First-principles Studies Of New-type Ferroelectric And Multiferroic Materials

Ferroelectric materials with stable and switchable polarization have been extensively exploited for applications in nonvolatile memories,capacitors,sensors and actuators.Multiferroics integrates more than one primary ferroic order in a single material,which not only simultaneously possesses ferroelectricity,magnetism,etc.but also cross-coupling among these multi-ferroic order would take place,expected to realize the novel application of the new device.The traditional ferroelectric or multiferroic materials are mostly bulk perovskite materials,such as typical ferroelectric material Ba Ti O₃,multiferroic material Bi Fe O₃,these materials generally have large band-gap and low mobility,moreover,their thin films will suffer from critical thickness effect,which is a challenge when they are applied for the multifunctional devices with higher requirements of miniaturization and integration.However,low-dimensional（two-,one-and zero-dimensional）materials have transformative technological potential due to their natural advantages,in addition,ultrahigh-density data storage and diversified multiferroic coupling can be realized in low-dimensional materials,and it has important significance for new magnetoelectric nanometer devices.Recent important theoretical and experimental advances also shed light on further basic or advanced research.In this paper,a series studies of room temperature robust ferroelectricity/multiferroicity are carried out,including not only two-dimensional（2D）multiferroic materials with strong magnetoelectric coupling,but also single-molecule ferroelectric designs with high storage density and writing speed.（1）Sodium bismuth dichalcogenides:candidates for ferroelectric high-mobility semiconductors for multifunctional applications.We show first-principles evidence of Na Bi S₂and Na Bi Se₂with large polarizations,moderate bandgaps and high electron-mobilities,but they have relatively small switching barriers.Their ferroelectricity can be robust under ambient conditions with enhanced polarization upon either application of a small tensile strain or ion doping,where distortion can be increased and multiferroics may also be obtained,they can be promising candidates for combining both functions of nonvolatile memory and efficient manipulation of signals,and realize theoretically proposed efficient ferroelectirc photovoltaics and thermoelectrics.（2）Room-temperature multiferroicity and diversified magnetoelectric couplings in 2D Cu Cr X₂（X=S/Se）materials.Ultrathin layer of Cu Cr S₂and Cu Cr Se₂are 2D room-temperature multiferroics,the vertical ferroelectricity is neither induced by an empty d shell nor spin-driven,giving rise to the mutual exclusions that hinder the coexistence of robust magnetism and ferroelectricity in traditional multiferroics can be resolved.Their ferromagnetism can be stabilized due to enhanced carrier density and orbital shifting by the vertical polarization.Layers with different thicknesses may exhibit diversified types of magnetoelectrics for efficient‘magnetic reading+electrical writing’.（3）Fullerene-based 0D ferroelectrics/multiferroics for ultrahigh-density and ultrafast nonvolatile memories.We show first-principles design of 0D ferroelectrics/multiferroics based on polar functionalized fullerene,the switching barriers of both modes,for the screened candidates with considerable magnetic moments and dipole moments,are all in the ideal range for working at ambient conditions.Moreover,compared with conventional ferroelectrics for data storage,they may be endowed with high areal density and high writing speed that are respectively more than 2 and 3 orders of magnitude higher.Our study opens up a new way to explore single-molecule ferroelectrics for nonvolatile memories.