Melting of Spatially Modulated Phases in La-doped BiFeO3 at Surfaces and Surface-Domain Wall Junctions

TL;DR

This study investigates how La-doping influences the formation and melting of spatially modulated phases at surfaces and domain wall junctions in BiFeO3, combining atomic-level microscopy with thermodynamic modeling.

Contribution

It demonstrates that La-doping alters polarization gradients, leading to the emergence and suppression of spatially modulated phases at surfaces and interfaces in multiferroic BiFeO3.

Findings

La-doping changes polarization gradient coefficients.

Spatially modulated phases are suppressed near domain wall junctions.

Melting of SMP is driven by electrostatic energy minimization.

Abstract

The interplay between the surface and domain wall phenomena in multiferroic LaxBi1-xFeO3 in the vicinity of morphotropic phase transition is explored on the atomic level. Scanning Transmission Electron Microscopy (STEM) has enabled mapping of atomic structures of the material with picometer-level precision, providing direct insight into the spatial distribution of the order parameters in this material and their behavior at surfaces and interfaces. Here, we use the thermodynamic Landau-Ginzburg-Devonshire (LGD) approach to explain the emergence of spatially modulated phases (SMP) in La0.22Bi0.78FeO3 films, and establish that the change of polarization gradient coefficients caused by La-doping is the primary driving mechanisms. The suppression, or "melting", of the SMP in the vicinity of the domain wall surface junction is observed experimentally and simulated in the framework of LGD theory. The melting originated from the system tendency to minimize electrostatic energy caused by long-range stray electric fields outside the film and related depolarization effects inside it. The observed behavior provides insight to the origin of surface and interface behaviors in multiferroics.