Unusual sign-changing Faraday effect in nanometer-thick magnetic films

TL;DR

This paper discovers a novel interface contribution to the Faraday effect in nanometer-thick magnetic films, causing the sign of Faraday rotation to depend on film thickness, challenging traditional bulk-only understanding.

Contribution

It introduces an analytical model accounting for both bulk and interface effects, revealing interface-driven Faraday rotation even without bulk contribution.

Findings

Sign of Faraday rotation depends on film thickness.

Interface contribution can dominate in nanometer-thick films.

Experimental and numerical confirmation of interface effects.

Abstract

It is generally believed that the magneto-optical Faraday effect appears in the the bulk of a magnetic material and its sign is fully determined by the sign of the non-diagonal permittivity element. Here we reveal an additional contribution to the Faraday effect from the film interfaces. It becomes notable for films with a thickness of a few tens of nanometers. As a result, in the absorption band of the film a novel feature of the Faraday effect is experimentally observed and numerically confirmed: 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 an interplay between the bulk and surface contributions which nicely describes the experimental data. Moreover, the Faraday rotation coming purely from interfaces without any bulk contribution is demonstrated. Possible applications of the observed unusual Faraday rotation behavior include hardware data encryption and other devices performing polarization control at nanoscale.