Enhanced Functionalities Achieved by a Vertically Aligned Nanocomposite Approach
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Vertically aligned nanocomposite (VAN) thin films have emerged as a new thin-film platform which is composed of at least one strongly correlated metal oxide coupled with another synergistically selected oxide. Self-assembled, heteroepitaxial VAN films form as a consequence of several key attributes, including the growth kinetics, thermodynamic stability, crystal chemistry and thin film epitaxial constraint. The VAN films have exhibited various morphologies depending on specific material system and growth parameters, such as nanomaze, nanocheckerboard, and vertical nanopillars embedded in a planar matrix. Owing to tunable vertical lattice strain and novel interface coupling, the VAN films have been exploited as a very effective platform for enhancing physical properties and exploring novel functionalities. In this dissertation, we have achieved highly textured growth of La0.7Sr0.3MnO3:ZnO (LSMO:ZnO) VAN film on the semiconductor silicon substrate with a SrTiO3 (STO)/TiN bilayer buffer. By tuning the film composition and associated spin-dependent tunneling and scattering across the structural boundaries, we have demonstrated enhanced and tunable low-field magnetoresistance (LFMR) effects. Different interface couplings between ferromagnetic (or ferrimagnetic)-antiferromagnetic (FM-AFM) spins have been created in the VAN structure. BiFeO3 (BFO) has been selected as the AFM, while CoFe2O4 (CFO) and LSMO have been selected as the ferrimagnet and ferromagnet, respectively. Either rotatable or pinned AFM spins have been formed at the vertical interfacial region of BFO:CFO and BFO:LSMO VAN film, respectively. As a result, enhanced perpendicular magnetic anisotropy and perpendicular magnetic exchange bias have been achieved from these two interface couplings, respectively. The magnetic exchange coupling at the vertical interfaces in the VAN architecture has been exploited to explore a novel way to control the magnetotransport property in VAN films. FM LSMO and AFM NiO have been selected to form the vertical FM-AFM exchange coupling in the prepared VAN architecture. A dynamic and reversible switch of the resistivity between two distinct exchange biased states has been achieved through a field cooling procedure with a magnetic field bias. Using BFO:CFO VAN films as a model system, we demonstrate an effective method to modulate the vertical heterointerface and the morphology of nanocomposite films by adjusting the laser repetition frequency during deposition. Both vertical and gradient interfaces have been obtained through the film thickness, which strongly correlates with strain tuning and interface coupling, and thus modifies the magnetic anisotropy, coercive fields and FE switching behavior. The studies in this dissertation demonstrate several examples of enhanced performance using the benefits of the unique VAN architecture. The huge vertical interfacial area for functional coupling and the effective vertical strain control independent of the substrate in the VAN films, as well as the simple self-assembly, provide a new dimension to tune the properties of metal oxides.
Zhang, Wenrui (2015). Enhanced Functionalities Achieved by a Vertically Aligned Nanocomposite Approach. Doctoral dissertation, Texas A & M University. Available electronically from