The discovery of a universal behavior in rare‐earth (RE)‐substituted perovskite BiFeO3 is reported. The structural transition from the ferroelectric rhombohedral phase to an orthorhombic phase exhibiting a double‐polarization hysteresis loop and substantially enhanced electromechanical properties is found to occur independent of the RE dopant species. The structural transition can be universally achieved by controlling the average ionic radius of the A‐site cation. Using calculations based on first principles, the energy landscape of BiFeO3 is explored, and it is proposed that the origin of the double hysteresis loop and the concomitant enhancement in the piezoelectric coefficient is an electric‐field‐induced transformation from a paraelectric orthorhombic phase to the polar rhombohedral phase.

We report on the discovery of a lead-free morphotropic phase boundary in Sm doped BiFeO 3 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 (FE) to antiferroelectric (AFE) transition at approximately Bi 0.86 Sm 0.14 FeO 3 with dielectric constant and out-of-plane piezoelectric coefficient comparable to those of epitaxial (001) oriented Pb(Zr,Ti)O 3 (PZT) thin films at the MPB. The discovered composition may be a strong candidate of a Pb-free piezoelectric replacement of PZT.

Structure-electromechanical property correlations in rare-earth ͑RE͒-substituted ͑001͒ BiFeO 3 ͑BFO͒ epitaxial thin films have been investigated. Quantitative piezoelectric coefficient ͑d 33 ͒ and dielectric constant ͑ 33 ͒ measurements, in conjunction with selected area electron diffraction, reveal that the enhancement in d 33 and 33 at the morphotropic phase boundary ͑MPB͒ of the RE-substituted films ͑RE= Dy 3+ , Gd 3+ , and Sm 3+ ͒ 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 =La 3+ case, we find the nanoscale phase coexistence to be less pronounced. This explains the lack of increase in d 33 for the La 3+-substituted BFO films, in contrast to the Dy 3+ , Gd 3+ , and Sm 3+-substituted films.

The direction of the ferroelectric polarization vector is a key factor infl uencing the properties of ferroelectric/piezoelectric [ 1 ] 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][4][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 ] Chemical substitution into perovskite BiFeO 3 , which displays room-temperature multiferroic properties, [ 2 , 7 ] has been a subject of much interest since the substitution results in improved ferroelectric properties and enhancement in piezoelectric and dielectric properties. [ 8 -10 ] One important consequence of substitution is that a symmetry-lowering structural phase transition from the rhombohedral phase for pure BFO to another structure takes place, displaying the characteristics of an MPB with enhanced dielectric and piezoelectric properties, as observed in Pb-based ferroelectrics. [ 3 , 4 ] Recently, we demonstrated [ 11 ] that substitution of rare earth elements (RE = Sm, Gd, and Dy) into the A-sites of BFO thin fi lms results in a ferroelectric rhombohedral to paraelectric orthorhombic structural transition, exhibiting a double hysteresis behavior in the polarization-electric fi eld (PE) hysteresis loop, and that the occurrence of this transition can be universally described by the averaged A-site cation radius regardless of the substituted rare-earth element. The piezoelectric coeffi cient d 33 and dielectric constant ε 33 are enhanced at the boundary, and the maximum d 33 reaches 110 pm V − 1 , [ 8 ] which is comparable to the value for epitaxial Pb(Zr 0.48 Ti 0.52 )O 3 thin fi lms at the MPB. [ 12 ] Based on the results of fi rst-principles calculations, it was proposed [ 11 ] that an electric fi eld-induced structural transformation from the nonpolar orthorhombic to the polar rhombohedral phase is the origin for the double hysteresis behavior and the concomitant enhanced properties at the boundary.Here, we demonstrate that the polarization vector in Smsubstituted BiFeO 3 (Sm-BFO) continuously rotates from the [111] direction for pure BFO toward the [001] direction as the Sm concentration is increased. (The pseudo-cubic indices are used in this study.) Although Sm substitution reduces the total amount of polarization from 110 μ C cm − 2 for pure BFO to 60 μ C cm − 2 at Sm 14% where the rhombohedral to orthorhombic structural transition takes place, the piezoelectric coeffi cient is increased by as much as 50% at the structural bou...

Bilayered Pb(Zr(1–x),Tix)O3 ferroelectric thin film heterostructures show complex ferroelastic nanodomain patterns. These ferroelastic nanodomains exist only in the upper layer, and hence are able to move under the application of an external electric field. Quantitative analysis reveals an enhanced piezoelectric coefficient of ≈220 pm V−1, rendering them attractive for a variety of electromechanical devices.

Growth of (111)-oriented BaTiO3-Bi(Mg0.5Ti0.5)O3 epitaxial films and their crystal structure and electrical property characterizations

Direct evidence of controlling the population of magnetic domains in BiFeO 3 thin films through electric field is reported using neutron diffraction. By fabricating BiFeO 3 thin films on vicinal SrTiO 3 substrates, we have achieved ferroelectric monodomains as confirmed by piezoresponse force microscopy. The application of an electric field between the bottom SrRuO 3 and the top electrode switches the ferroelectric domain state with concomitant changes in magnetic reflections observed with neutron diffraction, indicating changes in the antiferromagnetic domain populations. The observed magnetoelectric switching behavior by neutron diffraction is compared with the electric-field effect on the magneto-optical Kerr effect measurement on patterned pads of exchange coupled Co film deposited on top of the BiFeO 3 films. The present result shows possible new directions for the realization of magnetoelectric devices.

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.