A scanning near-field optical microscope for the imaging of magnetic domains in reflection

The design and implementation of a scanning near-field optical microscope expressly intended for the imaging of magnetic domains in reflection with subwavelength resolution is described. The microscope employs similar to 40 nm silver particles optically excited at the plasmon resonance as the near-field probes. Special attention is given to the Newton ring interferometer used to regulate the separation between the near-field probe and the sample. The results of two tests of the microscope performance are presented. The first is a measurement of the dependence of the near-field magneto-optic Ken effect on probe/sample spacing for a longitudinally magnetized metallic thin film. The effect decreases rapidly with increasing separation with a length scale comparable to the probe size. The second is a series of images of a domain in a perpendicularly magnetized multilayer metallic film at different probe/sample spacings. The images indicate a persistent effect with increasing separation but diminishing resolution. Fundamental differences between the near-field magneto-optic Ken effect for perpendicularly and longitudinally magnetized samples is discussed. An estimate is made as to the fundamental limit of resolution with this particular form of near-field microscopy. (C) 1996 American Institute of Physics.