Research On The Regulation Of Spin Transport Properties In Quasi-one-Dimensional Molecular Devices

Today,as we can see,it is a charismatic research territory to further explore and use the electronic spin degrees of freedom,whether it is in the basic theory or the practical application field.Applying this new degrees of freedom which elucidated by the quantum mechanics to the solid-state devices,resulting in a discipline so called spintronics which has already achieved a record-breaking gorgeous transition from theoretical ideas to practical applications only through a decade of development,and it is far away from showing its full potential power.The spintronics devices utilize the fact that the transport current is actually composed of carriers which have two different kinds of spins,encoding the information carried by the spin transport current according to different spin states,and is ultimately identify by its different coupling states with the magnetic electrodes.Therefore,when we use the spin and magnetic moment characteristics that we have in a miniaturized electronic device system more effectively,such device based on spintronics will have significantly better characteristics than conventional electronic devices.For example,higher integration,faster operating speed,extremely low power consumption and unique non-volatile,etc.At the same time,as electronic devices continue to be miniaturized in accordance with Moore's Law,more and more subtle process technology has made the size of today's transistor devices gradually approaching the molecular scale.And that has further promoted the development of molecular electronics.The basic theory and experimental techniques have demonstrated that the molecules often have weak spin-orbit coupling and abundant hyperfine interaction,and makes it possible to maintain spin coherence over longer relaxation times and distances in molecular electronic systems than with the conventional metal and semiconductor materials.Undoubtedly,these kinds of intrinsic quantum property will make it possible to make devices that combine the advantages of both molecular electronics and spintronics,and this so-called molecular spin electronics device has a very unique and attractive prospect.Here,we have analyzed the magnetic transport properties of several different molecular devices by using the non-equilibrium Green's function formalism combined with density functional theory.In our first job,we have investigated the effects of the redox reaction on the magnetic transport properties of a single anthraquinone(AQ)molecule connected to zigzag graphene nanoribbon electrodes.Two kinds of contacted types,isomeric AQ-14 and AQ-15,are considered in this work.The results show the excellent spin-filtering with 100%spin filtering efficiency can be found in AQ-14 molecular device.Redox reaction on the molecule doesn't affect its spin filtering behavior.After the contacted type changing,the spin-filtering behavior is only found in AQ-15 molecular device at the reduced state.When the molecule is oxidized,the a-spin current of the device is reduced dramatically leading to the absence of the spin-filtering behavior.More importantly,the on-off of the a-spin electronic conductance of AQ-15 induced by redox reaction can allow it be designed as a spin current switching.And in another research,we investigate the spin transport properties of a single phenalenyl or pyrene molecule connected to zigzag graphene nanoribbon electrodes by using the non-equilibrium Green's function formalism with density functional theory.We found the difference of the symmetry on these two molecules will bring a remarkable effect on the spin transport properties of the devices.The spin-resolved currents of the single pyrene molecular device are much lower than that of the single phenalenyl molecular device when they all connected to electrodes symmetrically.In addition,we found the change of the connected site will decrease the spin-resolved currents of the phenalenyl-based molecular device drastically,but had no longer any influence with the pyrene-based molecular device.The results will be helpful for us to futher understand the transfer of the spin-carriers in the spintronic systems.In the final work,we have investigated the spin transport properties of single porphycene molecular devices connected to ZGNRs electrodes using different connection types,and also have explored the effects of electrode width variations on them.In the single porphycene molecular device systems which employing a horizontal connection style,perfect spin-filtering characteristics with almost 100%spin-filtering efficiencies are exhibited and are less affected by changes in electrode width.At the meantime,its ?-spin transport currents exhibit a negative differential resistance,and an increase in the width of the electrode will intensify the peak-to-valley ratio of ?-spin transport current.Further analysis shows that the reason for the negative differential resistance characteristics is the decrease of the transmission pathways on the both sides of electrodes as the bias voltage rising up.However,the changing of connection type improves the transport of the two kinds of spin electrons in single porphycene molecular devices with a zigzag staggered connection,and also makes the spin filter effect in the device no longer apparent.At this time,the increase of the electrode width can significantly enhance the spin transport currents of these kind of devices,and further weaken the spin filtering effect in the device.The final results show that the connection mode in spintronic devices is also an important factor influencing the spin transport properties of the molecular device.