NEAR-FIELD MAGNETOOPTICS WITH POLARIZATION-SENSITIVE STOM

A scanning tunneling optical microscope (STOM) operating with polarize d light has been developed to study thin magnetic films. The magnetic film is deposited on the external face of a prism and illuminated in t otal reflection conditions with linearly polarized laser light. The ev anescent mode close to the magnetic film surface is detected with a ti p-ending monomode optical fiber connected at its other end to a light- polarization analyzer mounted at the entrance of a photomultiplier tub e. The polarization sensitivity of the whole system, which was found t o depend on the tip condition, was characterized on the bare prism wit h s- and p-polarized excitations. The magneto-optical effect in the ev anescent mode is measured through a lock-in amplifier by modulating th e magnetic field produced by a coil surrounding the tip. With this set -up we have studied two different systems, both exhibiting perpendicul ar magnetization. The first one is a dielectric garnet film. The image s, obtained on this sample by measuring the magneto-optical effect und er very low amplitude of the external magnetic field modulation, show up submicronic details due to magnetic domain wall motion. The second system is a metallic 25 nm Au/1 nm Co/4 nm Au sandwich with a large co ercive field(approximate to 1 kOe). The magneto-optical effect is here measured by modulating the field with an amplitude larger than the co ercive field so that the saturated magnetization is periodically flipp ed. In this system we have taken advantage of the possibility to excit e surface plasmon resonances in noble metal thin films with p-polarize d light. Near-field measurements performed with our microscope demonst rate that the intensity of the evanescent mode is strongly enhanced (t wo orders of magnitude) at resonance. Moreover, the interaction of the light electric field with the gold surface plasmon leads to a related amplification of the magneto-optical effects in the evanescent mode.