Theoretical Study Of Magnetic And Multiferroic Properties Of Two Dimensional Halides

For nearly two decades,two-dimensional（2D）van der Waals materials have been favored by researchers due to their broad application prospects in microelectronics and optoelectronics.Among all 2D materials,2D van der Waals materials with intrinsic ferroelectricity,ferromagnetism and ferroelasticity have gradually become the most attractive"star"materials.For these"star"materials,most of them are transition metal chalcogenides or halogen compounds,such as Cr I₃,Cr₂Ge₂Te₆and Fe₃Ge Te₂and so on.It is well known that transition metal compounds belong to strongly correlated systems,in which charges,spins and orbitals are coupled with each other,resulting in the competition of various phases and rich physical properties,such as colossal magnetoresistance effect,high temperature superconductivity phenomenon,magnetoelectric coupling,and multiferroicity.Therefore,the study of two-dimensional transition metal compounds not only has important physical significance,but also has important potential application value.From another point of view,although scientists have achieved plenty of research progress in the field of multiferroics,there are still huge challenges in seeking for intrinsic ideal two-dimensional ferroic materials.In addition,it is noteworthy that most of attentions are focused on compounds containing 3d electrons,but in fact,in addition to transition metal compounds,rare earth compounds also play a very important role in the investigation of modern materials.Due to the large magnetic moment,strong Hubbard correlation and magnetic anisotropy of rare earth compounds with 4f electrons,it can prompt us to search for some non-trivial magnetic and multiferroic materials.This paper mainly predicts and studies the magnetic,ferroelectric and multiferroic properties of three 2D halogen compounds through the calculation of density functional theory.The specific research contents are as follows:（1）Multiferroic study of monolayer halide VOF₂.2D VOF₂is an intrinsic ferroelectric ferromagnetic material with a tetragonal lattice.Through DFT calculations and Monte Carlo simulations,we have systematically studied its electronic structure,dynamic stability,ferroelectricity,and magnetic properties.The ferromagnetism of VOF₂is mainly derived from an unpaired electron in V⁴⁺,which can be found by projecting the density of states to occupy the d_xyorbital near the Fermi surface.Very interestingly,the ferroelectric origin of this material is contrary to the traditional d⁰rule,because the anisotropic orbitals makes the two empty orbitals of d_yzand d_xzhybridize with the p_zorbital of O to generate ferroelectricity.Compared with VOX₂（X=I,Br and Cl）monolayer,it has the largest ferroelectric polarization.By applying different Hubbard U values,VOF₂still maintains the ferromagnetic ground state.Meanwhile,we only need to apply a compressive stress of 1.3%to tune the magnetic ground state from FM to AFM-III ground state.In short,VOF₂monolayer is an intrinsic and robust ferroelectric ferromagnetic material,whose magnetism and ferroelectricity are both derived from V ions.（2）Magnetic properties and multiferroic properties of rare-earth halide Gd I₃monolayer.Two-dimensional Gd I₃,similar to Cr I₃,is a honeycomb lattice magnet.By theoretical calculations,Gd I₃monolayer possesses a simple-type antiferromagnetic ground state with a strong Hubbard correlation.By inserting Li atoms into the interstitial positions of hexatomic rings,magnetic ground state is transformed from Néel-type to Stripy-type antiferromagnetism.In addition to the magnetic transition,the introduction of electrons leads to strong electron-phonon coupling and produces a strong Peierls dimerization phenomenon.Simutaneously,the Peierls transition induces rich physical properties,including lattice distortion,magnetic change,metal-insulator transition and so on.Very interestingly,the Peierls transition also induces ferromagnetic distortion（～4%）,making the halide（Gd I₃）₂Li become a multiferroic system with ferroelasticity and antiferromagnetism.Besides,we implant Mg atoms into the vacancies,which induces a larger ferroelasticity distortion（～9%）,but the overall physical effect is analogous to that of（Gd I₃）₂Li.（3）Magnetic and electrical properties of rare-earth halide Gd Cl₃monolayer.Different from the hexagonal and triangular lattices of common 2D magnetic materials,the Gd Cl₃monolayer is a Zigzag-type antiferromagnet with an orthorhombic structure.Also,different from the common octahedral and triangular prism cage configurations,each Gd is surrounded by six Cl ions,forming a very rare hendecahedra.After implanting Li atoms into Gd Cl₃,the magnetic ground state changes from Zigzag-type antiferromagnetic phase to ferromagnetic phase.Synchronously,spontaneous structural reorganization occurs after relaxation,from the original high-symmetry phase（Pmmn）to low-symmetry phase（C2/m）.According to Monte Carlo simulations,magnetic transition temperature（T_C=48 K）can be acquired,which is higher than that of Cr I₃（45K）and Cr₂Ge₂Te₆（28K）.In addition to the magnetic transition,Li-atoms implanation results in a strong electrical transport anisotropy and induces interesting quantum tunneling and colossal magnetoresistance effect.Meanwhile,bilayer Gd Cl₃is constructed to explore effect of van der Waals interaction on the magnetic and electronic structure.Bilayer Gd Cl₃has an antiferromagnetic-V phase,whose magnetic ground state transform from antiferromagnetic-V to antiferromagnetic-II by Li-atom intercalation.