A study of polymorph dynamics in mixed‐phase BiFeO3 thin films via
AFM
and in‐situ
TEM
applications of external stimuli
Authors: Holsgrove Kristina , Duchamp Martial , Browne Niall , Edwards David , Bernier Nicolas , Mazumdar Dipanjan , Gregg Marty , Kumar Amit , Arredondo Miryam 
Published: December 2016

Epitaxially strained BiFeO
3
(BFO) thin films have seen an upsurge of interest over the past decade with the revelation of fascinating characteristics such as giant ferroelectric polarisation, exceptional piezoelectric and magnetoelectric responses to name a few.
[1]
The growth of BFO on substrates providing a large in‐plane compressive strain which facilitates structural alterations of the material to that of a tetragonal‐like phase (T‐phase) and distorted rhombohedral‐like phase (R‐phase) has opened the door to a plethora of opportunities in terms of controlling nanoscale interfaces and expanding our understanding of solid state physics. In a recent review by Nagarajan et al. (2016),
[2]
it was advised that there are still a number of exciting possibilities as well as pending questions which need to be addressed before the functionality of mixed‐phase BFO thin films can be driven to the next level.

Using both aberration‐corrected TEM and STEM we have performed a high resolution study of the lattice parameters which characterise the ‘R’ and ‘T’ phase (Figure 1) and mapped the strain evolution (via geometrical phase analysis (GPA)) between polymorphs using the LaAlO
3
substrate as reference. Having identified the metastable ‘T’ polymorph and the extent to which the thin film (and FIB‐prepared lamella) thickness can influence the ratio of ‘R’ to ‘T’ polymorphs, we demonstrate the thermal energy required to expand the dimensions of the highly strained ‘T’ polymorph through in‐situ heat cycling experiments (Figure 2), and explain the complexity involved in such a transition.

The relative ease through which transitions from one phase to another can be achieved varies depending on the application of stimuli selected. In this study we use a variation of techniques to explore the physical phenomena behind the functionality of mixed‐phase BFO. We highlight the benefits of reversibly reorganising phases via the application of external stimuli with an AFM probe tip. Through precise SEM and FIB techniques we have identified the pre‐written AFM areas and milled cross‐sectional lamellae across these areas for post‐AFM analysis using STEM. With this unique study we detect and compare structural differences between the as‐grown polymorphs in a native lamella and polymorphs written through external stimuli via an AFM tip. We show differences in lattice parameters and strain evolution in the newly written polymorphs via nano‐beam electron diffraction using a 2nm sized electron probe; this precision has allowed us to explicitly map the strain across a large area of these polymorphs. With the addition of EELS focusing on the Fe‐

L
2,3

and O‐
K
edges we show changes in ELNES features which suggest octahedral distortion variations between polymorphs.

By combining AFM and state‐of‐the‐art STEM techniques in this distinctive way we provide an insight into the link between exciting interfacial behaviour seen via AFM at the polymorph boundaries on the microscopic scale, and structural polymorph changes seen via STEM on the atomic scale.