Effects Of Length And Spin-orbit Coupling In Organic Spin Valves

Organic spintronics is a new field of spintronics,which owns the merit in future design of low-cost flexible spin devices.Magnetoresistance effect of organic spin valves is a hot topic in organic spintronics.Due to the weak spin-orbit coupling and hyperfine interaction of organic materials,the spin relaxation time in organic devices is much longer,which is beneficial to the spin transport.Therefore,based on different materials and different experimental techniques,various organic spin valves have been synthesized in recent years,such as LSMO/Alq₃/Co,Fe₃O₄/Rubrene/Co,LSMO/Cu Pc/Co,and LSMO/Ir（ppy）₃/Co,etc.Although a lot of research works on organic spin valves have been performed,there still exist some phenomena and questions remaining obscure.Firstly,in the measurement of length-dependence of the magnetoresistance in various organic spin valves,it has been found that the magnetoresistance shows different behaviors with the length of the central molecule.Although several calculations about specific systems have been reported,a deep physical understanding about the length dependence of magnetoresistance is still lack.Secondly,although the spin-orbit coupling in organic materials is weak,recent studies show that considerable spin-orbit coupling can be achieved in organic materials in some cases.On the one hand,many organic materials contain heavy elements,such as S atom in thiophene,Cu atom in copper phthalocyanine,and Al atom in 8-hydroxyquinoline aluminum.On the other hand,it is found that the structural modulation of organic molecules,such as the change of twist angle,will also lead to a significant increase of the spin-orbit coupling strength.Spin Hall effect caused by organic spin-orbit coupling has been measured experimentally.Therefore,it is necessary to explore the role of organic spin-orbit coupling in the spin valve magnetoresistance.Aiming at the above issues,this thesis has carried out the following research:（1）Based on Su-Schrieffer-Heeger model and Green’s function method,the length effect of organic molecules on the tunneling magnetoresistance was studied.In the regime of tunneling transport,the length effect is investigated under three mechanisms by considering three kinds of typical energy match between the Fermi surface of the electrode and the molecular energy levels,that is,barrier tunneling,resonance tunneling and transition from barrier tunneling to resonance tunneling.The results show that under the barrier tunneling mechanism,the electron transmission decreases with the length,and the magnetoresistance decreases exponentially or linearly with the increase of the chain length.In the latter two mechanisms,the magnetoresistance changes with the chain length in an oscillatory nonlinear manner.The research results are consistent with the experiments qualitatively.The results are further analyzed from the transmission spectrum.（2）Using the extended Su-Schrieffer-Heeger model and Green’s function method,the effects of spin-orbit coupling in organic spin valves on spin-polarized transport and magnetoresistance are calculated.The effect of softness of organic materials are fully considered,where the spin mixing and spin transport caused by spin-orbit coupling in rigid,dimerized and polaronic lattices are compared.It is found that compared with the rigid lattice,the spin mixing of electronic states can be enhanced by the dimerization and polaron lattice distortion at specific spin-orbit coupling strengths.In the case of antiparallel spin configuration of the ferromagnetic electrodes,the spin-orbit coupling has a stronger effect on the transmission spectrum.By calculating the spin-dependent current and magnetoresistance,it is found that the spin-orbit coupling can reduce the magnetoresistance,but the effect is related to the bias voltage and lattice configuration.