Optical analysis of magnetic microstructures

Abstract

Encoding information using magnetic microstructures is utilised e.g. in magnetic stripe cards or banknotes. In order to analyse these structures quickly and non-destructively a magneto-optical setup based on the Faraday effect can be used. The Faraday effect states that the plane of polarisation of polarised light is rotated when it passes a Faraday-active material and is subjected to an external magnetic field. Measuring the change of intensity of light passing a set of polarisers allows the calculation of the change of the polarisation angle, which in turn allows drawing conclusions on the magnitude of the external magnetic field. A first setup yielded very good results in the high-speed analysis of patterns with a structure size of about 50 μm. However, the setup's low amplitude resolution only allowed qualitative measurements. In order to find the limitations of this measurement principle with respect to amplitude, temporal, and spatial resolution as well as their interdependence a new setup was purpose-built for characterisation. Its components were examined closely and various methods of signal enhancement were evaluated. The measurements displayed long- and short-term temporal as well as spatial dependencies. The subsequent enhancement of the signal's amplitude resolution came with a loss in temporal or spatial resolution and vice- versa. The interdependence of amplitude, spatial, and amplitude resolution was characterised further and from this a generalised description of their lower bound for a given set of parameters was derived. This should serve as an estimate of the feasibility as well as a build guideline of a similar setup.