Magnetic Phase Formation in Self-Assembled Epitaxial
BiFeO[subscript 3]–MgO and BiFeO[subscript
3]–MgAl[subscript 2]O[subscript 4] Nanocomposite Films
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Citation
Kim, Dong Hun, XueYin Sun, Tae Cheol Kim, Yun Jae Eun,
Taeho Lee, Sung Gyun Jeong, and Caroline A. Ross. “Magnetic
Phase Formation in Self-Assembled Epitaxial BiFeO[subscript
3]–MgO and BiFeO[subscript 3]–MgAl[subscript 2]O[subscript 4]
Nanocomposite Films Grown by Combinatorial Pulsed Laser
Deposition.” ACS Applied Materials & Interfaces 8, no. 4
(February 3, 2016): 2673–2679. © 2016 American Chemical
Society
As Published
http://dx.doi.org/10.1021/acsami.5b10676
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American Chemical Society (ACS)
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Research Article
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Magnetic Phase Formation in Self-Assembled Epitaxial BiFeO3−MgO
and BiFeO3−MgAl2O4 Nanocomposite Films Grown by Combinatorial
Pulsed Laser Deposition
Dong Hun Kim,†,‡ XueYin Sun,†,§ Tae Cheol Kim,‡ Yun Jae Eun,‡ Taeho Lee,‡ Sung Gyun Jeong,‡
and Caroline A. Ross*,†
†
Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United
States
‡
Department of Materials Science and Engineering, Myongji University, Yongin 120-728, Republic of Korea
§
School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, People’s Republic of China
S Supporting Information
*
ABSTRACT: Self-assembled epitaxial BiFeO3−MgO and
BiFeO3−MgAl2O4 nanocomposite thin films were grown on
SrTiO3 substrates by pulsed laser deposition. A two-phase
columnar structure was observed for BiFeO3−MgO codeposition within a small window of growth parameters, in which
the pillars consisted of a magnetic spinel phase (Mg,Fe)3O4
within a BiFeO3 matrix, similar to the growth of BiFeO3−
MgFe2O4 nanocomposites reported elsewhere. Further, growth
of a nanocomposite with BiFeO 3 −(CoFe 2 O 4 /MgO/
MgFe2O4), in which the minority phase was grown from
three different targets, gave spinel pillars with a uniform
(Mg,Fe,Co)3O4 composition due to interdiffusion during
growth, with a bifurcated shape from the merger of neighboring pillars. BiFeO3−MgAl2O4 did not form a well-defined vertical
nanocomposite in spite of having lower lattice mismatch, but instead formed a two-phase film with in which the spinel phase
contained Fe. These results illustrate the redistribution of Fe between the oxide phases during oxide codeposition to form a
ferrimagnetic phase from antiferromagnetic or nonmagnetic targets.
KEYWORDS: thin film oxide, epitaxy, spinel, perovskite, multiferroic, combinatorial pulsed laser deposition,
self-assembled oxide nanocomposite, BiFeO3
■
INTRODUCTION
affects the magnetic and ferroelectric properties dramatically
and leads to magnetoelectric coupling.6,14−16
Interdiffusion between spinel and perovskite phases has been
reported to be small,12,13 but significant interdiffusion can occur
within spinel pillars during deposition, leading to a homogeneous pillar composition even if the composition of the arriving
flux was changed partway through growth.17 We showed
previously that interdiffusion in spinel pillars was prevented by
growing a diffusion barrier such as BFO partway through the
film, producing a film consisting of two separated layers of
spinel pillars with different compositions.17
In this paper, we report on the growth of nanocomposites
made by combinatorial pulsed laser codeposition of MgO or
MgAl2O4 (MAO) and a perovskite BFO, with a focus on
examining interdiffusion between the two phases during
growth, and the deposition conditions that determine which
Self-assembled nanocomposite thin films are columnar
structures comprising two (or more) codeposited crystalline
phases grown epitaxially on a single crystal substrate, and
exhibit a remarkable range of functional properties, which can
include high-temperature superconductivity, colossal magnetoresistance, ferroelectricity, magnetism, and multiferroicity.1−4
Most of the self-assembled nanocomposites reported to date
consist of pairs of oxides such as a perovskite (e.g., BiFeO3,
BFO) and a spinel (e.g., CoFe2O4, CFO), grown on single
crystalline substrates such as (001)-oriented SrTiO3 (STO). A
common morphology is for one phase to grow as pillars within
the other,5−8 such as spinel pillars in a perovskite matrix, with
both phases epitaxial with the substrate. There are also reports
of epitaxial nanocomposite thin films which incorporate other
oxide crystal structures such as MgO or ZnO.9−11 Elastic
interactions through the interfaces between the two phases are
very important in determining the properties of the nanocomposite. For example, strain transfer at the vertical interface
between piezoelectric BiFeO3 and magnetostrictive CoFe2O4
© 2016 American Chemical Society
Received: November 7, 2015
Accepted: January 11, 2016
Published: January 11, 2016
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Figure 1. (a, b, and c) Top view SEM images of BFO−MgO nanocomposites on (001) STO substrate fabricated by combinatorial PLD. BFO
content increases from left to right. (d, e, and f) SEM of etched BFO−MgO nanocomposites corresponding to panels a, b and c, respectively. (g, h
and i) XRD patterns of unetched and etched BFO−MgO nanocomposites grown by combinatorial PLD corresponding to panels a, b and c,
respectively.
process for CFO and MFO was reported elsewhere.17 A Bi-rich
Bi1.2FeO3 target was prepared by Plasmaterials, CA.
BFO−MgO and BFO−MAO nanocomposite films were deposited
by combinatorial pulsed laser deposition (CPLD) using a KrF excimer
laser (λ = 248 nm) on SrTiO3 (001) substrates (STO, abulk,STO = 3.905
Å) at 5 mTorr of oxygen pressure. The substrate temperature during
deposition was fixed at 650 °C, which is the optimal condition in our
chamber to make stoichiometric BiFeO3 from the Bi-rich target.
Nanocomposite thin films contained excess Bi at low deposition
temperatures,24 whereas the volume fraction of BiFeO3 was reduced at
higher deposition temperatures (>700 °C) due to Bi volatility. The
laser frequency was 10 Hz and the fluence at the target was 2.6 J/cm2.
CPLD was used to produce a set of samples with a range of
composition by alternately ablating two different targets to produce a
submonolayer film of tapered thickness from each target, as described
elsewhere.17,25,26 The five 5 mm by 5 mm STO substrates were placed
in a row with 18 mm offset between the center of the plume of
evaporated material and the substrate rotation axis, and the targetsubstrate distance was 7 cm. The films were made using cycles
phase forms. MgO is commonly used as a tunnel barrier
material for magnetic tunnel junctions.18,19 MAO has received
attention as an alternative barrier material,20,21 and an 8 nm
thick MAO layer served as a diffusion barrier preventing Ti
migration from an STO substrate into a CFO film.22 MgO and
MAO layers are quite well lattice matched to CoFe2O4 or
MgFe2O4, suggesting their possible utility as diffusion barriers
to be incorporated within nanocomposite films. CFO/MFO
nanocomposites have been reported previously.23
■
EXPERIMENTAL SECTION
MgO, MgAl2O4 (MAO, the eponymous spinel), CoFe2O4 (CFO,
cobalt ferrite), and MgFe2O4 (MFO, magnesium ferrite) targets were
prepared by a conventional oxide sintering method. The lattice
parameters of the final targets from X-ray diffraction (XRD) were aMgO
= 4.213 ± 0.002 Å and aMgAl2O4 = 8.081 ± 0.003 Å, which are close to
the bulk lattice parameters of abulk,MgO = 4.213 Å (JCPDS # 04-0829)
and abulk,MgAl2O4 = 8.083 Å (JCPDS # 21-1151). The target preparation
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Figure 2. (a) Top view SEM (b) θ-2θ XRD pattern of BFO−MgO nanocomposite grown at optimal deposition conditions. (c) In-plane and out-ofplane hysteresis loops of nanocomposite measured by VSM. (d) TEM image and elemental mapping of released pillars on a copper grid.
parameter of 4.209 ± 0.002 Å (Figure 1g), which is close to
that of bulk MgO. An alternative interpretation is that the
pillars consist of a spinel phase, MgxFe3−xO4 (MxFO) formed
by incorporating Fe from the arriving flux. This is plausible
because the lattice parameter of MgFe2O4 (MFO) is 8.375 Å,
making its (004) peak nearly coincident with that of MgO
(002). Peaks from (00l) pseudocubic BFO were not observed,
and instead a peak at 2θ = 32.1° corresponds to bismuth oxide
Bi2O3 or to BFO (011) that was removed by etching, Figure 1d.
The near-bulk lattice parameter of the MgO or MxFO indicates
that it is not coherent with the matrix, and the matrix did not
form the usual cube-on-cube BFO/STO (001).
In contrast, the sample in Figure 1b showed peaks from both
MgO (002) or MxFO (004) and BFO pseudocubic (00l),
Figure 1h. The pillar phase was under out-of-plane compressive
strain (c = 4.187 Å) compared to the sample of Figure 1(a) due
to epitaxy at the vertical interface with BFO. There were two
overlapping peaks near 2θ = 45° from the rhombohedral BFO
matrix corresponding to c = 4.065 Å and c = 3.995 Å
(compared to bulk pseudocubic abulk,BFO = 3.965 Å). This is
consistent with the SEM image in Figure 1b that indicates two
populations of BFO, the material surrounding the dense pillars,
and larger BFO regions that cover part of the surface.
In the film with the most BFO, Figure 1c, the diameter and
fraction of the pillar phase was much lower. From the X-ray
data, Figure 1i, the pillar phase peak could not be distinguished
due to its small volume fraction. The major peaks were from
rhombohedral BFO grown epitaxially on the STO with out-ofplane lattice parameter of 4.087 Å. A smaller peak from
tetragonal BFO also occurs around 39°, similar to what was
seen in low-temperature-deposited BFO−CFO nanocomposites.24 The lattice mismatch between the pillar phase and the
matrix may help stabilize the tetragonal BFO phase.
consisting of 500 laser pulses of BFO followed by 200 pulses of MgO
or MAO, repeated to build up the desired film thickness.
The phases present in each film were investigated by XRD
(PANalytical X’Pert Pro). Top view images of the surface morphology
were obtained from scanning electron microscopy (SEM, Helios
Nanolab 600). To observe the vertical growth of the nanostructured
composites, some samples were etched with 10% dilute hydrochloric
acid (HCl) to remove the BFO and reveal the pillars. Composition
analysis was carried out using energy dispersive spectroscopy (EDS) in
a high resolution transmission electron microscopy (HR-TEM, JEOL
JEM 2010F) instrument. Pillars from the nanocomposite were freed by
removing the BFO matrix then gathered on a copper mesh coated with
a carbon film using a syringe after dispersing them in isopropyl alcohol.
In-plane and out-of-plane magnetic hysteresis loops were measured
using vibrating sample magnetometry (VSM, ADE model 1660) at a
magnetic field of −10 to +10 kOe at room temperature. The
magnetization was calculated for the whole volume of the film instead
of normalizing for the volume of the pillar phase.
■
RESULTS AND DISCUSSION
Figure 1a−c shows top view SEM images of the BFO−MgO
series, which had thicknesses of 80 to 120 nm. As the BFO
content increases, the density and areal coverage of the ∼20−
40 nm diameter rectangular islands both decrease. Figure 1d−f
shows the corresponding top view SEM images after removing
the matrix phase in hydrochloric acid for 2 min.27 The inset in
Figure 1f is a 45° tilted image of the nanocomposite after 1 min
etch that shows free-standing pillars supported by the
remaining matrix. These images show that the BFO−MgO
codeposition produces a columnar nanocomposite of islands or
pillars in a Bi-rich matrix. Films were grown up to 200 nm
thickness and demonstrated columnar structures.
For the MgO-rich sample of Figure 1a, the XRD peak at 2θ =
43.0° corresponds to MgO(002) with an out-of-plane lattice
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Figure 3. Top view SEM image of (a) as-deposited BFO−(MFO/MgO/CFO) nanocomposite. (b) XRD pattern of BFO−(CFO/BFO/MFO)
trilayer nanocomposite. (c) etched nanopillars after removing BFO. (d) TEM image and elemental mapping of etched trilayer pillar. (e) In-plane and
out-of-plane hysteresis loops of trilayer pillar nanocomposite measured at room temperature.
phases on the substrate, although some additional peaks were
detected.
The in-plane and out-of-plane magnetic hysteresis loops
show soft, isotropic magnetic behavior with a saturation
magnetization of 20 emu cm−3 averaged over the volume of
the entire film. We propose that these nanopillars consist of
magnesium ferrite spinel, MxFO. MFO has the spinel structure
and shows a soft magnetic behavior28−30 with bulk saturation
magnetic moment of 130 emu/cm3,29 and the bulk lattice
parameter is 8.375 Å (JCPDS #00-017-0464), close to that of
the pillar peak in Figure 2b that corresponds to out-of-plane
spinel lattice parameter of 8.374 ± 0.004 Å. The volume
fraction of the pillar phase estimated from the area in the top
view SEM image is 18 ± 4% which implies a saturation moment
for the pillars of 117 ± 26 emu cm−3, close to that of bulk
MFO. The low magnetic anisotropy of MFO pillars is a result
of the low magnetostriction and low magnetic moment, which
produce low magnetoelastic and shape anisotropies, respectively.
Further evidence that the pillars are spinel not MgO comes
from their morphology. The pillar shape resembles those of
CFO−BFO nanocomposites. The rectangular prismatic shape
and top surface faceted structure with four tilted (111) planes is
characteristic of spinel pillars in a perovskite matrix, originating
from the lowest surface energy {111} planes of spinel.6 In
contrast, simulation results show that the lowest surface energy
planes of MgO are {100} planes.31
STEM-EDS elemental mapping also supports the identification of MxFO pillars. The top left image in Figure 2d shows
Unexpectedly, the BFO−MgO nanocomposites showed a
magnetic signal measured with VSM at room temperature
(Figure S1), though single phase BFO films deposited at the
same conditions did not show a magnetic moment (Figure S2).
The saturation magnetic moments were 17, 30, and 15 emu
cm−3 for the samples of Figure 1a,b,c respectively, calculated
over the volume of the film, i.e., not normalized to the pillar
volume fraction. The magnetic hysteresis loops were isotropic
with low coercivity (<100 Oe). This supports the idea that a
ferrimagnetic spinel is present because neither MgO nor BFO
has a magnetic signal when grown at this deposition condition
(Figure S2), BFO being a canted antiferromagnet. The
magnetic signal did not disappear on etching by HCl so is
attributed to the pillar phase (Figure S3). To investigate this
further, a BFO−MgO nanocomposite was made by alternating
ablation of MgO and BFO targets at the optimal ratio obtained
from combinatorial PLD. 100 and 250 pulses were incident on
the MgO and BFO targets respectively, repeated 200 times to
make a 60 nm thick film. In Figure 2a, rectangular nanopillars
can be seen with their edges aligned along the ⟨110⟩ directions
of STO substrate. This nanocomposite showed XRD peaks
corresponding to MgO (002) or MxFO (004), and BFO (002)
near 2θ = 45° with out-of-plane lattice parameter of 4.187 ±
0.002 for the pillars and 4.099 ± 0.003 Å for the BFO. These
values are closer to each other compared with those of the
nanocomposites grown by CPLD. The pillar phase was in outof-plane compression and the BFO in out-of-plane tension. φ
scans shown in Figure S4 demonstrated 4-fold symmetry of
BFO and spinel phases indicating epitaxial growth of both
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Figure 4. (a, b, and c) Top-view SEM image of BFO−MgAl2O4 nanocomposites grown by CPLD with increasing BFO content. (d, e, and f)
Corresponding etched films. (g, h, and i) Magnetic hysteresis loops of the BFO−MAO nanocomposites measured by VSM at room temperature.
We now discuss a “triple-layer” nanocomposite consisting of
CFO/MgO/MFO in a BFO matrix, designated BFO−(CFO/
MgO/MFO), which was fabricated to investigate the effects of
inserting an MgO layer within a spinel pillar. We have already
shown in nanocomposite BFO−(CFO/BFO/MFO), where
layers of CFO and MFO pillars were separated by a thin BFO
layer, that any direct contact between the CFO and MFO
pillars led to interdiffusion producing (Co1−xMgx)Fe2O4 pillars
of uniform composition.17 Therefore, it was interesting to see
whether MgO could fulfill the function of a diffusion barrier in
a spinel pillar.
First, a magnetically hard BFO−CFO nanocomposite was
grown on a STO (001) substrate, then 20 nm thick BFO−
MgO was grown, and finally a soft magnetic BFO−MFO
nanocomposite layer was grown. Samples were made in a single
run in the same chamber equipped with four targets without
breaking vacuum. Figure 3a shows the top view SEM image of
the BFO−(CFO/BFO/MFO) trilayer nanocomposite. The
XRD pattern shows BFO and a single peak for spinel and/or
MgO, Figure 3b.
the TEM images of agglomerated nanoparticles collected on a
TEM grid after etching in HCl solution, and the other panels in
Figure 2d show Mg, Fe, and O mapping of the same area. The
composition analysis reveals that there is no detectable Bi, but
Fe is present throughout the pillars. The small pillar size
compared to the beam size precluded an accurate measurement
of Mg:Fe ratio.
The pillar structure and the magnetic hysteresis of the BFO−
MgO nanocomposite are similar to those of BFO−MFO
nanocomposites reported previously.23 The spinel phase forms
in the BFO−MgO nanocomposite because MFO is thermodynamically stable,32 and MFO has also been observed to form at
the interface between an MgO substrate and an iron oxide thin
film.33 However, other nanocomposites containing MgO have
been grown, such as (La0.7Ca0.3MnO3)−MgO,34 CoFe2O4−
MgO,11 and ZnO−MgO.35 This suggests that the low stability
of BFO contributes to the formation of MxFO in the BFO−
MgO codeposition. Formation of MxFO would leave excess Bi,
but Bi or Bi2O3 peaks were not detected by XRD except in the
sample of Figure 1a, and it is likely that excess Bi is lost from
the film due to its volatility.
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of spinel, which may indicate a lower Fe content in the spinel
made at lower BFO fraction counteracting the higher volume
fraction to give similar average magnetization. The films
showed magnetic anisotropy, in comparison with the isotropic
behavior of the BFO−MgO (Figure S1). This is assumed to
originate from the different morphology, composition and/or
magnetostriction and lattice strain of the (Mg, Al, Fe)3O4 spinel
phase in the BFO−MAO nanocomposites compared to the
MxFO in the BFO−MgO nanocomposites.
In the plane view, faceted rectangular pillars line up along the
[110] directions of the STO substrate resembling Figure 2a.
However, after etching in HCl solution for 3 min (Figure 3c),
the nanopillars are seen to grow from two or three roots
connected by a wider midsection. The roots correspond to the
BFO−CFO growth, the midsection to BFO−MgO, and the top
to BFO−MFO. The BFO−MgO layer is assumed to grow as
BFO−MxFO as discussed above, resulting in a higher pillar
volume fraction that caused the small CFO pillars to expand
and merge. Then growth of the top CFO−MFO layer leads to
narrowing of the pillars. The growth sequence suggests that the
pillars should consist of three chemically different layers, but
compositional analysis with EDS mapping shows a uniform
cation distribution throughout the whole pillar to produce a
spinel phase of (Co1−yMgy)Fe2O4 (Figure 3d). This exhibited
magnetic hysteresis loops with intermediate magnetic behavior
between that of hard magnetic CFO and soft magnetic MFO
(Figure 3e),17 in which both shape and magnetoelastic
anisotropy promote an out-of-plane easy axis. The anisotropy
was lower than that of a BFO−CFO nanocomposite.
Therefore, growing a layer of BFO−MgO did not provide an
effective diffusion barrier within the spinel pillars due to the
formation of MxFO instead of MgO from the arriving Mg flux.
In the all-spinel pillar, cation interdiffusion took place as seen in
bilayer spinel pillars.17
We finally describe the growth of nanocomposites made
from BFO−MAO. Figure 4a,b,c shows top-view SEM images of
100 nm thick BFO−MAO nanocomposites grown by CPLD
using BFO and MAO targets and Figure 4d,e,f shows their
SEM images after etching. MAO did not form well-defined
pillars as seen in the BFO−CFO or BFO−MFO cases,17,23,31
even though the lattice mismatch between BFO (2abulk,BFO =
7.930 Å) and MAO (abulk,MAO = 8.083 Å) is smaller than in
BFO−CFO or BFO−MFO.
The MAO-rich nanocomposite (Figure 4a) showed micrometer-sized islands that had facets along STO {100}. XRD
patterns, Figure S5a, clearly showed one film peak corresponding to MAO (004) whose lattice parameter was 8.098 ± 0.004
Å, but no BFO or Bi2O3 peak. The etching had little effect on
the morphology indicating that BFO or Bi2O3 were not present
at the top surface.
As the BFO content increased, Figure 4b, the numbers of
islands increased, and etching removed some of the island
material to produce an irregular surface. XRD showed two
phases corresponding to MAO (8.088 ± 0.004 Å) and BFO
(3.947 ± 0.004 and 3.975 ± 0.002 Å), Figure S5b. In Figure 4c
with greater BFO content, BFO covered most of the surface
and small diameter pillars were present, revealed by etching.
The XRD pattern displayed BFO peaks that had out-of-plane
lattice parameters of 4.027 ± 0.003 and 3.952 ± 0.004 Å, Figure
S5c. In both samples of Figure 4b,c, the BFO peaks (Figures
S5b,c) did not disappear on etching suggesting interdiffusion
may have led to a more etch-resistant matrix phase.
The nanocomposites all showed magnetic phases with inplane easy axis even though the spinel unit cell parameters were
characteristic of nonmagnetic MAO rather than MxFO. We
presume the magnetism originates from a partly substituted
(Mg, Al, Fe)3O4 spinel phase. Consequently, the excess Mg or
Al is present in the perovskite phase, yielding for example
Bi(Fe,Al)O3 with higher etch resistance than BFO. (The end
member BiAlO3 can form a perovskite structure.36) The
averaged saturation magnetization for the three samples was
similar, 20−25 emu cm−3 despite the different volume fractions
■
CONCLUSION
In summary, self-assembled two-phase nanocomposite films
were made by codeposition of BFO−MgO and BFO−MAO
and phase formation was investigated. The BFO−MgO formed
a nanocomposite consisting of spinel MxFO pillars in a BFO
matrix within a very small window of deposition parameters,
with little or no rocksalt MgO. The pillars contributed a
magnetic signal consistent with the volume fraction and the
saturation magnetization of MFO. A multilayer nanocomposite,
BFO−(CFO/MgO/MFO), showed related behavior in which
the pillars had a spinel structure throughout, with interdiffusion
leading to a uniform composition of (Co,Mg,Fe)3O4. The
pillars had interesting bifurcated structures due to the merger of
smaller pillars into larger ones, and the magnetic properties
were intermediate between those of magnetically hard CFO
and soft MFO pillars. In contrast, codeposition of BFO with an
aluminum spinel, MAO, in which there is a lower lattice
mismatch between the pure bulk phases, grew as a two-phase
film but did not produce clearly defined crystallographic pillars
under the deposition conditions used here. Fe incorporation in
the MAO led to a magnetic moment, and incorporation of Al
or Mg into the BFO matrix increased its etch resistance. These
results show that intermixing of the arriving flux from two
targets leads to formation of phases not present in the targets,
and in particular the growth of Fe-containing spinel phases
leads to magnetic properties in nanocomposites made from
nonferromagnetic targets.
■
ASSOCIATED CONTENT
S Supporting Information
*
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acsami.5b10676.
Magnetic hysteresis data of BFO−MgO and BFO, and Xray diffraction data of BFO−MgO and BFO−MAO
samples (PDF).
■
AUTHOR INFORMATION
Corresponding Author
*C. A. Ross. E-mail: caross@mit.edu.
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
This work was supported by the 2015 Research Fund of
Myongji University and by FAME, a STARnet Center of the
Semiconductor Research Corporation funded by DARPA and
MARCO. It was also supported by the National Research
Foundation of Korea (NRF) grant funded by the Korean
government (Ministry of Science, ICT & Future Planning)
(No. 2015R1C1A1A01051656).
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(19) Ikeda, S.; Miura, K.; Yamamoto, H.; Mizunuma, K.; Gan, H. D.;
Endo, M.; Kanai, S.; Hayakawa, J.; Matsukura, F.; Ohno, H. A
Perpendicular-Anisotropy CoFeB−MgO Magnetic Tunnel Junction.
Nat. Mater. 2010, 9, 721−724.
(20) Chapline, M. G.; Wang, S. X. Room-Temperature Spin Filtering
in a CoFe2O4/MgAl2O4/Fe3O4 Magnetic Tunnel Barrier. Phys. Rev. B:
Condens. Matter Mater. Phys. 2006, 74, 014418.
(21) Sukegawa, H.; Miura, Y.; Muramoto, S.; Mitani, S.; Niizeki, T.;
Ohkubo, T.; Abe, K.; Shirai; Inomata, K.; Hono, K. Enhanced Tunnel
Magnetoresistance in a Spinel Oxide Barrier with Cation-Site Disorder.
Phys. Rev. B: Condens. Matter Mater. Phys. 2012, 86, 184401.
(22) Rebled, J. M.; Foerster, M.; Estradé, S.; Rigato, F.; Kanamadi,
C.; Sánchez, F.; Peiró, F.; Fontcuberta, J. Ti Diffusion in (001)
SrTiO3−CoFe2O4 Epitaxial Heterostructures: Blocking Role of a
MgAl2O4 Buffer. Phys. Chem. Chem. Phys. 2013, 15, 18274−18280.
(23) Kim, D. H.; Aimon, N. M.; Ross, C. A. Self-Assembled Growth
and Magnetic Properties of a BiFeO3-MgFe2O4 Nanocomposite
Prepared by Pulsed Laser Deposition. J. Appl. Phys. 2013, 113, 17B510.
(24) Kim, D. H.; Aimon, N. M.; Ross, C. A. Pillar Shape Modulation
in Epitaxial BiFeO3−CoFe2O4 Vertical Nanocomposite Films. APL
Mater. 2014, 2, 081101.
(25) Kim, D. H.; Bi, L.; Aimon, N. M.; Jiang, P.; Dionne, G. F.; Ross,
C. A. Combinatorial Pulsed Laser Deposition of Fe, Cr, Mn, and NiSubstituted SrTiO3 Films on Si Substrates. ACS Comb. Sci. 2012, 14,
179−190.
(26) Sun, X. Y.; Zhang, C.; Aimon, N. M.; Goto, T.; Onbasli, M.;
Kim, D. H.; Choi, H. K.; Ross, C. A. Combinatorial Pulsed Laser
Deposition of Magnetic and Magnetooptical Sr(GaxTiyFe0.34−0.40)O3−δ
Perovskite Films. ACS Comb. Sci. 2014, 16, 640−646.
(27) Kim, D. H.; Aimon, N. M.; Sun, X.; Kornblum, L.; Walker, F. J.;
Ahn, C. H.; Ross, C. A. Integration of Self-Assembled Epitaxial
BiFeO3−CoFe2O4 Multiferroic Nanocomposites on Silicon Substrates.
Adv. Funct. Mater. 2014, 24, 5889−5896.
(28) Wells, D. M.; Cheng, J.; Ellis, D. E.; Wessels, B. W. Local
Electronic and Magnetic Structure of Mixed Ferrite Multilayer
Materials. Phys. Rev. B: Condens. Matter Mater. Phys. 2010, 81, 174422.
(29) Morrish, A. H. The Physical Principle of Magnetism; Wiley-IEEE:
New York, 2001; p 507.
(30) Cheng, J.; Lazarov, V. K.; Sterbinsky, G. E.; Wessels, B. W.
Synthesis, Structural and Magnetic Properties of Epitaxial MgFe2O4
Thin Films by Molecular Beam Epitaxy. J. Vac. Sci. Technol., B:
Microelectron. Nanometer Struct.−Process., Meas., Phenom. 2009, 27,
148−151.
(31) Watson, G. W.; Kelsey, E. T.; de Leeuw, N. H.; Harris, D. J.;
Parker, S. C. Atomistic Simulation of Dislocations, Surfaces and
Interfaces in MgO. J. Chem. Soc., Faraday Trans. 1996, 92, 433−438.
(32) Rao, Y. K. Stoichiometry and Thermodynamics of Metallurgical
Processes; Cambridge University Press: Cambridge, 1985; p 135.
(33) Zimnol, M.; Graff, A.; Sieber, H.; Senz, S.; Schmidt, S.; Mattheis,
R.; Hesse, D. Structure and Morphology of MgFe2O4 Epitaxial Films
Formed by Solid State Reactions on MgO(100) Surfaces. Solid State
Ionics 1997, 101, 667−672.
(34) Moshnyaga, V.; Damaschke, B.; Shapoval, O.; Belenchuk, A.;
Faupel, J.; Lebedev, O. I.; Verbeeck, J.; van Tendeloo, G.; Mücksch,
M.; Tsurkan, V.; Tidecks, R.; Samwer, K. Structural Phase Transition
at the Percolation Threshold in Epitaxial (La0.7Ca0.3MnO3)1−x:(MgO)x
Nanocomposite Films. Nat. Mater. 2003, 2, 247−252.
(35) Das, S.; Chakrabarti, S.; Chaudhuri, S. Optical transmission and
photoluminescence studies of ZnO−MgO nanocomposite thin films. J.
Phys. D: Appl. Phys. 2005, 38, 4021−4026.
(36) Belik, A. A.; Wuernisha, T.; Kamiyama, T.; Mori, K.; Maie, M.;
Nagai, T.; Matsui, Y.; Takayama-Muromachi, E. High-Pressure
Synthesis, Crystal Structures, and Properties of Perovskite-like
BiAlO3 and Pyroxene-like BiGaO3. Chem. Mater. 2006, 18, 133−139.
REFERENCES
(1) Zheng, H.; Wang, J.; Lofland, S. E.; Ma, Z.; Mohaddes-Ardabili,
L.; Zhao, T.; Salamanca-Riba, L.; Shinda, S. R.; Ogale, S. B.; Bai, F.;
Viehland, D.; Jia, Y.; Schlom, D. G.; Wuttig, M.; Roytburd, A.;
Ramesh, R. Multiferroic BaTiO3-CoFe2O4 Nanostructures. Science
2004, 303, 661−663.
(2) Ma, J.; Hu, J.; Li, Z.; Nan, C. W. Recent Progress in Multiferroic
Magnetoelectric Composites: from Bulk to Thin Films. Adv. Mater.
2011, 23, 1062−1087.
(3) Yan, L.; Yang, Y.; Wang, Z.; Xing, Z.; Li, J.; Viehland, D. Review
of Magnetoelectric Perovskite−Spinel Self-Assembled Nano-Composite Thin Films. J. Mater. Sci. 2009, 44, 5080−5094.
(4) MacManus-Driscoll, J. L. Self-Assembled Heteroepitaxial Oxide
Nanocomposite Thin Film Structures: Designing Interface-Induced
Functionality in Electronic Materials. Adv. Funct. Mater. 2010, 20,
2035−2045.
(5) Li, J.; Levin, I.; Slutsker, J.; Provenzano, V.; Schenck, R. K.;
Ramesh, R.; Ouyang, J.; Roytburd, A. L. Self-Assembled Multiferroic
Nanostructures in the CoFe2O4 -PbTiO3 System. Appl. Phys. Lett.
2005, 87, 072909.
(6) Zheng, H.; Zhan, Q.; Zavaliche, F.; Sherburne, M.; Straub, F.;
Cruz, M. P.; Chen, L.-Q.; Dahmen, U.; Ramesh, R. Controlling SelfAssembled Perovskite−Spinel Nanostructures. Nano Lett. 2006, 6,
1401−1407.
(7) Zhan, Q.; Yu, R.; Crane, S. P.; Zheng, H.; Kisielowski, C.;
Ramesh, R. Structure and Interface Chemistry of Perovskite-Spinel
Nanocomposite Thin Films. Appl. Phys. Lett. 2006, 89, 172902.
(8) Aimon, N. M.; Kim, D. H.; Choi, H. K.; Ross, C. A. Deposition of
Epitaxial BiFeO3/CoFe2O4 Nanocomposites on (001) SrTiO3 by
Combinatorial Pulsed Laser Deposition. Appl. Phys. Lett. 2012, 100,
092901.
(9) Zhang, W.; Chen, A.; Khatkhatay, F.; Tsai, C.-F.; Su, Q.; Jiao, L.;
Zhang, Z.; Wang, H. Integration of Self-Assembled Vertically Aligned
Nanocomposite (La0.7Sr0.3MnO3)1−x:(ZnO)x Thin Films on Silicon
Substrates. ACS Appl. Mater. Interfaces 2013, 5, 3995−3999.
(10) Khatkhatay, F.; Chen, A.; Lee, J. H.; Zhang, W.; Abdel-Raziq,
H.; Wang, H. Ferroelectric Properties of Vertically Aligned Nanostructured BaTiO3−CeO2 Thin Films and Their Integration on
Silicon. ACS Appl. Mater. Interfaces 2013, 5, 12541−12547.
(11) Barber, S. M.; Lin, Q.; Zou, G.; Haberkorn, N.; Baily, S. A.;
Wang, H.; Bi, Z.; Yang, H.; Deng, S.; Hawley, M. E.; Civale, L.; Bauer,
E.; McCleskey, T. M.; Burrell, A. K.; Jia, Q.; Luo, H. Magnetic
Properties of Self-Assembled Epitaxial Nanocomposite CoFe2O4:SrTiO3 and CoFe2O4:MgO Films. J. Phys. Chem. C 2011,
115, 25338−25342.
(12) Zheng, H.; Straub, F.; Zhan, Q.; Yang, P. L.; Hsieh, W.-K.;
Zavaliche, F.; Chu, Y.-H.; Dahmen, U.; Ramesh, R. Self-Assembled
Growth of BiFeO3-CoFe2O4 Nanostructures. Adv. Mater. 2006, 18,
2747−2752.
(13) Dix, N.; Muralidharan, R.; Warot-Fonrose, B.; Varela, M.;
Sánchez, F.; Fontcuberta, J. Critical Limitations in the Fabrication of
Biferroic BiFeO3−CoFe2O4 Columnar Nanocomposites Due to
Bismuth Loss. Chem. Mater. 2009, 21, 1375−1380.
(14) Comes, R.; Liu, H.; Khokhlov, M.; Kasica, R.; Lu, J.; Wolf, S. A.
Directed Self-Assembly of Epitaxial CoFe2O4−BiFeO3 Multiferroic
Nanocomposites. Nano Lett. 2012, 12, 2367−2373.
(15) Zavaliche, F.; Zhao, T.; Zheng, H.; Straub, F.; Cruz, M. P.; Yang,
P.-L.; Hao, D.; Ramesh, R. Electrically Assisted Magnetic Recording in
Multiferroic Nanostructures. Nano Lett. 2007, 7, 1586−1590.
(16) Aimon, N. M.; Kim, D. H.; Sun, X.; Ross, C. A. Multiferroic
Behavior of Templated BiFeO3−CoFe2O4 Self-Assembled Nanocomposites. ACS Appl. Mater. Interfaces 2015, 7, 2263−2268.
(17) Kim, D. H.; Aimon, N. M.; Sun, X.; Ross, C. A. Compositionally
Modulated Magnetic Epitaxial Spinel/Perovskite Nanocomposite Thin
Films. Adv. Funct. Mater. 2014, 24, 2334−2342.
(18) Yuasa, S.; Nagahama, T.; Fukushima, A.; Suzuki, Y.; Ando, K.
Giant Room-Temperature Magnetoresistance in Single-Crystal Fe/
MgO/Fe Magnetic Tunnel Junctions. Nat. Mater. 2004, 3, 868−871.
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DOI: 10.1021/acsami.5b10676
ACS Appl. Mater. Interfaces 2016, 8, 2673−2679