Magnetoelectric and optoelectric effects in ferroelectric tunnel junctions

Ferroelectric tunnel junctions (FTJs) consisting of two electrodes separated by an ultrathin ferroelectric tunnel barrier can exhibit giant resistance switching by ferroelectric polarisation reversal and allow for non-destructive information readout. By exploiting the internal degree of freedom-ferroelectric domain structure and external structure design-stacking with ferromagnetic electrodes, FTJs can be further enhanced with electric memristive behaviours and the magnetic field controllable operation. More and more novel properties are expected to be studied in FTJs as most of the constituent materials are made from transition metal oxides which could accommodate a wide range of fascinating properties. Therefore, FTJs have established themselves as an intriguing platform for novel electronic applications. To fully exploit the potential of FTJs in order to meet the requirements of current information storage industry such as ever-higher data storage densities, lower energy consumption, multifunction features and even form factor such as flexible and stretchable mechanical properties, new physics and materials along with innovative device fabrication strategies for FTJ-based systems are urgently needed. In this work, we first explored the possibility of magnetoelectrically coupled ferroelectric tunnel memristor. By combining the transport results and the virtualised ferroelectric domain evolution upon a set of electric fields, we came up with a qualitative physical model which can well describe the observed coupling between the tunnelling electroresistance and tunnelling magnetoresistance. The second block of this work is the study of a hybrid FTJ and ferroelectric photovoltaic cell device which realised the reversible optic control of resistance switching in FTJs. The mechanism of controlling the FTJ resistance states by the tip-enhanced photovoltaic effect is explained. Operation parameters including writing speed and strength are discussed in detail. The final subject is devoted to the development of flexible ferroelectric tunnel memristors using transferable FTJ nanomembranes. We studied the epitaxial layer transfer method and managed to obtain ultrathin single-crystalline ferroelectric oxide membranes. By comprehensive structural investigations, we revealed that the well-preserved single-crystal structure and compressive strain state in ferroelectric nanomembranes are key for the existence of ferroelectricity. Flexible memristive FTJs were made by mounting ferroelectric nanomembranes on plastic substrates and displayed excellent functional properties. Overall, this work not only provides new insights into the unprecedented magnetoelectrically and optoelectrically coupled systems with ultrathin ferroelectric oxide layers but also offers the strategy to extend their applications to flexible systems