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Title: New ways to tune the multi-functionality of oxide thin films for memory applications
Author: Yun, Chao
ISNI:       0000 0004 7972 9565
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2019
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This thesis presents studies on engineering the electrical and magnetic properties in oxide thin films using vertically-aligned nanocomposites (VAN). The works aim to enhance their multi-functionality for various memory applications. First, electroforming-free resistive switching (RS) with high ON/OFF ratio is realized in a novel model VAN system based on self-assembled Sm-doped CeO2 and SrTiO3 films that allow separate tailoring of nano-scale ionic and electronic channels at a high density (1012 inch-2). These devices allow precise engineering of the resistance states, thus enabling large and tunable ON/OFF ratios and high stability for acting as resistive random access memory (RRAM) devices. Second, the ferromagnetic insulating (FMI) and metallic (FMM) properties of La0.9Ba0.1MnO3 (LBMO) are tuned using VAN. La0.9Ba0.1MnO3 is FMI in bulk but usually shows metallicity in plain films. By using VAN consisting of CeO2 nanocolumns embedded in a La0.9Ba0.1MnO3 matrix, the FMI property is maintained in thin film form. The CeO2 phase acts as strain-controlling nanocolumns and at the same time, produces light Ce doping of the LBMO and thus filling of intrinsic cation vacancies. Together these reduce the unwanted double exchange (DE) coupling. This is hard to realize in plain LBMO films which contain cation vacancies, and have several strain-relaxing LBMO phases which result in an enhanced Mn4+/Mn3+ ratio, and hence to DE coupling and metallicity. Besides, by varying the growth temperature of the LBMO-CeO2 VAN, the system is engineered from a FMI to a FMM and the magnetoresistance is highly tunable, which are correlated to the tuning of the lateral size of both phases. These effects are attributed to a dimension change-induced change in the electronic band structure. The tunable properties of LBMO-CeO2 VAN make it a good candidate as low-power-consumption, high-Tc FMI and FMM components in magnetic random access memory (MRAM) and spintronic devices. Last, in-situ electric field tuning of magnetic properties is studied in La0.9Ba0.1MnO3-ZnO VAN, a novel candidate for magnetoelectric random access memory (MERAM) devices. The M-H curves and the remanence are tuned by applying electric fields at a low temperature (10 K). The possible origins for the magnetic modifications are discussed rationally, which include Joule heating, piezoelectric strain, current-induced induction field and charge trapping/detrapping related to a resistive switching effect (which is found to be the most likely mechanism for a hysteretic tuning of the remanence). All these effects are correlated to the existence of the ZnO phase. This work helps to understand the charge doping effect in manganite-ZnO VAN systems.
Supervisor: Driscoll, Judith Sponsor: Cambridge Commonwealth ; European & International Trust
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
Keywords: Vertically aligned nanocomposite ; Functional oxide thin films ; Resistive random access memory ; Ionic and electronic conductivity ; Ferromagnetic insulator and ferromagnetic metal ; Magnetoelectrics and multiferroics