| Recently, as transistor dimensions reach quantum regime, the diminishing scaling benefit makes conventional semiconducting devices encounter insurmountable bottlenecks. A number of potential alternatives based on new materials, structures and properties of prototype devices, have been proposed and extensively studied to obtain more powerful and functional devices. Multiferroic tunnel junction (MFTJ), exploiting the capability to control ferromagnetic and ferroelectric orders via external electric and magnetic fields simultaneously, is one of such promising candidates, which have potential applications not only in multi-state data storage due to the coexsistence of tunneling magnetoresistance (TMR) and tunneling electroresistance (TER), but also in electric field controlled spintronics as a result of the interfacial magnetoelectric coupling effect. Due to the rich physics involved and potential electronic/spintronic applications, strong experimental efforts have been made and the results on quaternary resistance states and electrical control of the spin transport unambiguously prove the MFTJ working principle, supporting theoretical predictions. However, the reported TMR and TER ratios in MFTJs were still small and the four-state effect can be observed only at low temperatures, limiting its practical applications. This thesis mainly focuses on these issues, in order to achieve better four state device with larger amplitudes of TMR and TER ratios and higher operating temperature.The whole thesis is divided into six chapters.In chapter1, starting from the concept of spin dependent tunneling in magnetic tunnel junctions and ferroelectric related electrical transport in ferroelectric tunnel junctions, we discuss the recent demonstrations of coexsiting TMR and TER effects in magnetic tunnel junctions with a ferroelectric barrier, namely, MFTJs. In addition, alternative magnetoelectric coupling mechanisms at ferromagnetic/ferroelectric interfaces are briefly reviewed also. In chapter2, using piezoresponse force microscopy technique, we demonstrate the switchable ferroelectric properties in nanometer-thick (Ba, Sr)TiO3films on La0.7Sr0.3MnO3bottom electrodes. An upward polarization orientation was found in as grown BaTiO3films on SrTiO3(001) substrate and the size of a single ferroelectric domain was determined to be smaller than～50nm. We have observed ferroelectric hysteresis in BaTiO3film with thickness down to～2nm. And the BaTiO3film with La0.7Sr0.3MnO3top electrode, emulating real MFTJ geometry, also shows ferroelectric hysteresis curve. In addition, the fabrication process of MFTJs is introduced.In chapter3, the room temperature four state effect in Lao.7Sro.3MnO3/Bao.95Sro.o5Ti03/Lao.7Sro.3MnO3//SrTiO3(001) MFTJ were investigated. The non-linear current (Ⅰ)-voltage (Ⅴ) with a good fitting of the conductance curve with BDR model indicates that the electron tunneling dominates the transport process. The TMR effect with sharp switched magnetic parallel and antiparallel resistances and TER effect with the polarization reversal of the barrier have been observed even at room temperature. The entire resistance (R)-magentic field (H) loop shifts when the polarization of the Ba0.95Sr0.05TiO3barrier is reversed, resulting in four resistance states at room temperature. In addition, we have also studied the four state effect in MFTJs with pure BaTiO3barrier which show larger TMR and TER effects at low temperatures. The largest TMR effect reaches741%at5K, and the spin polarization of La0.7Sr0.3Mn03at BaTiO3La0.7Sr0.3MnO3interface is0.89.In chapter4, we demonstrate a giant TER effect in La0.7Sr0.3MnO3BaTiO3/La0.5Ca0.5MnO3/La0.7Sr0.3MnO3MFTJs where resistive switching itself is induced by the interfacial magnetoelectric response. The MFTJs exhibit TER that is enhanced up to～10,000%by a nm-thick La0.5Ca0.5MnO3interlayer inserted at one of the interfaces. The observed phenomenon originates from the ferromagnetic-metal to antiferromagnetic-insulator phase transition in La0.5Ca0.5MnO3due to the modulation of carrier density by ferroelectric polarization switching, which will change the tunneling parameters and increase the resistance difference significantly. The observed giant magnetoelectrically induced TER effect is robust and may serve as a viable route for novel electronic and spintronic applications. In addition, the TMR effects were also observed in this kind of MFTJs and a four state effect was obtained accordingly.In chapter5, the four state effect was studied in MFTJs using ferromagnetic Lao.7Cao.3Mn03and ferroelectric (Ba, Sr)TiO3as the electrode and barrier layers, respectively. The hysteresis non-linear I-V curves with different ferroelectric polarizations indicates the occurrence of resistance switching as a result of TER effect. The fitting parameters (barrier width, average barrier height) from the BDR tunneling model provide information in understand the band structure of the MFTJ. And the R-H loop suggests the exsisting of two resistance states, parallel and antiparallel, leading to TMR effect. We have also studied the MFTJs on different substrates and found that the MFTJs NdGaO3(110) are more conducting than on SrTiO3and the annealling in oxygen will increase the junction resistance of MFTJs grown on NdGaO3(110) substrates. In addition, we have observed the electric field assisted switching of magnetizations.In chapter6, the La2-xSrxCuO4(LSCO)(x=0.10,0.16, and0.20)/1.0wt%Nb-doped SrTiO3(SNTO) heterojunctions were constructed and the temperature dependencies of the rectifying Ⅰ-Ⅴ characteristic under magnetic fields up to14T were investigated. A tunneling picture (field emission (FE)/thermionic-field emission (TFE) model) was carried out to represent the transport behaviors of the LSCO/SNTO junctions. It is found that the junction resistance (Rj), diffusion voltage (Vd), and the FE/TFE fitting parameter (E00) show sudden jumps near superconducting transition temperature. The results are discussed by considering the effect of superconductivity and the inhomogeneous LSCO/SNTO Schottky contacts. |
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