▎ 摘　　要

Various nonreciprocal thermal emitters, which provide opportunities for higher energy conversion efficiency, have been proposed to completely violate the traditional Kirchhoff's law. However, the tunable nonreciprocal thermal emitters remain barely investigated. In this paper, by sandwiching a graphene monolayer between a top metallic grating and a bottom magneto-optical film backed with a metallic mirror, we achieve tunable nonreciprocal radiation effect. It is shown that strong nonreciprocal radiation for graphene under initial state is realized at wavelength around 14.845 mu m when the incident angle is 30 degrees and the external magnetic field is 3 T. The physical origin behind such phenomenon is disclosed by investigating both the distribution of magnetic field at the resonant peak and the coupled-mode theory. Besides, the nonreciprocal radiation performance remains perfectly within certain range of structure parameters, which is benefit for practical fabrication and applications. More importantly, the strong nonreciprocal radiation of the proposed scheme can be flexibly tuned by the gate voltage of graphene without redesigning and refabricating the structure. We believe that this work will provide new approaches for the design of novel energy harvesting systems and nonreciprocal thermal emitters.