Use of a bulk high Tc magnetometer for non-destructive evaluation

A novel use of high-temperature superconductors for detecting defects in ferromagnetic and nonferromagnetic materials is reported. The magnetometer uses the even harmonics generated in a sample of YBaCuO when it is driven by an audio-frequency magnetic field. These harmonics arise from the nonlinear modulation of the critical flux state as vortices are swept through the intergranular spaces of the superconductor, and their amplitude is strongly dependent on DC magnetic field. Two samples of YBaCuO of identical geometry are used and placed in coaxial coils in the form of a gradiometer and driven at 10 kHz. However, a defect (large change in μ_r) in a nearby piece of ferromagnetic material produces a distortion in the local magnetic field (either the earth´s field or 30-Hz modulation field) which can be detected in the second harmonic output. For nonferromagnetic conductors, defects become observable through the field distortions caused by eddy currents