Conventionally, the magneto-optical Faraday effect is considered as a bulk magneto-optical effect, and for the isotropic homogeneous thin films its sign is fully determined by the sign of the nondiagonal permittivity element. Here we experimentally reveal an additional contribution to the Faraday effect from the film interfaces. It becomes notable in the absorption band for films with a thickness of a few tens of nanometers. As a result, a novel feature of the Faraday effect is observed and numerically confirmed: the sign of the Faraday rotation at a fixed wavelength becomes dependent on the film thickness and therefore is ambiguously related to the sign of the gyration and magnetization of the film. We elaborated an analytical model taking into account the bulk and interfacial contributions and interference due to multiple internal reflections, which nicely reproduces the experimental data. Moreover, our joint analysis of Faraday rotation and ellipticity reveals an additional bulk contribution to the Faraday rotation: magnetic circular dichroism combined with multiple internal reflections produces a finite phase difference between the transmitted circularly polarized waves even when the conventional single-pass bulk contribution vanishes. Possible applications of the observed unusual Faraday rotation behavior include sensing as well as other devices performing polarization control at nanoscale.

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• Faraday effect
• Ferromagnetism
• Ferromagnets
• Magnetic insulators
• Magnetic thin films