Anoop R. Damodaran

Observation of polar vortices in oxide superlattices

The complex interplay of spin, charge, orbital and lattice degrees of freedom provides a plethora of exotic phases and physical phenomena. In recent years, complex spin topologies have emerged as a consequence of the electronic band structure and the interplay between spin and spin–orbit coupling in materials. Here we produce complex topologies of electrical polarization—namely, nanometre-scale vortex–antivortex (that is, clockwise–anticlockwise) arrays that are reminiscent of rotational spin topologies—by making use of the competition between charge, orbital and lattice degrees of freedom in superlattices of alternating lead titanate and strontium titanate layers. Atomic-scale mapping of the polar atomic displacements by scanning transmission electron microscopy reveals the presence of long-range ordered vortex–antivortex arrays that exhibit nearly continuous polarization rotation. Phase-field modelling confirms that the vortex array is the low-energy state for a range of superlattice periods. Within this range, the large gradient energy from the vortex structure is counterbalanced by the corresponding large reduction in overall electrostatic energy (which would otherwise arise from polar discontinuities at the lead titanate/strontium titanate interfaces) and the elastic energy associated with epitaxial constraints and domain formation. These observations have implications for the creation of new states of matter (such as dipolar skyrmions, hedgehog states) and associated phenomena in ferroic materials, such as electrically controllable chirality.

Nanoscale structure and mechanism for enhanced electromechanical response of highly Strained BiFeO3 thin films.

Author(s): Damodaran, Anoop R; Liang, Chen-Wei; He, Qing; Peng, Chun-Yen; Chang, Li; Chu, Ying-Hao; Martin, Lane W | Abstract: The presence of a variety of structural variants in BiFeO3 thin films give rise to exotic electric-field-induced responses and resulting electromechanical responses as large as 5%. Using high-resolution X-ray diffraction and scanning-probe-microscopy-based studies the numerous phases present at the phase boundaries are identified and an intermediate monoclinic phase, in addition to the previously observed rhombohedral- and tetragonal-like phases, is discovered. Copyright

Ferroelectric polarization reversal via successive ferroelastic transitions

Switchable polarization makes ferroelectrics a critical component in memories, actuators and electro-optic devices, and potential candidates for nanoelectronics. Although many studies of ferroelectric switching have been undertaken, much remains to be understood about switching in complex domain structures and in devices. In this work, a combination of thin-film epitaxy, macro- and nanoscale property and switching characterization, and molecular dynamics simulations are used to elucidate the nature of switching in PbZr(0.2)Ti(0.8)O3 thin films. Differences are demonstrated between (001)-/(101)- and (111)-oriented films, with the latter exhibiting complex, nanotwinned ferroelectric domain structures with high densities of 90° domain walls and considerably broadened switching characteristics. Molecular dynamics simulations predict both 180° (for (001)-/(101)-oriented films) and 90° multi-step switching (for (111)-oriented films) and these processes are subsequently observed in stroboscopic piezoresponse force microscopy. These results have implications for our understanding of ferroelectric switching and offer opportunities to change domain reversal speed.

Enhancement of Ferroelectric Curie Temperature in BaTiO3 Films via Strain‐Induced Defect Dipole Alignment

The combination of epitaxial strain and defect engineering facilitates the tuning of the transition temperature of BaTiO3 to >800 °C. Advances in thin-film deposition enable the utilization of both the electric and elastic dipoles of defects to extend the epitaxial strain to new levels, inducing unprecedented functionality and temperature stability in ferroelectrics.

Temperature and thickness evolution and epitaxial breakdown in highly strained BiFeO3 thin films

Author(s): Damodaran, AR; Lee, S; Karthik, J; MacLaren, S; Martin, LW | Abstract: We present the temperature- and thickness-dependent structural and morphological evolution of strain-induced transformations in highly strained epitaxial BiFeO 3 films deposited on LaAlO 3 (001) substrates. Using high-resolution x-ray diffraction and temperature-dependent scanning-probe-based studies, we observe a complex temperature- and thickness-dependent evolution of phases in this system. A thickness-dependent transformation from a single, monoclinically distorted, tetragonal-like phase to a complex mixed-phase structure in films with thicknesses up to ∼200 nm is the consequence of a strain-induced spinodal instability in the BiFeO 3/LaAlO 3 system. Additionally, a breakdown of this strain-stabilized metastable mixed-phase structure to nonepitaxial microcrystallites of the parent rhombohedral structure of BiFeO 3 is observed to occur at a critical thickness of ∼300 nm. We further propose a mechanism for this abrupt breakdown that provides insight into the competing nature of the phases in this system.

Epitaxial Ferroelectric Heterostructures Fabricated by Selective Area Epitaxy of SrRuO3 Using an MgO Mask

Illustration of a new high-temperature hard-mask process based on traditional lithography and selective wet-etching of MgO. The hard mask is compatible with standard nano-lithography techniques and heat treatments in excess of 1000 °C. Here, this technique is applied to produce temperature-stable contacts that give rise to low leakage, improved fatigue properties, and excellent high-temperature stability in ferroelectric thin-film capacitors.

Stationary domain wall contribution to enhanced ferroelectric susceptibility

In ferroelectrics, the effect of domain wall motion on properties has been widely studied, but non-motional or stationary contributions from the volume of material within the domain wall itself has received less attention. Here we report the measurement of stationary domain wall contributions to permittivity in PbZr(0.2)Ti(0.8)O₃ films. Studies of (001)-, (101)- and (111)-oriented epitaxial films reveal that (111)-oriented films, in which the motional domain wall contributions are frozen out, exhibit permittivity values approximately three times larger than the intrinsic response alone. This discrepancy can only be accounted for by considering a stationary contribution from the domain wall volume of the material that is 6-78 times larger than the bulk response, and is consistent with predictions of the enhancement of susceptibilities within 90° domain walls. This work offers new insights into the microscopic origin of dielectric enhancement and provides a pathway to engineer the dielectric response of materials.

Improved Pyroelectric Figures of Merit in Compositionally Graded PbZr1–xTixO3 Thin Films

Pyroelectric materials have been widely used for a range of thermal-related applications including thermal imaging/sensing, waste heat energy conversion, and electron emission. In general, the figures of merit for applications of pyroelectric materials are proportional to the pyroelectric coefficient and inversely proportional to the dielectric permittivity. In this context, we explore single-layer and compositionally graded PbZr1-xTixO3 thin-film heterostructures as a way to independently engineer the pyroelectric coefficient and dielectric permittivity of materials and increase overall performance. Compositional gradients in thin films are found to produce large strain gradients which generate large built-in potentials in the films that can reduce the permittivity while maintaining large pyroelectric response. Routes to enhance the figures of merit of pyroelectric materials by 3-12 times are reported, and comparisons to standard materials are made.

Unexpected Crystal and Domain Structures and Properties in Compositionally Graded PbZr1‐xTixO3 Thin Films

Synthesis of compositionally graded versions of PbZr(1-x)Ti(x)O3 thin films results in unprecedented strains (as large as ≈4.5 × 10(5) m(-1)) and correspondingly unexpected crystal structures, ferroelectric domain structures, and properties. This includes the observation of built-in electric fields in films as large as 200 kV/cm. Compositional and strain gradients could represent a new direction of strain-control of materials.

Effect of “symmetry mismatch” on the domain structure of rhombohedral BiFeO3 thin films

Considerable work has focused on the use of epitaxial strain to engineer domain structures in ferroic materials. Here, we revisit the observed reduction of domain variants in rhombohedral BiFeO3 films on rare-earth scandate substrates. Prior work has attributed the reduction of domain variants to anisotropic in-plane strain, but our findings suggest that the monoclinic distortion of the substrate, resulting from oxygen octahedral rotation, is the driving force for variant selection. We study epitaxial BiFeO3 DyScO3 (110)o heterostructures with and without ultrathin, cubic SrTiO3 buffer layers as a means to isolate the effect of symmetry mismatch on the domain formation. Two variant stripe domains are observed in films grown directly on DyScO3, while four-variant domains are observed in films grown on SrTiO3 buffered DyScO3 when the buffer layer is >2 nm thick. This work provides insights into the role of the substrate beyond just lattice mismatch in manipulating and controlling domain structure evolution in materials.