Varatharajan Anbusathaiah

Combinatorial discovery of a lead-free morphotropic phase boundary in a thin-film piezoelectric perovskite

We report on the discovery of a lead-free morphotropic phase boundary (MPB) in Sm doped BiFeO3 with a simple perovskite structure using the combinatorial thin film strategy. The boundary is a rhombohedral to pseudo-orthorhombic structural transition which exhibits a ferroelectric to antiferroelectric transition at approximately Bi0.86Sm0.14FeO3 with dielectric constant and out-of-plane piezoelectric coefficient comparable to those of epitaxial (001) oriented PbZr0.52Ti0.48O3 (PZT) thin films at the MPB. The discovered composition may be a strong candidate of a Pb-free piezoelectric replacement of PZT.

Microstructure-electromechanical property correlations in rare-earth-substituted BiFeO3 epitaxial thin films at morphotropic phase boundaries

Structure-electromechanical property correlations in rare-earth (RE)-substituted (001) BiFeO3 (BFO) epitaxial thin films have been investigated. Quantitative piezoelectric coefficient (d33) and dielectric constant (e33) measurements, in conjunction with selected area electron diffraction, reveal that the enhancement in d33 and e33 at the morphotropic phase boundary (MPB) of the RE-substituted films (RE=Dy3+, Gd3+, and Sm3+) is correlated with the presence of a competing intermediate antipolar phase with the rhombohedral ferroelectric and nonpolar orthorhombic phase. This leads to a complex nanoscale phase coexistence at the MPB. Extending the studies to RE=La3+ case, we find the nanoscale phase coexistence to be less pronounced. This explains the lack of increase in d33 for the La3+-substituted BFO films, in contrast to the Dy3+, Gd3+, and Sm3+-substituted films.

Chemical Substitution‐Induced Ferroelectric Polarization Rotation in BiFeO3

and multiferroic [ 2 ] materials. For instance, ferroelectric materials at morphotropic phase boundaries (MPB), where multiple structural phases with ferroelectric polarizations pointing in different crystallographic directions coexist, often display large piezoelectric coeffi cients. [ 3– 5 ] It is the ferroelectric distortions, which accompany the polarization rotation that leads to enhancements in the piezoelectric coeffi cient. In multiferroic BiFeO 3 (BFO), it has been shown [ 2 ] that the coupled antiferromagnetic order can be altered by switching the ferroelectric polarization vector. In fact, the ability of a material to display polarization rotation is recognized as an important precursor to occurrence of an MPB. [ 5 , 6 ]

Ferroelastic domain wall dynamics in ferroelectric bilayers

High-performance piezoelectric devices based on ferroelectric materials rely heavily on ferroelastic domain wall switching. Here we present visual evidence for the local mechanisms that underpin domain wall dynamics in ferroelastic nanodomains. State-of-the-art band excitation switching spectroscopy piezoforce microscopy (PFM) reveals distinct origins for the reversible and irreversible components of ferroelastic domain motion. Extrapolating the PFM images to case for uniform fields, we posit that, while reversible switching is essentially a linear motion of the ferroelastic domains, irreversible switching takes place via domain wall twists. Critically, real-time images of in situ domain dynamics under an external bias reveal that the reversible component leads to reduced coercive voltages. Finally, we show that junctions representing three-domain architecture represent facile interfaces for ferroelastic domain switching, and are likely responsible for irreversible processes in the uniform fields. The results presented here thus provide (hitherto missing) fundamental insight into the correlations between the physical mechanisms that govern ferroelastic domain behavior and the observed functional response in domain-engineered thin film ferroelectric devices.

Synthesis of Epitaxial Metal Oxide Nanocrystals via a Phase Separation Approach

Perovskite phase instability of BiMnO3 has been exploited to synthesize epitaxial metal oxide magnetic nanocrystals. Thin film processing conditions are tuned to promote the breakdown of the perovskite precursor into Bi2O3 matrix and magnetic manganese oxide islands. Subsequent cooling in vacuum ensures complete volatization of the Bi2O3, thus leaving behind an array of self-assembled magnetic Mn3O4 nanostructures. Both shape and size can be systematically controlled by the ambient oxygen environments and deposition time. As such, this approach can be extended to any other Bi-based complex ternary oxide system as it primarily hinges on the breakdown of parent Bi-based precursor and subsequent Bi2O3 volatization.

Ultrafast switching of ferroelastic nanodomains in bilayered ferroelectric thin films

The dynamic switching of ferroelastic nanodomains in ferroelectric PbZr0.3Ti0.7O3/PbZr0.7Ti0.3O3 bilayers was investigated. Synchrotron microdiffraction using a high-speed pulse generator reveals that electric field pulses as short as 200 ns can switch the ferroelastic domain. Multiples of random distribution analysis of the field-induced changes in diffraction peak intensities finds a dynamic strain change from 0.2 to 1% with increasing the pulse width. Raman spectroscopy shows considerable decreases in A1(1TO) soft mode intensity after applications of short pulses, confirming the ferroelastic switching. The results demonstrate that ferroelastic domains can indeed move at time scales of the order of hundreds of nanoseconds.

Ferroelastic domains in bilayered ferroelectric thin films

We investigate theoretically ferroelastic domain fractions in a heteroepitaxial bilayer consisting of (001) tetragonal PbZrxTi1−xO3 and (001) rhombohedral PbZr1−xTixO3 on a thick (001) passive substrate as a function of the lattice misfit strain between layers and the substrate. By considering the self-strain in each layer and the indirect elastic interaction between the layers, we provide a numerical analysis of the relative domain fractions in the tetragonal layer of a (001)PbZr0.2Ti0.8O3/(001)PbZr0.8Ti0.2O3 and (001)PbZr0.4Ti0.6O3/(001)PbZr0.6Ti0.4O3 bilayer structure as a function of the tetragonal layer thickness on (001)LaAlO3, (001)SrTiO3, and (001) MgO. It is found that the elastic coupling between the tetragonal and rhombohedral layers leads to an excess elastic energy in the tetragonal layer, resulting in a two to three times increase in the ferroelastic domain volume fraction of the tetragonal layer compared to single-layer films of similar thickness. These results show alternate ways of engine...

High-resolution piezoresponse force microscopy investigation of imprint in ferroelectric thin films

High-resolution piezoresponse force microscopy is used to visualize imprint in polycrystalline PbZr0.25Ti0.75O3 thin films. Three-dimensional domain images show the formation of a thin bright band (∼8nm in width) running along the grain boundary after local application of a negative bias. Such bands extend completely over the region under local bias thereby forming networks. Cross-section profile analysis reveals that these are not pinned regions, rather they are formed during the switching process. This demonstrates an active role of grain boundaries in pinning a preferential polarization state. Piezoresponse hysteresis loops confirm that these regions are imprinted.

Change in the magnetic structure of (Bi,Sm)FeO3 thin films at the morphotropic phase boundary probed by neutron diffraction

We report on the evolution of the magnetic structure of BiFeO3 thin films grown on SrTiO3 substrates as a function of Sm doping. We determined the magnetic structure using neutron diffraction. We found that as Sm increases, the magnetic structure evolves from a cycloid to a G-type antiferromagnet at the morphotropic phase boundary, where there is a large piezoelectric response due to an electric-field induced structural transition. The occurrence of the magnetic structural transition at the morphotropic phase boundary offers another route towards room temperature multiferroic devices.

Role of oxygen partial pressure and seed layer chemistry in flux mediated epitaxy of single phase multiferroic BiFeO3 thin films

Multiferroic BiFeO3 (BFO) thin films have been fabricated via flux mediated epitaxy with varying oxygen partial pressure and flux composition (Bi2O3:CuO) conditions. Transmission electron microscopy coupled with energy dispersive x-ray spectroscopy as well as piezoresponse force microscopy confirm, that with the correct flux and seed layer conditions, even at very low partial pressures (3mTorr) no secondary phases are formed. The study reveals the crucial role of the bottom seed layer and flux chemistry in epitaxy of BFO thin films and provides alternate routes to BFO epitaxy in oxygen-deficient environments.