Topological defects in ferroelectric or ferromagnetic materials : domain wall, vortices, skyrmions and beyond

Resume : The domain structure of (Na,K)NbO3 as a lead-free alternative is rather well investigated in bulk form, but little is known for thin films. Particularly the growth on lattice mismatched substrates has a decisive impact on the formation of ferroelectric phases and can yield new properties not observed in corresponding bulk crystals. In this study, 20-30 nm thick (K,Na)NbO3 films with pure ac-surface orientation were epitaxially grown on (110) NdScO3 crystalline substrates by MOCVD. Hence, they experience anisotropic lattice stress resulting in average in a slight compressive strain. X-ray diffraction data demonstrates that the (K,Na)NbO3 unit cells are monoclinically distorted along the [001]NSO in-plane direction. Piezoresponse force micrographs (PFM) reveal both a lateral and a vertical component of the electric polarization. The lateral PFM shows irregularly arranged 180° domains along [001]NSO superimposed by well-ordered ferroelastic subdomains with a lateral periodicity of 30-40 nm along [1-10]NSO. These ferroelastic domain walls cannot be described by low index crystallographic net planes. They rather incline a variable angle to the [001]NSO direction depending on chemical composition and in-plane monoclinic distortion of the film unit cells. Summarizing, the occurrence of such a domain structure is compatible only with monoclinic domains separated by S walls, which does not exist in tetragonal, rhombohedral or orthorhombic structures.

Resume : Over the last decade, there have been a number of significant breakthroughs in our understanding of ferroelectric domains and domain walls. The traditional view that ferroelectric domain walls are purely Ising-like has been challenged by recent high-resolution TEM data and theoretical calculations that predict a plethora of complex internal polarisation structures and even separate phase transitions within domain walls.Perhaps even more exciting have been the numerous discoveries of unexpected emergent behaviour at domains walls, not intrinsic to the bulk of the material.Enhanced domain wall conductivity, large photovoltaic responses, domain wall magnetism and the appearance ofpolarisation in otherwise centrosymmetric materials are just a few of the phenomena that bring hope of new technologies and fuel the ever-growing interest in the newly emerged field of domain-wall nanoelectronics. Collective responses of domain-walls are also being re-examined, particularly as research on nanoscale ferroelectrics has revealed that extremely dense, regular domain structures can be engineered in ultrathin films and heterostructures and are predicted to have interesting collective domain-wall dynamics. We have investigated the dielectric properties PbTiO3-based superlatticeswith such ultradense, nanoscale ferroelectric stripe domains. Dielectric impedance spectroscopy measurements in the 100 Hz-1MHz frequency range have been performedover a wide temperature range, from the cryogenic regime where domain walls are pinned to well above the Curie point where they no longer contribute to the dielectric response. The effect of domain structures and domain-wall motion on the dielectric response and the ferroelectric phase transition will be discussed.

Resume : Polar vortex structures and domain walls : Bloch line switching Switchable Bloch lines in SrTiO3 originate from polar dipoles inside twin walls. These vectors correlate as one-dimensional strings of switchable vectors in planar walls. These strings are called 'Bloch lines' in analogy to magentic excitations with a similar topology. They were first predicte by Houchmanzadeh et al (1991) and cofirmed theoretically by Conti et al. (2011). Detailed computer simulations by Zykova-Timan and Salje (2014) show that Bloch lines are not just polar but their polarity can be reversed by an applied field. Additional vortices between twin walls occur in virtually all ferroelectric/ferroelastic materials at finite temperatures, the vortices are found to flicker in time and space. Widely spaced ferroelastic twin boundaries nucleate vortices while dense twin boundaries suppress them. The time averaged number of vortices at any site decays exponentially, indicating the highly mobile dynamics of the vortex lattice. Applied electric fields break the rotational symmetry of vortices and finally destroy them. The observed vortex structures are akin to those observed in magnetic and superconducting disordered vortex lattices (Zhao et al. 2014). Zykova-Timan, T.,; Salje, E.K. H. APL 104,082907 (2014) Conti et al. JPCM 23, 142203 (2011) Houchmanzadeh et al. JPCM 3, 5163 (1991) Zhao et al. APL 105, 112906 (2014)