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A modified sol-gel process for multiferroic nanocomposite films
Ming Liu, Xin Li, and Jing Lou
Center for Microwave Magnetic Materials and Integrated Circuits, Department of Electrical
and Computer Engineering, Northeastern University, Boston, Massachusetts 02115, USA
Shijian Zheng and Kui Du
Shenyang National Laboratory for Materials Science, Institute of Metal Research,
Chinese Academy of Sciences, Shenyang 110016, China
Nian X. Suna兲
Center for Microwave Magnetic Materials and Integrated Circuits, Department of Electrical
and Computer Engineering, Northeastern University, Boston, Massachusetts 02115, USA
共Received 5 June 2007; accepted 29 August 2007; published online 24 October 2007兲
Multiferroic CoFe2O4–Pb共Zr, Ti兲O3共CFO-PZT兲composite films with nanoscale mixture of the two
phases were prepared by a modified sol-gel process, in which a mixed precursor solution of both
CFO and PZT was used. X-ray diffraction and transmission electron microscopy examinations
revealed the coexistence of perovskite PZT and spinel CFO that were mixed in nanoscale with mean
grain sizes of 5–10 nm. Magnetic properties of the CFO-PZT nanocomposite were examined,
which were consistent with their microstructures. The magnetoelectric coupling between CFO and
PZT was demonstrated by an external magnetic field induced electric polarization change. This
modified sol-gel processing provides an alternative for multiferroic composite films, which is
simpler and easier to control compared to the conventional layer-layer sol-gel process for
multiferroic composite films. © 2007 American Institute of Physics.关DOI: 10.1063/1.2800804兴
I. INTRODUCTION
Multiferroic composite materials consisting of both
ferro/ferrimagnetic and ferroelectric phases have drawn an
increasing amount of interest due to their capability of effi-
cient energy transfer between electric energy and magnetic
energy, and their potential applications in many multifunc-
tional devices.1–8Such materials can display the magneto-
electric 共ME兲effect, a dielectric polarization variation as a
response to an applied magnetic field, or an induced magne-
tization by an external electric field. Several synthesis meth-
ods have been developed for multiferroic composite materi-
als in the bulk form, such as eutectic unidirectional
solidification,9ceramic sintering,10 glue bonded laminates,11
tape casting,12 and hot molding,13 while pulsed laser deposi-
tion 共PLD兲,4,7,14–18 physical vapor deposition,19 and sol-gel
processing have been used for synthesizing multiferroic
composite film materials.20–23
Achieving strong magnetoelectric coupling in multifer-
roic composite films has been challenging. Zavaliche
et al.7,14 reported that multiferroic composite films with self-
assembled CoFe2O4spinel nanopillars in a BaTiO3perov-
skite matrix on single crystalline SrTiO3substrate was syn-
thesized by pulsed laser deposition, and the magnetoelectric
coupling was demonstrated by observing magnetization
change at the ferroelectric Curie temperature7and magneti-
zation reversal induced by applying electric field.14 Wan
et al.24 reported CoFe2O4–Pb共Zr, Ti兲O3共CFO-PZT兲multi-
ferroic composite films with a ME voltage coefficient of
220 mV/ cm Oe at magnetic frequency of 1 kHz, in which
the CFO and PZT phases were spin coated sequentially by
sol-gel process. Zhou et al. also reported layer by layer
CoFe2O4–Pb共Zr, Ti兲O3and Co0.9Zn0.1Fe2O4–Pb共Zr, Ti兲O3
multiferroic films, which were grown by PLD 共Ref. 15兲and
sol-gel,25 respectively. For reported sol-gel processes for
multiferroic composite films, the two phases 共ferrimagnetic
and ferroelectric phases兲were typically deposited from
different sources, and the nanocomposite multiferroic films
were formed by a spontaneous phase separation process
that occurred during the deposition or annealing
process.7,14,15,26–28 It was found that spontaneous phase sepa-
ration could occur in the sol-gel processed CFO-PZT nano-
composite film during the annealing step, leading to 0-3-type
nanocomposite film structure with the less-resistive CFO
phase embedded in PZT matrix. This 0-3-type microstructure
with less leakage is critical in achieving magnetoelectric
coupling in sol-gel processed ME composite films.
In this work, we report a modified sol-gel synthesis
method for nanocomposite multiferroic thin film with nano-
scale mixed CFO and PZT phase, in which the precursors of
both the ferrimagnetic and ferroelectric phases were mixed
before spin coating. The spin coating of the mixed precursor
solution of the ferrimagnetic and ferroelectric phases allows
for nanoscale mixing of the subsequent multiferroic compos-
ite phases which leads to a strong ME coupling.
II. EXPERIMENT
The nanocomposite thin films studied here were pre-
pared by a sol-gel process and spin-coating technique. A
0.2MPZT precursor with the molar ratio of 1:0.52:0.48 was
prepared by dissolving lead acetate Pb共CH3CO2兲23H2O, zir-
conium propoxide Zr共CH2CH2CH3O兲4, and titanium butox-
ide Ti共C4H9O兲4into 2-methoxyethanol. Also, cobalt acetate
a兲Phone: ⫹1-617-373-3351. Electronic mail: nian@ece.neu.edu
JOURNAL OF APPLIED PHYSICS 102, 083911 共2007兲
0021-8979/2007/102共8兲/083911/3/$23.00 © 2007 American Institute of Physics102, 083911-1
Downloaded 25 Oct 2007 to 129.10.56.199. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp
Co共CH3CO2兲2and iron acetate Fe共CH3CO2兲23H2O were
dissolved into 2-methoxyethanol to derive 0.1MCFO precur-
sor with molar ratio of 1:2. Then, both of precursor solutions
for PZT and CFO were mixed together with volume ratio of
1:2 and stirred continuously to form a mixed PZT-CFO pre-
cursor solution. The mixed solution was spin coated onto
Ru/SiO2/ Si substrate with a thickness of 0.5 mm at
2000 rpm for 20 s and subsequently baked at 90 °C for
5 min and 300 °C for 5 min. This spinning coating and bak-
ing procedures were repeated twice. After being calcined at
800 °C for 10 min in air, nanocomposite PZT-CFO films
with a thickness of 100 nm were obtained. It is expected that
the volumetric ratio between the CFO phase and PZT phase
is 0.52:0.48 in the final nanocomposite films.
III. RESULTS AND DISCUSSIONS
Phase structure characterization of the nanocomposite
thin films was performed with x-ray diffraction 共XRD兲using
Cu K
␣
radiation. Figure 1shows the typical XRD pattern of
nancomposite thin film. Perovskite PZT phase and the spinel
CFO phase can be identified without obvious preferential
crystallographic orientations. Clearly, mixing the precursor
solutions of the CFO phase and the PZT phase before spin
coating does not lead to obvious adverse effect on the result-
ant CFO phase and the PZT phase. High-resolution transmis-
sion electron microscopy 共TEM兲examination on these mul-
tiferroic films was performed to obtain further microstructure
information. Figure 2共a兲shows the cross section image of
nanocomposite thin film in which a well-defined interface
between substrate and PZT-CFO layer with a thickness of
⬃100 nm was observed. Both the PZT phase and CFO phase
can be identified, together with some amorphous phase共s兲,as
shown in Fig. 2共b兲, which is an indication of insufficient
annealing. The presence of the amorphous phase共s兲could
lead to reduced magnetoelectric coupling between ferroelec-
tric and magnetic phases. In addition, the PZT and CFO
phases show well-defined grains with their grain sizes in the
range of 5–10 nm with CFO nanograins nearly surrounded
by PZT nanograins, which indicates nanoscale mixture of the
CFO phase and PZT phase. This 5– 10 nm scale mixture of
the magnetic phase and the ferroelectric phase, which was
not reported before, could be due to the spontaneous phase
separation in the spin-coated mixed CFO and PZT precursor
layer during the short annealing process. This nanoscale mix-
ing of the CFO and PZT phases could lead to significantly
reduced eddy current loss and lowered leakage current path
in multiferroic films and, therefore, an enhanced magneto-
electric coupling.
Magnetic properties of the multiferroic nanocomposite
thin films were characterized with vibrating sample magne-
tometer with the applied magnetic fields perpendicular and
parallel to the thickness of the film at room temperature, as
shown in Fig. 3. The CFO-PZT multiferroic nanocomposite
films show very similar magnetic hysteresis loops for both
in-plane and out-plane orientations with an overall saturation
magnetization of 118 emu / cm3for the whole film. This
magnetization is relatively low even after considering the
volume fraction effect of CFO phase, which could be due to
the small particle sizes of the CFO phase within the PZT
matrix, as well as due to the existence of amorphous phase in
the film. The similarity in the magnetic hysteresis loops for
both in-plane and out-plane orientations is consistent with
the TEM observations that the CFO nanograins are dispersed
in the PZT phase to form isolated CFO nanograin islands
with a 0-3 connection.
The ferroelectric properties of the nanocomposite thin
films were also measured using a Radiant Precision LC test
system. Figure 4shows the polarization versus electric field
共P-E兲hysteresis loops of a PZT-CFO nanocomposite film 共at
zero magnetic field兲and a pure PZT film which was obtained
by sol-gel process under similar conditions. The saturation
polarization 共Ps兲and remanent polarization 共Pr兲of the PZT-
CFO film are 18.5 and 8.3
C/cm2, respectively, compared
FIG. 1. 共Color online兲X-ray diffraction pattern of multiferroic CFO-PZT
nanocomposite thin film.
FIG. 2. Cross-section scanning transmission electron microscopy 共STEM兲
image 共a兲and HRTEM image 共b兲of multiferroic CFO-PZT nanocomposite
thin film.
FIG. 3. 共Color online兲Magnetic hysteresis loops of CFO-PZT nanocompos-
ite thin film.
083911-2 Liu et al. J. Appl. Phys. 102, 083911 共2007兲
Downloaded 25 Oct 2007 to 129.10.56.199. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp
to a Psof 45
C/cm2and Prof 32.8
C/cm2for the pure
PZT film. The reduction of the Prand Psvalues of CFO-PZT
nanocomposite thin film is expected to be due to the pres-
ence of nonferroelectric and less-resistive CFO phase.
The magnetoelectric coupling in these CFO-PZT films
was demonstrated through the measurement of magnetic
field induced change of the electric polarization by measur-
ing the P-Ehysteresis under different magnetic fields, simi-
lar to what was done in Refs. 14 and 16. As shown in Fig. 4,
the P-Ehysteresis loops were changed with the application
of a 1200 Oe magnetic field perpendicular to the film sur-
face. The remnant polarization values were reduced by 22%,
with the presence of a 1200 Oe magnetic field applied per-
pendicular to the thin film. This remnant polarization reduc-
tion of 22% is much larger than the polarization measure-
ment error of ⬍2% for the P-Ehysteresis loop.
In summary, multiferroic CFO-PZT nanocomposite thin
film with the average grain size of 5–10 nm has been syn-
thesized with a modified sol-gel process in which a mixed
precursor solution of the CFO phase and the PZT phase was
used. Strong magnetoelectric coupling was observed in this
nanoscale mixed CFO and PZT nanocomposite film. This
modified sol-gel processing method provides a simpler alter-
native for synthesizing multiferroic composite films which is
easier to control compared to the conventional layer-layer
sol-gel process for multiferroic composite films.
ACKNOWLEDGMENT
This work is sponsored by NSF under Award No. DMR-
0603115.
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tiferroic CFO-PZT nanocomposite film and pure PZT film.
083911-3 Liu et al. J. Appl. Phys. 102, 083911 共2007兲
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