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

Abstract

Domain switching pathways fundamentally control performance in ferroelectric thin film devices. In epitaxial bismuth ferrite (BiFeO 3 ) films, the domain morphology is known to influence the multiferroic orders. While both striped and mosaic domains have been observed, the origins of the latter have remained unclear. Here, it is shown that domain morphology is defined by the strain profile across the film-substrate interface. In samples with mosaic domains, X-ray diffraction analysis reveals strong strain gradients, while geometric phase analysis using scanning transmission electron microscopy finds that within 5 nm of the film-substrate interface, the out-of-plane strain shows an anomalous dip while the in-plane strain is constant. Conversely, if uniform strain is maintained across the interface with zero strain gradient, striped domains are formed. Critically, an ex situ thermal treatment, which eliminates the interfacial strain gradient, converts the domains from mosaic to striped. The antiferromagnetic state of the BiFeO 3 is also influenced by the domain structure, whereby the mosaic domains disrupt the long-range spin cycloid. This work demonstrates that atomic scale tuning of interfacial strain gradients is a powerful route to manipulate the global multiferroic orders in epitaxial films.

The Experimentalist's Guide to the Cycloid, or Noncollinear Antiferromagnetism in Epitaxial BiFeO3

“…Another pertinent question is how the cycloid would respond to size reduction in two or three dimensions. For instance, upon nanostructuring [ 293,294 ] a thin film to dimensions at or below the cycloid period—in a similar way that domain walls influence the cycloid period [ 89,137 ] —one expects size effects to become increasingly important. The recent advances in scanning probe magnetometry techniques will prove indispensable in such cases.…”

“…[ 79 ] Under moderate compressive strain, c) CEMS (ref. [ 137 ] ) and d) neutron diffraction [ 256 ] evidence a cycloid with its plane close to (001). Under slight (0.1%) tensile strain, e) resonant elastic X‐ray scattering (REXS) and scanning NV magnetometry shows a type‐2 cycloid with propagation vector along .…”

“…c) Adapted with permission. [ 137 ] Copyright 2020, Wiley‐VCH. d) Reproduced under terms of the CC‐BY Creative Commons Attribution 4.0 International license ( https://creativecommons.org/licenses/by/4.0).…”

“…[ 256 ] (Figure 22d) and recently reanalysis of CEMS data considering such a tilting of the cycloid plane yielded a very good fit to the data (Figure 22c). [ 137 ]…”

“…In 25 nm thick (001)‐oriented BFO films grown on DyScO 3 , it was shown by CEMS that if the domain structure comprises well‐ordered stripe like domains, the cycloid is almost harmonic ( m = 0.5). [ 137 ] This is due to the fact that since the domain size is much larger than the cycloid period, the cycloid has sufficient “space” to propagate unperturbed by the domain walls. On the other hand, in samples with mosaic‐like domains with characteristic size around half the cycloid period, the cycloid becomes “domain like,” implying that for each cycloid period the cycloid straddles two domains.…”