First Principles Studies On Magnetic Spin Controlling Of Dilute Magnetic Semiconductor Nanowires And Silicenes

As a new academic subject,spintronics apply electron spin to electronic devices.Spintronic devices can process data more efficiently than traditional electronic devices.At present,there are three major challenges in developing spintronics,which are,spin-polarized carrier injection,long-distance transport of spins,and the detection and regulation of spins.The first step of solving these problems is to find or produce suitable spintronic materials.On the other hand,in order to meet the trend of increasing miniaturization and integration of electronic devices,study on low-dimensional spintronic materials is becoming more and more important.Thus,the first-principles studies of controlling the magnetic spin properties of traditional and new spintronic material,dilute magnetic semiconductor nanowires and silicene nanoribbons,are carried out in this dissertation.Our theoretical work can not only explain the magnetic properties of experimentally synthesized materials but also give an instruction of producing materials with desired magnetic properties.Firstly,the magnetic properties of wurtzite ZnS nanowires doped with five kinds of transition metal atoms?V,Cr,Mn,Fe and Co?are investigated.This system belongs to second-generation dilute magnetic semiconductors.However,in experimental studies of ZnS nanostructures doped with transition metal atoms,same doping systems often exhibit completely different magnetic properties in different experimentally synthesized samples.Thus,it is necessary to systematically study the magnetic coupling mechanism between transition metal atoms in ZnS nanostructures.In this chapter,four parts of work are carried out about ZnS nanowires doped with transition metal atoms:?1?The doping method that two transition metal atoms are doped in the interior of the nanowires has the lowest formation energy,which is selected as the investigated sample in the following part.?2?We calculated the energy difference between the ferromagnetic and antiferromagnetic states of these transition metal atoms doped ZnS nanowires.It is found that the ground state of the V or Cr-doped system is ferromagnetic while the ground state of Mn,Fe or Co doped system is antiferromagnetic.?3?We also introduced additional electrons or holes by replacing the S atom connecting the two transition metal atoms by a Cl or P atom.When co-doped with P,the ferromagnetism of the V or Cr doped systems is enhanced while the antiferromagnetism of the Fe or Co doped systems is enhanced.When co-doped with Cl,the ferromagnetism of the V or Cr doped systems is weakened while the antiferromagnetism of the Fe or Co doped systems is weakened.For Mn-doped system,both P and Cl dopants will enhance the ferromagnetism of the system.?4?The S atom connecting the two transition metal atoms is removed to investigate the effect of an S vacancy on the magnetic coupling.Above calculation results can be explained by the energy band coupling model under the wurtzite structure,and this model is modified from the one under the zinc-blende structure.In this chapter,we investigate the magnetic coupling mechanism between transition metal atoms which are doped in ZnS nanowires.Our study can provide an instruction to experimentally produced ZnS nanostructures with desired magnetic properties.Secondly,we calculate the electromagnetic properties of wurtzite GaN nanowires doped with Gd atoms,mainly the magnetic coupling mechanism in it and the realization of room temperature ferromagnetism by co-doping with Cu or C atoms.This system belongs to the third-generation dilute magnetic semiconductors.The Gd-doped GaN system has been widely discussed since it was experimentally found to have room temperature ferromagnetism and colossal magnetic moments.The origin of the room temperature ferromagnetism and colossal magnetic moments are still unclear.In order to investigate whether the system can exhibit new useful properties in low-dimensional materials,Gd atoms are doped into GaN nanowires by replacing Ga atoms with Gd atoms in this chapter.The energy difference between the ferromagnetic and antiferromagnetic states of the system is almost 0(?E=E_FM-E_AFM=0)when only Gd atoms are doped in,which is consistent with the previous calculation results in three-dimensional GaN doped Gd materials.In order to make the system have a stable ferromagnetic state and thus can be used as practical spintronic material,we try to enhance the ferromagnetic coupling in it by introducing holes into the system.We first introduce holes by replacing the Ga atoms with Cu atoms.Each Cu atom can introduce two hole states.The stability of the ferromagnetic state is significantly enhanced(?E=E_FM-E_AFM=40 meV)after co-doping in Cu atoms.By comparing the partial density of states and the energy band structure of the system before and after co-doping in Cu atoms,it can be seen that the electronic state of Gd-4f is located deeply below the Fermi level which means that it can hardly couple with free carriers to stabilize the ferromagnetic state.So Gd-4f electrons hardly participate in the magnetic coupling of the system.However,there is a strong p-d coupling between Cu-3d and N-2p states which pushes the N-2p state on the valence band maximum to the Fermi level and then an obvious asymmetry between the spin up and the spin down channel arises near Fermi level.A strong p-d coupling occurs between the changed N-2p state and the Gd-5d state which makes the ferromagnetic state gain energy thus more stable than the antiferromagnetic state.The total magnetic moment of this co-doped system is the sum of the magnetic moment of Gd atoms and the number of the holes introduced by the Cu atoms.We continue to increase the hole concentration of the system to further increase the energy difference until the ferromagnetic state can exist stably at room temperature.Then,we introduce hole state into GaN doped Gd nanowire by replacing the N atoms with C atoms.It is found that a strong p-d coupling between the Gd atom and the neighbor C atom which makes the system has a ferromagnetic state at room temperature.As the distance between the C atom and the Gd atom increases,the p-d coupling is rapidly weakened and so does the stability of the ferromagnetic state.By comparing the energy band structures of different doping methods,it can be seen that the unoccupied states introduced by C atoms are closely related to the doping position of C atoms.And the partial charge density tells us that these unoccupied states are mainly derived from C atoms while small amount comes from the N atoms.Finally,we calculate the magnetic properties on the zigzag edges of rectangular silicene nanoribbons?RZSiNRs?which is a new-type spintronic material.Silicene as a graphene-like material has similar geometric and electronic structures to graphene.It has caused extensive concern because it can be well compatible with silicon-based electronic devices that are widely used today.Many previous theoretical studies have shown that the infinitely long zigzag-edged silicene nanoribbons have a stable antiferromagnetic state,where the spin-up and spin-down magnetic moments are separated on the two zigzag edges.However,the size of the silicene nanoribbons prepared experimentally are always finite.In order to explore the influence of sample size on the magnetic properties and provide a reference for the experimental preparation of the silicene nanoribbons with desired magnetic properties,we calculate the magnetic properties of silicene nanoribbons with different size.We find that:?1?A stable antiferromagnetic state appears at the two zigzag edges when the size of the nanoribbon is larger than RZSiNRs[4,3].And the three magnetic states are ordered from low to high energy:antiferromagnetic state,ferromagnetic state and non-magnetic state;?2?Magnetic moments on the zigzag edges originate from boundary Si-pz states.?3?While increasing the length of the zigzag edge,the averaged magnetic moment of Si atoms on zigzag edges approaches a specific value—the averaged magnetic moment of Si atoms on zigzag edges of the infinitely long zigzag-edged ribbon with the same width.?4?Al atom dopants can effectively manipulate the magnetic moment distribution and the magnetic coupling state of the two zigzag edges.