Controlled self-assembly of multiferroic core-shell nanoparticles exhibiting strongmagneto-electric effectsGollapudi 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 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/104/5?ver=pdfcov Published by the AIP Publishing

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Controlled self-assembly of multiferroic core-shell nanoparticles exhibitingstrong 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)

1Physics Department, Oakland University, Rochester, Michigan 48309-4401, USA2Radiophysics Department, Taras Shevchenko National University of Kyiv, Kyiv 01601, Ukraine3Chemistry 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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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 �OH 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 cm�1 due to overlap with

peaks already present in the unfunctionalized nanoparticles.

A peak at 2036 cm�1, 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 cm�1 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 core�50 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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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 H¼ 3046 Oe parallel or

H¼ 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

4pMeff¼ 4pM�Ha, 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 c¼ 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 4pM¼ 450 G that compares well with the esti-

mate of 4pMeff from the FMR data. The measured

c¼ 3.3 GHz/kOe (corresponding to g¼ 2.36) is somewhat

higher than single crystal value of g¼ 2.2 for nickel ferrite26

and close to g¼ 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 (DH¼ 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 H¼ 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)

�P(H¼0)]/Pr (H¼0)¼ 3.8%, and Ec also shows a 3.8%

increase under H¼ 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 H¼ 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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141.210.134.98 On: Mon, 03 Feb 2014 16:59:31

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� 0.3� 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 H¼ 0–4 kOe. The f-dependence of the real

part of the relative dielectric constant er0 is shown in Fig.

5(a) for a series of H. The er0 vs f data are for sample-A. A

general decrease in er0 is observed with the application of H.

The fractional change in er0 under H¼ 4 kOe defined by

Der0/er0(0)¼[er

0(H)�er0(H¼0)]/er

0 (H¼0) as a function of f is

shown in Fig. 5(a). It ranges from �1% to �1.7%.

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 t¼ 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 MEVC¼ 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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vs bias field H data for a film of sample-A.

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