Monika Iwanowska
Ferroelectrics are materials with a spontaneous electrical polarization, which can be switched by an applied electric field between two or more stable orientations permitted by symmetry. The regions where the ferroelectric material is polarized in one direction, known as domains, are separated by interfaces called domain walls, which may exhibit a wide range of properties that differ from the host material. Driven by the aspirations for novel device concepts, tremendous on-going research is focused on domain walls in ferroelectrics, in particular bismuth ferrite, BiFeO3. The great interest in this material is explained by the very attractive combination of properties including multiferroicity, i.e. coexistence of coupled ferroelectric and antiferromagnetic orders. Furthermore, a rich spectrum of recently discovered electronic properties of domain walls in BiFeO3 shows promise for domain-wall based functional elements or even “domain-wall-based electronics”. Despite a number of spectacular findings, the electronic properties of domain walls in BiFeO3 prove to be an extremely complicated and challenging topic. In particular this is valid for the subject of domain wall conduction, where experimental data from different groups is sometimes contradictory and the physical mechanisms still require further elucidation. The present study explores charge transport through domain walls in BiFeO3 and attempts to gain an insight into the underlying physics using both previously established and newly developed experimental approaches. Use of high quality epitaxial layers of pure and La-doped BiFeO3 with regular arrays of 109o and 71o domain walls allowed for comparative analysis of conductive properties of domain walls of different types. Fresh insight into the domain wall conduction was achieved by using self-patterned nano-electrodes in combination with various scanning force microscopy techniques. It has been demonstrated that both interface-controlled injection and bulk mechanisms govern the transport properties of domain walls in BiFeO3-based films. In spite of the apparent difference between the conduction properties of 71° and 109° domain walls, both cases are shown to be consistent with the same scenario of injection through the interfacial barrier controlled by the spontaneous polarization. The time dependence of the conduction properties and its hysteretic behavior for 109o domain walls is explained by the sensitivity of the Schottky barrier to the polarization and to the degree of its screening. The particular conductive behavior of 71o domain walls is attributed to the presence of small metastable domains with the opposite direction of out-of-plane polarization component. In contrast to the widely accepted concepts, the commonly reported diode-like behavior of the domain walls is found to be a consequence of the strong asymmetrical geometry of the experiment. Specifically, it originates from the probe-sample asymmetry rather than the electronic properties of the domain walls or interfaces. Analysis of the polarization reversal kinetics in BFO films shows that both the Kolmogorov-Avrami-Ishibashi model and nucleation limited switching formalism fail to provide an adequate description of the switching process. The essential characteristic features influencing the polarization switching in BFO films are: (1) anisotropic domain growth, (2) substantial variation in the speed of the domain wall movement, and (3) blocked domains with the boundaries “frozen” by trapped defects.
EPFL2013
