Magneto-optical leakage flux testing of steel

A magnetic garnet film was studied as a leakage flux sensor for detecting defects in a steel surface. The modification of magnetization in the garnet film by the leakage was detected magnetooptically. The perpendicular anisotropy of the garnet film is sensitive only to the normal component of the stray field. The spatial resolution of the measurement was high as a result of the small effective lateral dimensions of the film, as determined by the size of the light beam (as small as 10 μm in the present experiment). A leakage field profile from an artificial groove in a magnetized steel was therefore measurable with high accuracy. Although the sensitivity and the spatial resolution with which defects were detected decreased when the effective film area was enlarged by increasing the spacing between the film and the defect, the groove width was evaluated with an accuracy better than 15 μm