In ferroelectric materials, domain walls separate regions with different polarisation orientation. Fundamentally, they provide an excellent model system of the rich physics of pinned elastic interfaces, whose behaviour is key for controlling domain size and stability in device applications. In addition, domain walls can present novel functional properties quite different from those of the parent phase. The extreme localisation of such properties at these intrinsically nanoscale features makes them potentially useful as active components in future miniaturized electronic devices. Particularly exciting has been the discovery of domain-wall-specific electrical conductivity, shown first in multiferroic BiFeO3, and assigned to a wide range of different microscopic mechanisms. I will present our generalisation of these observations to the simpler ferroelectric Pb(Zr,Ti)O3, using a range of scanned probe microscopy techniques at different time scales. Our measurements highlight the key role of surface adsorbates and oxygen vacancies, and show how their density and distribution can be modulated to reversibly control domain wall transport. In the same ferroelectric samples, we also observe an unusual mechanical response response, forbidden by symmetry in the parent phase but permitted at domain walls as a result of the local symmetry breaking. This enhanced shear response could be technologically important for ferroelectric based surface acoustic wave devices. Within the framework of pinned elastic interfaces, in both BiFeO3 and Pb(Zr,Ti)O3 we also explore the effecs of temperature, environmental boundary conditions, and defects on polarisation switching dynamics and domain wall roughness. In thermal quench measurements, we demonstrate out-of-equilibrium memory effects in the evolving roughness configuration. We also show suprisingly high values of the roughness exponent in intrinsic ferroelectric/ferroelastic stripe domains. Finally, using cumulative application of ultra-short voltage pulses, we observe unexpectedly long relaxation times of the sub-critical nucleus in the initial stages of polarisation reversal.
Prof. Patrycja Paruch
DQMP, Université de Genève, Switzerland
A close look at ferroelectric domain walls, roughly speaking: the crucial role of defects in polarisation switching, roughness, and conduction