic field of the tip. Therefore, the interpretation of the magnetic contrast in MFM images for such samples is difficult.This has led to the characterization of magnetic properties without a magnetic probe (Hoffmann et al, 1998). The technique uses a vibrating non-magnetic conducting tip to image the magnetic domains of a ferromagnetic sample.…”

“…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.…”

Magnetic interactions in 2D and 3D arrays

“…Dissipative forces are mainly due to air damping [Sader, 1998], which depends on tip-sample separation [Proksch et al, 1999;Schäffer et al, 2003]. Magnetic dissipation via eddy currents [Hoffmann et al, 1998] or switching of the tip or sample through minor hysteresis loops [Proksch et al, 1999] are expected to have smaller effects.…”