Theoretical Design Of Two-dimensional Intrinsic Ferromagnetic Materials And Construction Of Spin-dependent Heterostructures

The realization of magnetism in two-dimensional（2D）systems could revolutionize the development of information technology.However,as proposed by the Mermin-Wagner theorem,long-range magnetic ordering is prevented in 2D crystals due to the increased thermal fluctuations.Until 2017,the intrinsic ferromagnetism down to single-/few-layer limit was experimentally demonstrated in 2D ferromagnetic（FM）Cr₂Ge₂Te₆ and Cr I₃,which stimulates the discovery and research of various 2D magnetic materials.2D magnetic materials exhibit rich magnetic properties and excellent physical phenomena in the atomic limit of thickness,such as tunable surface-functionalized magnetism,multiferroic effect,quantum anomalous Hall effect and so on.In addition,these ultra-thin magnets and their heterostructures are expected to be integrated into spintronics,providing an ideal platform for exploring their underlying physics and interfacial effects.The discovery of magnetic materials at the 2D limit has made a significant breakthrough in the field of 2D magnetism,which is a true achievement of a landmark in recent years.However,the study of 2D magnets is still in its infancy.The number of 2D magnetic materials is still very limited.The deep understanding of spin coupling mechanisms and the discovering of excellent physical properties are still lacking as well.In addition,the exploration of the interfacial effects of magnetic heterointerfaces is still insufficient.The first-principles calculations and Monte Carlo（MC）simulations can not only be used to efficiently predict 2D magnetic materials and understand their microscopic magnetic mechanisms,but also explore novel interface effects by constructing heterostructures.It is significant for the possible functions and applications of spintronic devices in the future,through enlarging the family of 2D magnets,reducing the trial and error in experiments,and finally providing reasonable theoretical design and strategies of noval spin electronic devices.In order to enrich the types of 2D FM materials and their potential spintronics applications,exploring and understanding new FM materials with excellent properties and magnetic heterostructures with multifunctional properties are focused in this dissertation.Based on the first-principles calculations and MC simulations,the electronic structure and magnetic properties of two kinds of intrinsic antiferromagnetic（AFM）materials at their 2D limit have been systematically investigated,as well as the FM/FM heterostructure and magnetic/ferroelectric heterostructure,which can be servered for spin-optoelectronic devices,have been designed by using electronic and magnetic characteristics.The main research content and conclusions of this thesis are summarized as follows:（1）Chromium sulfide halide monolayers:intrinsic FM semiconductors with large spin polarization and high carrier mobility.2D ferromagnetic semiconductors（FMSs）are desirable for their potential to enhance the functionality of semiconductor devices via the utilization of spin degrees of freedom.Herein,a series of intrinsic FMS monolayers were predicted in the layered chromium sulfide halide Cr SX（X=Cl,Br,I）family with large spin polarization,large magnetic moments and high Curie temperatures（T_C）.From Cl to I,the T_C is predicted to be 150 K,160 K and 170 K,respectively.Subsequent experiment has verified that the T_C of single-layer Cr SBr is 146 K,which is similar to the predicted result.The distinct ferromagnetic exchange interactions derive from the difference between Cr-S-Cr and Cr-X-Cr bond angles.Among them,Cr SCl and Cr SBr monolayers also have high hole mobilities up to6.6×10³ and 5.3×10³ cm² V^-1 s^-1,respectively.Furthermore,these 2D monolayers exhibit excellent dynamic and thermal stabilities and a small exfoliation energy from the bulk.This work has been partially confirmed by experiments,thus these intrinsic FMSs with their high carrier mobilities are of great significance for the next-generation spintronics and electronics.（2）2D non-van der Waals（vd W）FM transition metal oxide with the coexistence of semiconducting ferromagnetics and piezoelectrics.Stable 2D FMSs with multifunctional properties have attracted extensive attention in device applications.Non-vd W transition-metal oxides with excellent environmental stability,if FM,may open up an unconventional and promising avenue for this subject,but they are usually AFM or ferrimagnetic.Herein,an FMS is predicted,monolayer Fe₂Ti₂O₉,which can be obtained from Li Nb O₃-type Fe Ti O₃ AFM bulk,with a large perpendicular magnetic moment（6μ_B/f.u.）and T_C up to 110 K.The intriguing magnetic properties are derived from the double exchange and negative charge transfer between O＿p orbitals and Fe＿d orbitals.In addition,a large in-plane piezoelectric coefficient d₁₁ of 5.0pm/V is observed.This work offers a competitive candidate for multifunctional spintronics and may stimulate further experimental exploration of 2D non-vd W magnets.（3）2D FMS heterostructures with spin-constrained optoelectronic functionalities.2D vd W engineering has brought about many extraordinary and new physics concepts and potential applications.Herein,a new type of spin-constrained optoelectronic device is proposed,using2D FMSs heterostructures.It is based on a photoexcited double-band-edge transition model,involved the coupling between the interlayer magnetic order and the spin-polarized band structure,i.e.,the reversible switch of band alignment can be achieved via the reversal of the interlayer magnetic orientation.Such a unique magnetic optoelectronic device can be realized in the Cr Br₃/Cr Cl₃ heterostructure and other 2D FMS heterostructures that have the same direction of the easy magnetization axis and a reconfigurable spin-ploarized band alignment.Combining the magnetic-field and light manipulation,the device is expected for applications in ultra-fast information processing,memory and logic switching.This study enables the possibility of fully vd W-based ultra-compact spintronics devices based on 2D vd W heterostructures.（4）2D multiferroic heterostructures with spin-constrained photoelectricities.Materials with tunable magnetoelectric orders enable the integration of sensing,data storage and processing into one single device to realize the low-power consumption and fast response.The multiferroic material plays an important platform,while the scarcity of single-phase multiferroics spurs extensive researches in pursuit of composite systems combining different types of ferroic materials.Herein,a 2D layered magnetic/ferroelectric heterostructure with spin-constrained photoelectric functionality is designed.The ground state of FM and AFM orderings in the magnetic layer is altered by the polarization direction of the ferroelectric layer.Specifically,the FM heterostructure exhibits a type-II band alignment.Due to the light-induced charge transfer,spin-polarized/unpolarized current arises from FM/AFM state,which can be recorded as"1"/"0"state and served for logic processing and memory applications.Such heterostructures can be integrated into spintronic device,holding the possibility of low-power electrical write operation and non-destructive optical read operation.Based on first-principles calculations,the Ni I₂/In₂Se₃ heterobilayer is demonstrated to be an ideal candidate to realize such spin-photoelectric functionalities.The reversible FM state（easy-axis magnetic anisotropy）and AFM state（easy-plane magnetic orientation）in Ni I₂ layer originates from interfacial charge transfer due to the proximity effect.This work offers a considerable potential of the integration of memory processing capability into one single device with 2D layered multiferroic heterostructures.