Structure and properties of edge dislocations in $\mathrm{BiFe}{\mathrm{O}}_3$

Edge dislocations are frequently found in epitaxial ${\mathrm{BiFeO}}_3$ multiferroic thin films and are expected to exhibit distinctive and localized magnetoelectric properties. However, an exhaustive characterization of these dislocations at the atomic level has to date been largely overlooked. Here, we use a combination of scanning transmission electron microscopy techniques, atomistic simulations obtained from classical molecular dynamics calculations, and real-space multiple-scattering theory to explore the chemical properties and the bonding characteristics of the atoms located at and near the dislocation cores. We find that in addition to Bi, small amounts of Fe atoms are present in the ${\mathrm{BiFeO}}_3$ dislocation cores which result in uncompensated Fe spins along the dislocations and give rise to a magnetic signal. Our results suggest that edge dislocations in ${\mathrm{BiFeO}}_3$ films could be efficiently used for realizing ${\mathrm{BiFeO}}_3$-based magnetic devices.

Figure 1

Representative STEM images of a BFO/LSMO bilayer grown on a $\mathrm{SrTi}{\mathrm{O}}_3\left(001\right)$ substrate. (a) HAADF-STEM Moiré image of the BFO/LSMO bilayer exhibiting a high density of edge dislocations (highlighted with red circles). (b) A single edge dislocation located in the middle of the BFO layer. The Burgers circuit is shown in yellow and the resulting Burgers vector is indicated with a green arrow. (c) Two pairs of edge dislocations located near the BFO/LSMO interface. Each Burgers circuit encloses a pair of parallel dislocations lying on perpendicular slip planes in close proximity. The resulting Burgers vectors are indicated with green arrows. The crystal orientations shown in (a) apply to all images.

Figure 2

Representative atomic resolution and chemical mapping images of a BFO/SRO bilayer grown on a $\mathrm{SrTi}{\mathrm{O}}_3\left(001\right)$ substrate. (a) HAADF-STEM image of the BFO/SRO bilayer along the [100] zone axis obtained as an average of a time series consisting of 15 frames. Two parallel edge dislocations lying on perpendicular slip planes are identified nearby the BFO/SRO interface. (b) Corresponding averaged ABF image of the same area. (c) Polarization map superimposed to the HAADF-STEM image. (d) Elemental maps calculated from the EDX spectrum image acquired in the dashed area specified in the HAADF and ABF images. The (purple) Bi and (yellow) Fe maps were obtained using the Bi-Mα1 and Fe-Kα peaks, respectively. Intensity line profiles averaged over seven pixels were extracted from the elemental maps along a line perpendicular to the slip plane (marked by a white dashed box); they uncover the presence of Fe atoms in the dislocation core column.

Figure 3

O-$K$ edge EEL spectra acquired across an edge dislocation core found in the BFO/SRO bilayer sample. The spectra were obtained from the areas indicated by the boxes. The core spectrum is highlighted in dark green, while the spectra immediately above and below the dislocation core are shown in light green. The gray spectra correspond to more distant positions from the dislocation core.

Figure 4

(a) Initial configuration for the dislocation model used for the molecular dynamics simulation. (b) Optimized configuration of the two dislocation cores obtained using the BV potential of $\mathrm{BiFe}{\mathrm{O}}_3$ [36]. (c)–(e) Optimized configurations of the dislocation core section for the three models obtained after molecular dynamics simulations. The Bi, Fe, and O atoms are shown in purple, yellow, and red, respectively.

Figure 5

Comparison between simulated HAADF-STEM images for (a) the three different structural models and (b) the experimental data. (c) Scheme of the edge dislocation with dashed lines indicating the (pink) Bi and (yellow) Fe atoms used to obtain the bulk-averaged HAADF intensities. (d) Averaged HAADF intensities obtained from the simulated images of each model (M1, M2, and M3) and the experimental data, calculated following the scheme given in (c). The Bi bulk is represented in pink, Fe bulk in yellow, and core in green.

Figure 6

Comparison of the O-$K$ edge EEL spectra obtained from the three different dislocation core models with dissimilar Bi/Fe atomic ratios together with the experimentally acquired O-$K$ edge spectra. For easier comparison with the calculated spectra, the experimental data were fitted with multiple Gaussians.

Figure 7

Energy-loss near-edge structures and projected density of states (PDOS) calculated for Model 3 at the dislocation core and “bottom” and “top” positions (two unit cells below and above the core, as defined in the text). Contributions from the different atomic orbitals for the O, Fe, and Bi atomic species are given in separate panels.

Figure 8

Off-axis electron holography investigation of a BFO/LSMO bilayer grown on a $\mathrm{SrTi}{\mathrm{O}}_3\left(001\right)$ substrate acquired with a tilt of $15.6^{\circ }$. (a) Electron hologram acquired at 370 V. (b) Mean inner potential and (c) magnetic contribution to the electron phase shift. (d) Rotation map superimposed to the atomic-resolution HAADF-STEM image showing the two dislocation cores at a distance of about 15 nm. This image was rotated with respect to the other panels in order to align the atomic columns with the edges of the image. (e) Profile of the magnetic phase shift along the direction connecting the two dislocation cores [marked in blue in (c)]. The positions of the cores are marked by the red lines. (f) Magnified view of the magnetic phase shift of the two dislocation cores and phase contours of the amplified (24x) phase. The two dislocation cores display a dipolarlike behavior.