Magnetoelectric effect of the multilayered CoFe2O4/BaTiO3composites fabricated by tape casting

Dongxiang Zhou Liangbin Hao Shuping Gong

Qiuyun Fu Fei Xue Gang Jian

Received: 13 January 2012 / Accepted: 24 March 2012 / Published online: 21 July 2012

Springer Science+Business Media, LLC 2012

Abstract This paper presents the structural, ferroelectric,

ferromagnetic, resonance and magnetoelectric (ME) prop-

erties of multilayered ME composites fabricated using tape

casting method. The compositions corresponding to

CoFe2O4 (CFO) with particle size of *150 nm andBaTiO3 (BTO) with particle size of *100 nm were chosenas ferromagnetic and ferroelectric phases, respectively.

Delamination was found at the interface between CFO and

BTO layers, which was related to the residual stress due to

the difference in thermal expansion coefficient between the

two layers. The largest direct magnetoelectric and converse

magnetoelectric coefficients of the multilayered ME com-

posite were, respectively, 36 lV/cm Oe at a bias magneticfield of 2,800 Oe and 1.16 9 10-3 G/V at a frequency of

30 kHz. In addition, the corresponding interfacial coupling

coefficient was calculated to be 3.2 9 10-5. For the mul-

tilayered ME composite, a resonance frequency of

4.96 MHz and a bandwidth of 40 kHz were obtained using

capacitance-frequency spectrum method.

1 Introduction

Magnetoelectric (ME) materials, due to their extrinsic ME

effect, have been focused increasingly for their potential

applications in inductors, sensors and filters [13]. The ME

effect is defined as an electric polarization response to an

applied external magnetic field, also called direct magne-

toelectric (DME) effect, or an induced magnetization

response to an applied external electric field, also called

converse magnetoelectric (CME) effect [4, 5]. In general,

the effect is quantitatively characterized by ME coefficient

[5]. The DME coefficient aE and CME coefficient aB couldbe described by the following expressions [5, 6]: aE = dE/(dHac) and aB = dB/(dV), where dE is the electric field,dHac is the amplitude of ac magnetic field, dV is the appliedvoltage, and dB is the magnetic induction. Up to date, thecoefficients for various ME materials have been reported

[7]. Among the ME materials, layered composites have

become a central issue, since they possess stronger ME

properties in comparison with the single-phase materials or

particulate composites [8, 9]. Most recently, the layered

ME composites are mainly fabricated by epoxy-bonding

method [8, 9] and deposition method [1012]. Neverthe-

less, these two methods have following drawbacks: (1) the

epoxy layer lessens the ME effect and results in aging [9,

13]; (2) the substrate clamping effect makes ME effect

weak [1416]. Alternatively, tape casting technique has

been used to obtain multilayered ME composites such as

0.2Pb(Zn1/3Nb2/3)-0.8Pb(Zr0.5Ti0.5)O3 (PZNT)/(Ni0.6Cu0.2Zn0.2) Fe2O3 (NCZF) and CoFe2O4 (CFO)/Pb(Zr0.52Ti0.48O3) (PZT) [6, 9]. Of the multilayers, however, little

literature has been published about the resonance fre-

quency and CME property of the multilayered ME com-

posite. Furthermore, the ME effect of the multilayered

CFO/BaTiO3 (BTO) composite is seldom investigated. In

this paper, we chose CFO and BTO as ferromagnetic and

piezoelectric phases, respectively. Multilayered CFO/BTO

composite was fabricated using tape casting method. The

structural, ferroelectric, ferromagnetic, resonance, DME

and CME properties of the composite were investigated in

detail.

D. Zhou L. Hao S. Gong Q. Fu (&) F. Xue G. JianDepartment of Electronic Science and Technology,

Huazhong University of Science and Technology,

1037 Luoyu Road, Hongshan District 430074,

Wuhan, Hubei, Peoples Republic of China

e-mail: fuqy@mail.hust.edu.cn

123

J Mater Sci: Mater Electron (2012) 23:20982103

DOI 10.1007/s10854-012-0706-9

2 Experimental

CFO powder was prepared through solid state reaction

method: raw materials of Co2O3 (AR, Aladdin Chemistry

Co., Ltd, Shanghai, China) and Fe2O3 (AR, Tianjin

Dengfeng Chemical Reagent Factory, Tianjin, China) were

mixed in molar ratios for 12 h, dried, sieved, and presin-

tered at 900 C for 2 h; then the synthesized particles wereball milled for 2 h, dried, and sieved to get a fine powder

with particle size of *150 nm. BTO powder with averageparticle size of *100 nm was commercially supplied(Hebei Kingway Chemical Industry Co., Ltd, Baoding,

China). The CFO and BTO powders were mixed with the

organic additives to prepare the slurries for tape casting. In

order to get the slurries, secondary ball milling and double

solvent additions methods were used. That is, the CFO and

BTO powders were mixed with solvents (ethyl alcohol and

trichloroethylene) and a dispersant (tributyl phosphate) in

ball mills for 3 h, then a plasticizer (dibutyl phthalate), the

foregoing solvents and a binder (polyvinyl butyral) were

added into the ball mills and milled for 3 h. After preparing

the CFO and BTO slurries, they were tape casted with

0.5 mm height of doctor blade. The tapes were dried at

temperature of 70 C for 10 min and alternately laminatedunder high pressure (56 Mpa) and temperature (50 C).The laminated green bodies were cut and heated at 280 Cfor burnout of organic components. Afterwards, the sin-

tering process was conducted at 1,250 C for 2 h with aheating rate of 1.7 C/min under a pressure of 3.5 kPa.Finally, electrical contacts were made with silver paste at

550 C for 15 min with a heating rate of 5 C/min, and thecomposites were polarized in silicon oil. To polarize the

composites, they were heated up to 140 C and cooleddown to room temperature under an electric field of

1.3 kV/mm and kept for 15 min at room temperature.

The microstructure and composition of the composite

were investigated by scanning electron microscopy (SEM)

and X-ray diffraction (XRD). The polarization hysteresis

loop was characterized by a ferroelectric test system

(Multiferroic, Radiant Technologies, Inc.). The piezoelec-

tric coefficient (d33, p) of the composite was measured by a

quasi-state d33 meter (ZJ-3A, Shanghai institute of acous-

tics). The magnetization of the composite was measured

using a vibrating sample magnetometer (VSM, Lakeshore

7400). The capacitance of the multilayered composite was

determined using an impedance analyzer (4294A, Agilent).

The DME effect of the multilayered composite was

investigated in transverse mode. A small ac magnetic filed

dHac with an amplitude of 12 Oe at a low frequency of1 kHz was generated by the Helmholtz coils driven by a

power amplifier (BP4610, NF, Yokohama, Japan). The

dHac superimposed onto a bias magnetic filed Hdc wasapplied parallel to the plane of the composite. The induced

voltage from the composite was measured with a digital

lock-in amplifier (SRS Inc., SR850, Sunnyvale, CA, USA)

under various bias magnetic field Hdc.

For CME effect measurement, the composite was placed

in a bias magnetic field Hdc = 1,000 Oe produced by an

electromagnet. A sine electric field with an amplitude of

3 V from a signal generator was applied to the sample.

Both directions of the magnetic and electric fields were

perpendicular to the surface of the sample. A search coil

around the sample was connected to an oscilloscope for

measuring the induced voltage due to the change of mag-

netic flux dB in the multilayered composite.

3 Results and discussion

Figure 1 shows the XRD pattern of the composite after

grinding into a powder form. The XRD pattern reveals

spinel structure CFO and perovskite structure BTO phases

without any impurity. After further analysis of the XRD

pattern, the tetragonal BTO (space group: P4 mm) and

cubic CFO (space group: Fd3 m) are found. Also the lattice

parameter for CFO is a = 8.3981 A, for BTO are

a = 4.0039 A and c = 4.0318 A. The micro-morphology

of the CFO/BTO composite is shown in Fig. 2. Figure 2a

displays the cross-sectional image of the composite, from

which it can be found that CFO and BTO layers are

alternately arranged. The thicknesses of CFO and BTO

layers are *50 and 65 lm except the middle layer,respectively. The microstructure of the interface between

CFO and BTO layers is shown in Fig. 2b. It is found that a

transition layer which is composed of interfacial delami-

nation exists at the interface. Figure 2c and d illustrate that

Fig. 1 XRD pattern of the multilayered CFO/BTO composite aftergrinding into a powder form

J Mater Sci: Mater Electron (2012) 23:20982103 2099

123

sintered BTO is much denser than CFO. And the particle

sizes of BTO and CFO phases are found to be, respectively,

*200 nm and 500 nm.The polarizationelectric field (PE) hysteresis loop of

the composite measured at room temperature is shown in

Fig. 3. The loop demonstrates typical ferroelectric char-

acteristic for the ME composite, in which a coercive field

(Ec) of 16 kV/cm is obtained. The remnant polarization

(Pr) of the loop is *0.4 lC/cm2, which is smaller than

those of both bulk BTO and CFO/BTO core-shell com-

posite [17]. In addition, the present CFO/BTO composite

exhibits a lower d33,p value of 16 pC/N.

Magnetization property of the ME composite measured

at room temperature with the applied magnetic field par-

allel to the plane of the composite (in-plane) is presented in

Fig. 4. Evident ferromagnetic property is observed. The

saturation magnetization (Ms), remnant magnetization (Mr)

and coercive field (Hc) are 91 emu/cm3, 43 emu/cm3 and

677 Oe, respectively.

ME effect will be enhanced significantly at resonance

frequency by contrast with the effect at nonresonance

frequencies [18], so the experiments of the ME effect are

often conducted near the resonance frequency [7, 18]. To

determine the resonance frequency of the layered ME

Fig. 2 a The cross-sectionalSEM image of the multilayered

CFO/BTO composite, b SEMimage of the interface between

CFO and BTO layers, c SEMimage of BTO layer, d SEMimage of CFO layer

Fig. 3 PE hysteresis loop of the multilayered CFO/BTO composite

2100 J Mater Sci: Mater Electron (2012) 23:20982103

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composite, the capacitance-frequency spectrum method has

been used [7]. For the multilayered CFO/BTO composite,

the capacitance as a function of frequency is shown in

Fig. 5. It shows that the electromechanical resonance fre-

quency is measured to be 4.96 MHz, which is twenty times

larger than that in PZT/CFO/PZT composite prepared by

using conventional ceramic processing [19]. To the best of

our knowledge, seldom report has been made on such high

resonance frequency in the range of electromechanical

resonance frequency. Also the antiresonance frequency is

measured to be 5 MHz. Therefore, the bandwidth of the

CFO/BTO composite is found to be 40 kHz, which is lar-

ger than that of the trilayered ME composite fabricated

using pressure assisted sintering [20].

In the process of measuring DME effect, an ac mag-

netic filed produced by a Helmholtz coil is needed [7].

Because of the impedance of the coil, the ME effect is

often investigated at low frequency [7]. Thus the fre-

quency of the ac magnetic filed is fixed at 1 kHz in this

study. Figure 6 shows the DME coefficient aE for theCFO/BTO composite. It is noteworthy that aE shows astrong dependence on bias magnetic field Hdc varying

from 27 to 7,200 Oe. In the range of 27400 Oe, aEdecreases with the increasing bias magnetic field. For

Hdc [ 400 Oe, aE first increases to a peak value under abias field of 2,800 Oe, then decreases with the increasing

bias magnetic field. Evidently the shape of aE curve issimilar to that of piezomagnetic coefficient of the BTO

films/CFO substrates as reported previously, since the Hdcdependence of ME coefficient aE tracks the Hdc depen-dence of the piezomagnetic coefficient [21]. Furthermore,

the maximal aE (Fig. 6) is 36 lV/cm Oe, and its corre-sponding induced voltage is 30 lV. Although the twovalues are comparable to those reported by Hrib et al.

[22] and Yang et al. [23], the maximal aE is two to threeorders of magnitude less than the value previously

reported in literature on the PZT/(Ni1xZnx)Fe2O4 thick-

film composite [24]. Besides, the maximal aE for theCFO/BTO composite is an order of magnitude smaller

than that for the NFO/BTO composite synthesized using

tape casting method [25]. This NFO/BTO composite

consists of 15 layers of BTO and 16 layers of CFO, and

its corresponding aE is estimated to be 0.8 mV/cm Oe intransverse mode [25]. Since the interfacial coupling

coefficient k is directly related to ME coefficient [9], and

the piezomagnetic coupling of CFO is larger than that of

NFO [6], it could be deduced that k of the CFO/BTO

composite should be much smaller than that of the

NFO/BTO composite. To confirm this, aE is given by[6, 25, 26]

Fig. 4 Magnetic hysteresis loop of the multilayered CFO/BTOcomposite

Fig. 5 Capacitance as a function of frequency for the multilayeredCFO/BTO composite

Fig. 6 DME coefficient aE as a function of bias magnetic field Hdcfor the multilayered CFO/BTO composite

J Mater Sci: Mater Electron (2012) 23:20982103 2101

123

where f is the volume fraction of the piezoelectric phase,pmsij are compliance coefficients for piezoelectric(magnetostrictive) phases, pd31 is the transverse piezo-

electric coefficient for piezoelectric phase, mqij is the

piezomagnetic coefficients, and peT33 is the effective per-mittivity. The material parameters for BTO, CFO and

NFO are listed elsewhere [6, 26], and f for CFO/BTO and

NFO/BTO composites are 0.66 and 0.48, respectively.

Substituting these corresponding parameters into Eq. (1),

the interfacial coupling coefficients k of the CFO/BTO

and NFO/BTO composites are calculated to be *3.2 910-5 and 2.3 9 10-3, respectively, demonstrating that the

result is consistent with our deduction.

As mentioned above, the DME coefficient aE and inter-facial coupling coefficient k of the multilayered CFO/BTO

composite are smaller than those of the NFO/BTO com-

posite. This could be attributed to two reasons. One is that

chemical reaction and interdiffusion at the interface may

degrade the ME properties [8]. Another is concerned with

residual stress generated at the interface of the multilayered

composite due to the difference in thermal expansion coef-

ficient (TEC) between the BTO and CFO phases. BTO has

the TECs of a1 = a2 = 15.7 9 10-6 K-1, and a3 = 6.4 9

10-6 K-1; and the TECs for CFO are a1 = a2 =a3 = 10 9 10

-6 K-1 [27]. Moreover, He [28] has reported

that the TEC of BTO is greater than 10 9 10-6 K-1 at a

temperature above 120 C (the Curie temperature of BTO).Therefore, when the multilayered composite is cooled down

after being sintered, residual stress will be produced at the

interface. According to [29], compressive residual stress

will be generated in CFO layers, while tensile residual stress

will appear in BTO layers. Although the compressive

residual stress in CFO layers is advantageous to piezomag-

netic coefficient and ME coupling effect [30], the residual

stress leads to cracking and delamination at the interface

[29]. Thus these defects could decrease the interfacial cou-

pling between BTO and CFO layers, thereby lowering the

DME effect. To clarify the existence of delamination, the

SEM image of the interface between BTO and CFO layers

was captured as shown in Fig. 2b. It shows that the delam-

ination does exist at the interface. Therefore, it could be

inferred that the interfacial delamination should be respon-

sible for the DME coefficient and interfacial coupling

coefficient of the CFO/BTO composite.

The frequency dependence of CME coefficient aB wasmeasured as shown in Fig. 7. The frequency is in the range

of 30200 kHz because of the limitation of the instruments.

The figure shows that, in general, aB decreases withincreasing frequency, thus it is expected that aB will beenhanced if the frequency is less than 30 kHz. The maxi-

mal value of aB here, 1.16 9 10-3 G/V at 30 kHz, is about

two orders of magnitude bigger than that of the CFO/BTO

coreshell composite [17]. This could be attributed to the

percolation and imperfect polarization in the coreshell

structure [17]. Furthermore, the largest aB for the multi-layered CFO/BTO composite is comparable to that for the

three-phase composite prepared by bonding piezoceramic,

metal cap and magnet [31]. Additionally, the phasic dif-

ference of the multilayered composite is larger than that of

PZT/Terfenol-D composite [7, 18].

Although the DME and CME effects exhibit reverse

characteristics [4, 5], both of the effects are achieved via

interfacial coupling in ME composites [9]. Consequently, a

low (high) interfacial coupling coefficient may result in

weak (strong) CME effect. As explained above, interfacial

coupling coefficient could be reduced owing to the inter-

facial interdiffusion and delamination. Therefore, the CME

coefficient could be further improved when the interfacial

coupling coefficient increases.

4 Conclusions

We have successfully fabricated multilayered CFO/BTO

composites by using tape casting method. XRD pattern

shows that no impurity other than CFO and BTO phases

Fig. 7 CME coefficient aB together with phasic difference asfunctions of frequency for the multilayered CFO/BTO composite

aE kf 1 f pd31mq11 mq21

ms11 ms21peT33kf ps11 ps21peT331 f 2pd312k1 f 1

2102 J Mater Sci: Mater Electron (2012) 23:20982103

123

exists. The SEM images illustrate that the CFO and BTO

layers are alternately arranged but with delamination at the

interface. The coexistence of ferroelectric and ferromag-

netic properties indicates that the multilayered composite is

magnetoelectric. The DME, CME and interfacial coupling

coefficients are found to be 36 lV/cm Oe under the biasmagnetic field of 2,800 Oe, 1.16 9 10-3 G/V at a fre-

quency of 30 kHz and 3.2 9 10-5, respectively. The inter-

facial coupling coefficient is influenced by, in addition to the

interfacial interdiffusion of the CFO/BTO composite, the

interfacial delamination caused by residual stress due to the

difference in TEC between BTO and CFO layers. Conse-

quently, it is concluded that the ME effect will be enhanced

if the interfacial property is optimized. Besides, the elec-

tromechanical resonance frequency of the multilayered

composite is measured to be 4.96 MHz, which may provide

the possibility of designing high frequency ME devices.

Acknowledgments The authors would like to thank National Nat-ural Science Foundation of China (Grant No. 60871017/f010612) and

Provincial Nature Science Foundation of Hubei in China. The authors

also acknowledge the support of Shaanxi Normal University and

Tsinghua University for the CME and DME coefficients measure-

ment, respectively.

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Magnetoelectric effect of the multilayered CoFe2O4/BaTiO3 composites fabricated by tape castingAbstractIntroductionExperimentalResults and discussionConclusionsAcknowledgmentsReferences