Giant magnetoelectric coupling effect in lead-free

Journal of Alloys and Compounds 712 (2017) 256e262

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Giant magnetoelectric coupling effect in lead-free perovskite BiFeO3/ Na0.5Bi4.5Ti4O15 composite films Jieyu Chen, Zhehong Tang, Shifeng Zhao* School of Physical Science and Technology, & Inner Mongolia Key Lab of Nanoscience and Nanotechnology, Inner Mongolia University, Hohhot, 010021, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 13 March 2017 Received in revised form 7 April 2017 Accepted 11 April 2017 Available online 12 April 2017

Lead-free BiFeO3/Na0.5Bi4.5Ti4O15 composite films were deposited on Pt(100)/Ti/SiO2/Si substrates using chemical solution deposition. The surface, phase and electric domain structure, as well as ferroelectric, leakage, piezoelectric, magnetic and magnetoelectric properties were investigated systematically. The good ferroelectric, leakage properties, piezoelectric and magnetic properties were obtained in composite films. More importantly, a giant magnetoelectric voltage coefficient is observed in BiFeO3/Na0.5Bi4.5Ti4O15 composite films with the maximum aE ¼ 136 mV/cm$Oe at Hbias ¼ 8.0 kOe, which is comparable with lead-based composites films. Such giant magnetoelectric effect is derived from good single-phase characteristics and similar crystal structure between BiFeO3 and Na0.5Bi4.5Ti4O15 film layers. The present work provides a feasible way of preparing magnetoelectric composite films and facilitating their applications in micro-electro-mechanical system and information storage devices and weak magnetic field detection. © 2017 Elsevier B.V. All rights reserved.

Keywords: Lead-free Composite films Piezoelectric properties Magnetoelectric effect

1. Introduction The magnetoelectric (ME) effect is defined as the induced dielectric polarization under an applied magnetic field or as the induced magnetization in the presence of an applied electric field [1]. The single-phase magnetoelectric materials are small amount of existing in nature without considerable amount of polarization and magnetization at room temperature, which possess a weak magnetoelectric (ME) coefficient at or above room temperature, making them inadequate for practical applications [2]. Alternatively and with great design flexibility, ME composites combining ferroelectric and ferromagnetic phases together have drawn significant interest in recent years [3]. The ME effect of the composite materials originates from the strain/stress, i.e., the strain induced in one constituent (either by magnetostriction in ferromagnetic phase or by converse piezoelectric effect in ferroelectric phase) is transferred to the others and alters its polarization or magnetization. There are several kinds of multiferroic heterostructures being investigated, which mainly includes three types. The first is (0e3) nanocomposites, which requires rather complicated process and is

* Corresponding author. E-mail address: [email protected] (S. Zhao). http://dx.doi.org/10.1016/j.jallcom.2017.04.093 0925-8388/© 2017 Elsevier B.V. All rights reserved.

difficult to choose component distribution due to the self-assembly nature [4]. The second is (1e3) columnar films, which suffer from large leakage due to the low resistivity of the constitutional magnetic phase or conductive interfaces [5e8]. The third is (2-2) type multilayers, which have been the most well investigated ME materials [9e13]. Nonetheless the ME response of these multilayers is limited to quite a low level due to the clamping from the stiff substrates [14]. The (2-2) type multilayers widely exist the relatively large substrate clamping effect, which will seriously reduce the magnetoelectric output in the thin films [15]. While the thickness of the film is relatively thicker, the substrate clamping effect could be partially or totally suppressed [16,17]. The traditional magnetoelectric composite films use lead-based materials as piezoelectric phases, which present well magnetoelectric coupling effect [18e21]. However, they are very toxic to the kidney, brain, and nervous system due to containing more than 60 wt% lead. Looking for new lead-free magnetoelectric composite films is imminent. Up to date, the lead-free layered perovskite materials has achieved relatively ideal results, in which, multiferroic materials are used as ferromagnetic phase [e.g., BiFeO3; Bi5Ti3FeO15] and layered perovskite materials as piezoelectric phases [e.g., Bi4Ti3O12; Bi0.5(Na0.85K0.15)0.5TiO3] [22e25]. The mutual coupling between layered perovskite materials has formed a good coupling effect. So it is attractive to find lead-free based

J. Chen et al. / Journal of Alloys and Compounds 712 (2017) 256e262

magnetoelectric composite films with more outstanding magnetoelectric properties. The Aurivillius phase Na0.5Bi4.5Ti4O15 (NBT), belonging to the family of bismuth layer-structured ferroelectrics, has received considerable interests because of its unique piezoelectric, electromechanical and ferroelectric properties [26]. Specifically, NBT was proposed to be an excellent replacement for the toxic Pb(Zr,Ti)O3, which is currently being extensively used in ferroelectric and piezoelectric devices [27]. As a piezoelectric phase, NBTO can provide fascinating piezoelectric properties helpful to obtain good magnetoelectric output. BiFeO3 (BFO) is multiferroic materials coexisting of ferroelectric and antiferromagnetic orders at room temperature, which has attracted interest because of their potential magnetoelectric coupling behaviors [28,29]. However, the weaker magnetoelectric effect with firstorder and second-order ME coefficients of 10e20 mV/cm.Oe does not satisfy the actual demands on magnetoelectric performance [30]. Therefore, bismuth based magnetoelectric composite films with similar structure (perovskite structure) are promised to obtain strong magnetoelectric effect. NBT and BFO respectively as ferroelectric phase and ferromagnetic phase form magnetoelectric composite films, which respectively have excellent piezoelectric performance and piezomagnetic performance. More importantly, BFO/NBT bilayer composite films are expected to obtain giant magnetoelectric effect due to the excellent crystal lattice matching, which makes lead-free magnetoelectric composite materials to replace lead-based magnetoelectric composite materials becoming a possibility. This work aims to prepare BFO/BTFO magnetoelectric composite films. The ferroelectric, piezoelectric, leakage, magnetic properties as well as giant magnetoelectric coupling effect are investigated. The origins of the giant magnetoelectric effect are discussed in detail. 2. Experimental procedure BFO/NBT composite films were prepared by chemical solution deposition and deposited on Pt/Ti/SiO2/Si(100) substrates. For preparing NBT solution, bismuth nitrate pentahydrate [Bi(NO3)3$5H2O], sodium nitrate [NaNO3] and butyl titanate [CH3(CH2)3O]4Ti were used as starting materials. Bismuth nitrate pentahydrate and sodium nitrate were added to the ethylene glycol solvent in proportions, with an excess 10% Bi and an excess 5% Na to compensate for its loss during annealing. Until the solution is clear transparent via stir, this precursor solutions was titrated to [CH3(CH2)3O]4Ti and C5H8O2 precursor solution to obtain NBT solution. The preparation method of the BiFeO3 precursor solutions are discussed in our previous works [31]. Firstly, NBT films were deposited on Pt/Ti/SiO2/Si(100) substrates by spin coating using NBTO precursor solutions onto the wafers at 4000 rpm for 30 s. Then the deposited films dried at 280 C in a thermostatic oven for 300s and annealed at 710 C for 5 min under an oxygen atmosphere with a flow of 1.5 L/min, respectively. The deposited films annealed at 710 C for 30 min after above steps for ten times. Secondly, BFO layers were deposited by spin coating BFO precursor solutions onto NBT/Pt/Ti/SiO2/Si(100) wafers at 3500 rpm for 30 s, Then the deposited films were dried at 280 C in a thermostatic oven for 300 s and annealed at 550 C for 5 min under an oxygen atmosphere with a flow of 1.5 L/min. The composite films were annealed at 550 C for 30 min after the above steps for 10 times. Such BFO/ NBT composite films were successfully prepared using this method. The X-ray diffraction (XRD) patterns were obtained by X-ray diffraction (XRD, Panalytical Empyrean) on a D/MAX-RA diffractometer using CuKa radiation. The microstructure and film thickness examinations for BFO/NBT composites films were conducted by scanning electron microscopy (SEM, Hitachi SU-3500) with an operating voltage at 15 kV. BFO/NBT composite films capacitors

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were fabricated using ion-sputtering Au top electrodes with a diameter of 0.2 mm on the surface of the films. The ferroelectric hysteresis loops and leakage properties were obtained by a multiferroic tester system (MultiFerroic100V, Radient Technology, USA). The domain structure and piezoelectric coefficients d33 were investigated using a piezoresponse force microscopy (PFM, Asylum Research CypherTM) with out-of-plane. Dielectric constant and dielectric loss were obtained by a precision impedance analyzer (Agilent E4990A). The magnetic properties were measured by a physical property measurement system (PPMS, Quantum Design). For the magnetoelectric coupling measurement of BFO/NBT composite films, a magnetic bias field Hbias together with a small alternating magnetic field Hac ¼ 8.53 Oe and frequency f ¼ 500 Hz was applied perpendicular to the film plane. The induced magnetoelectric voltage VME was recorded by a lock-in amplifier (SRS Inc., SR830). Above is all the measurements at room temperature.

3. Results and discussion 3.1. Structure investigations The XRD pattern of BFO/NBTO bilayer composite films sample shown in Fig. 1 clearly indicate the formation of the positions of all the indexed diffraction peaks for BFO and NBT films are in good agreement with that of the standard diffraction peaks (ICDD card No.01-071-2494, and PDF#52e1640), where the sharpness of the features indicates a high quality polycrystalline films. Importantly, All peaks of the composite films are indexed to a rhombohedral structure belong to the space group R3C [31] of BFO films and an orthorhombic structure belong to the space group [A21am] of NBT films in agreement with literature [32]. It can be seen that two kind of films are well crystallized and without preferred orientations and structural distortion. At the same time, without any chemical reaction between the two phases during the annealing process. The inset of Fig. 1 further reveals the cross-sectional SEM image of BFO/NBTO composite films. It is shown that the interface between BFO and NBTO layers is clear and no intermediate layer is observed. The thickness of BFO film layer is 879 nm and that of NBT film layer is 545 nm. Therefore, both single-phase BFO and NBT films can coexist without interface diffusion in the composite films. Such results agree with the XRD patterns very well.

Fig. 1. X-ray diffraction (XRD) patterns of BFO/NBTO composite films, the inset is the cross-sectional SEM image of the composite films.

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3.2. Ferroelectric domains Fig. 2 shows the surface morphology, amplitude and phase images of the as-grown BFO/NBT composite films simultaneously obtained using piezoresponse force microscopy (PFM). Fig. 2(a) gives the topographic images of the samples with 1.5 1.5 mm2 scanning area of BFO/NBTO composite films. The average grain size of BFO/NBT films is approximately 200 nm. Moreover, all grains of the films present a dense rod-shaped particles and the prominency of the film within 10 nm was observed from the chromatic aberration, which also suggests that the films has the smooth, crack-free surface morphology with well-distributed fine grains smooth surface, distinct grain boundaries and uniform grains. Fig. 2(b) and (c) show the PFM amplitude and phase images, in which different colors represent different response intensities and local polarization orientations, respectively. As can be seen, BFO/NBTO composite films have clear domain structures that exhibit a fractal growth habit with a great deal of domain boundaries coinciding with grain boundaries, which suggests that the grain boundaries confine the shape of domain boundaries and affect the domain structures [33]. The original amplitude images show that most particles of films of amplitude focused on purple area of color bar, which suggest the amplitude of films is about 0 nm in initial state. Meanwhile, The initial phase images is somewhat out of order, which suggests the domains with variety of orientation existing in BFO/NBTO films. Another, it has been proved that the large grain endows a poly domain configuration while only mono domains are found if the grain size becomes smaller [34]. Since the present BFO/ NBTO composite films possess larger domain size and less density of the domain walls, it is expected to obtain a lower leakage because the certain domain walls in BFO/NBTO composite films are much more conductive than the domains themselves [35]. Fig. 2(d), (e) and (f) show the domain switching by applying þ15 V DC bias voltage on the tip in the same region with the positive bias being applied on the probe, which respectively are the images of the topography, amplitude and phase. Compared the morphologies of (a) with (d), there is little evidence of specimen damage according to the nearly identical topography images. The surface remains its original state and the prominency of surface is not obvious change through comparison the color bar, which all

prove that surface is not damaged through applying þ15 V tip voltage. But the domain structures appear obvious change as shown in Fig. 2(e) and (f). The amplitude signal presents an obvious change comparison to the initial state, which suggest that a strong electrostriction appear on the surface by applying þ15 V tip voltage and good piezoelectric properties for the composite films. At the meanwhile, the phase signal also shows obvious difference from the initial state. Most of these domains switch to the consistent orientation after applying 15 V DC bias. Comparing Fig. 2(c) with (f) is more clearly presents the difference in PFM phase contrast between these two extreme conditions (0 and þ 15 V). For the domain switching, on one hand, powerful amplitude signal and phase contrast in Fig. 2(e) and (f) indicate most completely switching because piezoresponse signal is rather uniform within the grains. On the other hand, the polarization direction of most domains apparently switches while minor domains have not switched after poling, which is attributed to the following three reasons. Firstly, the mobile charged carriers move to the grain boundaries after poling, which results in the domain wall pinning and impeding domain switching [36,37]. Secondly, the domain was likely to be restricted by the strong strain conditions or some other configuration reasons, such as the structural incompatibilityinduced local stress across the grain boundary. Thirdly, grain boundary-like trenches are likely to reduce the mismatch of elastic distortions between individual domains, which limits the switching of the domains [38]. Nonetheless, the unknown but absolutely subsistent impurities would also be one of the non-negligible reasons. Under a large dc field, all the domains are supposed to be poled in one direction. However, the impurities are unable or very difficult to be reoriented by external field due to their immobile nature [38]. In some regions, the domains are restricted by any of these factors owned a higher ferroelectric coercive force, þ15 V was not sufficient to reverse all the ferroelectric domains. Therefore, ferroelectric and leakage properties of BFO/NBTO composite films can be predicted by the images of the topography, amplitude and phase. 3.3. Electric properties The ferroelectric polarization(P)-electric field (E) hysteresis

Fig. 2. (a) The typical surface topography, (b) PFM amplitude, (c) phase images with 1.5 1.5 mm2 scanning area of the as-grown BFO/NBTO composite films; (d) The typical surface topography, (e) PFM amplitude, (f) phase images of BFO/NBTO composite films after poling procedure with þ15 V tip electric bias.


loops of BFO/NBTO composite films at room temperature were shown in Fig. 3(a). It is shown that the hysteresis loops are up to standard at all electric field and do not show any remarkable asymmetric behavior and serious polarization degradation when subjected to symmetric bipolar voltage pulses. The remnant polarization (Pr), saturation polarization(Ps), and coercive fields (2Ec) at an applied field of 694 kV cm1 are 77.4 mC/cm2, 124.09 mC/cm2, and 480 kV/cm, respectively. Thus results show that BFO/NBTO composite films obtain more excellent ferroelectric properties compared with the single-phase BFO and NBTO films [31,32]. Such outstanding ferroelectric properties are attributed to the welldefined domain structure and domain switching under applying external electric field as discussed above. Besides, It was closely related to the decreased leakage current behaviors derived from the depression of the oxygen vacancies. Fig. 3(b) shows leakage current densities versus applied electric fields (J-E) for BFO/NBTO composite films. The measured leakage current density of BFO/NBTO composite films is 2.35 104 A/cm2 at an applied electric field of 421 kV/cm. It suggests that the leakage properties are improved obviously comparing with that of the pure BFO films or NBTO films [31,32], which are mainly attributed to the factors as follows. First is no channel for carriers moving cross the interface due to a potential barrier in the interface without interface diffusion. It is necessary for carriers to cross more potential barrier to reach the bottom or top electrode, therefore, the density of carrier reduces. Secondly, the good leakage current properties are related the leakage conduction mechanism. Inset of Fig. 3(b) plots the curves of log(J) versus log(E), which can definitely determine the nature of the conduction mechanism. A linearity of the curves is observed over the entire region of the applied electric field for BFO/NBTO composite films. The curves can be fitted well by linear segments with different slopes. At all electric field region, the slope is close to 2, indicating a linear SCLC conduction is dominated, which arises from the density of free carriers due to the carrier injection, becoming greater than the density of the volume-generated free carriers and was follows the SCLC current law [31], The solo SCLC conduction mechanism can make BFO/NBTO composite films obtain well leakage properties. Hence, the excellent leakage properties of BFO/NBTO composite films were demonstrated. In addition, the excellent ferroelectric and leakage performance is also associated with the dielectric properties of BFO/NBTO composite films, which is discussed as follows. The frequency dependent dielectric properties of BFO/NBTO composite films were measured in the frequency range of 102 Hze106 Hz, which are shown in Fig. 4 The high values of the dielectric constant 725 is obtain, which is attributed to the denser microstructure and larger grain size as shown in Fig. 2(a) Especially, the larger grain size usually results in larger polarization and higher

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Fig. 4. The relative dielectric constant (εr), the inset is the dissipation factor (tand) vs. frequency of BFO/NBTO composite films.

dielectric constant. The change of the dielectric constant is not obvious in the frequency range 100 Hz to 1 MHz, which is mainly due that the lack of the ability of dipoles not follow the a quick flip of the electric field. At low frequency the hopping of electron between Fe2þ and Fe3þ ion or Ti3þ and Ti4þ does not follow the applied electric field and hence does not contribute to dielectric constant [39]. At higher frequencies above 1 MHz, the decrease of the dielectric constant is attributed to the space charge polarization due to the presence of the oxygen vacancies [40]. The Inset of Fig. 4 shows the lower dielectric loss (tand) for BFO/NBTO composite films with a proximate value of and 0.0205, at 1 kHz. The low dielectric loss were observed in the present BFO/NBTO composite films, which is helpful to obtain a good ferroelectric and piezoelectric properties. The low dielectric loss for the films is well correlated with the low leakage current density. It has been widely proved that there is a strong correlation between dielectric loss and space charge components, such as oxygen vacancies. The large space charge components lead to a large dielectric loss in ferroelectric films. Since BFO/NBTO composite films obtain a good leakage features, it is not difficult to understand the lower tand. Another, a variety of factors determine of tand of BFO/NBTO composite films such as domain wall pinning, interfacial diffusion between the film and bottom electrode, space charge accumulation at grain boundaries [41]. The lower tand values may relate domain switching. Besides, a quick flip of the electric field led to sharply increase the tand values at high frequency, higher than 1 MHz,

Fig. 3. (a) The polarization vs. electric field (P-E) hysteresis loops, (b) leakage current of BFO/NBTO composite films; Inset of (b) plots the curves of lg(J) versus lg(E).

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Fig. 5. The applied voltage dependence of the piezoelectric response for BFO/NBTO composite films.

which is mainly due to the dipoles in BFO/NBTO composite films cannot follow rapid change of the electric field. The good dielectric properties are good for BFO/NBTO composite films obtaining an excellent electrical properties. Fig. 5 shows the curves of piezoelectric coefficient (d33) and surface displacement (d) vs. applied voltage (V) for BFO/NBTO composite films. The displacement was achieved by keeping the PFM tip fixed above the interesting point and applying a DC voltage from 15 to þ15 V while recording the piezo-response signals. BFO/NBTO composite films exhibit well-defined piezo-response. As observed in Fig. 5, a typical well-shaped displacement-voltage (DV) ‘butterfly’ curve is obtained with a displacement maximum of 4.06 nm appearing at 15 V, which shows a strain as high as 2.63‰ ratio. A piezoelectric hysteresis loop (d33-V) is calculated from the D-V curve based on the law of converse piezoelectric effect, with considering the unexpected shift of the D-V curve intersection from the origin [42]. The d33-V loop clearly shows that BFO/NBTO composite films are switchable and ferroelectricity is retained. The piezoelectric coefficient d33 of BFO/NBTO composite films derived from the slope of the displacement versus voltage curve is as high as 285 pm/V at 20 V, which suggests the strong piezoelectric effect for BFO/NBTO composite films. Such strong piezoelectric properties are attributed to the larger spontaneous polarization for the composite films. To be specific, the piezoelectric effect can be described by the following equation: [43]

d33 ¼ 2Qeff εP

Fig. 6. Magnetic hysteresis loops of BFO/NBTO composite films at room temperature.

3.4. Magnetic properties Fig. 6 shows the magnetic hysteresis loops of BFO/NBTO composite films measured with the magnetic fields up to 30 kOe applied parallel to the film plane (in plane). The well-defined magnetic hysteresis loops reveal well ferromagnetic properties for the composite films. Interestingly, a good magnetic hysteresis loop with a relatively strong saturation magnetization Ms of ~4.68 emu/cc observed for BFO/NBTO composite films, which are compare with the pure BFO films [23]. Meanwhile, The remnant magnetization (Mr) values of BFO/NBTO composite films were 0.3 emu/cc, and coercive fields are only Hc~52 Oe. Thus excellent magnetic properties are mainly originated from the strong ferromagnetic the film layer of BFO rather than that of NBTO layer. Ferromagnetic properties of BFO mainly originate from the rotation of the FeO6 octahedra and the polar displacements between the anion and cation sublattice [44]. And the local ferromagnetic Fe-O clusters and the existence of Feþ2 ions caused by the valence fluctuation (Feþ2/Feþ3) are also responsible for the ferromagnetic properties [45]. 3.5. Magnetoelectric investigations Fig. 7 shows the magnetoelectric voltage coefficient aE as a

(1)

where Qeff is the effective electrostrictive coefficient, P and ε are spontaneous polarization and permittivity, respectively. It is important determining d33 with P, ε and Qeff. Firstly, the high d33 of BFO/NBTO composite films is due to the large spontaneous polarization and the accompanying high permittivity, as discussed above, larger spontaneous polarization and larger Pr and Ps are obtained due to the well-defined domain switching. Secondly, It was observed obvious electrostrictive behaviors when applied an electric bias on the films, as shown in Fig. 2(e), which larger effective electrostrictive coefficient Qeff is beneficial to the large value of d33. Thirdly, higher relative dielectric constant (εr) and lower loss (tand) are beneficial for the piezoelectric coefficient d33. What is more, for the present BFO/NBTO composite films with the thickness of 1.4 mm, comparable with the ceramics, the influence of substrate clamping effect can be effectively be reduced, which makes the larger piezoelectric coefficient is reasonable.

Fig. 7. The magnetoelectric voltage coefficient as a function of bias magnetic field Hbias for BFO/NBTO composite films.


function of bias magnetic field (Hbias) for BFO/NBTO composite films with Hbias and dHbias applied perpendicular to film surface (inplane measurement mode), which is defined as aE ¼ dE/dH ¼ dV/ (t$dH), where t is the thickness of film at an AC magnetic field from 0Oe to 8000Oe. For the magnetoelectric coupling measurement of BFO/NBTO composite films, a magnetic bias field Hbias together with a small alternating magneticfield Hac ¼ 8.53 Oe and frequency f ¼ 500 Hz was applied perpendicular to the film plane. The aE value sharply increases with increase of the magnetic field, and then it increases slowly and reaches the maximum value. The maximum magnetoelectric voltage coefficient is as high as 136 mV/cm$Oe at 8000 Oe, which is larger than that of the reported lead-free composite film and comparable with the lead-based magnetoelectric films [17,20]. So giant magnetoelectric coupling effect is attributed the outstanding multiferroic properties. The magnetoelectric coefficient is expressed by the corresponding equations,

12 kd31 qm 11 vm p p p 2 m sm 11 þ s12 ε33 vp þ s11 þ s12 ε33 vm 2 d31 vm

mismatch and interface coupling effect between NBTO as piezoelectric phase and BFO as piezomagnetic phase. It obtain excellent ME effect for the composite films. A strong magnetoelectric response is observed in BFO/NBTO composite films, reaching the maximum aE ¼ 136 mV/cm$Oe at Hbias ¼ 8.0 kOe, which is comparable with lead-based composites films. The present work suggests an ideal avenue to prepare the magnetoelectric composite films, which facilitates their applications in memory devices, spin devices, and sensors. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 11564028, 11264026), and Inner Mongolia Science Foundation for Distinguished Young Scholars (Grant No. 2014JQ01). References

p

aE;31 ¼

261

(2)

where superscript m and p denotes magnetic and piezoelectric phase, and k, s, ε, d, q, v denotes coupling coefficient, elastic compliance constant, dielectric constant, piezoelectric coefficient, piezo-magnetic coefficient. It shows that aE,31 is proportional to the piezomagnetic coefficient q. The excellent domain flip feature, leakage characteristic and electrostrictive phenomenon exists in BFO/NBTO composite films, which can form excellent ferroelectric and piezoelectric properties. The excellent ferroelectricity and piezoelectricity is obtained, which suggest that composite films has a large q. The well dielectric properties make films obtain a relatively large dielectric constant. According Equation (1), the good electrical properties, large dielectric constant and well coupling effect between BFO phase and NBTO phase may produce a large magnetoelectric effect. NBTO and BFO has the similar structure (perovskite structure), which can make them form well coupling effect. The strain can be a good delivery through the interface, which can make piezoelectric phase and piezomagnetic phase form the perfect combination. From what has been discussed above, similar structure of BFO and NBTO can form a good coupling through interface. The good coupling of BFO and NBTO has good single features, good electrical properties and magnetic properties, which can make BFO/NBTO composite films gain a large ME effect. Meanwhile, as a kind of single-phase magnetoelectric materials, BFO also plays an important effect on the total magnetoelectric effect of BFO/NBTO composite films. The total magnetoelectric effect including two parts. First is BFO layer as piezomagnetic phase and NBTO as piezoelectric phase to obtain compound magnetoelectric effect. The other part of magnetoelectric effect comes from the contribution of single-phase multiferroic materials of BFO. The former originates the interface coupling between BFO and NBTO films, which is responsible for the magnetoelectric effect. The latter is caused by the intrinsic ferroelectricity and weak ferromagnetism as a spontaneous magnetoelectric effect. It occurs only for fractional wavenumbers and relates to secondary atomic displacements induced by an harmonic coupling to the magnetoelastic lattice modulation [46]. Therefore, the giant magnetoelectric effect is reasonable. 4. Conclusions In summary, lead-free BFO/NBTO composite films were prepared by chemical solution. BFO and NBTO film layer have similar structure (perovskite structure), which can form good lattice

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