Theoretical Investigation Of Two-dimensional Van Der Waals Magnets

Two-dimensional van der Waals（vd W）materials,such as Mn Se₂,Fe₃Ge Te₂（FGT）,and Fe₅Ge Te₂（F5GT）,have attracted extensive attention since their high Curie temperature,largely due to their potential application in high-temperature spin devices.The intralayer and interlayer magnetic couplings in the material play a critical role in the performance of the spin devices,such as spin filter magnetic tunnel junctions and giant magneto resistance.Therefore,controlling these two couplings is of great significance for the design of spin devices.In this work,we start from intralayer magnetic coupling,investigating the impacts of various modulation methods on intralayer magnetic coupling and its performance.Then,we extend our work to the interlayer magnetic coupling between vd W layers,trying to systematically explore the controllable magnetic coupling of two-dimensional vd W materials.Firstly,we focus on two-dimensional transition metal dichalcogenide Mn Se₂（2D-Mn Se₂）,which has a potential spintronic application in thin-film devices due to its Curie temperature of～300 K.We demonstrate theoretically a tunable magnetic transition of 2D-Mn Se₂ between anti-ferromagnetic（AFM）metal and ferromagnetic（FM）half metal as strain increases.The mechanism of that transition involves a competition between d-p-d through-bond and d-d direct interaction in 2D-Mn Se₂.Hole doping is an alternative way to enhance the stability of FM coupling.Adsorption（including Li,Na,Cl,and F）and vacancy（Mn and Se）studies confirm that the controllable magnetism of 2D-Mn Se2 is related to both interaction competition and charge doping.Tensile strains can greatly amplify through-bond interaction and exchange parameters,resulting in a sharp increase of Curie temperature.Magnetic orientation flips between in-and off-plane are observed as strains are applied.Next,we turn to the interlayer coupling in FGT vd W materials.Our experimental partners have firstly realized a method to intercalate protons,which can induce the exchange bias（EB）effect in the FGT system.Through first-principles calculations,we explore the impact of the vd W effect on interlayer coupling in FGT.The vd W effect significantly reduces the interlayer spacing of FGT and enhances interlayer chemical coupling.Besides,the insertion of H in the interlayer of FGT can further improve the interlayer chemical coupling.Our calculation shows the energy difference between the ferromagnetic and antiferromagnetic may reduce significantly after H insertion,leading to the co-existence of ferromagnetic（FM）layers and antiferromagnetic（AFM）layers.The formation of the FM/AFM interface in the system could result in the exchange bias effect,which explains the experimental observation.According to Maxwell Boltzmann distribution,we estimate that the concentration of H in the FGT can reach 4.75×10²²cm^-3 under the gate voltage of-4.5 V.We have also investigated the interlayer magnetic coupling of F5GT.Interestingly,the collaborated experimental work found that the gate voltage can lead to the disappearance and flip of the Hall resistance loop.The electron doping is introduced in F5GT by the gate voltages.Based on density functional theory,we find that the magnetic phase of F5GT changed from interlayer FM to interlayer AFM coupling after electron doping,which results in the disappearance of the Hall resistance loop.Moreover,our calculation shows that the Hall conductivity can reverse its sign under the impact of charge doping,in line with the experimental observation.In addition,we observe an interlayer magnetic coupling spike under 2.5%tensile strain.This spike is induced by the abrupt change of magnetic moment of the Fe5 atom from～0.1μ_B to～1.4μ_B.The mechanism behind it involves the electron transfer between spin-up and spin-down channels.Additionally,the interlayer coupling will in turn increase the magnetic moment of Fe5 atoms.Furthermore,we extend our study to the interlayer magnetic coupling of vd W heterojunctions.An Ising-type AFM-Fe PS₃/FM-Fe₅Ge Te₂ vd W heterojunction is experimentally and theoretically constructed.The lowest-energy structure and magnetic configuration exhibit large interlayer magnetic coupling,which can induce an EB effect.However,the insertion of H can significantly reduce the interlayer magnetic coupling from-17μe V/?² to 26μe V/?²,resulting in the disappearance of the EB effect.Moreover,the change of the magnetic order of the AFM layer can also disturb the interlayer magnetic coupling（Interlayer magnetic coupling changed from-17μe V/?²to 5μe V/?²）,resulting in the disappearance of the EB effect.In conclusion,we have investigated the intralayer and interlayer magnetic coupling of 2D magnetic van der Waals materials by strain,H intercalation,charge doping,and defects based on first-principles calculations.Our theoretical studies well explain various interesting phenomena observed in experiments,such as the disappearance and flip of the Hall resistance loop,and the controlling of the EB effect,et al.Our research proposes theoretical understandings and provides significant guidance to the design of two-dimensional magnetic van der Waals material spin devices.