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Importance of the interband contribution to the magneto-refractive effect in Co/Cu multilayers

The optical properties of Co/Cu multilayers are investigated theoretically using a multiband tight-binding model and the results are compared to experimental data on the magneto-refractive effect (MRE). The optical conductivity of both parallel and antiparallel configurations of Co/Cu multilayers is calculated using the Kubo–Greenwood formula. The Kramers–Kronig relations are utilized to obtain the imaginary part of the conductivity and the real part of the dielectric function. The conductivity is decomposed into the intraband and interband terms, so that their different contributions to the MRE may be analysed. In particular, we find that the competition between the intraband and interband contributions leads to a change of sign in the MRE as a function of frequency, a feature which is also observed experimentally in the infrared region. This is in contrast to the predictions of the Drude model, where only the intraband part of the conductivity is considered and the MRE curve always takes the same sign. Therefore, it is vital to include the interband contribution for a full spectral study of the origin of the infrared MRE data.