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Title: Strain effect in multiferroic BiFeO3 epitaxial thin films
Authors: Chen, Zuhuang.
Keywords: DRNTU::Engineering::Materials::Functional materials
DRNTU::Engineering::Materials::Microelectronics and semiconductor materials::Thin films
DRNTU::Science::Physics::Electricity and magnetism
Issue Date: 2012
Abstract: Multiferroic materials have gained considerable interest recently because of the intriguing fundamental physics and wide range of potential applications. Among them, BiFeO3 (BFO) is of particular interest because it is one of the very few known single phase multiferroic materials which simultaneously possesses ferroelectric and magnetic orders above room temperature. Strain engineering is a very effective technique to tune the behavior of functional oxide thin films. The goal of this project is to unveil aspects of the effect of epitaxial strain on the structural and ferroelectric properties of BFO thin films. To investigate the effect of the epitaxial strain, epitaxial BFO thin films were grown by pulsed laser deposition on various perovskite single crystal substrates which can impose different misfit strains. High-resolution synchrotron x-ray diffraction measurements, piezoelectric force microscopy, transmission electron microscopy and theoretical calculations were performed to investigate the crystal and domain structure of the strained films. We demonstrate that epitaxial strain has a dramatic effect on the phase stability, domain structure and ferroelectric properties of BFO thin films. Our results reveal that BFO films under small misfit strain adopt a monoclinically distorted rhombohedral (R-like) structure; and domain structure of the R-like films can be varied by appropriate choice of substrates and substrate vicinity. For BFO films grown on LaAlO3 (LAO) substrate, which imposes a large compressive strain of -4.3% on the films, a tetragonal-like (T-like) phase with a large c/a ratio of ~1.23 was found to be stable and a strain-driven morphotropic phase boundary (MPB) was observed. We determined the crystal and domain structures of this T-like phase and clarified its strain-induced polarization rotation path, for the first time. We reveal that this T-like phase is monoclinic MC phase. In addition, two triclinic phases were detected in the mixed-phase MPB films obtained at larger thicknesses where the strain-relaxation occurs. First-principles computations suggest that such triclinic phases form by phase separation of a single monoclinic state for elastic strain compatibility between the phase-separated states. Moreover, a unique shear strain relaxation mechanism for the T-like films is disclosed and a phase transition is revealed at ~100 °C from the T-like MC phase to a T-like monoclinic MA phase. We developed a planar electrode technique to directly measure the in-plane component of the spontaneous polarization in the films which facilitated the characterization of the effect of misfit strain on the polarization of epitaxial BFO thin films. The results demonstrate that the polarization vector in R-like phase is constrained to lie within (110) plane and rotates from [111] toward [001] direction with increasing compressive strain; and the in-plane component of the spontaneous polarization in the pure T-like phase in the film grown on LAO is as large as 44±4 μC/cm2, and that the resultant polarization vector, which lies on the (010) plane, is rotated away by around 16o from the [001] direction. Our results enrich the knowledge of the crystal and domain structure in epitaxial BFO films and contribute to a fundamental understanding of the influence of epitaxial strain on ferroelectric properties of BFO thin films to facilitate continued progress in the field of strain engineering. The large polarization in the strain-stabilized T-like phase and large piezoelectric response in the strain-driven MPB make BFO a prime candidate for next-generation non-volatile memories and lead-free actuators.
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