First-principles Study On The Controlling Of Spin Transport Properties Of Zinc Oxide Nanoribbons

Charge and spin are the two intrinsic properties of electrons.Traditional electronic devices,which rely on the manipulation of electronic charge to realize information processing,are facing increasing challenges in terms of device miniaturization and integration.In contrast to traditional electronic devices,spintronic devices can simultaneously utilize both intrinsic properties of the electrons.As a consequence,these devices exhibit excellent characteristics,such as high speed,high integration,and low power consumption,thereby representing the most promising devices in the post-Moore era.Developing novel materials for spintronic devices,exploring their spin-related effects,and achieving electron spin transport manipulation have become a research hotspot in recent decades.Zigzag zinc oxide(ZnO)nanoribbons exhibit a unique edge magnetic effect.Owing to this property,it is possible to prepare spintronic devices based on ZnO nanoribbons.In this paper,density functional theory was used in combination with non-equilibrium Green's functions to study the spin transport properties of ZnO nanoribbons under the influence of defects,doping,magnetic field,and electric field.Several interesting transport phenomena were observed,and the following conclusions can be drawn.(1)Edge defects can regulate the spin transport properties of ZnO nanoribbons.The spin-up and spin-down transmission spectra of ZnO nanoribbons with different widths under zero bias were observed to be asymmetrical,thus exhibiting the spin polarization phenomenon.This phenomenon is caused by the orbital splitting of O 2p and Zn 3d at the edge of the ZnO nanoribbons.To improve the spin filtering effect of ZnO nanoribbons,O and Zn edge defects were used to control the spin transport properties of ZnO nanoribbons.It was found that the spin-up transport channel of the ZnO nanoribbon regulated by edge Zn defects was destroyed,while the spin-down transport channel at the edge O defects remained conductive.The spin filter effect could reach 100%under low bias voltage,indicating that edge Zn defects can effectively regulate the spin transport properties of ZnO nanoribbons.(2)Doping with transition metal elements can be used to control the spin transport properties of ZnO nanoribbons.Since the formation energies of Mn,Fe,and Co-doped ZnO nanoribbons are all less than zero,the corresponding doping systems can exist stably,thus enabling the realization of stable magnetic properties for the ZnO nanoribbons.The magnetic properties of these doped systems mainly originate from the unpaired electrons of the 3d orbitals of the doping atoms.Furthermore,the doping position of the transition metal elements was found to have different effects on the electromagnetic properties of the ZnO nanoribbons.When the transition metal elements occupy the O edge of the ZnO nanoribbon,the Mn and Co-doped systems are found to be metallic,while the Fe-doped system is semi-metallic.When Mn,Fe,and Co occupy the middle of the ZnO nanoribbon or the Zn edge,the Mn and Fe-doped systems are observed to exhibit semi-metal properties,while the Co-doped system has metallic properties.Finally,when the transition metal element occupies the Zn edge of the ZnO nanoribbon,the two edges of the nanoribbon are ferromagnetic,and the spin filtering effect is close to 100%.(3)A magnetic field can be used to manipulate the spin transport properties of ZnO nanoribbons.It was demonstrated that the two edges of the ZnO nanoribbon can be converted from a ferromagnetic state to an antiferromagnetic state via an external magnetic field.The calculation results show that the energy required for this transition process is low,and that the ZnO nanoribbon is susceptible to changes in the edge coupling state under the effect of the external magnetic field.Under the applied magnetic field,the Fe-doped ZnO nanoribbon changed from exhibiting semi-metallic properties dominated by spin-down transport to exhibiting semi-metallic properties dominated by spin-up transport.This effect was found to introduce significant changes to the spin transport channel of the nanoribbon,resulting in the value of I_AFM being much smaller than that of I_FM.Indeed,Fe-doped ZnO nanoribbons exhibited a giant magnetoresistance effect,with the magnetoresistance reaching a value of the order of 10⁶.This effect became even more pronounced(of the order of 10¹²)under application of the external magnetic fields,which controls the electrons transport.Furthermore,the Co-doped ZnO nanoribbons exhibited an abnormal magnetoresistance effect under the influence of the electrons magnetic properties.(4)An electric field can be used to modulate the spin transport properties of ZnO nanoribbons.A gate electrode was added to the bipolar device,and the spin transport of the ZnO nanoribbon was regulated via the gate voltage.Under modulation of the ZnO nanoribbon in a longitudinal electric field,it was found that a negative gate voltage can effectively control the spin filter effect of the device.As the negative gate voltage increased,the spin filter effect gradually increased.When the gate voltage was set to-3 V,the spin filter effect of the device could be adjusted to be 100%.During the modulation of the ZnO nanoribbon via a lateral electric field,the gate voltage was found to have a strong ability to control the spin filter effect of the device.Spin-up or spin-down currents could be selected through changing the gate voltage,thus permitting the realization of a spin filter selector based on ZnO nanoribbons.