  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.
