Growth and Electric Field Control of Phase Separated Manganites

Perovskite Manganites have received numerous attentions due to exotic behaviors such as colossal magnetoreistance (CMR) and electronic phase separation (EPS). The purpose of my research is to answer fundamental questions about the growth properties of manganites and electric field control of the EPS properties. Experimental study was conducted on controlling the growth mode of La0.7Sr0.3MnO3[Lanthanum Strontium Manganese Oxide] thin films using pulsed laser deposition. Different thin film morphology, crystallinity and stoichiometry have been observed depending on growth parameters. To understand the microscopic origin, the thermodynamic processes were theoretically analyzed and a growth diagram was constructed. Three boundaries between highly and poorly crystallized, layer-by-layer and 3D, stoichiometric and non-stoichiometric growth were identified in the growth diagram. A good fit of our experimental observation with the growth diagram was found. This case study demonstrates that a more comprehensive understanding and the predicting of the growth mode in PLD is possible. Behaviors such as high Tc superconductivity, CMR, and the metal-insulator transition, have been tied to inherent electronic phases coexisting in a single crystal material. These phases offer the potential for creating new types of electronic devices based on tuning the finely balanced energetics stabilizing emergent phase domains. Here we demonstrate novel approaches to induce resistive electric field effect transitions based on the modification of the inherent electronic domain structures in single crystal materials. A phase separated manganite system confined to a scale which isolates a few electronic domains is controlled using laterally gated electrodes to tune percolative conduction channels which give repeatable resistive changes. This technique also makes it possible to create multistate switching devices from a single confined transport channel. Electro-resistance up to 400% is observed during the cooling process under static electric field. These findings provide an avenue to control inherent electronic phases as a means of creating novel nano-electronic devices. While manganites are the primary focus throughout this dissertation, both the growth diagram and spatially confined E-field techniques can be extended to understand fundamental growth phases in other epitaxial oxides materials and exploring the electric field effect on different oxides system

Growth diagram of La0.7Sr0.3MnO3 thin films using pulsed laser deposition

An experimental study was conducted on controlling the growth mode of La0.7Sr0.3MnO3 thin films on SrTiO3 substrates using pulsed laser deposition (PLD) by tuning growth temperature, pressure and laser fluence. Different thin film morphology, crystallinity and stoichiometry have been observed depending on growth parameters. To understand the microscopic origin, the adatom nucleation, step advance processes and their relationship to film growth were theoretically analyzed and a growth diagram was constructed. Three boundaries between highly and poorly crystallized growth, 2D and 3D growth, stoichiometric and non-stoichiometric growth were identified in the growth diagram. A good fit of our experimental observation with the growth diagram was found. This case study demonstrates that a more comprehensive understanding of the growth mode in PLD is possible

Growth diagram of La0.7Sr0.3MnO3 thin films using pulsed laser deposition

An experimental study was conducted on controlling the growth mode of La0.7Sr0.3MnO3 thin films on SrTiO3 substrates using pulsed laser deposition (PLD) by tuning growth temperature, pressure, and laser fluence. Different thin film morphology, crystallinity, and stoichiometry have been observed depending on growth parameters. To understand the microscopic origin, the adatom nucleation, step advance processes, and their relationship to film growth were theoretically analyzed and a growth diagram was constructed. Three boundaries between highly and poorly crystallized growth, 2D and 3D growth, stoichiometric and non-stoichiometric growth were identified in the growth diagram. A good fit of our experimental observation with the growth diagram was found. This case study demonstrates that a more comprehensive understanding of the growth mode in PLD is possible

Active control of magnetoresistance of organic spin valves using ferroelectricity

Organic spintronic devices have been appealing because of the long spin life time of the charge carriers in the organic materials and their low cost, flexibility and chemical diversity. In previous studies, the control of resistance of organic spin valves is generally achieved by the alignment of the magnetization directions of the two ferromagnetic electrodes, generating magnetoresistance.1 Here we employ a new knob to tune the resistance of organic spin valves by adding a thin ferroelectric interfacial layer between the ferromagnetic electrode and the organic spacer. We show that the resistance can be controlled by not only the spin alignment of the two ferromagnetic electrodes, but also by the electric polarization of the interfacial ferroelectric layer: the MR of the spin valve depends strongly on the history of the bias voltage which is correlated with the polarization of the ferroelectric layer; the MR even changes sign when the electric polarization of the ferroelectric layer is reversed. This new tunability can be understood in terms of the change of relative energy level alignment between ferromagnetic electrode and the organic spacer caused by the electric dipole moment of the ferroelectric layer. These findings enable active control of resistance using both electric and magnetic fields, opening up possibility for multi-state organic spin valves and shed light on the mechanism of the spin transport in organic spin valves