POLARIZATION-COUPLED TRANSPORT BEHAVIOR IN ULTRATHIN FERROELECTRIC HETEROSTRUCTURES

Ferroelectric polarization-coupled resistive switching behavior in ferroelectric tunnel junctions (FTJs), the tunneling electroresistance (TER) effect, is a recently predicted new phenomenon, which attracts interest due to potential application in the next generation non-volatile ferroelectric random access memories (FeRAMs). In this dissertation, we demonstrate the TER effect in FTJ devices by means of scanning probe microscopy (SPM) techniques. We have investigated several device configurations for enhancement of polarization stability and for demonstration of the resistive switching behavior: (i) using the SPM probe as a top electrode; (ii) using heterostructures with engineered interfacial atomic terminations; (iii) using metal electrodes; (iv) adding an additional polar molecular layer at the interface. Stable and switchable resistance states with a ratio over three orders of magnitude have been achieved in these FTJs. These results are promising for employing FTJs in non-volatile memory and logic devices. Furthermore, for the first time, ferroelectric polarization reversal by pure mechanical force---a fundamentally new phenomenon---predicted almost 50 years ago, has been demonstrated. We show that the strain gradient generated by an atomic force microscope (AFM) tip pressing onto an ultrathin ferroelectric film can result in polarization reversal in a nanoscale volume without application of an external bias due to the flexoelectric effect. The mechanically generated ferroelectric domains are more localized than electrically generated domains, enabling much higher density of data storage. Pure mechanical force can therefore be used as a dynamic tool for polarization control and enable applications in which memory bits are written mechanically and read electrically. Multiferroic materials and magnetoelectric coupling between ferroelectric and ferromagnetic order parameters are attracting great interest both due to the fundamental physical insight that these systems give and due to the vast potential for applications such as the electrical control of magnetism and vice versa. In this dissertation we also demonstrate the existence of a large room-temperature ferroelectric polarization modulation of magnetization at the ferroelectric-ferromagnetic interface upon polarization reversal. Adviser: Alexei Gruverma