A New Magnetooptical Effect Discovered on Magnetic Multilayers: The Magnetorefractive Effect
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J. C. JACQUET AND T. VALET THOMSON CSF, Laboratoire Central de Recherches, Domaine de Corbeville, 91 404 Orsay Cedex, France
ABSTRACT We show theoretically that the change in the magnetization structure of magnetic metallic multilayers under the application of a magnetic field shall be generally associated with a significant change of the refractive index. This constitutes a new magnetooptical effect : the magnetorefractive effect. Optical transmission measurements under an applied magnetic field through [Ni 80Fe 20/Cu/Co/Cu] multilayers, in the light wavelength region between 2 jim and 20 gim, clearly demonstrate the existence of the predicted effect and are found in reasonnable agreement with the theoretical calculations.
INTRODUCTION Metallic magnetic multilayers have been a subject of great interest in the last few years, since they exhibit unique physical properties : giant magnetoresistance [1,2], oscillatory interlayer exchange coupling [3], giant magnetothermal conductivity and giant magnetothermopower [4]. Their optical and magnetooptical properties at visible wavelengths have been also investigated, mainly in connection with magnetic data-storage applications. At such wavelengths the interband contribution to the optical constants is very significant. Thus, considering interband optical properties as resulting from a weighted average over all the electronic transitions between initial and final states, one expects some influence of the artificial layering on the optical and magnetooptical constants through quantum size effects. Reports of oscillations of the saturation Kerr rotation with the interlayer thickness in Fe/Cu/Fe [5], Fe/Au/Fe and Fe/Ag/Fe [6] sandwiches, due to the appearance of quantum well states, are among the recent demonstrations of such quantum size effects. Here we will be interested in a totally different kind of size effects appearing in the optical properties of metallic surfaces, thin films and multilayers, when either the skin depth ( SD, the decay length of the optical electric field in the metal ), or the layer thicknesses, become of the order of the conduction electron mean free path ( MFP ). These are quasiclassical effects, related to the non locality of the intraband optical conductivity on length scales shorter than the MFP. The anomalous skin effect, which was thoroughly analyzed by Reuter and Sondheimer [7], is a prototype of this second kind of size effects. When considering the specific case of an optical wave incident on a metallic multilayer, even in the normal skin effect regime MFP - 8SAL will be hardly reached at room temperature in the spectral domain of interest. From here on, we will thus assume that it is not the case. Consequently, we will further discuss the optical properties of multilayers as those of effective homogeneous metals of refractive index fiSAL. At first sight this implies a local relationship between the electric field and the induced current, nevertheless the underlying non locality of the conductivity remains a deciding factor through the spa
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