1. Controlled self-assembly of multiferroic core-shell nanoparticles exhibiting strong
magneto-electric effects
Gollapudi Sreenivasulu, Maksym Popov, Ferman A. Chavez, Sean L. Hamilton, Piper R. Lehto, and Gopalan
Srinivasan
Citation: Applied Physics Letters 104, 052901 (2014); doi: 10.1063/1.4863690
View online: http://dx.doi.org/10.1063/1.4863690
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2. Controlled self-assembly of multiferroic core-shell nanoparticles exhibiting
strong magneto-electric effects
Gollapudi Sreenivasulu,1
Maksym Popov,1,2
Ferman A. Chavez,3
Sean L. Hamilton,1
Piper R. Lehto,1
and Gopalan Srinivasan1,a)
1
Physics Department, Oakland University, Rochester, Michigan 48309-4401, USA
2
Radiophysics Department, Taras Shevchenko National University of Kyiv, Kyiv 01601, Ukraine
3
Chemistry Department, Oakland University, Rochester, Michigan 48309-4401, USA
(Received 9 December 2013; accepted 15 January 2014; published online 3 February 2014)
Ferromagnetic-ferroelectric composites show strain mediated coupling between the magnetic and
electric sub-systems due to magnetostriction and piezoelectric effects associated with the ferroic
phases. We have synthesized core-shell multiferroic nano-composites by functionalizing
10–100 nm barium titanate and nickel ferrite nanoparticles with complementary coupling groups
and allowing them to self-assemble in the presence of a catalyst. The core-shell structure was
confirmed by electron microscopy and magnetic force microscopy. Evidence for strong strain
mediated magneto-electric coupling was obtained by static magnetic field induced variations in the
permittivity over 16–18 GHz and polarization and by electric field induced by low-frequency ac
magnetic fields. VC 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4863690]
A multiferroic is a material that exhibits two or more of
the primary ferroic orderings such as ferromagnetism and
ferroelectricity.1,2
A composite made of ferromagnetic and
ferroelectric phases represents an engineered multiferroic in
which an applied magnetic or electric field produces a me-
chanical strain, leading to coupling between the electric and
magnetic sub-systems. The magneto-electric effect (ME) in
such composites is an induced polarization P in a magnetic
field H (direct ME effect) or an induced magnetization or an-
isotropy field in an electric field E (converse ME effect).3–8
Multiferroic composites studied so far include thick films,
thin film heterostructures, and bulk composites with ferrites,
lanthanum manganites, 3d-transition-metals, or rare-earth
metals and alloys for the ferromagnetic phase, and lead zir-
conate titanate (PZT), lead magnesium niobate-lead titanate
(PMN-PT), and lead zinc niobate-lead titanate (PZN-PT) or
barium titanate (BTO) for the ferroelectric phase.3–8
Several
composites show strong ME coupling that are orders of mag-
nitude stronger than in single phase multiferroics.1–9
The nature of ME interactions in nanostructured multi-
ferroic composites is of fundamental and technological im-
portance. Theories predict a strong ME coupling in
ferromagnetic-ferroelectric core-shell nanoparticles, nanopil-
lars, and nanowires.10
Such composites can be further
assembled into superstructures by surfactant-assisted or mag-
netic field or electric directed assembly techniques.11–13
Here, we report on chemical self-assembly of nickel ferrite
(NiFe2O4–NFO) and barium titanate (BaTiO3–BTO)
core-shell nanoparticles and evidence for strong
magneto-electric effects. Past efforts on core-shell nanocom-
posites include particles, tubes, and fibers.5,6,14
Synthesis of
core-shell particles of cobalt ferrite and BTO or PZT by a
surfactant-assisted thermal method and by hydrothermal and
annealing process were reported in the past.15,16
In a recent
study, 12–200 nm particles of BTO and cobalt ferrite were
functionalized by attaching carboxylate and amine groups,
respectively, and a core-shell particulate composite was
formed by bonding the two particles with the addition of a
coupling agent.11
A majority of works to-date dealt with ME
characterization of individual nanostructures by scanning
probe microscopy (SPM).14–16
This work is on “click-reaction” assisted chemical self-
assembly of core-shell multiferroic particulate composites
and ME characterization of the as-assembled composites.
We have synthesized 10–100 nm nickel ferrite-barium tita-
nate core-shell nanoparticles by attaching complementary
coupling groups to the nanoparticles and allowing them to
self-assemble in the presence of a catalyst. Scanning (SEM)
and transmission electron microscopy (TEM) and magnetic
force microscopy (MFM) studies show the anticipated core-
shell structure for the assembled clusters. The ferromagnetic
and ferroelectric nature of the composite was confirmed by
magnetization, ferromagnetic resonance (FMR), and P vs E
measurements. ME characterization by P vs E under static
magnetic field H reveal 3.8% to 4.9% change in the remnant
polarization, that is, indicative of strong ME coupling.
Additional evidence for ME coupling has been obtained
through influence of H on permittivity e vs f over 16–18 GHz
and by sample response to low-frequency ac magnetic fields.
Details on chemical self-assembly and characterization are
provided next.
The generation of hetero-assemblies between nanopar-
ticles using chemical means is a highly versatile approach.
The Cu(I)-catalyzed azide-alkyne cycloaddition (CuAAC)
reaction (commonly known as “click” chemistry) has
recently found application in producing hetero-assemblies
between nanoparticle surfaces.17
But application of the tech-
niques to assembly of larger structures such as nanoparticle-
nanoparticle linkages is much less explored.17–22
In this
work, we utilized the CuAAC reaction to self-assemble BTO
and NFO nanoparticles.23
Co-precipitation followed by
annealing at 400–1200 C was employed to synthesize
10–100 nm NFO particles24
whereas vendor supplied
50–100 nm BTO was used. We first modified the BTO ora)
Email: srinivas@oakland.edu
0003-6951/2014/104(5)/052901/5/$30.00 VC 2014 AIP Publishing LLC104, 052901-1
APPLIED PHYSICS LETTERS 104, 052901 (2014)
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3. NFO nanoparticles either with azide group or with alkyne
group as depicted in Fig. 1. Following this, CuAAC reaction
between azide- and alkyne modified nanoparticles was
performed.23
Azide modification was achieved using a procedure sim-
ilar to that reported in the literature.17
The 10–100 nm NFO
nanoparticles and BTO particles were immersed in 3-buten-
1-ol and irradiated with ultraviolet (UV) light (254 nm, $15
mW/cm2
) while being mechanically agitated to graft the
3-buten-1-ol molecules to the nanoparticle surface via the
alkene group. The terminal ÀOH groups were converted to
methanesulfonyl (mesyl) groups, which were subsequently
converted to azide groups by immersing the sample in a solu-
tion of supersaturated NaN3. Successful functionalization
was verified using FTIR (KBr pellet) spectroscopy. The
appearance of absorbance bands that are characteristic of
azide-bound onto the nanoparticles were not clearly
observed between 1380 and 1620 cmÀ1
due to overlap with
peaks already present in the unfunctionalized nanoparticles.
A peak at 2036 cmÀ1
, however, was clearly observed in the
FTIR spectra in Fig. 1(b) and is known to be associated with
the azide group.21
Functionalization of NFO or BTO nano-
particles with O-propargyl citrate groups was accomplished
by mixing O-propargylcitric acid with the nanoparticle sus-
pension in methanol.21,22
Attachment of the O-propargyl ci-
trate groups was verified using FTIR (KBr pellet)
spectroscopy. The appearance of absorbance bands that are
characteristic of O-propargyl citrate bound onto the nanopar-
ticles was observed at 1570 and 1412 cmÀ1
in the FTIR spec-
tra in Fig. 1(c).21
We synthesized 3 types of core-shell particles via the
CuAAC reaction: 100 nm diameter NFO coreÀ50 nm BTO
shell (sample-A), 50 nm BTO core–10 nm NFO shell (sam-
ple-B), and 100 nm BTO core–10 nm NFO shell (sample-C).
Structural and compositional studies on as-assembled par-
ticles were carried out by X-ray diffraction (XRD), SEM,
TEM, and SPM. The XRD pattern for sample-A is shown in
Fig. 2(a). The data show diffraction peaks expected for NFO
and BTO and all the three samples were free of any impurity
phases. Clusters of core-shell particles were characterized
with an SEM (JEOL JSM 6510). Representative SEM micro-
graph in Fig. 2(b) for sample-A clearly shows the core-shell
nature of the clusters. The chemical composition of the par-
ticles and clusters was examined by the energy dispersive
X-ray spectroscopy (EDS). The spectrum in Fig. 2(c) shows
all the elemental energy peaks which are present in BTO and
NFO phases, and it also shows the Si and Pt peaks from the
Pt coated quartz substrate on which the particles were
dispersed.
The particles were also characterized by transmission
electron microscopy (Morgagni 268). The TEM images for
samples-A and -B in Figs. 3(a) and 3(b) clearly show the
core and the surrounding shell. For sample-A, one estimates
FIG. 1. (a) Synthetic strategy for attaching O-propargyl citrate groups to
BaTiO3 and azide groups to NiFe2O4. Hetero-linkage between the function-
alized nanoparticles is achieved in the presence of Cu(I) catalyst leading to
formation of BaTiO3-NiFe2O4 nanocomposites. FTIR spectra of (b) pure/a-
zide-functionalized NiFe2O4 (NFO) nanoparticles and (c) pure/O-propargyl
citrate functionalized BaTiO3 (BTO) nanoparticles.
FIG. 2. (a) X-ray diffraction data for as-assembled core-shell particles with
100 nm NFO core and 50 nm BTO shell (sample-A). (b) SEM micrograph of
clusters of sample-A. (c) EDS data for sample-A.
FIG. 3. (a) TEM micrograph showing core-shell structures for sample-A. (b)
Similar TEM micrograph for clusters of 50 nm BTO core and 10 nm NFO
shell (sample-B). MFM (c) amplitude and (d) phase image of sample-B.
052901-2 Sreenivasulu et al. Appl. Phys. Lett. 104, 052901 (2014)
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4. eight 50 nm-BTO particles around the 100 nm-NFO core in
2D and 16 BTO particles around NFO in 3D. Figure 3(a)
shows approximately 6 to 8 particles in a shell around the
core for sample-A. Magnetic force microscopy measure-
ments were carried out (with Park Systems XE-100E) and
the results for sample-B are shown in Figs. 3(c) and 3(d).
The topography image in Fig. 3(c) shows a core-shell struc-
ture and the phase image in Fig. 3(d) shows both phases in
different contrasts.
Magnetic characterization on as-assembled clusters was
performed by magnetization (M) and FMR. Room-
temperature data on M vs H (measured with a Faraday
Balance) for dried powder of sample-A are shown in Fig.
4(a). A ferromagnetic behavior with hysteresis and remnants
and a saturation M of 11 emu/g are evident. Similarly,
samples-B and C with 10 nm NFO also showed a ferromag-
netic behavior. For sample-C saturation M of 9.5 emu/g,
remnant M-value of 0.1 emu/g and a coercive field of 5 Oe
were measured. These observations are in general agreement
with past studies that reported superparamagnetic character
in 5.7 nm NFO particles and a ferromagnetic behavior in
18 nm NFO particles.25
For an estimated ferrite weight frac-
tion of 25% in sample-A and 22% in sample-C, the measured
saturation M corresponds to 44 emu/g for ferrite-only sam-
ples and compares favorably with the reported value of
50 emu/g for polycrystalline nickel ferrites.26
As-assembled powder was then pressed into a thin rec-
tangular pellet and placed in a coplanar slot-line for FMR
measurements.27
Profiles of scattering parameter S21 vs f
were recorded for bias field H ¼ 3046 Oe parallel or
H ¼ 3933 Oe perpendicular to the sample plane. Such data in
Fig. 4(b) for sample-A show absorption of microwave power
due to FMR. Data on resonance frequency and field were
then used to calculate the effective saturation induction
4pMeff ¼ 4pMÀHa, where Ha is the anisotropy field. Using
the data in Fig. 4(b) for sample-A, we obtained 4pMeff
¼ 600 G and the gyromagnetic ratio c ¼ 3.3 GHz/kOe. Using
the saturation magnetization data in Fig. 4(a) and X-ray den-
sity of 5.4 g/cc and measured 40% porosity for the pellet,
one estimates 4pM ¼ 450 G that compares well with the esti-
mate of 4pMeff from the FMR data. The measured
c ¼ 3.3 GHz/kOe (corresponding to g ¼ 2.36) is somewhat
higher than single crystal value of g ¼ 2.2 for nickel ferrite26
and close to g ¼ 2.27 for epitaxial NFO film grown by chem-
ical vapor deposition.28
Kojima reported a similar result, an
apparent g value of 2.3–2.5 at 9.6 GHz, for polycrystalline
NFO with the g-value increasing with increasing porosity of
the sample and attributed this to an internal field Hi that
arises due to demagnetization associates with the pores,
resulting also in broadening of FMR absorption.29
In our
samples of as-assembled powders, the porosity was as high
as 40%–50% and one anticipates similar high values for
g-value as reported in Ref. 29. The frequency half-width of
$4 GHz (DH ¼ 1200 Oe) for sample-A (profile in Fig. 4(b))
with 40% porosity must be compared with 400–800 Oe for
polycrystalline NFO with porosity of 5%–15% reported by
Kojima.29
Ferroelectric characterization involved measurements of
P vs E. Figure 4 shows representative data (obtained with a
Radiant Ferroelectric Tester) for discs of compacted as-
assembled powders. Sample-A with 100 nm NFO core and
50 nm BTO shell shows a relatively small remnant polariza-
tion and coercive field (Fig. 4(c)) compared to sample-C
with 100 nm BTO core and 10 nm NFO shell (Fig. 4(d)).
This could be attributed to (i) smaller volume fraction of
BTO, $66%, in sample-A compared to 77% in sample-C
and (ii) 50 nm cubic BTO in sample-A is expected to have a
smaller P compared to 100 nm BTO with teragonal structure
in sample-C.30
The P vs E profile in Fig. 4(c) for sample-A is
indicative of a relatively small loss compared to the profile
in Fig. 4(d) for sample-C that shows a decrease in P for
E > 5 kV/cm and is indicative of a large leakage current in
the sample. This observation is in agreement with anticipated
smaller leakage current in sample-A with BTO shell (with
much higher resistivity) than for sample-C with 10 nm NFO
shell (with much smaller resistivity). Thus, one infers from
the data in Fig. 4 that the ferrite shell in sample-C gives rise
to a large leakage current. A similar, but a much higher leak-
age current, was evident from P vs E for films with nanofib-
ers of NFO and BTO.31
The nature of direct magneto-electric coupling in the
core-shell nanoparticles was studied by two techniques: (i)
static magnetic field H effects on ferroelectric order parame-
ters P and permittivity e and (ii) sample response to an ac
magnetic field. Results on P vs E under H are considered
first. Figures 4(c) and 4(d) show such data for H ¼ 0 and 2.5
kOe for samples-A and -C. Both samples show an H-induced
change in the remnant polarization Pr and the coercive field
Ec. From the data in Fig. 4(c) for sample-A, the H induced
fractional increase in Pr defined as DPr/Pr(0) ¼ [Pr(H)
ÀP(H¼0)]/Pr (H¼0) ¼ 3.8%, and Ec also shows a 3.8%
increase under H ¼ 2.5 kOe. The data in Fig. 4(d) for
sample-C show similar H induced changes in Pr and Ec. But,
unlike the case for sample-A, both parameters decrease by
FIG. 4. (a) Static magnetization M as a function of field H for sample-A. (b)
Profiles of scattering parameter S21 vs frequency f for sample-A showing
absorption of microwave power due to FMR at 11 GHz. Profiles are for
static magnetic field H parallel or perpendicular to the plane of a disc of
as-assembled particles. (c) Polarization vs E for a sample-A for static mag-
netic field H ¼ 0 and 2.5 kOe. (d) Similar data P vs E data under H for a disc
of as-assembled core-shell particles with 100 nm BTO core-10 nm NFO shell
(sample-C).
052901-3 Sreenivasulu et al. Appl. Phys. Lett. 104, 052901 (2014)
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5. 4.9% and 2.9%, respectively. These observations could be
attributed to possible switch in the magnetostriction induced
strain on BTO, from compressive to tensile, when the core
and shell are interchanged in the assembled nanoparticles.
Theory predicted either a decrease or increase in Pr and Ec
depending on the nature of strain, compressive, or tensile,
and was confirmed experimentally in PZT films.32,33
The magneto-dielectric effect (MDE), i.e., the H
induced variation in the permittivity or dielectric resonance
frequency, was investigated at high frequencies, from 16 to
18 GHz.34
The measurements were done using an Agilent
Vector Network analyzer and (85071E) materials measure-
ment software. A precision quarter-wavelength waveguide
section was used as the sample holder. A sample of dimen-
sions 10.7 Â 0.3 Â 0.89 mm3
was prepared from dried
as-assembled powder, so as to completely fill the cross sec-
tion of the transmission line without gaps at the fixture walls.
A static field H was applied along the wide wall of wave-
guide. The permittivity was estimated from a two-port mea-
surement of transmission and reflection coefficients with the
sample mounted in the waveguide. The above mentioned
sample dimensions and frequency interval, 16–18 GHz, were
chosen in such a way that neither FMR nor dielectric reso-
nance were present in the sample. The variation in e with H
was recorded for H ¼ 0–4 kOe. The f-dependence of the real
part of the relative dielectric constant er
0
is shown in Fig.
5(a) for a series of H. The er
0
vs f data are for sample-A. A
general decrease in er
0
is observed with the application of H.
The fractional change in er
0
under H ¼ 4 kOe defined by
Der
0
/er
0
(0)¼[er
0
(H)Àer
0
(H¼0)]/er
0
(H¼0) as a function of f is
shown in Fig. 5(a). It ranges from À1% to À1.7%.
We also studied the low-frequency ME response of the
sample by subjecting a film of as-assembled composite to a
bias field H and an ac magnetic field dH. The film was pre-
pared by mixing 25 mg of the composite with 2 ml solution
of a binder polyvinyl acetate (PVA) and then spreading it
between two parallel electrodes with a separation t ¼ 3 mm
on a glass slide. The composite with the binder was allowed
to dry at room temperature that resulted in a 20 lm thick
film. Measurements of ME response were carried out by
applying H and dH (¼1 Oe at 30 Hz) parallel to each other
and perpendicular to the electrodes and by measuring voltage
dV across the electrodes with a lock-in amplifier. The
magneto-electric voltage coefficients MEVC ¼ dV/(t dH)
were measured as a function of H and the results are shown
in Fig. 5(b) for films of sample-A. One observes several fea-
tures including (i) a zero-bias MEVC, (ii) a 100% increase in
MEVC with increasing H, (iii) a decrease in MEVC when H
is reversed, and (iv) hysteresis and remnance in MEVC vs H.
The zero-bias ME effect and the decrease in MEVC upon re-
versal of H direction indicate the possible presence of a uni-
axial built-in magnetic field in the film. Modeling efforts are
underway to understand the origin of such a field and the
overall ME response in Fig. 5(b).
Finally, we compare the data in Figs. 4 and 5 with ME
coupling reported for similar ferromagnetic-ferroelectric com-
posites. Past reports on ME characterization of nanostructured
composites were primarily on magnetic force or piezo force
microscopy of individual nanostructures. The MEVC for the
as-assembled BTO-NFO in Fig. 5(b) is much smaller than
reported values for bulk and thick film NFO-PZT and similar
composites.3
The relatively small MEVC could be attributed
to smaller than expected magnetostriction and piezoelectric
coupling coefficient and a large leakage current in the nano-
composites. It is also of interest to compare the H-induced
polarization and changes in e in Figs. 4 and 5 with results for
single phase multiferroics. Induced polarization and MDE in
single phase multiferroics, in general, was reported to be on
the order of 1% or less in fields of several Tesla whereas the
present system shows changes in e and P in the range 1%–5%
for fields on the order of 4 kOe.1–3
In conclusion, we have self-assembled ferrite-ferroelec-
tric core-shell nanoparticles and studied the nature of ME
interactions in as-assembled composites. Barium titanate
and nickel ferrite nanoparticles were functionalized with
O-propargylcitrate and alkyl azide groups, respectively.
Linkages between dissimilar nanoparticles were achieved
using a “click” chemistry approach leading to heterogeneous
nanostructured composites. Evidence for magneto-electric
coupling was obtained through H-induced polarization,
magneto-dielectric effects over 16–18 GHz, and low fre-
quency ME effects. It is of importance to use the core-shell
particles as building blocks and assemble superstructures of
chains, rings, 2D and 3D structures for investigations of ME
coupling through scanning probe microscopy techniques and
microwave and millimeter measurement techniques.3,11,35,36
Such ME superstructures are also key requirements for prac-
tical applications.
The research was supported by a grant from the Army
Research Office (Grant No. W911NF1210545). The efforts
were also supported in part by grants from the National
Science Foundation (Nos. DMR-0902701, CHE-0748607,
CHE-0821487, REU-PHY-1062836, and MRI-ECCS-
1040304).
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