Theoretical Studies On Hybrid Interface States And Spin Transport Properties In Molecular Magnetic Tunnel Junctions

The magnetic tunnel junction based on the magnetoresistance effect is one of the core structural units of the hard disk magnetic read head.It updates information storage technology and makes the hard disk develop rapidly in the direction of high storage density and miniaturization.With its continuous miniaturization,people began to pay attention to using organic molecules as the intermediate layer of the magnetic tunnel junction to construct molecular magnetic tunnel junctions.Compared with the intermediate layer of traditional inorganic materials,organic molecules have a smaller size,more modifiable ways,faster response speed,and lower cost and power consumption.In addition,organic molecules are mainly composed of carbon,hydrogen,and other light elements.Therefore,they usually possess the advantages of weak spin-orbit coupling and long spin relaxation time,which is conducive to the control and transport of spin.However,the mechanism of spin transport in molecular magnetic tunnel junctions is more complicated.One of the reasons is that orbital hybridization occurs when organic molecules are in chemical contact with ferromagnetic metals,resulting in hybrid interface states.The hybrid interface states are located near the Fermi level,which plays a decisive role in the spin transport properties of the device at low bias voltage.At the same time,its spin polarization is more sensitive to the interfacial contact configuration and molecular structure,and has strong controllability.Researchers have conducted a lot of investigations on ferromagnet/molecule spinterfaces.However,so far,the mechanism of the effect of hybrid interface states on spin transport properties in molecular magnetic tunnel junctions is still unclear.How to manipulate the hybrid interface states,and further modulate the spin transport properties of molecular magnetic tunnel junctions remains to be explored.In this thesis,using the nonequilibrium Green’s function method combined with density functional theory,the effects of molecular anchoring groups,protonation,molecular geometrical torsion angle,quantum interference,and functional groups on the hybrid interface states and spin transport properties of molecular magnetic tunnel junction have been systematically investigated,respectively.The main contents and relevant results of the researches are summarized as follows:The contact details between the molecule and the ferromagnetic metals have a significant impact on the hybrid interface states,and the atoms with different hybrid types in the molecule can hybridize with the 3d orbitals of the ferromagnetic metals in different forms.In molecular magnetic tunnel junctions,the anchoring group is a bridge connecting the molecule and the ferromagnetic electrode,which affects the contact and coupling between the molecule and the electrode.We have used anchoring atoms with different hybrid types and electronegativity to study their effects on the hybrid interface states and spin transport properties.The results show that the change of spin polarization of hybrid interface states with energy depends on the hybrid types of the anchoring atoms,and the anchoring atoms with greater electronegativity will make the spin polarization curves occur red shift.This red shift will lead to a significant change of spin polarization near the Fermi level.Further transport calculations show that the spin transport properties of molecular magnetic tunnel junctions are closely related to the hybrid interface states.When the anchoring atoms with the same hybrid type are used,the one with greater electronegativity will lead to the higher tunnel magnetoresistance of the device under low bias voltage.Brooke et al.observed the conductance switching phenomenon of 4,4’-vinylenedipyridine(44VDP)single-molecule junction caused by the change of solution p H value in the experiment.They pointed out that this phenomenon was caused by the protonation of the pyridine group in the 44 VDP molecule,which changed the bonding between the molecule and nickel electrode.But they did not give a detailed theoretical explanation.Based on the 44 VDP single-molecule device structure in the experiment,we have further considered the relative magnetization directions of the two nickel electrodes and studied the effect of protonation on the hybrid interface states and spin transport properties in 44 VDP molecular magnetic tunnel junction.The numerical results have clarified the switching mechanism in the experiment,namely,the protonation of the pyridine group in the 44 VDP molecule weakens the interface coupling strength between the molecule and the metal.More importantly,we have found that the protonation reduces the number of hybrid interface states and modifies the spin polarization at the ferromagnet/molecule interface,and further leads to an obvious change in the spin transport properties of the molecular magnetic tunnel junction,such as the inversion of tunnel magnetoresistance from positive to negative values,and the significant increase of current spin polarization.In the previous research works,we have studied the hybrid interface states and spin transport properties in the molecular magnetic tunnel junctions by changing the contact details between the molecule and the ferromagnetic electrodes.Then,a question is worth exploring.If the contact details between the molecule and the electrode are kept unchanged,and only the molecular structure is changed,whether the hybrid interface states in the molecular magnetic tunnel junction can be changed,and what is the effect on the spin transport properties? We have first investigated the effect of the molecular geometrical torsion angle on the hybrid interface states and spin transport properties of the biphenyl molecular magnetic tunnel junction.The calculation results reveal a different response mechanism of the hybrid interface states relative to the molecular orbitals as the change of torsion angle between the two phenyl rings in the biphenyl molecule.From the perspective of the density of states,the hybrid interface states hardly change,while the molecular orbitals will move obviously;From the perspective of the transmission spectrum,the position of the transmission peaks contributed by the hybrid interface states remains unchanged and the height of their transmission peaks changes,while the position of the transmission peaks contributed by the molecular orbitals shifts and the height of their transmission peak almost remains unchanged.Further transport calculations show that the spin-dependent current contributed by the hybrid interface states is suppressed by the increase of the torsion angle with a relation of cosine square of the torsion angle,which is consistent with the results in the nonmagnetic molecular junction without obvious effect of hybrid interface states.However,the tunnel magnetoresistance of the device can be enhanced by increasing the torsion angle at relatively high bias voltage.The dependence of the tunnel magnetoresistance on the torsion angle relies on the bias voltage and the detailed situation of the hybrid interface states,and the latter can be achieved by changing the anchoring groups.In molecular magnetic tunnel junctions,the hybrid interface states near the Fermi level are extremely important for the spin transport properties of the device.In addition to the hybrid interface states,another phenomenon that can effectively control transport near the Fermi level in the single-molecule junction is destructive quantum interference,that is,the transmission probability of electrons is strongly suppressed at certain energies.We have explored an effective method to manipulate the spin transport properties of molecular magnetic tunnel junctions by combining hybrid interface states with destructive quantum interference.It is found that the position of the anti-resonance feature of destructive quantum interference can be effectively tuned by substituting the carbon atoms at different positions of the central benzene ring of the meta-linked oligo-phenylene-ethynylene(OPE)molecule with a nitrogen atom.The change in the position of the anti-resonance feature of destructive quantum interference caused by the molecular structure has almost no effect on the density of states of the hybrid interface states,but it can selectively suppress the transmission probabilities contributed by the hybrid interface states.This finally leads to the enhancement or reduction of the current spin polarization,which realizes the modulation of the spin transport properties of the molecular magnetic tunnel junction.According to the relative position between the anti-resonance feature of destructive quantum interference and the hybrid interface states,three different patterns are specifically proposed.That is,the anti-resonance feature of destructive quantum interference is located in the middle,left or right of the two transmission peaks contributed by the hybrid interface states,which causes the current spin polarization in the order of high,medium,and low,respectively.In addition,we have also found that a similar phenomenon can be achieved by substituting the hydrogen atom at the same position in the central benzene ring of the OPE molecule with the side group.Previous research works have shown that only changing the molecular structure hardly affects the density of states of the hybrid interface states,but affects the transmission probabilities contributed by the hybrid interface states.Is this a general conclusion? We will further investigate this issue.Keeping the contact details between the molecule and the ferromagnetic electrodes unchanged,we have studied the effects of functional groups on the hybrid interface states and spin transport properties of Anthraquinone(AQ)molecular magnetic tunnel junction.The numerical results show that by substituting neutral,electron-withdrawing and electron-donating functional groups on the cross-conjugated sites of the AQ molecule,the hybrid interface states,and their spin polarization can be significantly modulated.Orbital analyses show that this is due to the distinct movement of the molecular orbitals caused by the functional groups,so as to overlap with the hybrid interface states.Further transport calculations show that the tunnel magnetoresistance is enhanced at low bias voltage and reversed at high bias voltage by using electron-withdrawing functional groups.This is due to the sudden change of spin polarization of electron states near the Fermi level.This thesis consists of the following eight chapters.The first chapter is the introduction,which mainly introduces the development process of molecular spintronics,as well as the research progress of molecular spintronic devices and ferromagnet/molecule hybrid interface states effects.The second chapter mainly introduces the theoretical calculation methods used in the subsequent relevant researches of this thesis: density functional theory,which is the commonly used electronic structure calculation method,and nonequilibrium Green’s function method,which is the calculation method of electronic transport properties in molecular devices.Chapters three to seven are the research works,which introduce the specific contents of the researches and the theoretical results in detail.Among them,in Chapter three,the effects of anchoring groups on hybrid interface states and spin transport properties of the benzene molecular magnetic tunnel junction have been studied.By using anchoring atoms with different hybrid types and electronegativity,the laws of the spin polarization of hybrid interface states and the tunnel magnetoresistance of molecular magnetic tunnel junctions vary with the anchoring atoms is found.In chapter four,the effects of protonation on hybrid interface states and spin transport properties of the 44 VDP molecular magnetic tunnel junction have been studied.We have not only explained the experimentally observed conductance switching phenomenon of the44 VDP single-molecule junction with the p H value of the solution,but also designed a multifunctional molecular magnetic tunnel junction with protonation-controlled inversion of tunnel magnetoresistance and enhancement of current spin polarization.In chapter five,the effects of molecular geometrical torsion angle on hybrid interface states and spin transport properties of the biphenyl molecular magnetic tunnel junction have been studied.It is found that the hybrid interface states and the molecular orbitals changed with the molecular geometrical torsion angle are completely different.In chapter six,the effects of quantum interference on hybrid interface states and spin transport properties of the OPE molecular magnetic tunnel junction have been studied.We have proposed a strategy to modulate the transmission probabilities contributed by the hybrid interface states and manipulate the current spin polarization by using the position-adjustable anti-resonance feature of destructive quantum interference.In chapter seven,by substituting functional groups with different properties on the cross-conjugation sites of the AQ molecule,it is found that when the contact details between the molecule and the electrodes in the molecular magnetic tunnel junction remain unchanged,the movement of the molecular orbitals caused by the substitution of functional groups in the molecule can also have a significant impact on the density of states of the hybrid interface states.Finally,chapter eight is the conclusion and prospect,which is a comprehensive and detailed conclusion of the research works of this thesis,and prospects on the development of molecular magnetic tunnel junctions in the future.