The First-principles Study On Low-dimensional Spintronic Materials And Devices

The successful discovery of giant magnetoresistance(GMR)effect reveals the importance of the electron spin in the field of the electronic information storage.By changing the magnetization direction of the magnet in the free layer,the huge magnetoresistive ratio can be observed in magnetic tunnel junctions or spin valves and other magnetic composite structures due to the spin-dependent scattering characteristics of the spin.Therefore,it can be widely applied in the fields of magnetic information storage.The goal of spintronics is to develop highly integrated,fast and operable spintronic devices based on the spin degree of freedom and the spin-related coupling mechanisms.On one hand,it can realize the function of information storage that the charge degree of freedom can realize.On the other hand,it can solve many important problems,such as,high power consumption,existing in the current integrated devices.How to effectively control or manipulate the spin is a great challenge in spintronics,especially to achieve the two ideal and important goals,i.e.,pure spin current with low power consumption and half-metallicity with full spin polarization.The emerging two-dimensional(2D)materials have attracted wide attention due to their unique electronic properties caused by quantum confinement effect,and they are considered as a kind of ideal candidates for the next-generation electronic devices.Under these two backgrounds,how to combine 2D materials with the above two ideal goals,namely,how to achieve pure spin current and half-metallicity with fully spin polarized transport in 2D materials,has become a hot topic in the field of spintronics in recent years.Therefore,in this thesis,we mainly aim at the scientific problems of how to regulate the spin polarized transport in 2D materials,and uses density functional theory and non-equilibrium Green's function methods to robustly achieve fully spin polarized transport and pure spin current through different strategies,such as thermal,optical and electrical stimuli.The followings are the main research contents of our studies:1.Realizing thermally induced spin current effect in silicon carbide nanoribbon by the edge co-doping mechanism.Based on the edge states features of ferromagnetic(FM)zigzag silicon carbide nanoribbons(ZSiCNRs),we propose an edge co-doping scheme to independently control the spin-polarized transport properties for different spin channels.It is found that the edge state transport channels can be separately turned off or kept open by specific edge doping.Interestingly,by replacing an edge C atom with a B atom and an edge Si atom with a P atom in the central region,the opposite slopes of electronic transmission for different spin channels around Fermi level are obtained("X" shape),which leads to a Seebeck coefficient with diferent signs for different spins.The subsequent thermoelectric field drives electrons of different spin channels in opposite directions,which leads unambiguously to a spin current.More importantly,by tuning the chemical potential and working temperature,pure spin current without net charge current can be achieved.2.Spin photogalvanic effect in silicene nanoribbons with asymmetric Hterminations.We propose an asymmetric hydrogenation scheme,namely,each edge Si atom at one edge is saturated by one H atom and each edge Si atom at the other edge is saturated by two H atoms,to achieve bipolar magnetic semiconducting behavior in zigzag silicene nanoribbons.Due to a fact that zigzag silicence nanoribbons with asymmetrical hydrogen passivated edges(H-2H ZSiNRs)have unique characteristics such as bipolar spin splitting around the Fermi level and the broken centrosymmetry,we further construct a spin optoelectronic device based on the H-2H ZSiNRs.It is found that,the flow direction of different spin channels,spin polarization and the magnitude of the photocurrent can be efficiently controlled by tuning the photon energy or polarization/helicity angle under the illumination of the linearly/elliptically polarized light.Interestingly,at a certain polarization/helicity angle or a certain photon energy,fully spin polarized current can be achieved by either linearly or elliptically polarized light.Moreover,when the two leads are in antiparallel magnetic configuration,pure spin current without an accompanying charge current can be robustly achieved,which is independent on the photon energy.These results suggest that spin-orbit coupling is not a necessary condition for producing spin-polarized transport with photogalvanic effect and H-2H ZSiNRs are promising candidates for designing novel spin optoelectronic devices in the future spintronic applications.3.Spin polarization antisymmetry driven pure spin current with photogalvanic effect.We propose a novel and robust scheme to generate pure spin current by photogalvanic effect(PGE)in 2D antiferromagnetic systems with spatial inversion symmetry.Because this kind of systems have the characteristics of spatial inversion symmetry of the atomic structure and spin density inversion antisymmetry,pure spin current without net charge current is guaranteed to be generated under the illumination of the polarized light,neither dependent of the photon energy nor dependent of the polarization feature of the light.Firstly,our scheme is successfully applied to a photoelectric device constructed with a zigzag graphene nanoribbon(ZGNR)which has intrinsic antiferromagnetic coupling between the two edges and spin degenerate band structure.It suggests that spin splitting is not a prerequisite for pure spin current generation.More interestingly,by further introducing external transverse electric fields to the two leads to lift the spin degeneracy,the device may behave multifunctionally,capable of producing fully spin polarized current or pure spin current,depending on whether the fields in the two leads are parallel or anti-parallel.Very importantly,our scheme of pure spin current generation with PGE is not limited to ZGNR and can be extended to other 2D centrosymmetric magnetic materials with spin polarization antisymmetry,suggesting a promising category of 2D platforms for PGE based pure spin current generation.4.Ferroelectric control of half-metallicity in A-type van de Waals antiferromagnets.2D A-type antiferromagnetic van der Waals(vdW)materials with intralayer ferromagnetic and interlayer antiferromagnetic coupling have recently attracted intensive attention due to their special spintronic properties.By first-principles simulations,we propose a vdW multiferroic heterostructure formed by sandwiching the bilayer 2H-VSe2 between two layers of 2D out-of-plane ferroelectric Sc2C02.It is found that the proposed heterostructure presents a half-metallic nature in the bilayer 2H-VSe2 and the conducting spin polarized state is localized in one layer of 2H-VSe2.Meanwhile,the spin polarity and the spatial location of the half-metallic state will be reversed with the reversal of the ferroelectric polarization.The mechanism giving rise to this peculiar feature originates cooperatively from the built-in electric field of the ferroelectric sandwich,and from the charge transfer selectively occurring only at one interface,Based on the above findings,two prototypes of nonvolatile memory devices are further proposed,in which two states("1" and "0")can be realized by switching the polarization direction of the ferroelectric layers.These results demonstrate that the use of 2D out-of-plane polarized ferroelectric materials to modulate A-type vdW antiferromagnets provides not only a fascinating way to achieve half-metallicity in 2D materials,but also a way to design a new type of nonvolatile ferroelectric memory device.5.Theoretical prediction of a 2D semiconductor tin dioxide(SnO2)with tunable magnetism by doping.Based on density functional theory calculation,we predicte a stable and new 2D phase of tin dioxide(P-4m2)that shows an auxetic behavior with an in-plane negative Poisson's ratio(NPR).The NPR in our predicted SnO2 originates from the synergistic interaction of the lattice symmetry and the SnO4 tetrahedron symmetry.Furthermore,our results show that,SnO2 is an indirect-gap semiconductor with a band gap of 3.7 eV,and its electron mobility is up to 103 cm2V-1s-1.Interestingly,the band structure of SnO2 presents double Mexican-hat-like band edges in the valence bands near the Fermi level.Due to such a unique band feature,a ferromagnetic phase transition takes place with a half-metallic ground state that can be induced by hole doping within a very wide concentration range.Such a magnetic phase can be well explained by the Stoner mechanism.These findings demonstrate that the predicted 2D phase of SnO2 is a rare example of p-type magnetism and a promising candidate for applications in nanomechanics and low-dimensional spintronics.