Magneto-optical patterned structure for magnetic field sensing

Abstract

In non-destructive testing, the magnetic yokes in combination with Hall sensors are frequently used. Such types of sensing circuits are suitable for defect specification with axial geometry lower limit about one hundred micrometers. For inspection of smaller cracks and breaks (less than 10 micrometers) ,the Bi-garnet strip detectors are available. These structures can bring the information related to defect position, but the specification of the defect shape seems to be problematic. One of the possible solutions of this problem can be the using of patterned magnetic thin film structure, which would allow to determine the quality of the inspected body (the number of defects) and in the same time to solve the problem of defect shapes. Such structure can consist of thin magnetic film with in-plane magnetic anisotropy. The anisotropy can be influenced by external magnetic field (for example by stray field from a crack) -the result is the rotation of magnetization in the film plane. This effect can be detected using magneto-optical techniques. When the shape of the crack is in question, the film patterned into 2D grating could respond by the changes of magneto-optical diffraction (polarization properties of the light in various diffraction orders are changed) . The paper is oriented on the experimental study of such structure performed by means of magneto-optical Kerr ellipsometry. Magnetic properties were studied by Kerr (reflection) magneto-optical ellipsometry at 670 nm using differential intensity detection. The measurements were realized in various diffraction orders and the results were compared with the magneto-optical data obtamed from continuous layer. Experimental results and theoretical models support the idea to apply periodical structures with magnetic ordering as sensor unit in diagnostic and measurement processes.