Evaluation of the characteristics of ferrite garnet films for magneto-optical control of materials

A setup based on a polarization microscope was developed to study the characteristics of two types of ferrite-garnet films with the chemical composition Lu1.51Ho0.56Bi0.93Fe4.1Al0.9O12. The parameters of the hysteresis loop, values of residual magnetization, and the saturation field of the studied films were determined experimentally. The domain structure period was estab-lished, and a compact device for visualizing defects in steel samples was created. Using this device, a series of experiments were conducted on test samples with cracks of various sizes, and magneto-optical images of the cracks were obtained. The hysteresis loop of the two types of ferrite-garnet films was constructed in the “magnetiza-tion coefficient-magnetic field induction” coordinates. The magnetization coefficient was cal-culated based on the relative change in the area of bright and dark domains when the external magnetic field, directed perpendicular to the film plane, was varied. The efficiency of using two types of films, grown under different technological conditions but having the same chemical composition, in non-destructive testing methods was analyzed. It was found that the investigated films had the same domain period, but their residual magnetization and saturation field values varied significantly. Diagrams of the experimental setup and the developed device, as well as the characteristics of some of their components, are presented. To generate the magnetic field in the test samples, a coil wound on a U-shaped ferrite core was used, pressed against the sample surface on the opposite side relative to the crack location. The experiments were conducted under constant magnetic fields of varying intensity. The device was tested on specially prepared test samples with pre-grown cracks of known sizes. The tests demonstrated that films with higher residual magnetization and saturation field values were more sensitive to detecting defects such as enclosed cracks.