Study Of Multiferroic BiFeO₃ Thin Films By Scanning Probe Microscopy

Due to its intriguing room-temperature multiferroic properties,BiFeO₃ demonstrates many superior physical phenomena with great application potentials in sensor,information storage,nanoelectronic and spintronic devices.The interfaces of low-dimensional BiFeO₃ thin films?heterostructures,domain walls and surface?has also generated many exotic functionalities,which is distinct from those of the bulk,due to the quantum confinement effect.Therefore,the associated interfaces provide a great platform to exploring novel physical properties of low-dimensional systems,as well as the corresponding device concepts for the next generation of multifunctional devices.In this thesis,we use the scanning probe microscopy to study the surface and heterostructure effects of the BiFeO₃ thin films.The main contents of the thesis are itemized as follows.1.Combining both PFM and c-AFM imaging techniques,we first discovered that the nature of the enhanced conductivity near the 71^o domain walls in BiFeO₃ can be attributed to the novel metastable nano-domains.With the application of electric field,the upward nano-domains form near the domain walls from the previously downward polarized domain regions.The phase-field simulation also confirms that with the application of the electric field below the coercive field,the metastable upward-polarized nano-domains are occurred at the acute-angle side of the domain walls,and the polarization switching mechanism is 71°rotation.We further demonstrate that the upward-polarized micro-domains also show dramatically enhanced conductivity,which is consistent with the nano-domain results.In order to obtain direct insight of the conduction mechanism for both,we analyzed the I-V curves for different regions,and the results clearly show that the enhanced conduction should be attributed to the lower Schottky barrier height at the interface between the tip and sample surface modulated by the upward ferroelectric polarization switching.These results provide the insight into the relationship between the dynamic conduction near the domain walls and the ferroelectric domain dynamic behavior.2.We note that the ferroelectric domain structure is strongly correlated with the heterostructures.Accordingly,we developed a new substrate thermal treatment method,which can create regular self-assembly two termination in the SrTiO₃?001?substrate.We note that the direct growth of BiFeO₃ thin films on the conventional single TiO₂terminated SrTiO₃ substrate will show up only uniform vertical ferroelectric polarization.However,using the newly developed two terminated substrates,the BiFeO₃ thin films show up periodically modulated vertical ferroelectric domain structures.Using the high-angle annular dark-field?HAADF?imaging technique,we directly measured the interface atomic stacking sequence between BiFeO₃ and SrTiO₃ and found that the out of plane ferroelectric polarization is indeed correlated with the interfacial internal fields,which are opposite for different stacking sequences.Using the scanning probe microscopy,we further revealed that the domains with different vertical polarization directions have different coercive fields.Accordingly,the macroscopic region displays an interesting tri-states ferroelectric switching behavior,which is studied by the phase-field simulations.The direct demonstration of the correlation between ferroelectric domain structure?and evolution?and the interface properties suggest a novel pathway to design and engineer the novel functionalities of ferroelectric devices.