UC Berkeley

This dissertation focuses on the thorough characterization of nanoscale interface regions - so-called domain walls - between structurally, electronically and magnetically homogeneous areas or domains in BiFeO3 thin films. These domain walls exhibit intriguing properties not found in the domains themselves and were determined using a combination of nanoscale characterization techniques such as transmission electron microscopy, atomic force microscopy, piezo-response force microscopy, scanning tunneling microscopy, X-ray absorption spectroscopy and photoemission electron microscopy. Enhanced net magnetic moments are observed at certain type of domain

walls (109◦) in BiFeO3, although the adjacent domain areas are antiferromagnetically ordered with only a very small net magnetic moment. The same type of the domain walls is found to be conducting at a semiconductor level while the domains are insulating and exhibit robust ferroelectricity. 2-D arrays of these domain walls exhibit intriguing magnetotransport behavior: when both current path and magnetic field are oriented parallel to the domain walls, about 60% of negative magnetoresistance is observed at low temperature (~10 K), which can be well modeled with electron variable hopping. Moreover, in highly strained BiFeO3 thin films electrically controllable magnetic moments are found localized in a highly distorted rhombohedral-like phase confined between the adjacent tetragonal-like phase. Those magnetic moments can be turned on and off by applying an electric field.