Ferroelectric materials, because of their robust spontaneous electrical polarization, are widely used in various applications. Recent advances in modelling, synthesis and characterization techniques are spurring unprecedented advances in the study of these materials. In this Review, we focus on thin-film ferroelectric materials and, in particular, on the possibility of controlling their properties through the application of strain engineering in conventional and unconventional ways. We explore how the study of ferroelectric materials has expanded our understanding of fundamental effects, enabled the discovery of novel phases and physics, and allowed unprecedented control of materials properties. We discuss several exciting possibilities for the development of new devices, including those in electronic, thermal and photovoltaic applications, and transduction sensors and actuators. We conclude with a brief survey of the different directions that the field may expand to over the coming years. Strain engineering can be used to control the properties of thin-film ferroelectric materials, which are promising for electronic, thermal, photovoltaic and transduction applications. This Review addresses fundamental aspects, novel ways to control materials properties and the development of new ferroelectric-based devices.

https://escholarship.org/content/qt51w3s3s1/qt51w3s3s1.pdf?t=p8f5p7

Thin-film ferroelectric materials and their applications

We review the recent developments in the electric field control of magnetism in multiferroic heterostructures, which consist of heterogeneous materials systems where a magnetoelectric coupling is engineered between magnetic and ferroelectric components. The magnetoelectric coupling in these composite systems is interfacial in origin, and can arise from elastic strain, charge, and exchange bias interactions, with different characteristic responses and functionalities. Moreover, charge transport phenomena in multiferroic heterostructures, where both magnetic and ferroelectric order parameters are used to control charge transport, suggest new possibilities to control the conduction paths of the electron spin, with potential for device applications.

https://iopscience.iop.org/article/10.1088/0953-8984/24/33/333201/pdf

Electric field control of magnetism in multiferroic heterostructures

Multiferroic bismuth ferrite-based materials for multifunctional applications: Ceramic bulks, thin films and nanostructures

Using epitaxy and the misfit strain imposed by an underlying substrate, it is possible to elastically strain oxide thin films to percent levels—far beyond where they would crack in bulk. Under such strains, the properties of oxides can be dramatically altered. In this article, we review the use of elastic strain to enhance ferroics, materials containing domains that can be moved through the application of an electric field (ferroelectric), a magnetic field (ferromagnetic), or stress (ferroelastic). We describe examples of transmuting oxides that are neither ferroelectric nor ferromagnetic in their unstrained state into ferroelectrics, ferromagnets, or materials that are both at the same time (multiferroics). Elastic strain can also be used to enhance the properties of known ferroic oxides or to create new tunable microwave dielectrics with performance that rivals that of existing materials. Results show that for thin films of ferroic oxides, elastic strain is a viable alternative to the traditional method of chemical substitution to lower the energy of a desired ground state relative to that of competing ground states to create materials with superior properties.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0883769414000013

Elastic strain engineering of ferroic oxides

In the past several years, Materials Genome Initiative (MGI) efforts have produced myriad examples of computationally designed materials in the fields of energy storage, catalysis, thermoelectrics, and hydrogen storage as well as large data resources that are used to screen for potentially transformative compounds. The bottleneck in high-throughput materials design has thus shifted to materials synthesis, which motivates our development of a methodology to automatically compile materials synthesis parameters across tens of thousands of scholarly publications using natural language processing techniques. To demonstrate our framework’s capabilities, we examine the synthesis conditions for various metal oxides across more than 12 thousand manuscripts. We then apply machine learning methods to predict the critical parameters needed to synthesize titania nanotubes via hydrothermal methods and verify this result against known mechanisms. Finally, we demonstrate the capacity for transfer learning by using machin...

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Materials Synthesis Insights from Scientific Literature via Text Extraction and Machine Learning

Recent reports on epitaxial BiFeO{sub 3} films show that the crystal structure changes from nearly rhombohedral (''R like'') to nearly tetragonal (''T like'') at strains exceeding {approx}-4.5%, with the T-like structure being characterized by a highly enhanced c/a ratio. While both the R-like and the T-like phases are monoclinic, our detailed x-ray diffraction results reveal a symmetry change from M{sub A} and M{sub C} type, respectively, at this R-like-to-T-like transition. Therefore, the ferroelectric polarization is confined to different (pseudocubic) planes in the two phases. By applying additional strain or by modifying the unit-cell volume of the film by substituting Ba for Bi, the monoclinic distortion in the T-like M{sub C} phase is reduced, i.e., the system approaches a true tetragonal symmetry. Therefore, in going from bulk to highly strained films, a phase sequence of rhombohedral (R)-to-monoclinic (R-like M{sub A})-to-monoclinic (T-like M{sub C})-to-tetragonal (T) is observed. This sequence is otherwise seen only near morphotropic phase boundaries in lead-based solid-solution perovskites (i.e., near a compositionally induced phase instability), where it can be controlled by electric field, temperature, or composition. Our results now show that this evolution can occur in a lead-free, stoichiometric material and can be induced by stress alone.

Stress-induced R − M A − M C − T symmetry changes in BiFeO 3 films

Recent reports on epitaxial BiFeO${}_{3}$ films show that the crystal structure changes from nearly rhombohedral (``$R$ like'') to nearly tetragonal (``$T$ like'') at strains exceeding \ensuremath{\approx}\ensuremath{-}4.5%, with the $T$-like structure being characterized by a highly enhanced $c$/$a$ ratio. While both the $R$-like and the $T$-like phases are monoclinic, our detailed x-ray diffraction results reveal a symmetry change from ${M}_{A}$ and ${M}_{C}$ type, respectively, at this $R$-like-to-$T$-like transition. Therefore, the ferroelectric polarization is confined to different (pseudocubic) planes in the two phases. By applying additional strain or by modifying the unit-cell volume of the film by substituting Ba for Bi, the monoclinic distortion in the $T$-like ${M}_{C}$ phase is reduced, i.e., the system approaches a true tetragonal symmetry. Therefore, in going from bulk to highly strained films, a phase sequence of rhombohedral ($R$)-to-monoclinic ($R$-like ${M}_{A}$)-to-monoclinic ($T$-like ${M}_{C}$)-to-tetragonal ($T$) is observed. This sequence is otherwise seen only near morphotropic phase boundaries in lead-based solid-solution perovskites (i.e., near a compositionally induced phase instability), where it can be controlled by electric field, temperature, or composition. Our results now show that this evolution can occur in a lead-free, stoichiometric material and can be induced by stress alone.

/pdf/stress-inducedr-ma-mc-tsymmetry-changes-in-bifeo3films-yf42orzj36.pdf

Stress-inducedR−MA−MC−Tsymmetry changes in BiFeO3films

Highly-strained BiFeO3 exhibits a "tetragonal-like, monoclinic" crystal structure found only in epitaxial films (with an out-of-plane lattice parameter exceeding the in-plane value by >20%). Previous work has shown that this phase is properly described as a M$_{C}$ monoclinic structure at room temperature [with a (010)$_{pc}$ symmetry plane, which contains the ferroelectric polarization]. Here we show detailed temperature-dependent x-ray diffraction data that evidence a structural phase transition at ~100C to a high-temperature M$_{A}$ phase ["tetragonal-like" but with a (1-10)$_{pc}$ symmetry plane]. These results indicate that the ferroelectric properties and domain structures of strained BiFeO$_3$ will be strongly temperature dependent.

Temperature Driven Structural Phase Transition in Tetragonal-Like BiFeO3

Highly strained BiFeO3 exhibits a "tetragonal-like, monoclinic" crystal structure found only in epitaxial films (with an out-of-plane lattice parameter exceeding the in-plane value by >20%). Previous work has shown that this phase is properly described as an MC monoclinic structure at room temperature [with a (010)pc symmetry plane, which contains the ferroelectric polarization]. Here, we show detailed temperature-dependent X-ray diffraction data that reveal a structural phase transition at ~100 °C to a high-temperature MA phase ["tetragonal-like" but with a (110)pc symmetry plane]. These results indicate that the ferroelectric properties and domain structures of the strained BiFeO3 are strongly temperature dependent.

https://iopscience.iop.org/article/10.1143/APEX.4.095801/pdf

Synchrotron and lab-scale x-ray diffraction shows that tetragonal-like T'-BiFeO3 films on miscut LaAlO3 substrates (miscut < 5 deg) exhibit (00l)-planes tilted away from those of the substrate as predicted by the "Nagai model" (except for miscut <0.2 deg). Tilts as large as 1 deg are achieved even in 100 nm thick films, strikingly larger than those observed in other perovskites. We attribute this to the large c/a ratio and the high crystalline coherency of the T'-BiFeO3/LaAlO3 interface. This coherency is possible through an observed "diagonal-on-diagonal" alignment. Interestingly, the substrate miscut does not influence the relative population of monoclinic domains.

Joong Hee Nam

Papers

Stress-induced R − M A − M C − T symmetry changes in BiFeO 3 films

Stress-inducedR−MA−MC−Tsymmetry changes in BiFeO3films

Temperature Driven Structural Phase Transition in Tetragonal-Like BiFeO3

Temperature Driven Structural Phase Transition in Tetragonal-Like BiFeO3

Unit cell orientation of tetragonal-like BiFeO$_3$ thin films grown on highly miscut LaAlO$_3$ substrates