Resistance Switching Behaviors In Complex Transition-metal Oxides

In resistive random access memory (RRAM) devices that are composed by the metal/insulator/metal sandwiched structures, the nonvolatile high or low resistance state (HRS or LRS) can be easily controlled by the different voltage/current pulses. Due to the merits of such as simple structure, excellent scalability, low-power, fast speed, and high density, the RRAM devices have increasingly attracted attentions, and have been regarded as one of the promising candidates for the next-generation nonvolatile memory technology. Nowadays, extensive application of such RRAM devices is unpractical, since the performance stability remains still an issue, and the mechanisms for the resistance switching (RS) are under debate. Hence, exploring the RS mechanisms for various materials is important to seek the high performance and good stability for future applications.Complex oxide materials are usually categorized as strongly correlated electron systems, in which many exotic phemonena such as superconductivity, colossal magnetoresistance (CMR), and colossal electroresistance (CER) effects are available, and hence become the promising candidate materials for RRAM devices. In this dissertation, we focuse on the resistive switchng performances and the switching mechanisms of several manganites and ferroelectric oxides. This thesis is organized as following: In Charpter Ⅰ, a review on the research background and the latest development of RRAM materials and devices is given. After a brief introduction of several new-types of nonvolatile memory devices that are based on resistance switching, we describe in details the principle, structure and integration scenarios, switching mechanisms, and numerous materials issues of RRAM devices. Finally, we highlight our current understanding of CMR manganites.In Charpter Ⅱ, the resistance switching and relaxation behaviors of bulk polycrystalline La0.5Ca0.5Mn0.95Fe0.05O3and La0.225Pr0.4Ca0.375MnO3are addressed. Upon imposing periodical current pulses, one observes the overshooting relaxation behaviors in both materials. For La0.225Pr0.4Ca0.375Mn03, there are several stable nonvolatile resistance states, and the different current pulses can be used to control these different states reversibly. It is revealed that the reversible RS effect is the outcome of local thermal-cycle assisted rearrangement of the ferromagnetic metal (FMM) phase in the charge-ordered insulator (COI) matrix instead of the result of current-induced intrinsic transition of COI into FMM phase.In Charpter Ⅲ, the evolutions of electronic phase separation in La0.225Pr0.4Ca0.375MnO3are investigated by the specific temperature and magnetic-field cycling experiments. It is found that the electronic phase separation state at low temperature can be tuned substantially by the temperature and/or magnetic-field cycles. The surprising fact that the initial FMM nuclei can impede the growth of these nuclei during the cooling process, implies that there must coexist more than two phases which take part in the complex first-order phase transitions. The charge disordered insulating (CDI) phase is possibly one of the parent phases transiting into the FMM phase at low temperature. In addition, the accommodation strain is suggested to control the nucleation and growth of FMM domains.In Charpter Ⅳ, steady unipolar resistive switching of Pt/YMn1-δO3/Pt and Au/BaTi0.95Co0.05O3/Pt structures are investigated. The resistance ratio (>104) of high resistance state (HRS) over low resistance state (LRS) and long retention (105s) have been achieved. Besides, the endurance over105cycles and the switching time shorter than10ns/70ns for the SET/RESET are realized in the Au/BaTi0.95Co0.05O3/Pt structure. It is demonstrated that the electric field induced migration of oxygen vacancies/ions gives rise to the formation of conductive filaments, while the large Joule heating thermally drives those oxygen vacancies/ions to rupture the conductive filaments. Both the mechanisms contributes to the unipolar resistive switching. It is also suggested that the easily varied valence of metal ions, the pre-existing oxygen vacancies with sufficiently high density, and the local itinerant electrons introduced by doping, all favor the local metal-insulator phase transition and therein the formation/rupture of conductive filaments, responsbible for the stable resistive switching.At last, a summary of this thesis and the perspectives are presented in Charpter V.