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

Abstract

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.

Full text

Mater. Res. Lett., 2016
Vol. 4, No. 3, 168–173, http://dx.doi.org/10.1080/21663831.2016.1160260
www.tandfonline.com/toc/tmrl20/current
Coherent Growth of α-Fe2O3in Ti and Nd Co-doped BiFeO3Thin Films
Huairuo Zhanga, Daniel M. Marincelb†, Susan Trolier-McKinstryb,W.MarkRainforth
aand
Ian M. Reaneya∗
aDepartment of Materials Science & Engineering, University of Sheffield, Sheffield S1 3JD, UK; bDepartment of
Materials Science and Engineering and Materials Research Institute, Pennsylvania State University, University
Park, PA 16802, USA
(Received 1 November 2015; final form 25 February 2016)
Coherent dendritic α-Fe2O3precipitates 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 Fe2O3dendrites 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-
Fe2O3interface, ∼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-Fe2O3rather than the formation of the incoherent
Fe2O3particulate second phase frequently reported in BiFeO3-based thin films.
Keywords:Fe2O3Precipitate, BiFeO3Thin Film, Aberration-Corrected Scanning Transmission Electron Microscopy,
High-Angle Annular Dark Field Imaging
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.
BiFeO3has attracted much attention in the last 10
years due to its unique combination of magnetic
(TN=370°C) and ferroelectric (TC=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 TCas well as improve
the electrical properties for potential applications.[3–
9] For example, various BiFeO3-based thin films fab-
ricated via pulsed laser deposition (PLD) have been
developed to enhance the breakdown strengths with
respect to bulk materials.[10–16] One of the com-
mon issues encountered in BiFeO3-based thin films
is the appearance of parasitic second phases, that
is, iron-oxides such as α-Fe2O3and γ-Fe2O3.[13–
15] In a previous study, an unusual Fe2O3-rich per-
ovskite 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 (Bi0.75Nd0.25 )(Fe0.97Ti0.03 )O3(BNFO)
perovskite film on SrRuO3/SrTiO3substrate.[16]The
Fe2O3-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/SrRuO3/SrTiO3sample. In this study, dendritic
α-Fe2O3precipitates are also reported at the surface
region of the BNFO film. However, unlike the other
BiFeO3-based epitaxial films in which the parasitic
Fe2O3phase 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. Atomic-
resolved STEM analysis revealed that the α-Fe2O3
phase grows coherently with the BNFO matrix. At the
α-Fe2O3dendrite/BNFO matrix interface, a thin region
© 2016 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group.
This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/
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Mater. Res. Lett.,2016
(∼2 pseudocubic unit cells) of coherent Fe2O3-rich
perovskite structure was observed to promote the epitax-
ial growth of the α-Fe2O3rather than incoherent Fe2O3
particulate second phase. The new structure may exist in
many BiFeO3-based single-phase films reported before
in which the crystal structures of the films were charac-
terized solely using XRD, and may in part, explain the
ferromagnetic response observed at room temperature.
A BNFO ceramic PLD target was prepared accord-
ing to the process described in [7,8]. Films were
deposited on (001) SrTiO3(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 elec-
tron 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 low-
voltage conditions to reduce ion-beam damage. A dou-
ble 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 num-
ber than the surrounding BNFO matrix. The precipi-
tates grow obliquely upwards inside the BNFO film,
rather than the growth of the Fe2O3-rich perovskite
nanophase perpendicular to the substrate found in a pre-
vious 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 energy-
dispersive spectroscopy (EDS) line-scan analysis per-
formed 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 com-
position 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 thin-
ning 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
Figure 1. (Colour online) (a) ABF-STEM and (b) corresponding HAADF-STEM images of a cross-sectional sample showing the
needle-shaped precipitates in the top of the BNFO film, as illustrated by the black arrows. (c) An HAADF-STEM image showing
the position of a region of interest for the EDS line scan and (d) the corresponding composition profiles of Fe, Nd and Bi across
the BNFO matrix and precipitate.
169

Mater. Res. Lett.,2016
Figure 2. (Colour online) (a) HAADF-STEM and (b) corresponding ABF-STEM images of a plan-view sample with local
enlarged images (insets) showing the dendritic as well as dispersed round-shaped precipitates in the surface of the BNFO film.
(c) EELS spectra acquired from the BNFO matrix and precipitate, respectively. A reference spectrum of Fe2O3from Gatan EELS
Atlas is plotted for comparison.[17]
surface of the BNFO film. The nanoprecipitates assem-
ble 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 con-
trast 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 end-
on. 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 pre-
cipitate, respectively. A reference spectrum of Fe2O3
from the Gatan EELS Atlas is also plotted in Figure 2(c)
for comparison.[17] In contrast to the strong Ti-L2,3, and
Nd-M4,5 peaks from the BNFO matrix, only weak Nd-
M4,5 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-Kand Fe-L2,3 edges
of the precipitate are similar to those for Fe2O3in the
reference spectrum. It should be pointed out that it is
not clear whether the reference spectrum was collected
from α-Fe2O3or from γ-Fe2O3. Nonetheless, α-Fe2O3,
γ-Fe2O3and Fe3O4have very similar ELNES of O-K
and Fe-L2,3 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 coher-
ently 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 B-
site 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
170

Mater. Res. Lett.,2016
Figure 3. (Colour online) (a) Atomic-resolved HAADF-STEM and (b) corresponding ABF-STEM plan-view images showing
the coherent growth of α-Fe2O3and BNFO matrix. The red dash-lines are added as a guide to the eye to identify the change of
the lattices across the BNFO matrix and precipitate. (c) The polyhedral model showing face and edge-sharing FeO6octahedra of a
α-Fe2O3unit cell viewed along [241] zone-axis and (d) the corresponding projection of the unit cell. Oxygen anions are omitted
and only Fe cations are shown for the sake of clarity. The black dash lines illustrate the dumbbell structure of the Fe–Fe cations in
neighboring FeO6octahedra.
cation to form a dumbbell structure. Atomic resolution
images of the precipitate are readily interpreted as the
[241] zone-axis projection of rhombohedral α-Fe2O3.
Figure 3(c) shows a polyhedral model of the α-Fe2O3
unit cell viewed along the [241] zone-axis, illustrating
the neighboring FeO6octahedra with face and edge shar-
ing, resulting in the Fe cations approaching each other in
the [241] projection, as shown in Figure 3(d).
Atomic resolution HAADF-STEM imaging was
used to study the mechanism of epitaxial growth at the
α-Fe2O3/BNFO interface. Figure 4shows 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-Fe2O3unit cell viewed along
the [241] zone-axis is overlaid on the image for com-
parison. 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 tran-
sition zone (blue dashed lines) at the interface region
between the BNFO matrix and α-Fe2O3precipitate is
observed, ∼2 unit-cells wide, with a perovskite struc-
ture in which the intensity of the A-site columns is
substantially reduced, suggesting Bi-deficiency. In a
previous study, a Fe2O3-rich perovskite phase within
BNFO films was reported which exhibits a high con-
centration of Fe3+-ions on the A-site.[16] It is therefore
reasonable to speculate that the BNFO perovskite/α-
Fe2O3transition zone is composed of the newly dis-
covered Fe2O3-rich perovskite phase which in turn is
responsible for epitaxial growth of the Fe2O3dendrites,
due to the similar crystal chemistry of Fe2O3-rich per-
ovskite phase and α-Fe2O3.[20] It should be noted that
there are additional atomic columns superposed on the
perovskite lattice in the Fe2O3-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 BiFeO3films and ceramics.[21–23]
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-Fe2O3precipitate is
favored to be formed in the BiFeO3films grown by
PLD at low oxygen partial pressure, for example below 5
171

Mater. Res. Lett.,2016
Figure 4. Enlarged HAADF-STEM image of the lower right
section of Figure 3(a) showing the atomic structure of the inter-
face of α-Fe2O3and the BNFO matrix. A structural model
of the projection of a α-Fe2O3unit cell viewed along [241]
zone-axis is overlaid on the image for comparison. Only Fe
cations are shown since oxygen anions are invisible in the
HAADF image due to their weak scattering power. Yellow
dash-dot-dot lines are added as a guide to the eye. Blue dash–
dot-dot lines illustrate a transition zone with Bi-deficiency.
Red arrows show additional atomic columns from a Nd-rich
precipitate.
mTorr.[13–15] Recently, maghemite-like regions
(γ-Fe2O3) were also evidenced at crossing of two
antiphase boundaries in (Bi0.85Nd0.15 )(Fe0.9Ti0.1 )O3
ceramics synthesized in air.[24] Although α-Fe2O3is the
favored precipitate with the oxygen partial pressure used
in this work (40 mTorr) which is much higher than those
for γ-Fe2O3formation in the growth of BiFeO3films,
whether there is the presence of γ-Fe2O3precipitate in
this Nd- and Ti-codoped BiFeO3film is still not clear.
In summary, coherent α-Fe2O3forms at the surface
of epitaxial films of Ti- and Nd-codoped BiFeO3grown
by PLD. A Fe2O3-rich perovskite-structured transition
zone of ∼2 unit-cells wide at the BNFO/α-Fe2O3inter-
face promotes epitaxial growth of the α-Fe2O3rather
than the incoherent Fe2O3particulate second phase.
The new structure may exist in many BiFeO3-based
single-phase films reported before, and may in part,
explain the ferromagnetic response observed at room
temperature.
Acknowledgements H.Z., W.M.R. and I.M.R. acknowl-
edge the Engineering and Physical Sciences Research Council
for funding this work via grant EP/I038934/1. D.M.M. and
S.T.M. acknowledge the financial support from National Sci-
ence Foundation Grant 1005771, as well as a National Secu-
rity Science and Engineering Faculty Fellowship. The authors
thank the Kroto Centre for High Resolution Imaging & and
Analysis for access the JEM-Z3100F-R005 STEM/TEM.
Disclosure statement No potential conflict of interest was
reported by the authors.
ORCID
Huairuo Zhang http://orcid.org/0000-0002-1984-1200
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