Coherent Growth of α-Fe2O3 in Ti and Nd Co-doped BiFeO3 Thin Films

Coherent dendritic α-Fe2O3 precipitates were observed to form at the surface of epitaxial films of (Bi0.75Nd0.25)(Fe0.97Ti0.03)O3 (BNFO) grown by pulsed laser deposition. The Fe2O3 dendrites are assemblages of nanosized particles with an approximate length of 500 nm. Through the use of atomic resolution scanning transmission electron microscopy, a transition zone at the BNFO/α-Fe2O3 interface, ∼2 unit-cells wide, was observed to be Fe2O3-rich with the perovskite structure. It is proposed that the formation of the Fe2O3-rich perovskite structure encourages epitaxial growth of the α-Fe2O3 rather than the formation of the incoherent Fe2O3 particulate second phase frequently reported in BiFeO3-based thin films. Impact Statement The discovery of the new structure may explain the abnormally high ferromagnetic response observed in the BiFeO3-based single-phase epitaxial films, and will benefit the communities of materials science. GRAPHICAL ABSTRACT

BiFeO 3 has attracted much attention in the last 10 years due to its unique combination of magnetic (T N = 370°C) and ferroelectric (T C = 830°C) phase transitions above room temperature. [1,2] Extensive investigations of doping on the A-and B-sites have been carried out to lower the T C as well as improve the electrical properties for potential applications. [3][4][5][6][7][8][9] For example, various BiFeO 3 -based thin films fabricated via pulsed laser deposition (PLD) have been developed to enhance the breakdown strengths with respect to bulk materials. [10][11][12][13][14][15][16] One of the common issues encountered in BiFeO 3 -based thin films is the appearance of parasitic second phases, that is, iron-oxides such as α-Fe 2 O 3 and γ -Fe 2 O 3 . [13][14][15] In a previous study, an unusual Fe 2 O 3 -rich perovskite nanophase was observed by aberration-corrected scanning transmission electron microscopy (STEM) *Corresponding author. Email: i.m.reaney@sheffield.ac.uk † Present address: Department of Chemistry, Rice University, Houston, TX 77005, USA. in an epitaxial (Bi 0.75 Nd 0.25 )(Fe 0.97 Ti 0.03 )O 3 (BNFO) perovskite film on SrRuO 3 /SrTiO 3 substrate. [16] The Fe 2 O 3 -rich perovskite nanophase grows coherently with the BNFO matrix in the upper relaxed region of the BNFO film, observed in parts of a cross-sectional BNFO/SrRuO 3 /SrTiO 3 sample. In this study, dendritic α-Fe 2 O 3 precipitates are also reported at the surface region of the BNFO film. However, unlike the other BiFeO 3 -based epitaxial films in which the parasitic Fe 2 O 3 phase is readily identified by X-ray diffraction (XRD), here no second phase peaks were identified in the BNFO film by this method, due to the total volume of the fine nanoprecipitate beings not sufficient to produce a peak above the noise of a XRD pattern. Atomicresolved STEM analysis revealed that the α-Fe 2 O 3 phase grows coherently with the BNFO matrix. At the α-Fe 2 O 3 dendrite/BNFO matrix interface, a thin region ( ∼ 2 pseudocubic unit cells) of coherent Fe 2 O 3 -rich perovskite structure was observed to promote the epitaxial growth of the α-Fe 2 O 3 rather than incoherent Fe 2 O 3 particulate second phase. The new structure may exist in many BiFeO 3 -based single-phase films reported before in which the crystal structures of the films were characterized solely using XRD, and may in part, explain the ferromagnetic response observed at room temperature.
A BNFO ceramic PLD target was prepared according to the process described in [7,8]. Films were deposited on (001) SrTiO 3 (STO) substrates using a home-built PLD system with a 248 nm KrF excimer laser. The detailed film growth process is described in [16]. Cross-sectional and plan-view transmission electron microscopy (TEM) samples were prepared with a dual beam FIB/SEM FEI Quanta 3D 200. Plan-view TEM samples were prepared by thinning the substrate side. Samples were finished in low-angle and lowvoltage conditions to reduce ion-beam damage. A double aberration-corrected microscope JEM-Z3100F-R005 STEM/TEM operated at 300 keV was used to perform STEM analysis. Figure 1(a) shows an annular bright field (ABF) STEM image of a cross-sectional sample which reveals some bright needle-shaped precipitates in the upper relaxed region of the BNFO film. The precipitates are dark in the corresponding high-angle annular dark field (HAADF) STEM image (Figure 1(b)). The contrasts in HAADF-STEM and ABF-STEM images suggest that the precipitates have a lower average atomic number than the surrounding BNFO matrix. The precipitates grow obliquely upwards inside the BNFO film, rather than the growth of the Fe 2 O 3 -rich perovskite nanophase perpendicular to the substrate found in a previous work. [16] Figure 1(c) shows an enlarged HAADF-STEM image containing a precipitate, in which a white line illustrates the position for an X-ray energydispersive spectroscopy (EDS) line-scan analysis performed to check the chemical distribution in the BNFO matrix and the dark precipitate. Figure 1(d) shows the profiles of the relative composition (in atomic percentage at%) of the Fe, Nd and Bi cations (Ti was omitted due to the very weak Ti peaks which were barely above noise levels). The chemical profiles show that the cation composition of the BNFO matrix is close to the stoichiometry of the BNFO ceramic target. In contrast the precipitate is rich in Fe and deficient in Nd and Bi. In the center of the precipitate, the Fe content is as high as 96 at%, which suggests that the precipitate is an iron-oxide.
Plan-view samples were further prepared by thinning the substrate side to study the distribution of the iron-oxide precipitate in the BNFO film. Figure 2(a) shows a HAADF-STEM image of a plan-view sample, which reveals dark nanoprecipitates ( ∼ 10 nm) in the  (c) EELS spectra acquired from the BNFO matrix and precipitate, respectively. A reference spectrum of Fe 2 O 3 from Gatan EELS Atlas is plotted for comparison. [17] surface of the BNFO film. The nanoprecipitates assemble into dendrites with an approximate length of 500 nm. The precipitates are bright in the corresponding ABF-STEM image (Figure 2(b)). The shape and contrast as well as the electron energy-loss spectroscopy (EELS) analysis of the precipitates in the surface of the BNFO film suggest that they are the needle-shaped precipitates in the cross-sectional sample viewed endon. The iron-oxide composition of the precipitate was confirmed by the EELS analysis. Figure 2(c) shows the EELS spectra acquired from the BNFO matrix and precipitate, respectively. A reference spectrum of Fe 2 O 3 from the Gatan EELS Atlas is also plotted in Figure 2(c) for comparison. [17] In contrast to the strong Ti-L 2,3 , and Nd-M 4,5 peaks from the BNFO matrix, only weak Nd-M 4,5 peaks were attained from the precipitate, indicating that it is Ti and Nd deficient with respect to the BNFO matrix. In addition, the electron energy-loss near-edge fine structure (ELNES) of the O-K and Fe-L 2,3 edges of the precipitate are similar to those for Fe 2 O 3 in the reference spectrum. It should be pointed out that it is not clear whether the reference spectrum was collected from α-Fe 2 O 3 or from γ -Fe 2 O 3 . Nonetheless, α-Fe 2 O 3 , γ -Fe 2 O 3 and Fe 3 O 4 have very similar ELNES of O-K and Fe-L 2,3 edges, and are difficult to differentiate by EELS with a normal energy resolution ( ∼ 1 eV) which is limited by the instrument. [18] The crystal form of the iron-oxide can be readily identified with EELS when the energy resolution is better than 0.4 eV, [19] which is not the case for the shown spectra.
Atomic-resolved HAADF-STEM and ABF-STEM analysis was further performed to identify the crystal structure of the dendritic precipitates. Figure 3(a) shows an atomic-resolution HAADF-STEM image of a coherently grown precipitate obtained along the pseudocubic 001 zone-axis of the BNFO matrix. The precipitate shows a darker contrast than the surrounding BNFO. The bright contrast of the atomic columns of the cations in Figure 3(a) is reversed in the corresponding ABF-STEM image in Figure 3(b). The BNFO matrix reveals distinct differences in intensity and size of the A-and Bsite cations, but the precipitate generally shows identical contrast for all the cation columns. The uniform contrast is consistent with the above analysis of the precipitate having primarily an iron-oxide composition. The red dash-lines across the BNFO matrix and precipitate reveal the precipitate has a rhombic lattice ( ∼ 4°deviation from right angle) and the central cation approaches another  Atomic resolution HAADF-STEM imaging was used to study the mechanism of epitaxial growth at the α-Fe 2 O 3 /BNFO interface. Figure 4 shows an enlarged HAADF-STEM image in the lower right portion of Figure 3(a). A structural model of the projection of the Fe cations of a α-Fe 2 O 3 unit cell viewed along the [241] zone-axis is overlaid on the image for comparison. The structural model matches well with the experimental image. Yellow dash-dot-dot lines are added as a guide to the eye to illustrate the interface. A transition zone (blue dashed lines) at the interface region between the BNFO matrix and α-Fe 2 O 3 precipitate is observed, ∼ 2 unit-cells wide, with a perovskite structure in which the intensity of the A-site columns is substantially reduced, suggesting Bi-deficiency. In a previous study, a Fe 2 O 3 -rich perovskite phase within BNFO films was reported which exhibits a high concentration of Fe 3+ -ions on the A-site. [16] It is therefore reasonable to speculate that the BNFO perovskite/α-Fe 2 O 3 transition zone is composed of the newly discovered Fe 2 O 3 -rich perovskite phase which in turn is responsible for epitaxial growth of the Fe 2 O 3 dendrites, due to the similar crystal chemistry of Fe 2 O 3 -rich perovskite phase and α-Fe 2 O 3 . [20] It should be noted that there are additional atomic columns superposed on the perovskite lattice in the Fe 2 O 3 -rich transition zone in a local area, as illustrated by the arrows in Figure 4. The positions of these additional atomic columns are similar to Nd-rich nanoprecipitates recently observed by atomic resolution EELS spectrum-imaging analysis in Nd-and Ti-codoped BiFeO 3 films and ceramics. [21][22][23] The presence of some Nd-rich nanoprecipitates in the transition zone suggests that Nd was exsolved from the matrix and precipitated heterogeneously during the PLD process.
It should be noted that γ -Fe 2 O 3 precipitate is favored to be formed in the BiFeO 3 films grown by PLD at low oxygen partial pressure, for example below 5 mTorr. [13][14][15] Recently, maghemite-like regions (γ -Fe 2 O 3 ) were also evidenced at crossing of two antiphase boundaries in (Bi 0.85 Nd 0.15 )(Fe 0.9 Ti 0.1 )O 3 ceramics synthesized in air. [24] Although α-Fe 2 O 3 is the favored precipitate with the oxygen partial pressure used in this work (40 mTorr) which is much higher than those for γ -Fe 2 O 3 formation in the growth of BiFeO 3 films, whether there is the presence of γ -Fe 2 O 3 precipitate in this Nd-and Ti-codoped BiFeO 3 film is still not clear.
In summary, coherent α-Fe 2 O 3 forms at the surface of epitaxial films of Ti-and Nd-codoped BiFeO 3 grown by PLD. A Fe 2 O 3 -rich perovskite-structured transition zone of ∼ 2 unit-cells wide at the BNFO/α-Fe 2 O 3 interface promotes epitaxial growth of the α-Fe 2 O 3 rather than the incoherent Fe 2 O 3 particulate second phase. The new structure may exist in many BiFeO 3 -based single-phase films reported before, and may in part, explain the ferromagnetic response observed at room temperature.