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Bi0.5(Na0.85K0.15)0.5TiO3 multiferroic composite films
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Magnetoelectric coupling effect of Ga0.8Fe1.2O3/Bi0.5(Na0.85K0.15)0.5TiO3 multiferroic composite films Ruonan Tian, Jieyu Chen, Qingshan Lu, Shifeng Zhao
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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 BS T RAC T
Keywords: Lead-free Magnetoelectric coupling Multiferroic Composite films
Lead-free Ga0.8Fe1.2O3/Bi0.5(K0.15Na0.85)0.5TiO3 (GFO/BKNT) bilayer multiferroic composite films were fabricated on Pt(100)/Ti/SiO2/Si substrates via sol-gel methods. The microstructure, domain structure, ferroelectric, piezoelectric, magnetic properties as well as magnetoelectric coupling effect were investigated for the composite films at room temperature. Well-defined interfaces between GFO and BKNT layers and clear electric domain structures are observed. A strong magentoelectric effect is obtained with magnetoelectric voltage coefficient of αE=30.89 mV/cm Oe, which is attributed to excellent ferroelectric, piezoelectric, and magnetic properties, as well as the coupling interaction between ferromagnetic GFO and ferroelectric BKNT phases for lead-free bilayer composite films. Besides, GFO and BKNT demonstrate the similar perovskite structure with well lattice matching, which endows the outstanding coupling and fascinating magnetoelectric properties. The present work opens up the opportunity of lead-free magnetoelectric composite films for both further fundamental studies and practical device applications such as sensors, transducers and multistate memories.
1. Introduction The magnetoelectric (ME) materials consisting of piezoelectric and magnetostrictive phases have attracted research interest in recent years in view of their potential applications in various electronic devices including sensors, actuators and multistate memories [1,2]. In general, ME materials are broadly classified into two primary categories, including single phase and composite systems, respectively. However, the coexisting simultaneously magnetic ordering and electric ordering in single phase ME materials are rare and typically exhibit a weak ME effect in nature, which have been synthesized in the laboratory such as BiMnO3, BiFeO3, TbM2O5, in which less of the materials have ferromagnetic or ferrimagnetic properties, yet most of the materials are antiferromagnetism, such as BiFeO3 [3–5]. These ME materials also possess low ferroelectric and anti/ferromagnetic transition temperatures far below room temperature, which is not appropriate for practical applications [6,7]. Therefore, in order to improve the weak ME effect in single phase ME materials, it is particularly urgent to study alternatively artificial ME composites. Compared with the single phase ME materials, composites exhibit the strong ME effect by excellent coupling combining a ferroelectric phase and a ferromagnetic phase at room temperature [8,9]. ME composite materials have opened
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the door to practical devices allowing for magnetic field tuning of electrical properties, which has led to breakthroughs in applications such as non-volatile memory, energy harvesting, magnetic field sensing, and potentially many other devices [10–12]. Nevertheless, surpassing ME effect of composite materials depends on prominent ferroelectric and ferromagnetic phase. So far, many investigations have been performed on ME effect in composites, which combine perovskite ferroelectric oxides e.g., PbZr0.52Ti0.48O3 (PZT) with ferromagnetic materials e.g., (CoFe2O4, R-Fe) [13–15]. It is well known that large volatility of toxic lead fumes causes critical environmental problems during processing, which makes PZT compounds dangerous to handle. To address those issues, considerable ideas have been proposed by the development of lead-free ME materials with excellent single phase properties. The composite multiferroic materials with ME effect can also be classified as multi-layered films, bulk composites and so on. Compared with bulk ME composites, multiferroic composite films have unique superiorities such as the facile modulation of their thickness, composition, connectivity, and orientation at nanoscale, in which different phases could be combined at atomic-level, and by precise control of the lattice matching, promoting understanding of ME coupling at atomic scale [16,17]. From the above investigation, restrictions on using lead materials have motivated search for lead-
Corresponding author. E-mail address: [email protected] (S. Zhao).
http://dx.doi.org/10.1016/j.ceramint.2017.05.065 Received 10 April 2017; Received in revised form 7 May 2017; Accepted 9 May 2017 0272-8842/ © 2017 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Tian, R., Ceramics International (2017), http://dx.doi.org/10.1016/j.ceramint.2017.05.065
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free ME composite films. Bi0.5(K0.15Na0.85)0.5TiO3 (BKNT) with well ferroelectric, piezoelectric and leakage properties, which can be used in ME composite films as a piezoelectric phase, is considered to be an excellent candidate to replace lead-based piezoelectric films [18]. From a structure perspective, excellent piezoelectric properties of BKNT films come from the rhombohedral and tetragonal morphotropic phase boundary, which as a piezoelectric phase can provide good piezoelectric properties for magnetoelectric composite films. Recently, GaFeO3 with non-centrosymmetric orthorhombic structure (point group Pc 21 n ) is one of the few single-phase multiferroics that show excellent magnetic behavior and magnetostrictive properties [19]. Thus, a good strategy is to make Ga0.8Fe1.2O3 (GFO) as the ferromagnetic phase and BKNT as the piezoelectric phase, respectively. This type of coupling could lead to an optimizing microstructure and provide the outstanding magneticmechanical-electric coupling with strain (stress) conduction. Besides, GFO and BKNT have similar perovskite structures and good interface connecting. Therefore, GFO/BKNT lead-free bilayer composite films are expected to display the strong ME properties, which provides a feasible method for application of lead-free of magnetoelectric composite films and following-up studies. The present work investigate not only the ferroelectric and ferromagnetic properties, but also the magnetoelectric (ME) coupling effect for GFO/BKNT lead-free composite films synthesized by sol-gel methods. Simultaneously, the corresponding processing of this study is discussed in detail.
Fig. 1. XRD patterns of GFO/BKNT films, the inset is the cross-sectional SEM image of GFO/BKNT composite films.
GFO/BKNT composite films was characterized by SU-4800 scanning electron microscope with 10 kV voltage. Their ferroelectric hysteresis loops and leakage current density were recorded by the MultiFerroic100V multiferroic test system. Experiments on piezoelectric responses and domain structures measured by the piezoresponse force microscopy (PFM). The magnetic properties for GFO/BKNT composite films as well as FC and ZFC magnetization of the pure GFO films were studied by physical property measurement system (PPMS) at room temperature and at different temperatures up to 5 K, respectively. The measurement of ME coupling for GFO/BKNT composite films was observed by SR830, SRS Inc.
2. Experimental procedure The multiferroic GFO/BKNT composite thin films were prepared by a typical sol-gel method. All the reagents are analytically pure, used without further purification and treatment in this experiment. Pt(100)/ Ti/SiO2/Si substrate can not only make the films endow good crystallinity during the annealing process, but also contribute to the growth of the grains. Gallium nitrate hydrate [Ga(NO3)3], iron nitrate nonahydrate [Fe(NO3)3·9H2O] were used as precursors for Ga and Fe, respectively, for the formation of 0.2 mol/L solution of Ga0.8Fe1.2O3. Gallium nitrate hydrate and iron nitrate nonahydrate dissolved ethylene glycol with Fe3+, Ga3+ ion ratio of 0.8:1.2. Then, the solution was continuously stirred until it was transparent and placed at room temperature for a week to form a stable GFO precursor solution. In preparation process of BKNT precursor solution, bismuth nitrate pentahydrate [Bi(NO3)3·5H2O], sodium nitrate [NaNO3], potassium nitrate [KNO3], butyl titanate [CH3(CH2)3O]4Ti were used as original materials, ethylene glycol monomethyl ether used as solution, acetylacetone used as stabilizing agent. It is noteworthy that bismuth nitrate pentahydrate [Bi(NO3)3·5H2O] was 10% excess and sodium nitrate [NaNO3], potassium nitrate [KNO3] were 3% excess in the starting materials to compensate for Bi, K, and Na vaporization during the thermal annealing. The mixed precursor solution was placed on the magnetic stirrer to be stirred continuously until it was clarified and formed stable yellow solution with the molar concentration of 0.25 mol/L. Then the precursor solutions of BKNT and GFO were spin-coated on the Pt(100)/Ti/SiO2/Si substrates at a spinning rate of 450 rpm for 15 s and 4000 rpm for 40 s, separately. The gelution films were dried at 300 °C for 3 min. Next, BKNT films were placed in a rapid thermal processor (RTP-500) for annealing treatment at the temperature of 700 °C for 300 s while GFO films were annealed by the same method at 650 °C both with a oxygen flow density of 1.5 L/min. Finally, rapid thermal processor was performed at 650 °C for 1200 s to get the desired phase and density. The X-ray diffraction (XRD) pattern was characterized by Panalytical D/MAX-RA diffractometer, whose measuring parameter is λ of 0.15406 nm with CuKα radiation. In order to facilitate the electrical test, a uniform layer of Au electrodes as top electrodes whose diameter is 0.2 mm by a small magnetron sputtering device. Next a series of the properties of the GFO/BKNT composite films were characterized by the existing equipments. The cross section of the
3. Results and discussion The XRD pattern of the GFO/BKNT composite films is shown in Fig. 1. The distinct diffraction peaks are indexed to two groups of peaks. Namely, one group of peaks pertains to GFO single phase of orthorhombic structure with space group Pc 21 n according to the ICSD 35079, and the other of peaks exhibits a polycrystalline perovskite BKNT films with the reported data (ICSD 98047), indicating that neither impurity nor secondary phases are detected. It is worth noting that the main diffraction peaks of GFO (221) and BKNT (110) are almost overlapped, which means the well lattice matching between the two phase. The inset of Fig. 1(a) exhibits the SEM image of the cross section of GFO/BKNT composite films, indicating the dense and well crystalline nature. Obviously, it is difficult to identify specifically the interface between the GFO film and the BKNT film, as both GFO and BKNT films have similar perovskite structures. This phenomenon suggests that the good interface coupling between the GFO and BKNT single-phase is formed, which is the conducive to reduce the leakage in the composite films and enhance ME coupling effects of the composite films [20]. In addition, it is found that the thickness of GFO and BKNT layers are about 353 nm and 607 nm, respectively. As shown in Fig. 2(a), the polarization-electric field (P-E) hysteresis loops of GFO/BKNT composite films are measured under different applied electric fields at room temperature. The hysteresis loops of GFO/BKNT composite films are up to be saturated at various electric fields. At the same time, the saturation polarization Ps and the remnant polarization Pr increase with applied electric fields. When an electric field of amplitude 964 kV/cm is applied, Ps and Pr reach 114 μC/cm2 and 60 μC/cm2, respectively. The following reasons can account for so excellent ferroelectric properties of GFO/BKNT composite films. Firstly, the large polarization is obtained for the composite films because of the similar structure between GFO and BKNT derived from a consummate interface and the tensile stress, respectively. [21]. Then, the rhombohedral-tetragonal morphotropic phase boundary derived 2
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Fig. 2. (a) P-E hysteresis loops of GFO/BKNT films at room temperature, (b) the leakage current density (J) versus electric field (E) characteristics plots of GFO/BKNT films at room temperature, insert of Fig. (b) is the curves of Ln(J) versus Ln(E).
from the BKNT films plays a key role in the increase of Ps and Pr values for GFO/BKNT composite films. Last, a good domain structure is also one of the necessary conditions for the excellent ferroelectric properties. In addition, it is worth noting that the hysteresis loops are mildly dissymmetry, indicating that there is an internal bias electric field between the electrode and the film at the interface, i.e., the hysteresis loops can be forward and reverse saturated. This is caused by GFO layer, because each Fe site is formed around the octahedron and deviates from the center of the octahedron, which leads to electric polarization bias along b axis [19]. Fig. 2(b) shows the curves of the leakage current density (J) versus electric field (E), which were recorded with a voltage step width of 3.3 V for Au/GFO/BKNT/Pt film capacitors measured at room temperature. It is shown that the leakage current density for GFO/BKNT composite films is 3.1×10−5 A/cm2 at an applied field of 607 kV/cm, which suggests that the relatively excellent leakage performances for the composite films are obtained. The inset of Fig. 2(b) displays ln E – ln J curves for GFO/BKNT composite films, which demonstrably confirms the conduction mechanism. As shown, the fitted curves are straight and the linear slope value of 1.2, approximating to 1 in the whole electric field, which exhibits that the mechanism of the leakage current is Ohmic conduction. When the density of the free charge carrier in GFO/ BKNT composite film is less than or equal to that of the volume generated charge carrier, the conduction mechanism is clearly demonstrated by Ohmic conduction behavior. [22]. The leakage current density is calculated according to Ohmic conduction by using the following equation,
domains in the same grains, which are caused by the existence of different polarization directions in the same grains. Through the observation of the domain structures of GFO/BKNT composite films, one phenomenon can be seen that the domains are pinned in the grain boundaries. It can be seen that a large number of grain boundaries coincide with the domain boundaries combining the AFM images and the phase images, which indicates that the grain boundaries influence the domain structures while determining the shape of domain boundaries [23]. For the present GFO/BKNT composite films, the smaller grains are obtained and polydomain structures are induced. Due to ferroelastic domain displacement/creation, the inbuilt electric field between these domain walls and the change in the strain conditions affect the ferroelectric polarization switching. In the following study, when GFO/BKNT composite film applied to the 15 V tip electric bias with the scanning region of 1×1 µm2, the surface morphology, amplitude and phase images are gained by PFM. By comparison of Fig. 3(a) and Fig. 3(d), it is explained that the scanning areas of two patterns are coincident. Meanwhile, the comparison of the phase images of the PFM in Fig. 3(c) and (f) reveals that the change and reversal of ferroelectric domain are controlled by the applied electric field, in which the domain boundaries and the grain boundaries are basically coincident. But, compared with the above two images, there are obvious changes on the domain structures. After poling procedure, domains are pinned in grain boundaries. At the same time, the orientation of domains in the grain boundaries tends to be consistent. Those results show that the concentration of the domain walls of GFO/BKNT composite film is lower than that of the non poled domain walls. In addition, the ferroelectric domains of the poled GFO/ BKNT composite films exhibit the phenomenon that the domain size is similar to the grain size, unlike the original GFO/BKNT composite film as disorderly growth. Also, it is observed that the morphologies of the poled films has not changed at all with comparison to those of the asgrown GFO/BKNT composite films, which further reveals that the original surface morphology was not destroyed after applied to the tip electric bias. However, as shown in Fig. 3(e) and (f), the domain structures change obviously. The piezoelectric responses tend to be consistent in the corresponding grain regions, illustrating that domains of GFO/BKNT composite films possess good properties of domain switching. On the other hand, although the polarization direction of most of domains is changed after poling procedure, a small number of domains do not switch. When the bias voltage is applied to GFO/BKNT composite film, the domain wall pinning effect derived from charge carriers moving to grain boundaries limits domain switching [24,25]. It is well known that the domains are the physics foundation of the properties and applications, the switching of which significantly influence ferroelectric properties of GFO/BKNT composites films. Therefore, ferroelectric domains of GFO/BKNT composite films endow
J = qμNe E where μ is the ion mobility, J the leakage current density and Ne the electron density of thermal emission. Furthermore, the current density of single Ohmic mechanism is linear with the external field, which is beneficial to the excellent leakage characteristics of the composite films. By means of piezoresponse force microscopy (PFM) methods, the local polarization structure and static ferroelectric domain images of the present composite films are obtained, as displayed in Fig. 3. Fig. 3(a) presents the topographic image of GFO/BKNT composite films scanned by PFM with the scanning area of 1×1 µm2. Fig. 3(b) and (c) show the PFM amplitude and phase images of the as-grown GFO/ BKNT composite films without an applied electric field, respectively. Different response intensities and opposite polarity appear as regions of varied colors as shown in PFM amplitude and phase images. As demonstrated in Fig. 3(a), GFO/BKNT composite films display the slick, uniform and no crack of the surface morphology with distinct grain boundaries and well-distributed fine grains. Also it can be found that the domain walls appear in disorder and there are different 3
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Fig. 3. (a) The typical surface topography, (b) PFM amplitude, (c) phase images with 1×1 µm2 scanning area of the as-grown GFO/BKNT composite films; (d) The typical surface topography, (e) PFM amplitude, (f) phase images of GFO/BKNT composite films after poling procedure with 15 V tip electric bias.
well reversal feature, which makes the composite films get larger ferroelectric polarization. Fig. 4 represents the curves of piezoelectric coefficient (d33) and surface displacement (d) versus applied voltage (V) for GFO/BKNT composite films. The displacement displays complete butterfly shaped piezoresponse curves under the external electrical field. The induced surface displacement reaches 2.56 nm at the applied 15 V voltage. The piezoelectric hysteresis loops (d33-V) are calculated from the D-V curves based on the law of converse piezoelectric effect, which suggests that all films are switchable and the ferroelectricity is retained. The corresponding piezoelectric coefficient d33 calculated from the displacement-voltage curves reaches 230 pm/V. Such excellent piezoelectric properties are originated from the larger spontaneous polarization. Actually, the piezoelectric effect of poly-domain ferroelectric materials
can be divided into two parts. One of part is intrinsic piezoelectric response coming from the piezoelectric deformation within each elementary cell, the other extrinsic effect originates from additional deformation due to the motion of non-180 domain walls [26]. The part deriving from intrinsic piezoelectric response can be depicted as follow,
d33 = 2Qeff εP where Qeff represents the effective electrostrictive coefficient, P is the spontaneous polarization and ε is the dielectric constant. The P, ε and Qeff are the three important parameters of calculation of d33. In particular, the higher polarization value P implies the stronger piezoelectric response. As discussed previously in this article, GFO/BKNT composite films possess larger spontaneous polarization and larger Pr and Ps, which is attributed to the good domain switching. The tilting the polar vector of domain is easily aroused by the electric field, which creates strong piezoelectric activities [27]. On the other hand, the latter, i.e., the extrinsic piezoelectric responses, mainly originates from the interface interaction. GFO and BKNT layers have similar structures, indicating that the faultless interface coupling and tensile stress were obtained on the clearly boundary, which can effectively transmit mechanical deformation under the applied DC electric field. Fig. 5(a) shows the results of magnetization (M) measurements on GFO/BKNT composite films at room temperature under an external field (H) varying up to 3 T. The present composite films exhibit hysteresis behavior in the M-H curve, indicative of ferromagnetism. At the same time, the magnetic hysteresis loops shows a high saturation magnetization Ms ~57.38 emu/cc, remanent magnetization Mr ~25.82 emu/cc, and coercivity Hc ~1883.73 Oe of GFO/BKNT composite films. Thus strong ferromagnetic properties originate from GFO film layer. It is known that GFO have a ferrimagnetic structure, where the easy magnetic axis is along c-axis due to its Pc 21 n space groups [28,29]. For the present composite films, the excellent ferromagnetic properties mainly originate from the magnetic moment of Fe3+ positive ions occupied Ga2 sites, which is owing to the excess Fe3+
Fig. 4. The applied voltage dependence of the piezoelectric response for GFO/BKNT composite films.
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Fig. 5. (a) The ferromagnetic magnetic hysteresis loops of GFO/BKNT composite films, (b) ZFC and FC magnetization as a function of temperature for GFO films at H=500 Oe.
positive ions having priority to occupy Ga2 sites, while the rate of Ga1 sites occupied is very low [19]. It would be expected that greater antisite disorder between the Fe sites and the Ga sites would lead to a stronger ferromagnetic moment. The lager magnetic polarizations in GFO/BKNT composite films such as Ms and Mr are due to the influence of the cationic stoichiometry and their occupancy in the specific crystallography sites within GFO layers. In order to further study the magnetic properties of GFO/BKNT composite films, FC and ZFC curves of GFO films are measured and analyzed by follow-on work. To record the curves of FC magnetization versus temperature in the temperature range of 300–5 K, the composite films were cooled in the presence of an applied magnetic field of 500 Oe, and the data were recorded during the rise of the temperature. Then the composite films were again cooled to 5 K in the absence of field, and the data for ZFC curve were recorded during the increase of temperature. Fig. 5(b) shows zero field cooling (ZFC) and field cooling (FC) under applied magnetic field 500 Oe for GFO/BKNT composite films. As can be seen, the temperature at which the FC-ZFC curves are bifurcated is called the blocking temperature (TB). The blocking temperature of GFO/BKNT composite films is just above 300 K. As can be seen from the FC curve, the magnetization gradually decreases with the increase of temperature. The magnetic phase transition temperature (TC) is the temperature when the magnetization of the samples is zero, that is, the temperature of the materials from the ferromagnetic phase to the paramagnetic phase. So the TC value of GFO/BKNT composite films should be higher than 300 K. Thus raised TC value is attributed to the fact that the lower TC value should be ascribed to the higher Fe occupation at the Ga1 site [19]. In summary, the transition temperature is higher than room temperature, which is great significance for the application of multiferroic materials. Such result also leads to the results that the ME coupling effects of GFO/ BKNT composite films become stronger. Fig. 6 presents the ME voltage coefficient αE as a function of bias magnetic field Hbias for GFO/BKNT composite films. According to the dynamic magnetic field method, the ME coupling coefficient of GFO/ BKNT composite films can be determined [30]. The ME voltage coefficient can be used to represent the ME effect, which can be expressed by a formula, i.e., αE =δE/δHac=δV/tδHac. In this formula, t represents the thickness of films as well as E and Hac are induced electric field and an applied alternating current (AC) magnetic field, respectively. The ME coupling measurements of GFO/BKNT composite films were applied to a direct current (DC) bias magnetic field Hbias from 0 to 8000 Oe while an AC magnetic field Hac=8.24 Oe superimposed onto it at a frequency of f=600 Hz. The AC magnetic field and the DC bias magnetic field which were applied in measurements of GFO/BKNT composite films were perpendicular to the film plane. The ME voltage coefficient increases first and then remains stabilization after reaching the maximum ME voltage coefficient with the increase of
Fig. 6. The ME voltage coefficient as a function of bias magnetic field Hbias for GFO/ BKNT composite films.
the applied DC bias magnetic field. The maximum αE value of GFO/ BKNT composite films is obtained to be 30.89 mV/cm Oe when Hbias at near 7200 Oe. In general, the ME coupling is mainly to achieve the ME effect via the conduction of the interfacial stress between the ferromagnetic phase and the piezoelectric phase [31]. When applied an external magnetic field, the ferromagnetic phase of composite films produced the magnetostrictive effect, which results in internal displacement and stress conditions changing. The stress transfer to the ferroelectric phase through the coupling effect of the interface inducing piezoelectric effect in the ferroelectric phase which have promoted electric polarization inside the composite films [32]. In the present composite films, the dynamic magneto-elastic coupling is mainly obtained by the magnetostrictive effect originating from the GFO phase, while piezoelectricity arises from the ferroelectric effect of BKNT phase. Therefore, the interface between the ferromagnetic and ferroelectric phase induces the connection between magnetostrictive effect of the GFO phase and piezoelectric effect of the BKNT phase, which achieves stress transfer between the ferromagnetic and ferroelectric phase leading to the ME effect of GFO/BKNT composite films [33]. According to the ME coupling between the GFO and BKNT layers, the theoretical ME coefficient can be estimated [34],
αE =
−2pd31mq11 (1 − n ) (ms11 + ms12 ) p ε33 n + ( ps11 + ps12 ) p ε33 (1 − n ) − 2( p d31)2 (1 − n )
Where ms11 and mq11 denote the equivalent compliance coefficient and equivalent piezomagnetic coefficient, ε, d denoting dielectric constant and permeability, respectively. Furthermore, coexistence of ferromagnetic and ferroelectric phases in ME composite films has brought about 5
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Srinivasan, Multiferroic magnetoelectric composites: historical perspective, status, and future directions, J. Appl. Phys. 103 (2008) 031101. [33] G.V. Duong, R. Groessinger, M. Schoenhart, D.B. Basques, The lock-in technique for studying magnetoelectric effect, J. Magn. Magn. Mater. 316 (2007) 390–393. [34] G. Srinivasan, E. Rasmussen, B. Levin, R. Hayes, Magnetoelectric effects in bilayers and multilayers of magnetostrictive and piezoelectric perovskite oxides, Phys. Rev. B 65 (2002) 134402. [35] K. Ahadi, K. Cadien, Ultra low density of interfacial traps with mixed thermal and plasma enhanced ALD of high-κ gate dielectrics, RSC Adv. 6 (2016) 16301–16307. [36] K. Ahadi, A. Nemati, S.M. Mahdavi, Conductor–insulator transition and electronic structure of Ca-doped BiFeO3 films, Mater. Lett. 83 (2012) 124–126. [37] J.X. Zhang, J.Y. Dai, H.L.W. Chan, Interfacial engineering and coupling of electric and magnetic properties in Pb(Zr0.53Ti0.47)O3/CoFe2O4 multiferroic epitaxial multilayers, J. Appl. Phys. 107 (2010) 104105. [38] J.X. Zhang, J.Y. Dai, W. Lu, H.L.W. Chan, Room temperature magnetic exchange coupling in multiferroic BaTiO3/CoFe2O4 magnetoelectric superlattice, J. Mater. Sci. 44 (2009) 5143–5148. [39] J.P. de la Cruz, E. Joanni, P.M. Vilarinho, A.L. Kholkin, Thickness effect on the dielectric, ferroelectric, and piezoelectric properties of ferroelectric lead zirconate titanate thin films, J. Appl. Phys. 108 (2010) 114106.
a well-defined coupling to control electric and magnetic ordering parameters through each other [35,36]. From what has been discussed above, we may safely draw the conclusion that the excellent ferromagnetic and ferroelectric phases form a good coupling, which is a necessary condition for the composite films to obtain outstanding ME effect. For GFO/BKNT composite films, the ferroelectric and ferromagnetic phases have presented prominent single phase properties, i.e., BKNT layers show well ferroelectric and piezoelectric properties, while GFO layers exhibit well magnetism and piezomagnetic properties, which can generate excellent coupling between ferroelectric and ferromagnetic layers. These factors are conducive to obtain larger ME output. On the other hand, GFO and BKNT have similar structure (perovskite structure), which can form good coupling effect. The stress can be efficiently transferred at the interface between BKNT and BFO layers, which is another important reason of a good ME effect for GFO/ BKNT composite films. In spite of these, following two factors will also affect the ME output of GFO/BKNT composite films. One is the substrate holding effect, the other is interfacial charges accumulation at the interface arising from relatively large lattice and symmetry mismatch [14,37,38]. However, if the thickness of the films is relatively thick, the substrate clamping effect as well as the lattice and symmetry mismatch will be partially or completely suppressed [39]. The thickness of GFO/BKNT composite films prepared in this work is close to 1 µm, which is relatively thick. So the clamping effect will be weakened. At the same time, the impact of the lattice and symmetry mismatch on the GFO/BKNT composite films will be reduced. Consequently, GFO/ BKNT composite films can effectively prevent the external effect, which helps to acquire good ME effect. 4. Conclusions In brief, GFO/BKNT composite films were fabricated by sol-gel methods. It is proved that the composite films exhibit coexistence of excellent ferroelectric properties with ferromagnetic properties. To be specific, the present composite films show the perfect hysteresis loops, the lower leakage current density and the good domain switching characteristics, while terrific ferromagnetic properties are obtained at room temperature. In particular, it is worth mentioning that GFO/ BKNT composite films have strong ME effect with the maximum αE value of 30.89 mV/cm Oe, which exceeded the other lead-free ME composite films. The present composite films as the novel functional films possess not only ferroelectric and ferromagnetic properties, but also ME effect, which provide a new idea for potential applications in sensors, transducers, multistate memories. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (Grant nos. 11564028, 11264026 and 51202103), and Inner Mongolia Science Foundation for Distinguished Young Scholars (Grant no. 2014JQ01), the Youth Science and Technology Talents Foundation of Inner Mongolia (Grant no. NJYTB02). References [1] W. Eerenstein, N.D. Mathur, J.F. Scott, Multiferroic and magnetoelectric materials, Nature 442 (2006) 759–765. [2] J.F. Scott, Applications of magnetoelectrics, J. Mater. Chem. 22 (2012) 4567–4574. [3] S.T. Zhang, M.H. Lu, D. Wu, Y.F. Chen, N.B. Ming, Larger polarization and weak ferromagnetism in quenched BiFeO3 ceramics with a distorted rhombohedral crystal structure, Appl. Phys. Lett. 87 (2005) 262907. [4] Y.H. Lin, Q.H. Jiang, Y. Wang, C.W. Nan, Enhancement of ferromagnetic properties in BiFeO3 polycrystalline ceramic by La doping, Appl. Phys. Lett. 90 (2007) 172507. [5] R. Mazumder, P. Sujatha Devi, D. Bhattacharya, A. Sen, Ferromagnetism in nanoscale BiFeO3, Appl. Phys. Lett. 91 (2007) 062510. [6] C.W. Nan, M.I. Bichurin, S.X. Dong, D. Viehland, G. Srinivasan, Multiferroic magnetoelectric composites: historical perspective, status, and future directions, J.
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Magnetoelectric coupling effect of Ga0.8Fe1.2O3/Bi0.5(Na0.85K0.15)0.5TiO3 multiferroic composite films Ruonan Tian, Jieyu Chen, Qingshan Lu, Shifeng Zhao
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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 BS T RAC T
Keywords: Lead-free Magnetoelectric coupling Multiferroic Composite films
Lead-free Ga0.8Fe1.2O3/Bi0.5(K0.15Na0.85)0.5TiO3 (GFO/BKNT) bilayer multiferroic composite films were fabricated on Pt(100)/Ti/SiO2/Si substrates via sol-gel methods. The microstructure, domain structure, ferroelectric, piezoelectric, magnetic properties as well as magnetoelectric coupling effect were investigated for the composite films at room temperature. Well-defined interfaces between GFO and BKNT layers and clear electric domain structures are observed. A strong magentoelectric effect is obtained with magnetoelectric voltage coefficient of αE=30.89 mV/cm Oe, which is attributed to excellent ferroelectric, piezoelectric, and magnetic properties, as well as the coupling interaction between ferromagnetic GFO and ferroelectric BKNT phases for lead-free bilayer composite films. Besides, GFO and BKNT demonstrate the similar perovskite structure with well lattice matching, which endows the outstanding coupling and fascinating magnetoelectric properties. The present work opens up the opportunity of lead-free magnetoelectric composite films for both further fundamental studies and practical device applications such as sensors, transducers and multistate memories.
1. Introduction The magnetoelectric (ME) materials consisting of piezoelectric and magnetostrictive phases have attracted research interest in recent years in view of their potential applications in various electronic devices including sensors, actuators and multistate memories [1,2]. In general, ME materials are broadly classified into two primary categories, including single phase and composite systems, respectively. However, the coexisting simultaneously magnetic ordering and electric ordering in single phase ME materials are rare and typically exhibit a weak ME effect in nature, which have been synthesized in the laboratory such as BiMnO3, BiFeO3, TbM2O5, in which less of the materials have ferromagnetic or ferrimagnetic properties, yet most of the materials are antiferromagnetism, such as BiFeO3 [3–5]. These ME materials also possess low ferroelectric and anti/ferromagnetic transition temperatures far below room temperature, which is not appropriate for practical applications [6,7]. Therefore, in order to improve the weak ME effect in single phase ME materials, it is particularly urgent to study alternatively artificial ME composites. Compared with the single phase ME materials, composites exhibit the strong ME effect by excellent coupling combining a ferroelectric phase and a ferromagnetic phase at room temperature [8,9]. ME composite materials have opened
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the door to practical devices allowing for magnetic field tuning of electrical properties, which has led to breakthroughs in applications such as non-volatile memory, energy harvesting, magnetic field sensing, and potentially many other devices [10–12]. Nevertheless, surpassing ME effect of composite materials depends on prominent ferroelectric and ferromagnetic phase. So far, many investigations have been performed on ME effect in composites, which combine perovskite ferroelectric oxides e.g., PbZr0.52Ti0.48O3 (PZT) with ferromagnetic materials e.g., (CoFe2O4, R-Fe) [13–15]. It is well known that large volatility of toxic lead fumes causes critical environmental problems during processing, which makes PZT compounds dangerous to handle. To address those issues, considerable ideas have been proposed by the development of lead-free ME materials with excellent single phase properties. The composite multiferroic materials with ME effect can also be classified as multi-layered films, bulk composites and so on. Compared with bulk ME composites, multiferroic composite films have unique superiorities such as the facile modulation of their thickness, composition, connectivity, and orientation at nanoscale, in which different phases could be combined at atomic-level, and by precise control of the lattice matching, promoting understanding of ME coupling at atomic scale [16,17]. From the above investigation, restrictions on using lead materials have motivated search for lead-
Corresponding author. E-mail address: [email protected] (S. Zhao).
http://dx.doi.org/10.1016/j.ceramint.2017.05.065 Received 10 April 2017; Received in revised form 7 May 2017; Accepted 9 May 2017 0272-8842/ © 2017 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Tian, R., Ceramics International (2017), http://dx.doi.org/10.1016/j.ceramint.2017.05.065
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free ME composite films. Bi0.5(K0.15Na0.85)0.5TiO3 (BKNT) with well ferroelectric, piezoelectric and leakage properties, which can be used in ME composite films as a piezoelectric phase, is considered to be an excellent candidate to replace lead-based piezoelectric films [18]. From a structure perspective, excellent piezoelectric properties of BKNT films come from the rhombohedral and tetragonal morphotropic phase boundary, which as a piezoelectric phase can provide good piezoelectric properties for magnetoelectric composite films. Recently, GaFeO3 with non-centrosymmetric orthorhombic structure (point group Pc 21 n ) is one of the few single-phase multiferroics that show excellent magnetic behavior and magnetostrictive properties [19]. Thus, a good strategy is to make Ga0.8Fe1.2O3 (GFO) as the ferromagnetic phase and BKNT as the piezoelectric phase, respectively. This type of coupling could lead to an optimizing microstructure and provide the outstanding magneticmechanical-electric coupling with strain (stress) conduction. Besides, GFO and BKNT have similar perovskite structures and good interface connecting. Therefore, GFO/BKNT lead-free bilayer composite films are expected to display the strong ME properties, which provides a feasible method for application of lead-free of magnetoelectric composite films and following-up studies. The present work investigate not only the ferroelectric and ferromagnetic properties, but also the magnetoelectric (ME) coupling effect for GFO/BKNT lead-free composite films synthesized by sol-gel methods. Simultaneously, the corresponding processing of this study is discussed in detail.
Fig. 1. XRD patterns of GFO/BKNT films, the inset is the cross-sectional SEM image of GFO/BKNT composite films.
GFO/BKNT composite films was characterized by SU-4800 scanning electron microscope with 10 kV voltage. Their ferroelectric hysteresis loops and leakage current density were recorded by the MultiFerroic100V multiferroic test system. Experiments on piezoelectric responses and domain structures measured by the piezoresponse force microscopy (PFM). The magnetic properties for GFO/BKNT composite films as well as FC and ZFC magnetization of the pure GFO films were studied by physical property measurement system (PPMS) at room temperature and at different temperatures up to 5 K, respectively. The measurement of ME coupling for GFO/BKNT composite films was observed by SR830, SRS Inc.
2. Experimental procedure The multiferroic GFO/BKNT composite thin films were prepared by a typical sol-gel method. All the reagents are analytically pure, used without further purification and treatment in this experiment. Pt(100)/ Ti/SiO2/Si substrate can not only make the films endow good crystallinity during the annealing process, but also contribute to the growth of the grains. Gallium nitrate hydrate [Ga(NO3)3], iron nitrate nonahydrate [Fe(NO3)3·9H2O] were used as precursors for Ga and Fe, respectively, for the formation of 0.2 mol/L solution of Ga0.8Fe1.2O3. Gallium nitrate hydrate and iron nitrate nonahydrate dissolved ethylene glycol with Fe3+, Ga3+ ion ratio of 0.8:1.2. Then, the solution was continuously stirred until it was transparent and placed at room temperature for a week to form a stable GFO precursor solution. In preparation process of BKNT precursor solution, bismuth nitrate pentahydrate [Bi(NO3)3·5H2O], sodium nitrate [NaNO3], potassium nitrate [KNO3], butyl titanate [CH3(CH2)3O]4Ti were used as original materials, ethylene glycol monomethyl ether used as solution, acetylacetone used as stabilizing agent. It is noteworthy that bismuth nitrate pentahydrate [Bi(NO3)3·5H2O] was 10% excess and sodium nitrate [NaNO3], potassium nitrate [KNO3] were 3% excess in the starting materials to compensate for Bi, K, and Na vaporization during the thermal annealing. The mixed precursor solution was placed on the magnetic stirrer to be stirred continuously until it was clarified and formed stable yellow solution with the molar concentration of 0.25 mol/L. Then the precursor solutions of BKNT and GFO were spin-coated on the Pt(100)/Ti/SiO2/Si substrates at a spinning rate of 450 rpm for 15 s and 4000 rpm for 40 s, separately. The gelution films were dried at 300 °C for 3 min. Next, BKNT films were placed in a rapid thermal processor (RTP-500) for annealing treatment at the temperature of 700 °C for 300 s while GFO films were annealed by the same method at 650 °C both with a oxygen flow density of 1.5 L/min. Finally, rapid thermal processor was performed at 650 °C for 1200 s to get the desired phase and density. The X-ray diffraction (XRD) pattern was characterized by Panalytical D/MAX-RA diffractometer, whose measuring parameter is λ of 0.15406 nm with CuKα radiation. In order to facilitate the electrical test, a uniform layer of Au electrodes as top electrodes whose diameter is 0.2 mm by a small magnetron sputtering device. Next a series of the properties of the GFO/BKNT composite films were characterized by the existing equipments. The cross section of the
3. Results and discussion The XRD pattern of the GFO/BKNT composite films is shown in Fig. 1. The distinct diffraction peaks are indexed to two groups of peaks. Namely, one group of peaks pertains to GFO single phase of orthorhombic structure with space group Pc 21 n according to the ICSD 35079, and the other of peaks exhibits a polycrystalline perovskite BKNT films with the reported data (ICSD 98047), indicating that neither impurity nor secondary phases are detected. It is worth noting that the main diffraction peaks of GFO (221) and BKNT (110) are almost overlapped, which means the well lattice matching between the two phase. The inset of Fig. 1(a) exhibits the SEM image of the cross section of GFO/BKNT composite films, indicating the dense and well crystalline nature. Obviously, it is difficult to identify specifically the interface between the GFO film and the BKNT film, as both GFO and BKNT films have similar perovskite structures. This phenomenon suggests that the good interface coupling between the GFO and BKNT single-phase is formed, which is the conducive to reduce the leakage in the composite films and enhance ME coupling effects of the composite films [20]. In addition, it is found that the thickness of GFO and BKNT layers are about 353 nm and 607 nm, respectively. As shown in Fig. 2(a), the polarization-electric field (P-E) hysteresis loops of GFO/BKNT composite films are measured under different applied electric fields at room temperature. The hysteresis loops of GFO/BKNT composite films are up to be saturated at various electric fields. At the same time, the saturation polarization Ps and the remnant polarization Pr increase with applied electric fields. When an electric field of amplitude 964 kV/cm is applied, Ps and Pr reach 114 μC/cm2 and 60 μC/cm2, respectively. The following reasons can account for so excellent ferroelectric properties of GFO/BKNT composite films. Firstly, the large polarization is obtained for the composite films because of the similar structure between GFO and BKNT derived from a consummate interface and the tensile stress, respectively. [21]. Then, the rhombohedral-tetragonal morphotropic phase boundary derived 2
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Fig. 2. (a) P-E hysteresis loops of GFO/BKNT films at room temperature, (b) the leakage current density (J) versus electric field (E) characteristics plots of GFO/BKNT films at room temperature, insert of Fig. (b) is the curves of Ln(J) versus Ln(E).
from the BKNT films plays a key role in the increase of Ps and Pr values for GFO/BKNT composite films. Last, a good domain structure is also one of the necessary conditions for the excellent ferroelectric properties. In addition, it is worth noting that the hysteresis loops are mildly dissymmetry, indicating that there is an internal bias electric field between the electrode and the film at the interface, i.e., the hysteresis loops can be forward and reverse saturated. This is caused by GFO layer, because each Fe site is formed around the octahedron and deviates from the center of the octahedron, which leads to electric polarization bias along b axis [19]. Fig. 2(b) shows the curves of the leakage current density (J) versus electric field (E), which were recorded with a voltage step width of 3.3 V for Au/GFO/BKNT/Pt film capacitors measured at room temperature. It is shown that the leakage current density for GFO/BKNT composite films is 3.1×10−5 A/cm2 at an applied field of 607 kV/cm, which suggests that the relatively excellent leakage performances for the composite films are obtained. The inset of Fig. 2(b) displays ln E – ln J curves for GFO/BKNT composite films, which demonstrably confirms the conduction mechanism. As shown, the fitted curves are straight and the linear slope value of 1.2, approximating to 1 in the whole electric field, which exhibits that the mechanism of the leakage current is Ohmic conduction. When the density of the free charge carrier in GFO/ BKNT composite film is less than or equal to that of the volume generated charge carrier, the conduction mechanism is clearly demonstrated by Ohmic conduction behavior. [22]. The leakage current density is calculated according to Ohmic conduction by using the following equation,
domains in the same grains, which are caused by the existence of different polarization directions in the same grains. Through the observation of the domain structures of GFO/BKNT composite films, one phenomenon can be seen that the domains are pinned in the grain boundaries. It can be seen that a large number of grain boundaries coincide with the domain boundaries combining the AFM images and the phase images, which indicates that the grain boundaries influence the domain structures while determining the shape of domain boundaries [23]. For the present GFO/BKNT composite films, the smaller grains are obtained and polydomain structures are induced. Due to ferroelastic domain displacement/creation, the inbuilt electric field between these domain walls and the change in the strain conditions affect the ferroelectric polarization switching. In the following study, when GFO/BKNT composite film applied to the 15 V tip electric bias with the scanning region of 1×1 µm2, the surface morphology, amplitude and phase images are gained by PFM. By comparison of Fig. 3(a) and Fig. 3(d), it is explained that the scanning areas of two patterns are coincident. Meanwhile, the comparison of the phase images of the PFM in Fig. 3(c) and (f) reveals that the change and reversal of ferroelectric domain are controlled by the applied electric field, in which the domain boundaries and the grain boundaries are basically coincident. But, compared with the above two images, there are obvious changes on the domain structures. After poling procedure, domains are pinned in grain boundaries. At the same time, the orientation of domains in the grain boundaries tends to be consistent. Those results show that the concentration of the domain walls of GFO/BKNT composite film is lower than that of the non poled domain walls. In addition, the ferroelectric domains of the poled GFO/ BKNT composite films exhibit the phenomenon that the domain size is similar to the grain size, unlike the original GFO/BKNT composite film as disorderly growth. Also, it is observed that the morphologies of the poled films has not changed at all with comparison to those of the asgrown GFO/BKNT composite films, which further reveals that the original surface morphology was not destroyed after applied to the tip electric bias. However, as shown in Fig. 3(e) and (f), the domain structures change obviously. The piezoelectric responses tend to be consistent in the corresponding grain regions, illustrating that domains of GFO/BKNT composite films possess good properties of domain switching. On the other hand, although the polarization direction of most of domains is changed after poling procedure, a small number of domains do not switch. When the bias voltage is applied to GFO/BKNT composite film, the domain wall pinning effect derived from charge carriers moving to grain boundaries limits domain switching [24,25]. It is well known that the domains are the physics foundation of the properties and applications, the switching of which significantly influence ferroelectric properties of GFO/BKNT composites films. Therefore, ferroelectric domains of GFO/BKNT composite films endow
J = qμNe E where μ is the ion mobility, J the leakage current density and Ne the electron density of thermal emission. Furthermore, the current density of single Ohmic mechanism is linear with the external field, which is beneficial to the excellent leakage characteristics of the composite films. By means of piezoresponse force microscopy (PFM) methods, the local polarization structure and static ferroelectric domain images of the present composite films are obtained, as displayed in Fig. 3. Fig. 3(a) presents the topographic image of GFO/BKNT composite films scanned by PFM with the scanning area of 1×1 µm2. Fig. 3(b) and (c) show the PFM amplitude and phase images of the as-grown GFO/ BKNT composite films without an applied electric field, respectively. Different response intensities and opposite polarity appear as regions of varied colors as shown in PFM amplitude and phase images. As demonstrated in Fig. 3(a), GFO/BKNT composite films display the slick, uniform and no crack of the surface morphology with distinct grain boundaries and well-distributed fine grains. Also it can be found that the domain walls appear in disorder and there are different 3
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Fig. 3. (a) The typical surface topography, (b) PFM amplitude, (c) phase images with 1×1 µm2 scanning area of the as-grown GFO/BKNT composite films; (d) The typical surface topography, (e) PFM amplitude, (f) phase images of GFO/BKNT composite films after poling procedure with 15 V tip electric bias.
well reversal feature, which makes the composite films get larger ferroelectric polarization. Fig. 4 represents the curves of piezoelectric coefficient (d33) and surface displacement (d) versus applied voltage (V) for GFO/BKNT composite films. The displacement displays complete butterfly shaped piezoresponse curves under the external electrical field. The induced surface displacement reaches 2.56 nm at the applied 15 V voltage. The piezoelectric hysteresis loops (d33-V) are calculated from the D-V curves based on the law of converse piezoelectric effect, which suggests that all films are switchable and the ferroelectricity is retained. The corresponding piezoelectric coefficient d33 calculated from the displacement-voltage curves reaches 230 pm/V. Such excellent piezoelectric properties are originated from the larger spontaneous polarization. Actually, the piezoelectric effect of poly-domain ferroelectric materials
can be divided into two parts. One of part is intrinsic piezoelectric response coming from the piezoelectric deformation within each elementary cell, the other extrinsic effect originates from additional deformation due to the motion of non-180 domain walls [26]. The part deriving from intrinsic piezoelectric response can be depicted as follow,
d33 = 2Qeff εP where Qeff represents the effective electrostrictive coefficient, P is the spontaneous polarization and ε is the dielectric constant. The P, ε and Qeff are the three important parameters of calculation of d33. In particular, the higher polarization value P implies the stronger piezoelectric response. As discussed previously in this article, GFO/BKNT composite films possess larger spontaneous polarization and larger Pr and Ps, which is attributed to the good domain switching. The tilting the polar vector of domain is easily aroused by the electric field, which creates strong piezoelectric activities [27]. On the other hand, the latter, i.e., the extrinsic piezoelectric responses, mainly originates from the interface interaction. GFO and BKNT layers have similar structures, indicating that the faultless interface coupling and tensile stress were obtained on the clearly boundary, which can effectively transmit mechanical deformation under the applied DC electric field. Fig. 5(a) shows the results of magnetization (M) measurements on GFO/BKNT composite films at room temperature under an external field (H) varying up to 3 T. The present composite films exhibit hysteresis behavior in the M-H curve, indicative of ferromagnetism. At the same time, the magnetic hysteresis loops shows a high saturation magnetization Ms ~57.38 emu/cc, remanent magnetization Mr ~25.82 emu/cc, and coercivity Hc ~1883.73 Oe of GFO/BKNT composite films. Thus strong ferromagnetic properties originate from GFO film layer. It is known that GFO have a ferrimagnetic structure, where the easy magnetic axis is along c-axis due to its Pc 21 n space groups [28,29]. For the present composite films, the excellent ferromagnetic properties mainly originate from the magnetic moment of Fe3+ positive ions occupied Ga2 sites, which is owing to the excess Fe3+
Fig. 4. The applied voltage dependence of the piezoelectric response for GFO/BKNT composite films.
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Fig. 5. (a) The ferromagnetic magnetic hysteresis loops of GFO/BKNT composite films, (b) ZFC and FC magnetization as a function of temperature for GFO films at H=500 Oe.
positive ions having priority to occupy Ga2 sites, while the rate of Ga1 sites occupied is very low [19]. It would be expected that greater antisite disorder between the Fe sites and the Ga sites would lead to a stronger ferromagnetic moment. The lager magnetic polarizations in GFO/BKNT composite films such as Ms and Mr are due to the influence of the cationic stoichiometry and their occupancy in the specific crystallography sites within GFO layers. In order to further study the magnetic properties of GFO/BKNT composite films, FC and ZFC curves of GFO films are measured and analyzed by follow-on work. To record the curves of FC magnetization versus temperature in the temperature range of 300–5 K, the composite films were cooled in the presence of an applied magnetic field of 500 Oe, and the data were recorded during the rise of the temperature. Then the composite films were again cooled to 5 K in the absence of field, and the data for ZFC curve were recorded during the increase of temperature. Fig. 5(b) shows zero field cooling (ZFC) and field cooling (FC) under applied magnetic field 500 Oe for GFO/BKNT composite films. As can be seen, the temperature at which the FC-ZFC curves are bifurcated is called the blocking temperature (TB). The blocking temperature of GFO/BKNT composite films is just above 300 K. As can be seen from the FC curve, the magnetization gradually decreases with the increase of temperature. The magnetic phase transition temperature (TC) is the temperature when the magnetization of the samples is zero, that is, the temperature of the materials from the ferromagnetic phase to the paramagnetic phase. So the TC value of GFO/BKNT composite films should be higher than 300 K. Thus raised TC value is attributed to the fact that the lower TC value should be ascribed to the higher Fe occupation at the Ga1 site [19]. In summary, the transition temperature is higher than room temperature, which is great significance for the application of multiferroic materials. Such result also leads to the results that the ME coupling effects of GFO/ BKNT composite films become stronger. Fig. 6 presents the ME voltage coefficient αE as a function of bias magnetic field Hbias for GFO/BKNT composite films. According to the dynamic magnetic field method, the ME coupling coefficient of GFO/ BKNT composite films can be determined [30]. The ME voltage coefficient can be used to represent the ME effect, which can be expressed by a formula, i.e., αE =δE/δHac=δV/tδHac. In this formula, t represents the thickness of films as well as E and Hac are induced electric field and an applied alternating current (AC) magnetic field, respectively. The ME coupling measurements of GFO/BKNT composite films were applied to a direct current (DC) bias magnetic field Hbias from 0 to 8000 Oe while an AC magnetic field Hac=8.24 Oe superimposed onto it at a frequency of f=600 Hz. The AC magnetic field and the DC bias magnetic field which were applied in measurements of GFO/BKNT composite films were perpendicular to the film plane. The ME voltage coefficient increases first and then remains stabilization after reaching the maximum ME voltage coefficient with the increase of
Fig. 6. The ME voltage coefficient as a function of bias magnetic field Hbias for GFO/ BKNT composite films.
the applied DC bias magnetic field. The maximum αE value of GFO/ BKNT composite films is obtained to be 30.89 mV/cm Oe when Hbias at near 7200 Oe. In general, the ME coupling is mainly to achieve the ME effect via the conduction of the interfacial stress between the ferromagnetic phase and the piezoelectric phase [31]. When applied an external magnetic field, the ferromagnetic phase of composite films produced the magnetostrictive effect, which results in internal displacement and stress conditions changing. The stress transfer to the ferroelectric phase through the coupling effect of the interface inducing piezoelectric effect in the ferroelectric phase which have promoted electric polarization inside the composite films [32]. In the present composite films, the dynamic magneto-elastic coupling is mainly obtained by the magnetostrictive effect originating from the GFO phase, while piezoelectricity arises from the ferroelectric effect of BKNT phase. Therefore, the interface between the ferromagnetic and ferroelectric phase induces the connection between magnetostrictive effect of the GFO phase and piezoelectric effect of the BKNT phase, which achieves stress transfer between the ferromagnetic and ferroelectric phase leading to the ME effect of GFO/BKNT composite films [33]. According to the ME coupling between the GFO and BKNT layers, the theoretical ME coefficient can be estimated [34],
αE =
−2pd31mq11 (1 − n ) (ms11 + ms12 ) p ε33 n + ( ps11 + ps12 ) p ε33 (1 − n ) − 2( p d31)2 (1 − n )
Where ms11 and mq11 denote the equivalent compliance coefficient and equivalent piezomagnetic coefficient, ε, d denoting dielectric constant and permeability, respectively. Furthermore, coexistence of ferromagnetic and ferroelectric phases in ME composite films has brought about 5
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a well-defined coupling to control electric and magnetic ordering parameters through each other [35,36]. From what has been discussed above, we may safely draw the conclusion that the excellent ferromagnetic and ferroelectric phases form a good coupling, which is a necessary condition for the composite films to obtain outstanding ME effect. For GFO/BKNT composite films, the ferroelectric and ferromagnetic phases have presented prominent single phase properties, i.e., BKNT layers show well ferroelectric and piezoelectric properties, while GFO layers exhibit well magnetism and piezomagnetic properties, which can generate excellent coupling between ferroelectric and ferromagnetic layers. These factors are conducive to obtain larger ME output. On the other hand, GFO and BKNT have similar structure (perovskite structure), which can form good coupling effect. The stress can be efficiently transferred at the interface between BKNT and BFO layers, which is another important reason of a good ME effect for GFO/ BKNT composite films. In spite of these, following two factors will also affect the ME output of GFO/BKNT composite films. One is the substrate holding effect, the other is interfacial charges accumulation at the interface arising from relatively large lattice and symmetry mismatch [14,37,38]. However, if the thickness of the films is relatively thick, the substrate clamping effect as well as the lattice and symmetry mismatch will be partially or completely suppressed [39]. The thickness of GFO/BKNT composite films prepared in this work is close to 1 µm, which is relatively thick. So the clamping effect will be weakened. At the same time, the impact of the lattice and symmetry mismatch on the GFO/BKNT composite films will be reduced. Consequently, GFO/ BKNT composite films can effectively prevent the external effect, which helps to acquire good ME effect. 4. Conclusions In brief, GFO/BKNT composite films were fabricated by sol-gel methods. It is proved that the composite films exhibit coexistence of excellent ferroelectric properties with ferromagnetic properties. To be specific, the present composite films show the perfect hysteresis loops, the lower leakage current density and the good domain switching characteristics, while terrific ferromagnetic properties are obtained at room temperature. In particular, it is worth mentioning that GFO/ BKNT composite films have strong ME effect with the maximum αE value of 30.89 mV/cm Oe, which exceeded the other lead-free ME composite films. The present composite films as the novel functional films possess not only ferroelectric and ferromagnetic properties, but also ME effect, which provide a new idea for potential applications in sensors, transducers, multistate memories. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (Grant nos. 11564028, 11264026 and 51202103), and Inner Mongolia Science Foundation for Distinguished Young Scholars (Grant no. 2014JQ01), the Youth Science and Technology Talents Foundation of Inner Mongolia (Grant no. NJYTB02). References [1] W. Eerenstein, N.D. Mathur, J.F. Scott, Multiferroic and magnetoelectric materials, Nature 442 (2006) 759–765. [2] J.F. Scott, Applications of magnetoelectrics, J. Mater. Chem. 22 (2012) 4567–4574. [3] S.T. Zhang, M.H. Lu, D. Wu, Y.F. Chen, N.B. Ming, Larger polarization and weak ferromagnetism in quenched BiFeO3 ceramics with a distorted rhombohedral crystal structure, Appl. Phys. Lett. 87 (2005) 262907. [4] Y.H. Lin, Q.H. Jiang, Y. Wang, C.W. Nan, Enhancement of ferromagnetic properties in BiFeO3 polycrystalline ceramic by La doping, Appl. Phys. Lett. 90 (2007) 172507. [5] R. Mazumder, P. Sujatha Devi, D. Bhattacharya, A. Sen, Ferromagnetism in nanoscale BiFeO3, Appl. Phys. Lett. 91 (2007) 062510. [6] C.W. Nan, M.I. Bichurin, S.X. Dong, D. Viehland, G. Srinivasan, Multiferroic magnetoelectric composites: historical perspective, status, and future directions, J.
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