Surface magneto-optics in yttrium iron garnets

We report a study of surface reconstruction effects in yttrium iron garnets using density functional theory. This work responds to the need to explain the physical basis of recent experimental results showing a significant enhancement in Faraday rotation in iron garnets. These materials are extensively used in the telecom industry for nonreciprocal device applications. Understanding the physical basis of the heightened Faraday response at the surface is important for the development of ultrathin nonreciprocal devices. Our results show that the bandgap near the surface is significantly reduced compared to that of the bulk, and that spin-orbit coupling effects become more important near the surface. We find that the decrease in band gap results in an enhancement and change in direction in the Faraday rotation in the visible range all the way to the deep red. Electronic transition matrix elements for surface-sensitive ultra-thin layers are calculated and compared to those of bulk samples, leading to the conclusion that the octahedrally-oxygen-coordinated iron sublattice in these ferrimagnetic materials is more strongly affected by surface reconstruction than the other, anti-ferromagnetically coupled, tetrahedral sublattice. We explain how these changes contribute to the enhancement in magneto-optic response near the surface as compared to the bulk.