L. J. McGilly

Mesoscale flux-closure domain formation in single-crystal BaTiO3.

Over 60 years ago, Charles Kittel predicted that quadrant domains should spontaneously form in small ferromagnetic platelets. He expected that the direction of magnetization within each quadrant should lie parallel to the platelet surface, minimizing demagnetizing fields,and that magnetic moments should be configured into an overall closed loop, or flux-closure arrangement. Although now a ubiquitous observation in ferromagnets, obvious flux-closure patterns have been somewhat elusive in ferroelectric materials. This is despite the analogous behaviour between these two ferroic subgroups and the recent prediction of dipole closure states by atomistic simulations research. Here we show Piezoresponse Force Microscopy images of mesoscopic dipole closure patterns in free-standing, single-crystal lamellae of BaTiO3. Formation of these patterns is a dynamical process resulting from system relaxation after the BaTiO3 has been poled with a uniform electric field. The flux-closure states are composed of shape conserving 90° stripe domains which minimize disclination stresses.

Controlling domain wall motion in ferroelectric thin films

Domain walls in ferroic materials have attracted significant interest in recent years, in particular because of the unique properties that can be found in their vicinity. However, to fully harness their potential as nanoscale functional entities, it is essential to achieve reliable and precise control of their nucleation, location, number and velocity. Here, using piezoresponse force microscopy, we show the control and manipulation of domain walls in ferroelectric thin films of Pb(Zr,Ti)O₃ with Pt top electrodes. This high-level control presents an excellent opportunity to demonstrate the versatility and flexibility of ferroelectric domain walls. Their position can be controlled by the tuning of voltage pulses, and multiple domain walls can be nucleated and handled in a reproducible fashion. The system is accurately described by analogy to the classical Stefan problem, which has been used previously to describe many diverse systems and is here applied to electric circuits. This study is a step towards the realization of domain wall nanoelectronics utilizing ferroelectric thin films.

Negative-pressure-induced enhancement in a freestanding ferroelectric.

Ferroelectrics are widespread in technology, being used in electronics and communications, medical diagnostics and industrial automation. However, extension of their operational temperature range and useful properties is desired. Recent developments have exploited ultrathin epitaxial films on lattice-mismatched substrates, imposing tensile or compressive biaxial strain, to enhance ferroelectric properties. Much larger hydrostatic compression can be achieved by diamond anvil cells, but hydrostatic tensile stress is regarded as unachievable. Theory and ab initio treatments predict enhanced properties for perovskite ferroelectrics under hydrostatic tensile stress. Here we report negative-pressure-driven enhancement of the tetragonality, Curie temperature and spontaneous polarization in freestanding PbTiO3 nanowires, driven by stress that develops during transformation of the material from a lower-density crystal structure to the perovskite phase. This study suggests a simple route to obtain negative pressure in other materials, potentially extending their exploitable properties beyond their present levels.

Compliant ferroelastic domains in epitaxial Pb(Zr,Ti)O3 thin films

Ordered patterns of highly compliant ferroelastic domains have been created by use of tensile strained epitaxial Pb(Zr,Ti)O3 thin films, of very low defect density, grown on DyScO3 substrates. The effect of 180° switching on well-ordered a/c 90° domain patterns is investigated by a combination of transmission electron microscopy, piezoelectric force microscopy, and X-ray diffraction. It is shown that ferroelastic a-domains, having an in-plane polarization, can be created and completely removed on a local level by an out-of-plane electric field. The modifications of the ferroelastic domain pattern can be controlled by varying the parameters used during switching with a piezoresponse force microscope to produce the desired arrangement.

Piezoelectric enhancement under negative pressure

Enhancement of ferroelectric properties, both spontaneous polarization and Curie temperature under negative pressure had been predicted in the past from first principles and recently confirmed experimentally. In contrast, piezoelectric properties are expected to increase by positive pressure, through polarization rotation. Here we investigate the piezoelectric response of the classical PbTiO3, Pb(Zr,Ti)O3 and BaTiO3 perovskite ferroelectrics under negative pressure from first principles and find significant enhancement. Piezoelectric response is then tested experimentally on free-standing PbTiO3 and Pb(Zr,Ti)O3 nanowires under self-sustained negative pressure, confirming the theoretical prediction. Numerical simulations verify that negative pressure in nanowires is the origin of the enhanced electromechanical properties. The results may be useful in the development of highly performing piezoelectrics, including lead-free ones.

Superdomain Structure in Epitaxial Tetragonal PZT Thin Films Under Tensile Strain

The a/c domain pattern of tetragonal PbZr0.10Ti0.90O3 thin films under tensile misfit strain is investigated by piezoresponse force microscopy and X-ray diffraction. The results show a hierarchical ordering of the dense a/c domain structure into larger superstructures. The latter exhibit a preferred orientation and occasionally form distinct patterns such as flux closure loops of the net polarization. Additionally, the residual strain is measured with reciprocal space maps and the a-domain fraction is determined by theoretical calculations and correlated with results from rocking curve scans.

Room temperature concurrent formation of ultra-dense arrays of ferroelectric domain walls

Properties of ferroelectric domain walls are attractive for future nano- and optoelectronics. An important element is the potential to electrically erase/rewrite domain walls inside working devices. Dense domain wall patterns, formed upon cooling through the ferroelectric phase transition, were demonstrated. However, room temperature domain wall writing is done with a cantilever tip, one domain stripe at a time, and reduction of the inter-wall distance is limited by the tip diameter. Here, we show, at room temperature, controlled formation of arrays of domain walls with sub-tip-diameter spacing (i.e., inter-wall distance down to approximate to 10 nm). Each array contains 100-200 concurrently formed walls. Array rewriting is confirmed. The method is demonstrated in several materials. Dense domain pattern formation through a continuous electrode, practical for potential device applications, is also demonstrated. A quantitative theory of the phenomenon is provided

Polarization Switching and Domain Wall Motion in Circular and Ring Capacitor Structures in PZT Thin Films

Ferroelectric switching in circular and ring capacitors has been performed using a stroboscopic mode of piezoresponse force microscopy. A simple geometric model incorporating the characteristic domain wall motion is sufficient to describe the switching of circular capacitors but which however breaks down when attempting to describe the ring switching. Analysis of the switching dynamics implies that the domain wall moves faster along the perimeter of both the circle and ring electrode structures.

Controlled creation and displacement of charged domain walls in ferroelectric thin films

Charged domain walls in ferroelectric materials are of high interest due to their potential use in nanoelectronic devices. While previous approaches have utilized complex scanning probe techniques or frustrative poling here we show the creation of charged domain walls in ferroelectric thin films during simple polarization switching using either a conductive probe tip or patterned top electrodes. We demonstrate that ferroelectric switching is accompanied - without exception - by the appearance of charged domain walls and that these walls can be displaced and erased reliably. We ascertain from a combination of scanning probe microscopy, transmission electron microscopy and phase field simulations that creation of charged domain walls is a by-product of, and as such is always coupled to, ferroelectric switching. This is due to the (110) orientation of the tetragonal (Pb,Sr)TiO3 thin films and the crucial role played by the limited conduction of the LSMO bottom electrode layer used in this study. This work highlights that charged domain walls, far from being exotic, unstable structures, as might have been assumed previously, can be robust, stable easily-controlled features in ferroelectric thin films.

Dynamics of ferroelectric 180° domain walls at engineered pinning centers

The interaction between domain walls and pinning centers in ferroelectrics is of great interest from both fundamental and practical points of view. In this work, we show that, counter to intuition, the apparent velocity of domain walls can increase as the defect density increases. However, when we closely investigate the propagating front of the domain wall, we find that it is not unified but can be rough, indicating the presence of multiple nucleated domains in advance of the primary wall. We therefore ascribe the increased apparent velocity with defect density to actually derive from nucleation-aided motion. To further investigate the effect of engineered pinning centers, we spatially confined the defect regions and then propagated domain walls in that direction. We found that, given a sufficiently high defect density, walls can be pinned indefinitely at sub-threshold voltages. Finally, we outline a method to create domain wall propagation channels in which the wall is confined to a low defect region bo...