Near-field magneto-optics with polarization sensitive STOM

A scanning tunneling optical microscope (STOM) operating with polarized light has been developed to study thin magnetic films. The magnetic film is deposited on the external face of a prism and illuminated in total reflection conditions with linearly polarized laser light. The evanescent mode close to the magnetic film surface is detected with a tip-ending monomode optical fiber connected at its other end to a light-polarization analyzer mounted at the entrance of a photomultiplier tube. The polarization sensitivity of the whole system, which was found to depend on the tip condition, was characterized on the bare prism with s- and p-polarized excitations. The magneto-optical effect in the evanescent mode is measured through a lock-in amplifier by modulating the magnetic field produced by a coil surrounding the tip. With this set-up we have studied two different systems, both exhibiting perpendicular magnetization. The first one is a dielectric garnet film. The images, obtained on this sample by measuring the magneto-optical effect under 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 coercive field (≈ 1 kOe). The magneto-optical effect is here measured by modulating the field with an amplitude larger than the coercive field so that the saturated magnetization is periodically flipped. In this system we have taken advantage of the possibility to excite surface plasmon resonances in noble metal thin films with p-polarized light. Near-field measurements performed with our microscope demonstrate that the intensity of the evanescent mode is strongly enhanced (two 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.