Surface Enhanced Magneto-Optics in Noble Metal / Ferromagnetic Metal Multilayers
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de Physique de la Mati~re Condens6e, Ecole Polytechnique,
91128 Palaiseau Cedex, France. **Institut d'Optique Th6orique et Appliqure, Centre Universitaire, Bat. 503, 91403 Orsay Cedex, France. ABSTRACT We present an electromagnetic enhancement mechanism for the magneto-optical response of noble metal / ferromagnetic metal multilayer thin films. When such a structure is illuminated in total reflection condition, the resonant coupling of light with the noble metal surface plasmons gives rise to an amplification of the magneto-optically induced component of the light electric field. The experimental results obtained on a 30nm-thick Au / Co / Au model system show that this resonant feature observed in the Kerr rotation and ellipticity corresponds to a strong enhancement of the magneto-optical figure of merit and signal-to-noise ratio.
INTRODUCTION Enhancing magneto-optical (MO) effects stimulates a wide interest as it may promote new capabilities of optical techniques for studying magnetic material properties and because of the possible applications to information storage. Any approach to enhance the MO response (i. e. the MO figure of merit) of a structure involving a given magnetic material must account for two necessary conditions : the media must provide a high optical response (reflectivity) and a 12 large (Kerr) rotation or ellipticity. , Here we show that these two required conditions can be satisfied in simple systems such as noble metal / ferromagnetic metal multilayer thin films which are already known to exhibit large MO effects. 3 Our experimental and theoretical results obtained on a model Au / Co / Au structure4 evidence that a resonance-like characteristic feature is observed in the MO effects at the angle of incidence beyond the total reflection limit where the noble metal (gold) surface plasmon modes 5 couple with the p-component of the evanescent light electric field. This feature results in a strong enhancement of the Kerr rotation and ellipticity and above all of the MO figure of merit and of the signal-to-noise ratio. EXPERIMENT The sample is a Au / Co / Au film of total thickness d = 30 nm, grown on a glass plate under UHV conditions. 6 The thicknesses of the substrate and cap gold layers are respectively d' = 25 nm and d" = 4 nm. The I= 1 nm-thick cobalt layer has its easy-magnetization axis perpendicular to the film plane and exhibits a square hysteresis loop. In our experiment,4 the glass plate is optically coupled to a half-cylindrical glass prism 499 Mat. Res. Soc. Symp. Proc. Vol. 384 ©1995 Materials Research Society
with adapting refractive index liquid. Linearly polarized light of wavelength 647 nm (the red line of a krypton-ion laser) illuminates the sample through the prism at a variable incident angle 0. The intensity and polarization of the reflected beam are analyzed. The MO measurements are performed by specific modulation technics. The saturated magnetization of the Co-layer is periodically flipped by applying an alternative pulsed magnetic field perpendicular to the film plane
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