Designer defect stabilization of the super tetragonal phase in >70-nm-thick BiFeO₃ films on LaAlO₃ substrates

Abstract

The stability of the “T-like” (T′) phase in BiFeO3 films grown on LaAlO3(001) is investigated. We show that the T′ phase can be stabilized for thicknesses >70 nm under ultralow incident flux conditions in pulsed laser ablation growth. This low flux results in a low growth rate; thus, the sample is held at high temperatures (>600 °C) for much longer than is typical. Transmission electron microscopy and X-ray diffraction analysis suggest that such growth conditions favor the formation of nanoscale “defect pockets”, which apply a local compressive strain of ∼1.8%. We propose that the cumulative effect of local stresses induced by such “designer defects” maintains macroscale strain coherence mechanical boundary conditions, which then preserves the T′ phase to thicknesses beyond conventional wisdom. Finally, by intentionally introducing an amorphous phase at the film-substrate interface, it is shown that the mixed-phase proportion can be tuned for a given thickness.

Superior polarization retention through engineered domain wall pinning

“…This mixed-phase BFO system is a fertile ground for intriguing physical properties including large piezoelectric responses 34,35 and fieldinduced strains 36 , electrochromic effects 37 , non-zero magnetic moments 38 , electrical conductivity [39][40][41] , interesting mechanical properties 42,43 , and is a suitable system for reducing the polarization retention loss 30 . Herein, we demonstrate that by intentionally introducing designer defects, macroscale strain coherence is maintained throughout the thickness of the film, which leads to the preservation of the T' BFO phase and thus a complete suppression of the mixed-phase structures throughout the film 44,45 . These defects pervade the entire film uniformly and apply a local compressive strain.…”

Structural, magnetic, and ferroelectric properties of T-like cobalt-doped BiFeO3 thin films

“…This effect is ascribed to the presence of a strain-stabilizing Bi 2 O 3 phase. 35,40 More specifically, it has recently been found that fine tuning of growth conditions can favour the formation of "stacking fault" like nanoregions in our T films: these regions appear to help maintain the strain required for T BFO up to larger thicknesses. To determine the structural symmetry, XRD RSMs around the (103) and (113) reflections [Figs.…”

“…The results presented here may suggest that cobalt doping is a possible route to suppress the presence of the mixed phase, i.e., the sustaining of pure T phase for relatively large film thicknesses, and is in agreement with recent studies. 34,35 It is probable that the cobalt doping alone is not responsible for the suppression of the mixed-phase regions, as growth conditions can also be fine-tuned to favour nominally pure T phase films. 35,44 Detailed ferroelectric switching measurements find domain dynamics similar to typical R-like BFO films, while conductive AFM mapping evidences strong conductivity at the domain walls between polarization variants, opening up encouraging possibilities for domain wall-based nanoelectronics using T BFO.…”

“…Structural and magnetic characterizations show that the properties of our films remain nominally identical to undoped T BFO samples, with the notable exception of a lack of mixed-phase regions. 35 The high film quality allows us to demonstrate almost perfect interconversion between the T and R polymorphs using a local applied electric field provided by a conductive atomic force microscopy (AFM) tip. Furthermore, spectroscopic ferroelectric switching measurements show that the threshold field for conversion from T to R is lower than the coercive field required to switch the polarization, and we show that the domain walls between two polarization variants of T BFO are strongly conductive, which is beneficial for the design of nano-devices based on epitaxial T BFO (Ref.…”

Interfacial Strain Gradients Control Nanoscale Domain Morphology in Epitaxial BiFeO3 Multiferroic Films

“…Three growth chambers were used, the respective growth conditions of which can be found in Refs. [ 47,68,69 ] .…”