Low-Symmetry Monoclinic Phases and Polarization Rotation Path Mediated By Epitaxial Strain in Multiferroic BiFeO3 Thin Films (1011.0893v3)

Abstract: A morphotropic phase boundary driven by epitaxial strain has been observed in a lead-free multiferroic BiFeO3 thin films and the strain-driven phase transitions were widely reported to be iso-symmetric Cc-Cc ones by recent works. In this paper, we suggest that the tetragonal-like BiFeO3 phase identified in epitaxial films on (001) LaAlO3 single crystal substrates is monoclinic MC. This MC phase is different from MA type monoclinic phase reported in BiFeO3 films grown on low mismatch substrates, such as SrTiO3. This is confirmed not only by synchrotron x-ray studies but also by piezoresponse force microscopy measurements. The polarization vectors of the tetragonal-like phase lie in the (100) plane, not the (110) plane as previously reported. A phenomenological analysis was proposed to explain the formation of MC Phase. Such a low symmetry MC phase, with its linkage to MA phase and the multiphase coexistence open an avenue for large piezoelectric response in BiFeO3 films and shed light on a complete understanding towards possible polarization rotation paths and enhanced multiferroicity in BiFeO3 films mediated by epitaxial strain. This work may also aid the understanding of developing new lead-free strain-driven morphotropic phase boundary in other ferroic systems.

• The paper demonstrates that advanced synchrotron x‑ray diffraction and PFM techniques reveal the stabilization of a monoclinic MC phase in BiFeO₃ thin films.
• It shows that epitaxial strain drives polarization rotation paths, which enhance the film’s piezoelectric response.
• The research challenges previous phase assumptions by identifying a distinct low‑symmetry phase transition critical for next‑generation device applications.

An Analysis of Low-Symmetry Monoclinic Phases and Polarization Rotation in Strain-Mediated BiFeO₃ Thin Films

The paper investigates the distinct structural phases in epitaxially strained BiFeO₃ (BFO) thin films, specifically clarifying the nature of the phases induced by epitaxial strain. By employing advanced characterization tools such as synchrotron x-ray diffractometry and piezoelectric force microscopy (PFM), the research delineates the structural and ferroelectric domain peculiarities inherent to these films.

BiFeO₃ has been recognized for its room-temperature multiferroicity, making it a promising candidate for spintronic, piezoelectric, and memory devices. However, the transition from bulk to thin film form introduces complex structural scenarios due to strain exerted by varying substrate lattice constants. This strain often stimulates phase transitions that markedly impact the material's ferroelectric properties.

Recent works have reported iso-symmetric phase transformations in strained BFO films grown on substrates like LaAlO₃, traditionally assuming the presence of a monoclinic MA phase. In this rigorous investigation, the researchers compellingly propose that the stabilizing phase on LaAlO₃ substrates is actually monoclinic MC, distinguishing it from previously identified MA phases. X-ray diffraction and reciprocal space mapping (RSM) substantiate this claim, revealing a unique three-peak split characteristic of another MC phase in the (H0L) plane.

Experimental evidence from piezoelectric force microscopy corroborates the inferred monoclinic MC symmetry. By scanning different domains within the films, a consistent in-plane contrast was observed, attributing to the MC phase's polarization vector orientation within the (010) plane in contrast to previous findings.

Beyond identifying the MC phase, the paper evaluates the potential implications for the phases' piezoelectric response. The researchers suggest that the low-symmetry MC phase, combined with the possibility of multiphase coexistence near morphotropic phase boundaries (MPBs), enables significant polarization rotation paths. Such rotations are pivotal for enhanced piezoelectric responses, indicating promising avenues for device applications requiring high sensitivity and tunability.

The paper further explores the evolution of the c-lattice parameter across different substrates, illustrating a shifting trend attributable to misfit strain. This characterization reveals that BFO films undergo a distinct transition from rhombohedral-like MA to monoclinic MC phases under certain strain conditions, challenging previous assumptions of direct Cc-Cc phase transitions. Furthermore, films with large compressive strains, as induced by LAO substrates, transitioned markedly from R-like to T-like phases, demonstrating an entirely unique structural adaptation path.

This research not only advances the understanding of epitaxially strained BFO films but also sets a precedent for explorations of phase stabilization mechanisms in other lead-free multiferroics. The insights derived from this paper could inform the design of ferroelectric and multiferroic devices where strain mediates functional properties. Future research may leverage first-principles calculations to refine understanding of the mechanisms underpinning phase stability and to articulate comprehensive paths for polarization rotation. Such endeavors could revolutionize the tunability and applicability of multiferroic materials in next-generation electronic devices.