First-Principles Study of Large Gyrotropy in MnBi for Infrared Thermal Photonics

Nonreciprocal gyrotropic materials have attracted significant interest recently in material physics, nanophotonics, and topological physics. Most of the well-known nonreciprocal materials, however, only show nonreciprocity under a strong external magnetic field and within a small segment of the electromagnetic spectrum. Here, through first-principles density functional theory calculations, we show that due to strong spin-orbit coupling manganese-bismuth (MnBi) exhibits nonreciprocity without any external magnetic field and a large gyrotropy in a broadband long-wavelength infrared regime (LWIR). Further, we design a multi-layer structure based on MnBi to obtain a maximum degree of spin-polarized thermal emission at 7 $\mu$m. The connection established here between large gyrotropy and the spin-polarized thermal emission points to a potential use of MnBi to develop spin-controlled thermal photonics platforms.