Band structure reconfiguration and surface Faraday rotation in Bi-substituted iron garnets

Abstract

Bismuth-substituted lutetium iron garnets have exhibited a remarkable enhancement in Faraday rotation (FR) for films thinner than 50 nm. A sevenfold amplification in the magneto-optic gyrotropy was found to occur within 2 nm of the air-surface interface at 532 nm wavelength. The present study delves into the underlying physical mechanisms contributing to such amplification. Near-surface changes in band structure in these materials and their connection to the magneto-optic response are explored. Density functional theory is employed to investigate the changes in density of states and overall band structure reconfiguration of surface atoms. The transition dipole matrix (TDM) model is then applied to both bulk and surface states, correctly predicting a Faraday rotation enhancement at the surface as a result of overall surface band structure reconfiguration and resulting bandgap reduction. Surface versus bulk FR spectral response is extended beyond prior studies over the full visible and the near-infrared spectral ranges, predicting significant amplification across the telecom band. Experimental analysis through X-ray photoelectron spectroscopy (XPS) and UV-Vis spectroscopy reveal a reduction in bandgap as films are thinned down from 200 nm to 40 nm. By providing a deeper physical understanding of the origin of enhanced Faraday rotation at the surface, this work opens up avenues for more efficient miniaturized Faraday rotation applications. Knowledge of the band structure information thus uncovered may be used to demonstrate novel and more advanced applications.