| Spintronics is a new field of research exploiting the intrinsic spin of the electron and its associated magnetic moment,in addition to its fundamental electronic charge,on electrical conduction and information processing in solid-state devices.The property that electric spin is used for information transportation brings great advantages in information technology,such as faster data processing,higher circuit integration and lower energy consumption.There have been a number of spintronic devices,like spin valves,nonvolatile memory devices,read heads,and so on,which are widely used.However,in the spintronics device,three aspects:how the spin is generated and injected,the spin relaxation and dephasing in the process of spin transfer,and the control and detection of spin,have always been the great challenges for the development of spintronics.In order to solve these problems,people try to find the ideal spintronics materials.Currently the most widely used spintronic materials include ferromagnetic metals,magnetic semiconductors,diluted magnetic semiconducors,but these materials are not ideal as a result of the low-efficiency spin injection of ferromagnetic metal,difficult synthesis of magnetic half-metallic semiconductor,low ferromagnetic transition tempareture of diluted magnetic semiconductors,and so on.With the increasing degree of miniaturization of integrated circuit devices,the scale has reached the nano level where quantum coherence effects can happen,and the bulk materials become more and more unable to meet the requirements.At this time,the two-dimensional materials with atomic thinkness turn out to be very superior,due to not only the intrinsinc properties of two-dimensional materials,such as the light and thin nature,good flexibility and higher degree of stability than bulk materials in nanoscale,but also the convenience of integrating that the combination of two dimentional materials into van der Waals heterostructures that does not have the requirement of lattice matching and specific stacking angle.Among various 2-D materials,graphene has been identified as an ideal candidate for spintronics due to its long mean free path for spin transport as a result of its intrinsically weak spin-orbit coupling.In addition,graphene has many other superior properties for spintronics such as high carrier mobility and high thermal conductivity.However,pristine graphene is by itself intrinsically nonmagnetic,so certain modifications have to be made before it can be applied for spintronics.Based on the previous work on the magnetism of graphene and encouraged by recent experimental breakthroughs,here we propose a new graphene-based material-half-hydrogenated graphene nanoroads(HHGNRs),which consists of a half-hydrogenated graphene nanoroad embedded in a fully hydrogenated graphene sheet.Using first-principles density functional theory calculations,HHGNRs are fully spin-polarized and show a nature of bipolar ferromagnetic semiconductor.More importantly,as a result of areal magnetization enabled by half-hydrogenation,the overall magnetism of such a nanoroad is very robust against a variation of either its width or orientation,in sharp contrast with most of previously proposed designs of graphene-based magnetic materials that rely on the edge effect and are vulnerable to chemical contamination or structural defects.We have also proposed a design of an HHGNR-based device to examine the predicted bipolar ferromagnetic semiconducting nature through an all-electric controlled approach.Multiferroics are defined as materials that exhibit more than one of the primary ferroic properties at the same time,which are ferroelectroncity,ferromagnetism and ferroelasticity.Among various multiferroics,magnetoelectric multiferroic materials have attracted a lot of attention due to the potential technological use of controlling polarization with an magnetic field or magnetism with an electric field as a result of the magnetoelectric coupling,especially the property of the control of magnetic properties by electric fields can allow a valtage pulse to replace a magnetic-field-generating electric current in the reading and writing of a magnetic bit,which will reduce the size and energy loss of devices,also cut down the response time.However,in the more numerous and old type-I multiferroics,magnetoelectric coupling is very weak,mainly due to the independent nature of ferroelectricity and magnetic ordering.It is very difficult to realize controlling polarization with a magnetic field or magnetism with an electric field in this kind of materials.In type-II multiferroics,the ferroelectricity is caused by the magnetic ordering,and the magnetoelectric coupling is quite strong because of the nature between ferroelecticity and magnetic ordering.But the orderings of ferroelectricity and magnetism usually coexist at very low temperatures,and small net magnetization due to antiferromagnetic spin-spiral structures and low polarization is exhibited.In addition,the control of magnetism by electric field is limited.To date,the realization of multiferroics with strong magnetoelectric coupling still remains difficult.Based on the advantages of two-dimensional materials mentioned above,one always expects to realize more amazing properties in two-dimensional materials.Although the researches on multiferroic materials have been continued for many years,there are few strudies on magnetoelectric two-dimensional multiferroic materials.Here,we propose a design of a new class of 2D mutiferroic materials based on a 2D ferroelectric material In2Se3 with magnetic doping,which promise a strong magnetoelectric coupling.In the ferroelectric In2Se3,there exist two inequivalent In sites with different local symmetries for the magnetic dopants to substitute while they are interconvertible,by which the magnetic dopants at different In layers have different magnetic orders and the magnetic property of the system can be altered by flipping the electric polarization of the system with an applied external electric field.In this way,at a certain temperature,the same sample can be switched between ferromagnetic state and paramagnetic state by adjusting the electric field.Using density functional theory calculations,we have identified Mn and Co as good candidates for magnetic doping in such a 2D mutiferroic system.This research will provide a way to study the 2D magnetoelectric multiferroic materials.Therefore,in the first chapter of this paper,spintronics,two-dimensional materials and single-phase multiferroic materials were introduced.In the second chapter,the calculated method used in this paper was briefly stated.The third chapter mainly introduced the theoretical design of robust ferromagnetism and bipolar semiconducttivity in graphene-based nanoroads.The fourth chaper described the the the theoretical design of two-dimentional multiferroic materials based on two-dimensional ferroelectric In2Se3.Finally,the summary and outlook were contained in chapter five. |
