Computational studies on magnetism and ferroelectricity

Magnetics,ferroelectrics,and multiferroics have attracted great attentions because they are not only extremely im-portant for investigating fundamental physics,but also have important applications in information technology.Here,recent computational studies on magnetism and ferroelectricity are reviewed.We first give a brief introduction to magnets,fer-roelectrics,and multiferroics.Then,theoretical models and corresponding computational methods for investigating these materials are presented.In particular,a new method for computing the linear magnetoelectric coupling tensor without applying an external field in the first principle calculations is proposed for the first time.The functionalities of our home-made Property Analysis and Simulation Package for materials(PASP)and its applications in the field of magnetism and ferroelectricity are discussed.Finally,we summarize this review and give a perspective on possible directions of future computational studies on magnetism and ferroelectricity.

Interplay of Strain and Magnetism in FeSe Monolayers

Superconductivity and its relationship with strain remains elusive in the monolayer FeSe superconductor. Based on first-principles calculations and model studies, we investigate the magnetic properties of FeSe and FeTe monolayers and find that tensile strain induces changes to magnetic phases for both materials. Furthermore, we reveal that electron doping will decrease the difference of effective magnetic interactions between the a and b directions in an FeSe monolayer and hence suppress its nematicity. We suggest that the overall effect of tensile strain combined with electron doping hinders the appearance of both magnetic and nematic orders in an FeSe monolayer,which paves the way for the emergence of superconductivity.

Interplay between Kitaev interaction and single ion anisotropy in ferromagnetic CrI_(3) and CrGeTe_(3) monolayers

Magnetic anisotropy is crucially important for the stabilization of two-dimensional(2D)magnetism,which is rare in nature but highly desirable in spintronics and for advancing fundamental knowledge.Recent works on CrI3 and CrGeTe_(3) monolayers not only led to observations of the long-time-sought 2D ferromagnetism,but also revealed distinct magnetic anisotropy in the two systems,namely Ising behavior for CrI3 versus Heisenberg behavior for CrGeTe_(3).Such magnetic difference strongly contrasts with structural and electronic similarities of these two materials,and understanding it at a microscopic scale should be of large benefits.Here,firstprinciples calculations are performed and analyzed to develop a simple Hamiltonian,to investigate magnetic anisotropy of CrI3 and CrGeTe_(3) monolayers.The anisotropic exchange coupling in both systems is surprisingly determined to be of Kitaev-type.Moreover,the interplay between this Kitaev interaction and single ion anisotropy(SIA)is found to naturally explain the different magnetic behaviors of CrI3 and CrGeTe_(3).Finally,both the Kitaev interaction and SIA are further found to be induced by spin–orbit coupling of the heavy ligands(I of CrI3 or Te of CrGeTe_(3))rather than the commonly believed 3d magnetic Cr ions.

Tunable spin textures in polar antiferromagnetic hybrid organic–inorganic perovskites by electric and magnetic fields

The hybrid organic–inorganic perovskites(HOIPs)have attracted much attention for their potential applications as novel optoelectronic devices.Remarkably,the Rashba band splitting,together with specific spin orientations in k-space(i.e.,spin texture),has been found to be relevant for the optoelectronic performances.In this work,by using first-principles calculations and symmetry analysis,we study the electric polarization,magnetism.

Nature of spin-lattice coupling in two-dimensional CrI_(3) and CrGeTe_(3)

Spin-lattice (SL) coupling plays an important role in spintronic applications given its effects on magnetic,ferroelectric,optical,and thermodynamic properties.Experiments and theoretical calculations have revealed a large SL coupling effect in CrGeTe_(3) and CrI_(3) monolayers.However,the microscopic origin of SL coupling in these systems is still unclear.In this work,we develop a systematic method to explore the atomistic mechanism of SL coupling based on the density functional theory.We find that the first-and second-order SL couplings in ternary system CrGeTe_(3) are considerably stronger than those in binary system CrI_(3).For the first-order SL coupling,the Cr ions of the magnetic pair and Ge ions positively contribute to the strain enhancement of ferromagnetism in CrGeTe_(3).However,the Cr ions provide a negative contribution in CrI_(3).Furthermore,our tight-binding analysis suggests that the p-d hopping in CrGeTe_(3) gradually decreases with the tensile strain,rapidly enhancing the ferromagnetism under the tensile strain.The large frequency shifts in CrGeTe_(3) are caused by the large second-order exchange derivatives (one type of second-order SL coupling) of the Cr ions of the magnetic pair.