Magnetic susceptibility imaging as a new approach towards characterization and testing of para- and diamagnetic materials
Magnetic nondestructive testing (NDT) methods are mainly applied to characterize and inspect ferromagnetic or, at least, electrically conductive materials. However, all materials interact with a magnetic field to some extent. This interaction is generally quantified by the magnetic susceptibility X, which is the proportionality factor between magnetization M and applied magnetic field strength H. In the group of non-ferro- and non-ferrimagnetic materials, one can distinguish between para- (0 < X < 1) and diamagnetic (-1 < X < 0) behavior. This group comprises (besides many others) all kinds of plastics, glass, carbon and non-ferrous metals. It is legitimate to assume that there is a correlation between local X value and local anomalies of material composition, deformation and stress, which makes it interesting to explore related sensor principles with a potential for NDT application. The challenge is that for para- or diamagnetic materials, c can be ten orders of magnitude smaller than in the case of ferromagnetic materials, which significantly restricts the choice of sensors. The most sensitive options are either SQUIDs (superconducting quantum interference devices) that require cryogenic cooling and are expensive, or precision balances that measure the magnetic force acting upon the substance under test when it is exposed to a magnetic field gradient. The effect in principle is well known and has already been used for levitation experiments. This contribution however concerns a novel magnetic force based sensor for laterally resolved susceptibility measurement and demonstrates the possible range of NDT applications on the example of different non-ferromagnetic material samples.