基于γ-石墨炔分子磁隧道结对称性依赖的输运性质

Symmetry-Dependent Transport Properties ofγ-Graphyne-based Molecular Magnetic Tunnel Junctions

利用非平衡格林函数和密度泛函理论,研究不同类型γ-石墨炔分子磁隧道结(MMTJ)自旋极化输运特性的影响。磁隧道结以铁磁性的锯齿形石墨烯纳米带作电极。随着纳米带宽度变化,考虑γ-石墨炔的两种接触点,我们构造了8种有代表性的且具有不同对称性的隧道结。通过计算我们发现,对称性对磁隧道结的自旋输运起决定性作用。对于偶数碳链的锯齿形石墨烯纳米带,石墨炔的接触点位居于正中,这种结构的自旋极化输运性质远优于其它结构。比如在非常宽的偏压范围内都能达到100%的自旋极化率,且隧穿磁阻(TMR)高达3.7×10^(5),这表明该结构在自旋滤波器和自旋阀器件方面的应用潜力最大。与之形成对比的是,当耦合位置偏离锯齿形石墨烯纳米带的中心时,输运性质迅速变为普通电输运,相应的巨磁阻效应比最优对称结构约小4个数量级。

The molecular magnetic tunnel junction(MMTJ)with high tunnel magnetoresistance(TMR)is an important component for devices such as computers and electronic storage.With the rapid development of the modern electronics industry,the decrease of device size and the increase of area density,it is important to improve TMR technology.In addition,the computing process faces huge challenges.As the size of electronic devices decreases,small changes may cause completely different transmission characteristics,therefore the minute details of the device must be carefully controlled.In this paper,in order to find large TMR values and explore the role of symmetry on spin-polarized transport properties,γ-graphyne nanodots(γ-GYND)coupled between ferromagnetic(FM)metallic zigzag graphene nanoribbon(ZGNR)electrodes were used.Depending on the widths of the ZGNR and two types of contact positions between the ZGNR andγ-graphyne nanodots(γ-GYND),eight ZGNR/γ-GYND/ZGNR MMTJs with different symmetries were constructed.By using Keldysh non-equilibrium Green’s function(NEGF)and density functional theory(DFT),the I–V curve,the spin-injection efficiency(SIE)and TMR of MMTJs were calculated.We found that the transport properties of these MMTJs differed substantially.For absolute symmetric MMTJs,due to the wave functions corresponding to the band structure near the Fermi energy having different parity,the electron transport between the wave functions with different parity is prohibited,so we can see that the spin-down current is always zero.This implies that these absolutely symmetrical structures have 100%spin injection efficiency over a wide range of bias voltages.In addition,the calculation results also show that these absolutely symmetric structures also have large TMR at low bias,up to 3.7×10^(5),indicating that these devices have a large magnetoresistance effect and high magnetic field sensitivity,which can be used in the read head of computer hard disks,MRAM,and various magnetic sensors.However,for these asymmetric MMTJs,since there is no limitation of the wave function parity of the left and right electrodes,the spin-up current and spin-down current fluctuated as the bias voltage increased,so perfect SIE does not appear.In addition,the calculation results showed that the TMR of asymmetric MMTJs were four orders of magnitude smaller than with symmetric MMTJs.Thus the symmetry of MMTJs has a great influence on the spin-polarized transport properties of the device.These absolutely symmetrical MMTJs have spin-polarized transport properties that are far superior to other MMTJs.This is conducive to the manufacture of spin filters,rectifiers,and various magnetic sensors.Finally,these excellent characteristics can be explained by the transmission coefficient,local density of states(LDOS)and band structure.