Regulation Of Magnetism By Ferroelectricity In Two-dimensional Heterostructure

As the carrier of modern electronic information technology,high-density memory has become an inevitable developing trend.To meet the growing demand for information storage technology,it is vital to develop fast and efficient data storage devices.At present,there are two new types of memory devices,magnetic random access memory（MRAM）and ferroelectric random access memory（Fe RAM）.The former possess the superior features of high read speed and high storage density,but with high energy consumption in data writing as well.While,Fe RAM is very energy-efficient,and only a small current is required to write the data,but the complicacy and destructiveness when reading post a great challenge to its flexibility and service life.However,the superiority of integrating MRAM with Fe RAM not only can remain its speed and storage density but also deter from energy-consuming.Therefore,electrically written and magnetically read storage technology can be realized by the coupled ferromagnetic（FM）and ferroelectric（FE）materials.In other words,multiferroic materials with magnetoelectric coupling has becoming a key toward future study.Another factor about dimensionality reduction has been crucial to the trend of device miniaturization.According to the Mermin-Wagner theorem from the traditional theoretical framework,the existence of thermal fluctuations at finite temperature may easily destroy the two-dimensional（2D）magnetism based on the isotropic Heisenberg model,failing to realize the long-range magnetic ordering.Also,size effects generated from depolarization fields can invalidate ferroelectricity when the system down to nano-thickness.Therefore,the above barriers toward low-dimensional system make the search of 2D multiferroic materials even more difficult.Fortunately,since the discovery of graphene,extensive research on 2D materials has been carried out rapidly.The physical behaviors of three-dimensional materials have been increasing theoretically and experimentally presented in the form of 2D.For example,the successful synthesis in experiment of 2D FE materials,such as Sn Te,In₂Se₃,CuInP₂S₆,demonstrated that it is sufficient to depress depolarization field effect because of the relative displacement between atoms,making the system finally exhibit ferroelectricity.Another,the exfoliation of 2D magnetic materials,such as CrI₃,Fe₃GeTe₂,and Cr₂Ge₂Te₆,enable the fact that magnetic anisotropy energy（MAE）plays an important role in stabilize the long-range magnetic ordering.As a result,it provides great confidence to explore multiferroic materials with magnetoelectric coupling in the field of 2D atom-thick systems.Magnetic skyrmions are novel topologically protected whirling spin texture.Their nanoscale dimensions,high stability,and easy manipulation nature,together are promising for future spintronics applications.It is considered as a potential information carrier for the next generation of track memory,thus overcoming the limitations of current data storage and greatly increasing the storage speed and capacity.In light of this,effective control of skyrmion through the external field is one of a crucial strategy to meet the storage demand.Magnetic skyrmions have emerged as forms of Néel-type,Bloch-type,and the recently discovered bimeron in varies materials in recent years,even in van der Waals（vd W）magnet that have been at the frontier of material research.This atom-thick 2D magnets make the most of large surface area and reduced dimensionality,pushing the study of magnetic skyrmion to the atomically thin limit accordingly.In particular,the integrated skyrmion in 2D multiferroics can be effectively regulated using magnetoelectric coupling effect but also greatly reduce the energy consumption in memory devices.In this thesis,perovskite oxides and 2D vd W materials were mainly adopted as platforms to investigate the effective methods and underlying physical mechanisms of magnetoelectric coupling in FM/FE multiferroic heterostructures（HS）based on density functional theory（DFT）and micromagnetic simulation,which provide a theoretical basis for the design of future data storage and logic devices.On the one hand,we utilized the perovskite oxide HS as a platform to study Pb TiO₃/BiFeO₃ and BiFeO₃/LaFeO₃ HSs.Among them,by replacing the FE or FM part of the HS with the single-phase multiferroic material BiFeO₃,FE domain wall can be introduced at the interface,which greatly enriched the tunable parameters of HSs.Our first-principles calculations reveal that the charge transfer caused by the competition between polarization discontinuity and interfacial polar discontinuity in HSs is the key to realize magnetoelectric coupling.Also,the octahedral distortion occurred at the interface can significantly improve magnetoelectric coupling.In addition,setting different terminations and thickness of the system can make a huge contribution to the physical properties of perovskite multiferroic HSs,including 2D electron gas,electrical conductivity,magnetic susceptibility,magnetism as well as magnetic moment.In addition,atom-thick 2D FeI₂/In₂Se₃ HS has also been investigated.It is found that in this system the valence states of Fe ions transform from+2 to+3 by reversing the direction of ferroelectric polarization.This will lead to the magnetic interaction variation between the direct exchange and I ion mediated Fe-I-Fe superexchange according to the Goodenough–Kanamori–Anderson（GKA）rule.Therefore,magnetism can be robustly manipulated by reversing polarization.This work reveals that the existence of vd W interlayer spacing is not a barrier against magnetoelectric coupling but provide the possibility to realize magnetoelectric coupling in 2D vd W HS system.On the other hand,we investigated 2D vd W multiferroic HSs with integrated magnetic skyrmion to realize nonvolatile control via the magnetoelectric coupling effect in LaCl/In₂Se₃ and WTe₂/CrCl₃ HS.Firstly,for LaCl/In₂Se₃ HS,we found that the induced bimerons in this system can be generated or annihilated by In₂Se₃ FE polarization switching,proving that bimerons can be driven by stable current in HS.Another,in the presence of a perpendicular magnetic field,a skyrmion–bimeron–ferromagnetic phase transition cycle can be realized in our WTe₂/CrCl₃ HS,and accompanied by inversion of the topological number Q as well.Simulations of spin dynamics show that,the Néel-type skyrmion gains a higher velocity yet larger skyrmion Hall angle,in comparison to the bimeron.In addition,in LaBr₂ bilayers,stacking engineered multiferroics were constructed by fully considering the structural characteristics of vd W materials,and on the basis of this design,the evolution of polarization and magnetic textures in Moirésuperlattices has been accordingly investigated.This work sheds light on a novel approach to the design and control of magnetic skyrmions on 2D van der Waals materials.