Magnetic domain imaging with a scanning Kerr effect microscope

A method for imaging magnetic domains with spatial resolution of less than 0.5 µm is described. The method employs the magnetooptic Kerr effect and is applicable for observing surface domain structures, even if the magnetic surface of interest is covered by a thick transparent overcoat. Polarized laser light, incident at an oblique angle, is focused to a diffraction limited spot onto a surface whose magnetization is modulated at a fixed frequency by a magnetic field. The modulation in the polarization of the light due to the modulation of the magnetization beneath the spot is measured with the use of a lock-in amplifier. An image of the magnetization changes is then constructed by scanning the spot. Domains are revealed because areas of domain wall motion have larger magnetization changes than areas of magnetization rotation within domains. Since this approach minimizes noise and eliminates nonmagnetic signals, the domain image contrast is greatly enhanced. With 0.3 mW of laser power on a spot and a detection bandwidth of 10 Hz, signal to noise ratios of 65 db and a magnetization rotation sensitivity of 0.06° have been obtained in permalloy. Results of domain images in thin-film inductive heads are presented. This technique is ideally suited for the study of the dynamics of wall motion and magnetization rotation in magnetic films.