Emergent nanoscale functionality in uniaxial ferroelectrics

Abstract

This thesis has been focused on the characterization and manipulation of the electronic nanoscale properties of domains and domain walls in uniaxial ferroelectrics. Two classes of uniaxial ferroelectrics have been investigated, that is, improper and proper systems, represented by the hexagonal manganites (h-RMnO3) and lead germanate (Pb5Ge3O11), respectively. In h-RMnO3, going beyond previous studies on acceptor doping, this work studied the effect of donor doping on both the A- and B-site. The measurements indicate a strong correlation between the applied doping and oxygen stoichiometry, highlighting the importance of defect chemistry for the electronic response at domain walls in h-RMnO3. A comprehensive analysis of the temperature dependence of I(V )-curves extracted from conductive atomic force microscopy (cAFM) data revealed that Poole-Frenkel conduction is the dominant DC conduction mechanism in domains and at domain walls. In addition to the effect of ionic defects on electronic domain wall transport, it was studied how stoichiometric oxygen defects can be utilized to control conductivity on the nanoscale - without the need of functional domain walls. It was found that electric fields could be used to locally enhance the conductance by up to almost four orders of magnitude in h-RMnO3. This conductance enhancement occurs independently of the ferroelectric order, and advanced structures can be written with nanoscale spatial resolution. The written structures persist on a timescale of years, and under a range of experimentally accessible thermal and chemical conditions. In Pb5Ge3O11, piezoresponse force microscopy confirmed the presence of nominally charged domain walls. A comprehensive scanning probe microscopy investigation did not reveal any signature of free or bound charges at these walls. Additional experiments revealed unconventional dynamics, both under electric fields and under thermal treatments.