Study On Spin Transport Mechanism In Organic Multiferroic Tunnel Junction

The research of organic multiferroic tunnel junctions(OMFTJs)involves spintronics,organic electronics and ferroelectrics related materials,structures and their rich physical properties,which have attracted the interest of many researchers at home and abroad.Compared with inorganic spintronics,spin valves or magnetic tunnel junctions based on organic materials have attracted much attention due to their advantages such as long spin life,simple preparation,low cost,and rich functional group adjustability.Among them,ferroelectric organic polymers,such as polyvinylidene fluoride(PVDF)and its copolymers,have attracted wide attention due to their ability to form high-quality ordered layers and exhibit greater ferroelectric properties.However,the transport properties of organic multiferroic tunnel junction devices,especially the spin-polarized electron transport in organic ferroelectric materials as the intermediate layer,have not been fully understood.For these reasons,in this article,we studied the temperature dependence of the transport properties in OMFTJ based on La_0.6Sr_0.4MnO₃/PVDF/Co/Au structure.The specific research contents are as follows:(1)Based on the structure of La_0.6Sr_0.4MnO₃/PVDF/Co,the samples of organic multiferroic tunnel junction were prepared,and the process of OMFTJ with this structure was optimized.First of all,in order to obtain the best PVDF performance,the improvement of the preparation process of PVDF film was explored.We annealed the PVDF film at different temperature,and adjusted the thickness of the film by adjusting the concentration of PVDF,so as to optimize the preparation process of PVDF film.The results show that the PVDF film with the concentration of 20mg/ml after spin coating was annealed at 150?to form the best performance..Under the observation of atomic force microscope(AFM),the surface roughness is low and the crystal characteristics of ferroelectric?phase are obvious.Through the detection of piezoelectric microscope(PFM),it can carry out clear ferroelectric control,and under the measurement of conductive AFM(C-AFM),it was found that the damage behavior caused by pinhole or leakage current will not occur within the range of±3V polarization voltage.Then the diffusion behavior of Co from the top electrode layer to the middle organic layer in the preparation of La_0.6Sr_0.4MnO₃/PVDF/Co/Au device was investigated.The structure and local chemical composition of La_0.6Sr_0.4MnO₃/PVDF/Co/Au device were analyzed by scanning transmission electron microscopy(STEM)and electron energy loss spectroscopy(EELS).We found that the grown Co electrode diffused very limited to the PVDF organic layer.(2)Study on magnetoelectric transport properties in La_0.6Sr_0.4MnO₃/PVDF/Co structure.We selected the optimized organic multiferroic tunnel junction samples.First,we studied its electrical transport performance at different temperatures.It was found that the resistance changes with temperature at different bias voltages are different.For zero voltage bias,the resistance changes in three temperature ranges,while at+0.5V bias,the resistance almost monotonously increases with the increase of temperature.Then the changes of tunneling magnetoresistance and tunneling electroresistance in different temperature and polarization state were studied,the results show that the positive and negative polarization of PVDF will lead to different tunneling magnetoresistance(TMR)signals,resulting in a four resistance state,and the value of TMR will decrease with the increase of temperature.The tunneling electroresistance(TER)also has an extremely complex variation trend in the three temperature ranges.Finally,we analyzed the different transport mechanisms of TER in three different temperature ranges in detail.When the temperature is lower than 60K,the resistance of the two polarization decreases with the increase of temperature.The main reason is that the transition behavior of electrons in the organic barrier layer is dominated by the increase of temperature.Between 60K and 120K,the resistance increases with the increase of temperature,which is mainly due to the increase of electron scattering caused by thermal strain.From 120K to room temperature,the resistance decreases rapidly with the increase of temperature.It was found that the thermal fluctuation of ferroelectric domain plays an important role in the electric transport.