Magnetoelectric Coupling And Resistive Switching Effects In Multiferroic Heterostructure

In the past decades, with the rapid development of spintronics/electronics, it has become much harder for conventional electronics devices to meet the demands for device miniaturization, read-write speed, multi-functionalization and low energy consumption, owing to that the conventional semiconducting devices are approaching quantum limits, causing the restriction for the development of electronics device technologies. Thus it becomes more important to seek new materials with two or even more functional features for designing multi-functional devices to push the industry revolution. Multiferroic materials have attracted much attention since these materials have ferroelectric, ferromagnetic, unique capability to control ferroelectric and ferromagnetic via external electric and magnetic fields simultaneously. However, in single phase multiferroic materials the magnetoelectric coupling effect is very weak and only exists at low temperature. Therefore, much more attention has been paid to the multiferroic heterostructures with excellent magnetoelectric coupling effect, in which we have more choices in materials and configuration. Even now great progress has been made in multiferroic heterostructures, some pending issues need to address urgently, such as how to realize stable and strong room-temperature magnetoelectric coupling effect in multiferroic heterostructures, the effects of magnetic lanthanum manganites electrode on the variations of dielectric and ferroelectric properties in magnetic fields, and how to combine the domain wall engineering and macroscopic conduction to design multi-state resistive switching prototype memory device.Aiming at these problems, the effects of magnetic lanthanum manganite electrodes on the variations of dielectric and ferroelectric properties are studied systematically. The dissertation also focus on the origins of the tri-state resistive switching behavior involving the ferroelectric domain wall and interface effects, providing fresh ideas for multi-state resistive switching prototype memory device.The whole dissertation is divided into six chapters.In chapter 1, starting with a brief introduction to multiferroic materials,including the origins of multiferroic in single phase multiferroic materials and the advantages of composite multiferroic materials, we discuss the magnetodielectric and magnetoelectric coupling in ferroelectric/ferromagnetic multiferroic heterostructures. In addition, the mechanisms of resistive switching behavior in multiferroic heterostructures are also briefly reviewed.In chapter 2, the ferroelectric properties of the BiFeO3/La0.625Cao.375Mn03 (BFO/LCMO) heterostructures were investigated using different bottom electrode configurations at different magnetic fields and temperatures. It is found that the apparent coercive voltage (Vac) increases linearly with the increase of LCMO resistances for different electrodes, and the extrinsic relative contribution from different LCMO electrodes to the variation of Vac caused by magnetic fields were quantitatively analyzed based on the scenario of voltage drop model. The magnetic field and temperature dependences of the heterostructure coercive voltage (Vac0) obtained by subtracting the voltage drop on LCMO from Vac are closely related to the interface behaviors. These findings not only further elucidate the physics of magnetoelectric coupling in multiferroic heterostructures, but also are helpful for designing artificial prototype device.In chapter 3, we investigated the dielectric properties of the Au/BFO/LCMO heterostructures using different LCMO bottom electrode configurations at different magnetic fields and temperatures. It is found that the real part of the capacitance increases with the increase of LCMO resistances for different electrodes, and the dielectric relaxation shifts to higher frequencies. Meanwhile, the colossal magnetocapacitance (MC) of the heterostructures will be enhanced by the increase of LCMO resistance, but the dielectric loss also inreases. On the basis of the discussion about the dielectric relaxation process using different bottom electrode configurations, the dielectric relaxation is closely related to the electron hopping or transferring between Mn3+ and Mn4+. These findings are helpful for understanding the MC in ferroelectric/ferromagnetic multiferroic heterostructures, also instructive to the design of prototype device.In chapter 4, we use the inductively coupled plasma atomic emission spectrometry to identify the element contents in BiFeO3 film. Via inductively coupled plasma atomic emission spectrometry it can be concluded that there no existence of Bi loss in the BiFeO3 film, which is crucial for the determination of the type of semiconducting BiFeO3 film and will play important role in analyzing the electrical transport properties of the Au/BiFeO3/La0.6Sr0.4MnO3 (Au/BFO/LSMO) heterostructure. In addition, with the aid of scanning transmission electron microscopy, we reveal the "dead layer" at the interface of Au/BFO, and figure out the nature of the interface of BFO/LSMO, where obvious ion interdiffusion is found. These findings are useful for finding out the origins of the resistive switching behaviors in Au/BFO/LSMO.In chapter 5, we report a repeatable and stable tri-state resistive switching behavior manipulated by electric and magnetic fields for the Au/BFO/LSMO heterostructure. By changing the direction of the ferroelectric polarization, the resistances of the heterostructure can be switched among three stable states without an electroforming process. It is found that the non-volatile resistive switching behaviors in our sample can be attributed to co-contributions of Schottky-like barrier in the Au/BFO interface, the resistance changes of BFO/LSMO interface, and the domain wall density related resistance manipulated by polarizations. Most interestingly, the room-temperature current peaks near coercive voltage owning to the high conductivity of ferroelectric domain walls prove the negative magnetoresistance properties of ferroelectric domain walls macroscopically. The above findings are helpful for us to understand the effects of interfaces and the ferromagnetism of ferroelectric domain walls in multiferroic heterostructures on the non-volatile RS behaviors, which may give a possibility for combining resistive switching device with spintronics.In chapter 6, we investigate the in-plane in-plane anisotropic transport and magnetic properties of the Co film grown on different substrates. The resistance of easy axis is always larger than that of hard axis for any case. The physics behind the anisotropic magnetoresistance is intriguing.