Magneto-optical imaging and current profiling on superconductors

Magneto-optical imaging (MOI) is a useful and highly versatile technique for the investigation of magnetic and current-carrying properties of superconductors. High-speed imaging is particularly important as superconductors often exhibit interesting magnetic behaviour over short timescales, such as the behaviour of individual flux vortices during magnetic flux penetration. The following method was developed to facilitate the high-speed acquisition of magneto-optical images of superconducting samples, and the determination of magnetic field and current flow data from these images. This method may be the first to allow current mapping of short timescale magnetic events in superconductors. Magneto-optical images were acquired using a high-speed camera while monitoring sample conditions. These images were calibrated to obtain quantitative magnetic field data, which was then analysed to create images of the supercurrent distribution in the sample. For this purpose, an inverse Biot-Savart law process was applied, followed by an iterative in-plane field correction. The most efficient and accurate mathematical formulae for current calculation were derived from critical analysis and testing of formulae described in the literature. Image acquisition, sample monitoring, quantification and current calculation were all undertaken using purpose-built LabVIEW programs. The imaging procedure was then applied to model images and to samples of YBa2Cu3O7-δ to test the method developed. Upon application, magnetic fields in the samples were observed and analysed, and current behaviour was mapped visually from the magnetic field images.