Study On Interfacial Effects And Related Devices Of Multiferroic Oxide Heterostructures

Multiferroic materials have been one of the research hotspots in the field of materials due to the magnetoelectric coupling effects.Among the single-phase multiferroic materials,BiFeO3 has attracted much attention for the coupling effect between ferroelectric and antiferromagnetic.However,it is difficult for BiFeO3 film to obtain excellent ferroelectric due to leakage current,which greatly weaken the coupling effect.In the composite multiferroic materials,the classical system with magnetoelectric coupling is ferroelectric/ferromagnetic structure.The coupling effect at the interface is very complicated and the abundant physical connotation deserve to be studied.In addition,the development of oxide heterojunction devices offers new possibilities for the electronic field.The oxide p-n junctions and p-i-n junctions are playing an important role in the basic research and designing of future oxide devices.Besides,the oxide multiferroic tunnel junction(MFTJ)can realize stable four resistance states in a single cell,which can bring about a revolutionary increase in high-density storage.Our works focus on the interfacial effects of multiferroic oxide heterostructures and related research of devices,and systematic study have been realized.Our main research works are as follows:(1)The ferroelectric,dielectric and magnetic properties of BiFeO3 are improved by doping and composite films.Based on doped BiFeO3 film Bi0.9La0.1FeO3(BLF),multilayer heterostructures are prepared with buffer layers.The interfacial effects between BLF and buffer layers are introduced to suppress the impurity phases and defects.The leakage current is reduced and the ferroelectric,dielectric,and magnetic properties are improved.(2)In the study of interfacial coupling mechanism in the classical ferroelectric/ferromagnetic system,we introduce an insulating layer MgO between the ferroelectric PbZr0.2Ti0.8O3(PZT)and the ferromagnetic oxide La0.7Sr0.3MnO3(LSMO).A huge modulation of the magnetic properties with electric field at room temperature is realized.At the same time,the origin of the magnetoelectric coupling at the interface is systematically studied,and the charge carrier of the interface coupling is verified to be oxygen vacancy by fitting analysis of the conducting behaviors.(3)The oxide p-n junction device with perovskite structure is prepared using different doped manganese oxides LSMO and La0.7Te0.3MnO3(LTMO)as p-type and n-type electrodes.Significant rectification effect is obtained at low temperature,and the conducting characteristics under different bias regions show opposite trends with rising temperature.Using the fitted I-V behaviors,we study the transport properties of the p-n junction influenced by the carrier transition in LTMO with the rising temperature.(4)Based on study of the oxide p-n junction,the oxide p-i-n junctions are prepared by introducing the ferroelectric PZT layers.Due to the ferroelectric insulating layers,the conducting behaviors of p-i-n junction are significantly different from that of the p-n junctions.The carrier transition in LTMO with the rising temperature plays an important role in the transport properties of p-i-n junctions.At the same time,the ferroelectric of PZT also show a significant effect on the ?-? behavior.(5)Based on the oxide p-n junction and p-i-n junction,we design an asymmetric oxide multiferroic tunnel junction LSMO/PZT/LTMO with different doped electrodes LSMO and LTMO.The TMR ratios and the rectification effect are modulated by ferroelectric reversal of PZT barrier.The different resistances shown as high and low states with opposite reading bias are caused by the intrinsic diode effect,which is coupled with TMR and TER effects.Two different groups of four resistance states are realized with the reading biases of different polarities.