| Data record and storage are the foundation for human civilization.Magnetic hard disk is cheap and stable,becomes the domain media for data storage nowadays,in which the binary data 1 or 0 are represented by the magnetic orientation up or down.Conventional the data record i.e.the magnetic reversion is accomplished by the magnetic field generated by current,commiting heats and energy consumptions.This dissertation considers two type of magnetism controlling via non-current route including electric field and spin-current.Based on density functional theory(DFT)calculations,we predict the electrical controlling of magnetism in several famous nano-systems including phosphorene,hexagonal boron nitride and monolayer magnesium oxide.Many magnetic properties e.g.,magnetic moment,exchange coupling,Curie temperature,half-metalicity etc.are amenable to external electric field.And some systems even support the nonmagnetic-magnetic transition via electric field.Besides,we predicted a new type of two-dimensional topological insulator which consist of lead and tin.We demonstrate that it can work under room temperature and owns exceedingly large bulk gap,which is promising for suppressing the bulk leakage and enhancing the efficiency of spin-transfer-torque(STT)based magnetic random access memory(MRAM)technology.Not only the findings in this dissertation shed light on the future applications in spintronics,but also give us deeper physical understanding on the magnetic and spintronic structures in low-dimensional systems.In chapter 1,we give a brief introduction to magnetic modulations.For the case of electric-field controlling of magnetism,we classify the phenomenon reported in literature according to the spatial inversion symmetry of systems.And we discuss the mechanism behind these phenomena.Besides,we introduce the technology of MRAM based on SOT effect,as well as discuss and comparing the several conventional routes to generate SOT.Finally,we summarize the challenges and difficulties in these magnetic controlling mechanisms,and propose our solutions.In chapter 2,we introduce several approximate theory for quantum many-body system.Firstly,we discuss Hatree-Fock(HF)model which is based on the many-body wave functions.With the tool of perturbation theory,we describe the physical picture of HF model and point out the reason of deficiencies.After that,we discuss the density functional theory including Thomas-Fermi-Dirac(TFD)model which is based on total electron density and Khon-Sham(KS)model which is based on single electron density.Since most work in this dissertation is done on KS theory,we pay much attention on its key points,including the form of several approaches for exchange-correlation functionals,and the pseudo-potential based on projected augmented wave(PAW)method,and the implementation of self-consistent iterative optimization technique.Finally,we compare the feature of each scheme and give some outlook for the futu re development of DFT.In chapter 3,we investigate the magnetic edge states and electrical contronling of magnetism in phosphorene nanoribbons with bare zig-zag edge(ZPNRs).Based on hybrid functional calculations,we demonstrated that the Peierls-like edge distortion modifies the edge magnetism dramatically but the anti-ferromagnetic(AFM)order is still reserved.The coupling between opposite edges is carried by the wave-function-tails invaded into the inner zone of nanoribbons.Due to the local nature of edge states,the wave-function-tails met in the center are negligible if the nanoribbon is wide enough,inducing the energy degeneration for serveral anti-ferromagnetic orders.When a transverse electric field is applied,the quasi-C2v symmetry of ZPNRs may be broken or not according to the explicit AFM order.Some can achieve semiconducting-metalic transition while others may achieve semiconducting-half metal transition.Most importantly,once the metalic phase appears,polar charged accumulated in boundaries will prevent the furture influence of external electric field,stabilize the half metal state.Not only these findings help us know more about the magnetism in nano-systems,but also expand the application potential of ZPNRs in future spintronic applications.In chapter 4,we demonstrated that the polar charges and locality of edge states in hexagonal BN nanoribbons(BNNRs)play key roles in the behavior of edge magnetism M.By applying a transverse electric field,the magnetism in both zigzag and armchair edges can be regulated sufficiently.In particular,the magnetic on/off switches can be realized in armchair edges.To verify the chemical stability of magnetic switches,we consider various edge decorations including hydrogenation,fluoridation and hydroxylation,and find the preservation of electrical magnetic switches in all of they.Therefore,this kind of magnetic phase transition via electric field is robust,suggusting the excellent feasibility in expriments.We further express the DFT ground state under Wannier representation,and construct an effective Hubbard model.The mean-field solution of this interacting model also demonstrates the nonmagnetic-magnetic transition under biased-voltage.In order to investigate the magnetic behavior under finite temperature,we implement the phenomenological Landau-Ginsburg Hamiltonian,and a series of Monte Carlo simulations based on it demonstrate that the Curie temperature is also dependent on the biased-voltage.Finally,we find that uniaxial tensile strains can also produce significant magnetic modulation in Z-BNNRs,which stem from the piezoelectricity.All these suggest that h-BNNRs are ideal platforms for ME coupling researching and promising candidates for functional spintronics application.And this work suggests a new kind of mechanism that allows the electric field induces magnetism in nonmagnetic nano-system,sheding light on the realization of electrical controlling of magnetism.In chapter 5,we report on a study of electromagnetic(ME)properties of zigzag magnesium oxides nanoribbons(Z-MgONRs).We propose that the polar charges and the spin polarization are the two key factors for edge magnetism.We demonstrate that both O-and Mg-edges are magnetic and their magnetic moments all can be efficiently modulated via external electric field.And the edge magnetism is further studied in the framework of effective tight-binding model,which provides a starting point for the calculation of one particle Green s function and the determination of exchange interactions.Utilizing the linear response model,we find that Z-MgONRs exhibit two kinds of exchange interaction with extremely different natures.The magnetism in O-edge is strongly localized with ferromagnetic order while the magnetism in Mg-edge is itinerant with Ruderman-Kittel-Kasuya-Yosida(RKKY)like interactions.And these two couplings can also be modulated by electric field,giving rise to the electrical modulations of spin-wave-density(SWD).Furthermore,we improved the conventional Wolff algorithm for Monte Carlo simulation,incorporating long-range interactions and antiferromagnetic interactions.The improved algorithm owns sufficiently small dynamical factors near the transition temperature.A series Monte Carlo simulation based on this kind of algorithms show that the biased-voltage can also modulate the Curie temperature of the O-edge.All these suggest that z-MgONRs are ideal platforms for ME coupling and appealing candidates for manifold applications.These findings also give a clear description about the magnetic behavior in Z-MgONRs,expanding our understanding of low-dimensional magnetic systems.In chapter 6,we predicted that the two-dimensional(2D)binary compound of lead and tin(PbSn)in a buckled honeycomb framework can be tuned into a topological insulator with huge a band gap and structural stability via hydrogenation or growth on special substrates.This heavy-element-based structure is sufficiently ductile to survive the 18ps molecular dynamics(MD)annealing to 400K,and the band gap opened by strong spin-orbital-coupling(SOC)is as large as 0.7 eV.These characteristics indicate that hydrogenated PbSn(H-PbSn)is a stable platform for QSH realization at room-temperature.Since the bulk leakge will be sufficiently suppressed due to the large bulk gap,this platform is promising in future magnetic random access memory devices.Furthermore,to calculate the edge states in H-PbSn under variouse conditions,we improved the algorithm of calculation of surface Green's function.The new algorithm supports the consideration of surface absorption or reconstruction,and keeps high efficiency,will surely enhance the accurate of prediction based on surface state calculations. |
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