Manipulating Different Magnetization Configurations At Honeycomb Lattice Nanoribbons

Graphene-like materials have already become the major interest due to their unique electronic structure,density of states,and transport in the honeycomb lattice,including graphene,silicene,hafnium crystalline layers on layers on Ir(111),BN,and MoS2 etc.The magnetism in these samples is one of the concerned issues.The spin near the zigzag edges may be polarized as suggested by the experimental measure-ments and the first principle calculation.Generally,the spin is polarized oppositely at the opposite edges,i.e.,the state with antiferromagnetic correlation between the two edges(AFCE).Recent scanning tunneling spectra measurements reported a possible phase transition from AFCE state to the state with ferromagnetic correlation between the two edges(FMCE)upon expanding the nanoribbon width at the room temperature.Understanding the mechanism and further manipulating the phase transition are highly expected.On the other hand,generating and controlling the magnetism in nanoscale by means of the electric method are of course general interest,especially in graphene-like materials.We will focus on the two issues.The advances in these field will promise for future spintronics application,such as the magnetic field sensors,and the information storage technologyIn Chapter 1,we briefly review the microscopic structure and magnetism in graphene and graphene-like samples,together with some experimental techniques in-cluding scanning tunneling microscopy,scanning transmission electron microscopy,electron energy-loss spectroscopy,annular dark field imaging and electron energy-loss near-edge structure.In Chapter 2,we study the magnetism in the zigzag edged honeycomb lattice.For the undoped system,the ground state is always the AFCE state due to the constraint of the Lieb's theorem in bipartite lattices.For slightly doping,a robust magnetic con-figuration oscillation between the AFCE and FMCE state upon the finite size of the nanoribbon is unveiled.It was found that the magnetic phase oscillations are highly associated with the coherence between the two edges.We proposed a simple Landau theory to account for the oscillation by analysis the free energy.The intrinsic reason is that the edge magnetization is enhanced by acquiring the positive coherence with the other edge,which is realized by adjusting the edge magnetic correlations.Therefore,it is the quantum confinement(finite size)and interference(coherence)drives the phase oscillation.We further proposed that the phase transition may be manipulated in some degree by introducing the in-plance external magnetic field.In Chapter 3,we study the role of the external potential field in controlling the magnetism in graphene nanoribbons.The inversion symmetry is broken by the intro-duced field,which dramatically changes the coherence condition between the two edges and therefore manipulates the magnetic configurations of the nanoribbon with the fixed width.The system evolves into a magnetic state with the local magnetic momentum at only one side when the external potential field increases further.We therefore provide an easy and accurate access to control the magnetism in graphene-like nanoribbons.In case of strong electric field,the regular interval of magnetism phase upon the mag-nitude of field is unveiled,together with the movement of the dominating area of the magnetism.The magnetism originates from the itinerant electrons and is associated with the special 3/4-filling in the honeycomb lattice.The local magnetic momentum emerges when the 3/4-filling locates between the nearest neighbors sublattice from the different zigzag chain.We expect the robust magnetism can be measured experimen-tally and will be applied as the magnetic switch in future spintronics.In Chapter 4,the conclusions are summarized,together with some prospects.