| Two-dimensional（2D）van der Waals materials have been widely studied since the discovery of graphene in 2004.Up to now,thousands of 2D materials have been reported,and the reported 2D materials show a variety of properties.Multiferroic materials exhibit more new physical effects through magnetoelectric interaction,piezoelectric interaction,or magnetoelastic interaction.The multiferroics with ferroelectric and ferromagnetic order are of great significance for the basic research of condensed matter physics and practical application.2D materials have promising application prospects in magnetism,ferroelectricity and multiferroicity due to their unique structure and rich properties.In recent years,the research on 2D ferroelectric（FE）-ferromagnetic（FM）multiferroics has been gradually carried out.At present,2D FE-FM multiferroics with high Curie temperature and strong magnetic anisotropy are rare,and the magnetism of most 2D FE-FM multiferroics is mostly arise from the partial occupation of d orbitals.Recent theoretical studies reported that monolayer X2NO2（X=In,Tl）is a FE-FM multiferroics with room temperature d0 ferromagnetism,and monolayer Si N is an intrinsic p-orbital FE-FM multiferroics.In this paper,using first-principles calculations based on density functional theory,we theoretically predict the stability of monolayer GeN and SnN with buckled hexagonal lattice structure,and investigate the electronic structure,magnetic properties and ferroelectric properties of monolayer GeN and SnN.The main research results are as follows:The calculated results of the coherent energy and phonon spectrum show that the monolayer GeN and SnN with buckled hexagonal lattice structure can be synthesized and are dynamically stable.Further calculations show that the monolayer GeN and SnN are both I-type FE-FM multiferroic semiconductors.For the monolayer GeN,its magnetization strength is about 0.623μB/cell,which is mainly provided by the p-orbital of N atom and Ge atom,the spontaneous polarization intensity is 0.62μC/cm2,direction perpendicular to the plane of the monolayer,and its easy magnetization axis is in the plane.For the monolayer SnN,its magnetization strength is about 0.685μB/cell,which is mainly provided by the p-orbital of N atom and Ge atom,the spontaneous polarization intensity is 0.66μC/cm2,direction perpendicular to the plane of the monolayer,and its easy magnetization axis is in the plane.More Importantly,The Curie temperatures of the monolayer GeN and SnN are 320 K and 330 K,respectively,which is beyond the room temperature,and the magnetic anisotropy of the monolayer SnN reaches-0.38 me V/unit cell. |
