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LCMO bilayer heterostructures
Applied Surface Science 509 (2020) 145314
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Full Length Article
Substrate-imposed strain engineering of multiferroic properties in BCZT/ LCMO bilayer heterostructures Mingzhe Hua, Songbin Lib, Chuanbin Wangb, a b
T
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School of Mechatronics Engineering, Guizhou Minzu University, Guiyang 550025, Guizhou, China State Key Lab of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Strain engineering BCZT/LCMO Electrical properties Ferromagnetic properties Magnetoelectric properties
Ba0.85Ca0.15Zr0.9Ti0.1O3/La0.67Ca0.33MnO3 (BCZT/LCMO) bilayer epitaxial heterostructures were deposited on LaAlO3 (LAO), (LaAlO3)0.3(Sr2AlTaO6)0.7 (LSAT) and SrTiO3 (STO) single-crystalline substrates by using pulsed laser deposition, to investigate the substrate-imposed strain engineering of multiferroic properties. The epitaxial growth and the strain state of the as-deposited bilayer heterostructures were determined by X-ray reciprocal space mapping analysis. Results showed that the substrate-imposed strain engineering could turn the multiferroic properties by modulating the polarization switching and domain wall motion of the bilayer heterostructures. The optimal dielectric, ferroelectric and piezoelectric performances were obtained in the bilayer heterostructure deposited on STO substrate while an improved saturated magnetization was achieved in the bilayer heterostructure deposited on LAO substrate. The maximum magnetoelectric coupling was obtained in the bilayer heterostructure deposited on STO substrate with αE31 value of 61.2 mV/cm·Oe.
1. Introduction Multiferroic magnetoelectric materials exhibit ferroelectricity and ferromagnetism, and simultaneously have a coupling effect between the two [1,2]. This coupling effect is known as the magnetoelectric (ME) coupling effect that can realize the mutual regulation of electric field and magnetic field, has drew increasing attention because of its exotic physical properties and applications potential as multifunctional devices [3-7]. In terms of phase compositions, multiferroic materials are usually divided into single-phase multiferroics with intrinsic ME effect and artificial multi-phase composite multiferroics with extrinsic ME effect. Since the ME coupling effect of the single-phase multiferroic materials [8,9] are very weak or only exist at very low temperatures, the theoretical and experimental research is presently prioritized to the artificial multiferroic composites containing ferroelectric and ferromagnetic phases. Consequently, the artificial ME composites have been developed and are able to achieve convincing extrinsic ME coupling effect at room temperature, showing closely relationship with the interplay of the ferroelectric and ferromagnetic ordering [10]. With the development of advanced thin film growth techniques and theoretical computing science, researches on ME composite thin films are beginning to flourish in recent past [6,11]. In contrast with ME bulk composites, ME composite thin films show some exclusive advantages. For example, the interface characteristics of adjacent ferroelectric⁎
ferromagnetic interfaces can be flexible designed and precise controlled by strain engineering and interface engineering to realize the strain coupling at the atomic scale, thereby revealing the ME coupling physical mechanisms and further improving the ME effect. For layered ME composite thin films, strain engineering and interface engineering are expected to be an effective way to regulate the ME effect, achieving by changing the stacking order, layer thickness, number of layers, etc. of the ferroelectric layer and the ferromagnetic layer [12–14]. One of the primary routes to turn strain engineering in epitaxial thin films is choosing the appropriate lattice-mismatched substrates which can engineer the properties of epitaxial films by altering the strain state varying between large compressive strain to large tensile strain. The choice of the substrates makes a great difference in altering the strain state and film orientation of the epitaxial heterostructures because the clamping effect stemming from the substrate would restrain the strainmediated ME coupling effect. Previous studies on BaTiO3/ La0.67Sr0.33MnO3 [12] and La0.7Sr0.3MnO3/BaTiO3 [15] heterostructures have demonstrated that the substrate orientations possess the ability to modulate the interfacial coupling and multiferroic properties. However, there are few works focus on the substrate-imposed strain engineering of multiferroic properties in layered ME epitaxial films by changing the substrate types. More work needs to focus on the ME coupling effect influenced by constraint stress imposed by different types of lattice-mismatched substrates.
Corresponding author. E-mail address: [email protected] (C. Wang).
https://doi.org/10.1016/j.apsusc.2020.145314 Received 15 September 2019; Received in revised form 10 December 2019; Accepted 7 January 2020 Available online 09 January 2020 0169-4332/ © 2020 Elsevier B.V. All rights reserved.
Applied Surface Science 509 (2020) 145314
M. Hu, et al.
converted from LAO to STO. In order to investigate the strain state in the BCZT/LCMO heterostructures grown on LAO, LSAT and STO substrates, reciprocal space mapping (RSM) around the symmetric (0 0 2) and asymmetric (1 0 3) reflections were recorded as shown in the insets of Fig. 1(a)–(c). According to the RSM (0 0 2) symmetrical reflection, the aligned Qx coordinate of the scattered X-rays intensities confirms that the LCMO and BCZT layers of all bilayer heterostructures are epitaxially grown on the corresponding substrates. It is evident from the RSM patterns around the asymmetric (1 0 3) reflection that the RSM peaks of the LCMO layers for all bilayer heterostructures are aligned with the corresponding substrate peaks while those of the BCZT layers offset to the left, which means the fully strain of the LCMO layer and the partial relaxed of BCZT layers for all heterostructures. The lattice constants of the LCMO and BCZT layers for all bilayer heterostructures are presented in Table 1, determined based on the corresponding RSM peaks. In addition, the out-of-plane (εc) and in-plane (εa) misfit strains of the bilayer heterostructures are followed by εc = c-c0/c0 and εa = a-a0/a0, respectively. Where a0, c0 are the LCMO (a0 = c0 = 3.858 Å) or BCZT bulk values (a0 = 4.007 Å, c0 = 4.025 Å;) [16,20]; a, c are the determined lattice constants of the BCZT layer or LCMO layer. The calculated values of εa and εc of the LCMO and BCZT layers for the bilayer heterostructures grown on different substrates are marked in Table 1. According the positive/negative sign of the calculated results, we can deduce that the LCMO layer of BCZT/LCMO/LSAT and BCZT/LCMO/ STO bilayer heterostructures suffered form an in-plane tensile strain while that of the BCZT/LCMO/LAO bilayer heterostructure is under an in-plane compressive strain, and the BCZT layers of all bilayer heterostructures are subjected to an in-plane compressive strain in descending order of LAO > LSAT > STO. Above all, we successfully turn strain engineering in the bilayer epitaxial heterostructures by grown on different lattice-mismatched substrates. Fig. 2(a)–(c) displays the AFM images of the BCZT/LCMO bilayer heterostructures grown on LAO, LSAT and STO substrates, respectively, which clearly disclose the uniform film growth and smooth surfaces of the heterostructure films. The root-mean-square (RMS) roughness of the BCZT/LCMO/LAO, BCZT/LCMO/LSAT and BCZT/LCMO/STO bilayer heterostructures are 0.814 nm, 0.800 nm and 0.763 nm, respectively, while the average grain size both are approximately 20.0 nm with no significant difference. It seems like that the smaller the in-plane compressive strain in the BCZT layer is, the smoother morphology of the bilayer heterostructures.
In this paper, the ferroelectric layer of the ME heterostructure is the environmentally-friendly Ba0.85Ca0.15Zr0.9Ti0.1O3 (BCZT), is regarded as one of the most promising lead-free piezoelectric materials with equivalent piezoelectric properties to lead-based piezoelectric materials [16]. Besides, perovskite lanthanum manganite La0.67Ca0.33MnO3 (LCMO) exhibits relatively large magnetostrictive coefficient and compatible lattice parameters to BCZT ferroelectric layer [17,18], and is thereby chosen as the ferromagnetic layer of the ME heterostructure. More importantly, LCMO has metallic conductivity and can be used as a bottom electrode, eliminating the process of introducing an external electrode [19]. As mentioned above, we present a detail study on the substrate-imposed strain engineering of multiferroic properties in Ba0.85Ca0.15Zr0.9Ti0.1O3/La0.67Ca0.33MnO3 (BCZT/LCMO) bilayer heterostructures, which were grown on LAO, LSAT and STO three different single-crystalline substrates with (0 0 1) orientation. 2. Experimental BCZT/LCMO bilayer heterostructures with bottom LCMO layer (~50 nm) and top BCZT layer (~100 nm), were epitaxial grown on (0 0 1) LaAlO3 (LAO, 3.788 Å), (0 0 1) (LaAlO3)0.3(Sr2AlTaO6)0.7 (LSAT, 3.868 Å) and (0 0 1) SrTiO3 (STO, 3.905 Å) single-crystalline substrates, using pulsed laser deposition technique with a laser beam of 248 nm wavelength from a KrF excimer laser. During deposition, the substrate temperature of 650 ℃ and laser frequency of 10 Hz were maintained. The laser energy and oxygen pressure of the LCMO layer were 350 mJ and 20 Pa, respectively, while those of the BCZT layer were 300 mJ and 15 Pa. The crystal structure and reciprocal space mapping (RSM) measurements of the bilayer heterostructures were performed by using a PANalytical Empyrean four-circle diffraction system. The film surface morphology and corresponding piezoresponse force microscope (PFM) were recorded by using atomic force microscope (AFM, Dimension Icon Atomic Force Microscope, Bruker). The amplitude and phase of the outof-plane piezoresponse were obtained from the AFM vertical domains signal. The cantilever used was Pt/Ir coated SCM-PIT with a stiffness value of 28 N/m. An impedance analyzer was utilized to test the dielectric properties at room temperature. The room temperature ferroelectric hysteresis loops were measured by using a ferroelectric analyzer with frequency fixed at 1 kHz. Ferromagnetic hysteresis loops were measured at 10 K by using a physical property measurement system. The dc magnetic field dependence of ME coefficient αE31 was recorded by using a Quantum Design ME measurement system.
3.2. Electrical properties 3. Results and discussion The frequency dependent dielectric constant (εr) for the BCZT/ LCMO bilayer heterostructures grown on LAO, LSAT and STO substrates measured at room temperature are displayed in Fig. 3. As can be observed that εr decreases monotonically with the increase in frequency for all bilayer heterostructures. At low frequencies, the εr decreases sharply with a slight increase in measuring frequency, which may be caused by the charge polarization resulted from the inhomogeneous dielectric structure and interfacial polarization [21]. The εr decreases sharply higher values of the measuring frequency should be resulted from the lack of ability of dipoles to follow the fast-changing applied electric field at higher frequency, similar phenomenon has also been found in BCZT/CoFe2O4 bilayer film [22]. Moreover, the εr values at 1 kHz are 581.2, 697.0 and 932.9 for the BCZT/LCMO/LAO, BCZT/ LCMO/LSAT and BCZT/LCMO/STO bilayer heterostructures, respectively. The order of the εr values for the bilayer heterostructures is contrary to the order of the in-plane compressive strain in BCZT layers, revealing that the diminution of in-plane compressive strain in the BCZT layers results in the enhancement of εr. Some related reports [23–25] have pointed out that the two-dimensional compression with the direction applied normal to the axis of polarization could lead to a reduction in the dielectric constant of ferroelectric materials. This is
3.1. Structure analysis Fig. 1(a)–(c) present the X-ray diffraction (XRD) patterns of the BCZT/LCMO bilayer heterostructures deposited on LAO, LSAT and STO substrates, respectively. Only exists the (0 0 l) diffraction peaks belonged to the bilayer heterostructures and corresponding substrates indicate that all bilayer heterostructures display single-crystal structure with preferred orientation along the substrate reflections. Fig. 1(d) displays the rocking curves of the LCMO layer and BCZT layer for the BCZT/LCMO bilayer heterostructures deposited on different substrates. The obtained full width at half-maximum (FWHM) of the bilayer heterostructures is collected in Table 1. The FWHM values of the LCMO layers are 0.15°, 0.07° and 0.11° while those of the BCZT layers are 0.33°, 0.17° and 0.13° for the bilayer heterostructures deposited on LAO, LSAT and STO substrates, respectively. For LCMO layer, the minimum FWHM value of the BCZT/LCMO/LSAT bilayer heterostructure suggests the best crystallinity of the LCMO layer, attributing to the lowest lattice mismatch between the LSAT substrate and LCMO layer. For BCZT layer, the increased FWHM value indicates that the crystallinity of the BCZT layer is getting better when the substrate is 2
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Fig. 1. XRD θ–2θ scans and corresponding XRD RSMs of the BCZT/LCMO bilayer heterostructures grown on (a) LAO, (b) LSAT and (c) STO substrates. (d) XRD rocking curves of the LCMO and BCZT layers for BCZT/LCMO bilayer heterostructures grown on different substrates.
largest remnant polarization (Pr) at 1000 kV/cm of the BCZT/LCMO/ LAO, BCZT/LCMO/LSAT and BCZT/LCMO/STO bilayer heterostructures are 7.9μC/cm2, 16.6 μC/cm2 and 25.1 μC/cm2, respectively, and their corresponding electric coercive field (EC) are 148.2 kV/cm, 183.9 kV/cm and 364.2 kV/cm. The Pr of the bilayer heterostructures is in ascending order of LAO < LSAT < STO, which should be closely related to the improving domain wall motion that could be affected by several factors (e.g., grain size, clamping effect) [26,27]. As described above, the BCZT layer in all bilayer heterostructures shows similar grain sizes but different substrate-imposed in-plane compressive strains. Previous works [28,29] have found that polarization switching and the domain wall motion would be hampered by the pinning effect resulted from the substrate-imposed strain. As a result, the BCZT/LCMO/STO bilayer heterostructure exhibits the largest Pr value due to the reduction of the domain wall pinning. The polarization switching behaviors of the BCZT/LCMO bilayer heterostructures deposited on LAO, LSAT and STO substrates were illustrated in Fig. 5(a)–(c). The out-of-plane PFM images of all samples show a box-in-box switching with a positive + 8 V (1 μm × 1 μm) and negative −8 V (3 μm × 3 μm) probe bias switching. The uniform yellow tone of the PFM images reveals that the out-of-plane polarization states point upwards in all bilayer heterostructures, and therefore the purple tone represents the polarization states point downwards. The writing box-in-box patterns using opposite tip bias demonstrates that
Table 1 Structure parameters and strains of the BCZT/LCMO bilayer heterostructures deposited on LAO, LSAT and STO substrates. Samples
FWMH/° a/Å c/Å c/a εa/% εc/%
BCZT/LCMO/LAO
BCZT/LCMO/LSAT
BCZT/LCMO/STO
LCMO
BCZT
LCMO
BCZT
LCMO
BCZT
0.15 3.790 3.911 1.032 −1.76 1.37
0.33 3.996 4.048 1.013 −0.28 0.57
0.07 3.866 3.853 0.997 0.21 −0.13
0.17 4.002 4.044 1.010 −0.13 0.47
0.11 3.904 3.831 0.981 1.19 −0.70
0.13 4.004 4.039 1.009 −0.08 0.35
because the strain in the ferroelectric thin films can modify the ionic positions and vibrations, which are coupled to the polarization mechanism in the ferroelectric materials. The polarization versus electric field (P-E) hysteresis loops of the BCZT/LCMO bilayer heterostructures deposited on LAO, LSAT and STO substrates are displayed in Fig. 4(a)–(c), respectively, measured at different electric field amplitude but fixed at 1 kHz. The P-E hysteresis loops of all bilayer heterostructures are gradually getting saturated and fully developed as the applied electric field increases, which show the typical ferroelectric hysteresis loops, confirming the ferroelectric nature of all samples at room temperature. Additionally, the determined 3
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Fig. 2. AFM images of the BCZT/LCMO bilayer heterostructures grown on (a) LAO, (b) LSAT and (c) STO substrates.
downward polarization switching, and thereby hinders upward polarization rotation. In addition, the effective piezoelectric coefficient d33 determined from the amplitude-voltage butterfly loops are 29.8 ± 5 pm/V, 36.9 ± 5 pm/V and 42.2 ± 5 pm/V for the BCZT/ LCMO/LAO, BCZT/LCMO/LSAT and BCZT/LCMO/STO bilayer heterostructures, respectively. The piezoelectric performance of the bilayer heterostructures, is in ascending order of LAO < LSAT < STO, which is analogous to its ferroelectric behaviors. This is because of the minimum compressive strain in the BCZT layer of the BCZT/LCMO/STO bilayer heterostructure, which show improved domain wall motion compared to the other two samples.
3.3. Ferromagnetic and magnetoelectric properties Fig. 7 presents the magnetic hysteresis (M−H) loops of the BCZT/ LCMO bilayer heterostructures deposited on LAO, LSAT and STO substrates. The well-shaped M−H loops confirm the ferromagnetic nature of all bilayer heterostructures. It can be found that the saturated magnetization (MS) of the bilayer heterostructures is in descending order of LAO > LSAT > STO, while the magnetic coercive field (MC) is in descending order of LAO > STO > LSAT. Since the BCZT layer is a ferroelectric phase, the ferromagnetic properties of the bilayer heterostructures should stem from the ferromagnetic LCMO layer, indicating that the substrate-imposed strain has an impact on the ferromagnetic properties as a result of the strain-induced distortion of MnO6 octahedra in the LCMO layer. In most cases, the in-plane compressive strain increases the Mn-O bond angle which would improve the electronic hopping amplitude and in turn enhance ferromagnetism, while the tensile strain is reversed [32,33]. As mentioned in the XRD analysis, the LCMO layer of BCZT/LCMO/LSAT and BCZT/LCMO/STO bilayer heterostructures suffered form an in-plane tensile strain while that of the BCZT/LCMO/LAO bilayer heterostructure is under an in-plane compressive strain. As a result, the BCZT/LCMO/LAO bilayer heterostructure shows the largest MS, while the BCZT/LCMO/STO bilayer heterostructure exhibits the minimum MS. It should also be noted that the increasing in lattice distortion in LCMO layer could cause stronger
Fig. 3. Frequency dependent dielectric constant of the BCZT/LCMO bilayer heterostructures grown on LAO, LSAT and STO substrates.
the out-of-plane polarization of all bilayer heterostructures is electrically switchable. Furthermore, the contrast between outside unpoled region and the positively poled region is minimal in the BCZT/LCMO/ STO bilayer heterostructure, suggesting that the domains are primarily aligned upwards at their virgin states. Fig. 6(a)–(c) display the local PFM hysteresis loops of amplitude and phase for the BCZT/LCMO bilayer heterostructures grown on LAO, LSAT and STO substrates, respectively. It is obvious that the butterfly loops are well-shaped and the piezoresponse phases are almost 0-180° reversibly switchable, revealing the good switching behavior for all bilayer heterostructures. Notably, the amplitude-voltage butterfly loops with varying degrees of asymmetry behavior could be observed in all bilayer heterostructures, which has also been reported in the previous works [30,31]. This phenomenon should be ascribed to the electrode self-poling effect caused built-in electric field, which in turn improves
Fig. 4. P-E hysteresis loops of the BCZT/LCMO bilayer heterostructures grown on (a) LAO, (b) LSAT and (c) STO substrates. 4
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Fig. 5. Out-of-plane PFM images of the BCZT/LCMO bilayer heterostructures grown on (a) LAO, (b) LSAT and (c) STO substrates.
pinning centers for the magnetic domains, and thus result in a reinforcement in the coercivity [34]. As can be deduced from the Table 1, the LCMO layer of the BCZT/LCMO/LAO bilayer heterostructure is subjected to the largest residual strain, that is, the largest lattice distortion. Therefore, the BCZT/LCMO/LAO bilayer heterostructure displays the maximum MC. The magnetic field as a function of the transverse ME coefficient (αE31) for the BCZT/LCMO bilayer heterostructures deposited on LAO, LSAT and STO substrates are displayed in Fig. 8, which were performed at room temperature with the applied magnetic field parallel to the heterostructure surface. Results show that the αE31 value increases monotonically with increasing applied magnetic field, and then get saturated once at a certain magnetic field. The certain magnetic field at saturation is approximately 7000 Oe, 4000 Oe and 6000 Oe for the BCZT/LCMO/LAO, BCZT/LCMO/LSAT and BCZT/LCMO/STO bilayer heterostructures, respectively. This should be explained by the free magnetostriction of the LCMO layer limited by the substrate-imposed clamping effect. The greater the substrate-imposed strain in the LCMO layer is, the larger the applied magnetic field is required to get saturated. Consequently, the BCZT/LCMO/LAO bilayer heterostructure is in need of the largest applied magnetic field. In addition, the maximum values of αE31 are 26.1, 40.4 and 61.2 mV/cm·Oe for the BCZT/LCMO/ LAO, BCZT/LCMO/LSAT and BCZT/LCMO/STO bilayer heterostructures, respectively. As we all known, the ME coupling effect is a cross-coupling effect between the piezoelectricity of ferroelectric phase and magnetostriction of ferromagnetic phase, which is very sensitive to the strain state of the BCZT and LCMO layers imposed by the substrate [35,36]. As a result, the BCZT/LCMO/LAO bilayer heterostructure shows the smallest αE31 value due to the largest substrate-imposed strain in LCMO layer and the minimum piezoelectric coefficient, while the BCZT/LCMO/STO bilayer heterostructure exhibits the largest αE31 value attributed to the strongest piezoelectric effect.
Fig. 7. M−H loops of the BCZT/LCMO bilayer heterostructures deposited on LAO, LSAT and STO substrates. The inset displays the partially enlarged loops at low field region.
4. Conclusions In summary, epitaxial bilayer heterostructures of BCZT/LCMO were grown on various single-crystalline substrates by using pulsed laser deposition technique. The XRD and RSM analysis verified that all bilayer heterostructures were epitaxial grown on substrates with varying strain states. Results showed that the substrate-imposed strain can modulate the polarization switching and domain wall motion, and thus modified the electrical, ferromagnetic as well as ME properties of the bilayer heterostructures. The optimal electrical properties were achieved in the BCZT/LCMO bilayer heterostructure deposited on STO substrate while the greatest saturated magnetization was obtained in the BCZT/LCMO bilayer heterostructure grown on LAO substrate. Furthermore, the BCZT/LCMO bilayer heterostructure deposited on
Fig. 6. Local amplitude-voltage butterfly loops and phase-voltage hysteresis loops of the BCZT/LCMO bilayer heterostructures grown on (a) LAO, (b) LSAT and (c) STO substrates. 5
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Fig. 8. Magnetic field as a function of the αE31 coefficients of the BCZT/LCMO bilayer heterostructures grown on (a) LAO, (b) LSAT and (c) STO substrates.
STO substrate displays the largest magnetoelectric coefficient with αE31 of 61.2 mV/cm·Oe. These findings reveal that substrate-imposed strain engineering in ME heterostructure can turn the multiferroic properties toward to application for the new type multiferroic functional devices. CRediT authorship contribution statement Mingzhe Hu: Conceptualization, Methodology, Writing - original draft. Songbin Li: Data curation, Formal analysis. Chuanbin Wang: Funding acquisition, Writing - review & editing. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement This work was financially supported by the National Natural Science Foundation of China (51972252, 51567017), 111 Project (B13035), the National Science Foundation of Guizhou Minzu University (GZMU [2019]YB20) and the Fundamental Research Funds for the Central Universities (WUT: 2019III029). References [1] L.W. Martin, R. Ramesh, Multiferroic and magnetoelectric heterostructures, Acta Mater. 60 (6–7) (2012) 2449–2470. [2] J. Ma, J.M. Hu, Z. Li, C.W. Nan, Recent progress in multiferroic magnetoelectric composites: from bulk to thin films, Adv. Mater. 42 (18) (2011) 1062–1087. [3] M. Naveed-Ul-Haq, V.V. Shvartsman, H. Trivedi, S. Salamon, S. Webers, H. Wende, U. Hagemann, J. Schröder, D.C. Lupascu, Strong converse magnetoelectric effect in (Ba, Ca)(Zr, Ti)O3-NiFe2O4 multiferroics: a relationship between phase-connectivity and interface coupling, Acta Mater. 144 (2018) 305–313. [4] X. Yang, Z. Zhou, T. Nan, Y. Gao, G.M. Yang, M. Liu, N.X. Sun, Recent advances in multiferroic oxide heterostructures and devices, J. Mater. Chem. C 4 (2) (2016) 234–243. [5] M. Fiebig, T. Lottermoser, D. Meier, M. Trassin, The evolution of multiferroics, Nat. Rev. Mater. 4 (2) (2019) 146. [6] R. Ramesh, N.A. Spaldin, Multiferroics: progress and prospects in thin films, Nat. Mater. 6 (1) (2007) 21–29. [7] G. Schileo, C. Pascual-Gonzalez, M. Alguero, I.M. Reaney, P. Postolache, L. Mitoseriu, K. Reichmann, M. Venet, A. Feteira, Multiferroic and magnetoelectric properties of Pb0.99[Zr0.45Ti0.47(Ni1/3Sb2/3)0.08]O3-CoFe2O4 multilayer composites fabricated by tape casting, J. Eur. Ceram. Soc. 38 (4) (2018) 1473–1478. [8] G. Zhang, S. Dong, Z. Yan, Y. Guo, Q. Zhang, S. Yunoki, E. Dagotto, J.M. Liu, Multiferroic properties of CaMn7O12, Phys. Rev. B 84 (17) (2011) 174413. [9] X.X. Shi, X.Q. Liu, X.M. Chen, Readdressing of magnetoelectric effect in bulk BiFeO3, Adv. Funct. Mater. 27 (12) (2017) 1604037.
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Full Length Article
Substrate-imposed strain engineering of multiferroic properties in BCZT/ LCMO bilayer heterostructures Mingzhe Hua, Songbin Lib, Chuanbin Wangb, a b
T
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School of Mechatronics Engineering, Guizhou Minzu University, Guiyang 550025, Guizhou, China State Key Lab of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Strain engineering BCZT/LCMO Electrical properties Ferromagnetic properties Magnetoelectric properties
Ba0.85Ca0.15Zr0.9Ti0.1O3/La0.67Ca0.33MnO3 (BCZT/LCMO) bilayer epitaxial heterostructures were deposited on LaAlO3 (LAO), (LaAlO3)0.3(Sr2AlTaO6)0.7 (LSAT) and SrTiO3 (STO) single-crystalline substrates by using pulsed laser deposition, to investigate the substrate-imposed strain engineering of multiferroic properties. The epitaxial growth and the strain state of the as-deposited bilayer heterostructures were determined by X-ray reciprocal space mapping analysis. Results showed that the substrate-imposed strain engineering could turn the multiferroic properties by modulating the polarization switching and domain wall motion of the bilayer heterostructures. The optimal dielectric, ferroelectric and piezoelectric performances were obtained in the bilayer heterostructure deposited on STO substrate while an improved saturated magnetization was achieved in the bilayer heterostructure deposited on LAO substrate. The maximum magnetoelectric coupling was obtained in the bilayer heterostructure deposited on STO substrate with αE31 value of 61.2 mV/cm·Oe.
1. Introduction Multiferroic magnetoelectric materials exhibit ferroelectricity and ferromagnetism, and simultaneously have a coupling effect between the two [1,2]. This coupling effect is known as the magnetoelectric (ME) coupling effect that can realize the mutual regulation of electric field and magnetic field, has drew increasing attention because of its exotic physical properties and applications potential as multifunctional devices [3-7]. In terms of phase compositions, multiferroic materials are usually divided into single-phase multiferroics with intrinsic ME effect and artificial multi-phase composite multiferroics with extrinsic ME effect. Since the ME coupling effect of the single-phase multiferroic materials [8,9] are very weak or only exist at very low temperatures, the theoretical and experimental research is presently prioritized to the artificial multiferroic composites containing ferroelectric and ferromagnetic phases. Consequently, the artificial ME composites have been developed and are able to achieve convincing extrinsic ME coupling effect at room temperature, showing closely relationship with the interplay of the ferroelectric and ferromagnetic ordering [10]. With the development of advanced thin film growth techniques and theoretical computing science, researches on ME composite thin films are beginning to flourish in recent past [6,11]. In contrast with ME bulk composites, ME composite thin films show some exclusive advantages. For example, the interface characteristics of adjacent ferroelectric⁎
ferromagnetic interfaces can be flexible designed and precise controlled by strain engineering and interface engineering to realize the strain coupling at the atomic scale, thereby revealing the ME coupling physical mechanisms and further improving the ME effect. For layered ME composite thin films, strain engineering and interface engineering are expected to be an effective way to regulate the ME effect, achieving by changing the stacking order, layer thickness, number of layers, etc. of the ferroelectric layer and the ferromagnetic layer [12–14]. One of the primary routes to turn strain engineering in epitaxial thin films is choosing the appropriate lattice-mismatched substrates which can engineer the properties of epitaxial films by altering the strain state varying between large compressive strain to large tensile strain. The choice of the substrates makes a great difference in altering the strain state and film orientation of the epitaxial heterostructures because the clamping effect stemming from the substrate would restrain the strainmediated ME coupling effect. Previous studies on BaTiO3/ La0.67Sr0.33MnO3 [12] and La0.7Sr0.3MnO3/BaTiO3 [15] heterostructures have demonstrated that the substrate orientations possess the ability to modulate the interfacial coupling and multiferroic properties. However, there are few works focus on the substrate-imposed strain engineering of multiferroic properties in layered ME epitaxial films by changing the substrate types. More work needs to focus on the ME coupling effect influenced by constraint stress imposed by different types of lattice-mismatched substrates.
Corresponding author. E-mail address: [email protected] (C. Wang).
https://doi.org/10.1016/j.apsusc.2020.145314 Received 15 September 2019; Received in revised form 10 December 2019; Accepted 7 January 2020 Available online 09 January 2020 0169-4332/ © 2020 Elsevier B.V. All rights reserved.
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converted from LAO to STO. In order to investigate the strain state in the BCZT/LCMO heterostructures grown on LAO, LSAT and STO substrates, reciprocal space mapping (RSM) around the symmetric (0 0 2) and asymmetric (1 0 3) reflections were recorded as shown in the insets of Fig. 1(a)–(c). According to the RSM (0 0 2) symmetrical reflection, the aligned Qx coordinate of the scattered X-rays intensities confirms that the LCMO and BCZT layers of all bilayer heterostructures are epitaxially grown on the corresponding substrates. It is evident from the RSM patterns around the asymmetric (1 0 3) reflection that the RSM peaks of the LCMO layers for all bilayer heterostructures are aligned with the corresponding substrate peaks while those of the BCZT layers offset to the left, which means the fully strain of the LCMO layer and the partial relaxed of BCZT layers for all heterostructures. The lattice constants of the LCMO and BCZT layers for all bilayer heterostructures are presented in Table 1, determined based on the corresponding RSM peaks. In addition, the out-of-plane (εc) and in-plane (εa) misfit strains of the bilayer heterostructures are followed by εc = c-c0/c0 and εa = a-a0/a0, respectively. Where a0, c0 are the LCMO (a0 = c0 = 3.858 Å) or BCZT bulk values (a0 = 4.007 Å, c0 = 4.025 Å;) [16,20]; a, c are the determined lattice constants of the BCZT layer or LCMO layer. The calculated values of εa and εc of the LCMO and BCZT layers for the bilayer heterostructures grown on different substrates are marked in Table 1. According the positive/negative sign of the calculated results, we can deduce that the LCMO layer of BCZT/LCMO/LSAT and BCZT/LCMO/ STO bilayer heterostructures suffered form an in-plane tensile strain while that of the BCZT/LCMO/LAO bilayer heterostructure is under an in-plane compressive strain, and the BCZT layers of all bilayer heterostructures are subjected to an in-plane compressive strain in descending order of LAO > LSAT > STO. Above all, we successfully turn strain engineering in the bilayer epitaxial heterostructures by grown on different lattice-mismatched substrates. Fig. 2(a)–(c) displays the AFM images of the BCZT/LCMO bilayer heterostructures grown on LAO, LSAT and STO substrates, respectively, which clearly disclose the uniform film growth and smooth surfaces of the heterostructure films. The root-mean-square (RMS) roughness of the BCZT/LCMO/LAO, BCZT/LCMO/LSAT and BCZT/LCMO/STO bilayer heterostructures are 0.814 nm, 0.800 nm and 0.763 nm, respectively, while the average grain size both are approximately 20.0 nm with no significant difference. It seems like that the smaller the in-plane compressive strain in the BCZT layer is, the smoother morphology of the bilayer heterostructures.
In this paper, the ferroelectric layer of the ME heterostructure is the environmentally-friendly Ba0.85Ca0.15Zr0.9Ti0.1O3 (BCZT), is regarded as one of the most promising lead-free piezoelectric materials with equivalent piezoelectric properties to lead-based piezoelectric materials [16]. Besides, perovskite lanthanum manganite La0.67Ca0.33MnO3 (LCMO) exhibits relatively large magnetostrictive coefficient and compatible lattice parameters to BCZT ferroelectric layer [17,18], and is thereby chosen as the ferromagnetic layer of the ME heterostructure. More importantly, LCMO has metallic conductivity and can be used as a bottom electrode, eliminating the process of introducing an external electrode [19]. As mentioned above, we present a detail study on the substrate-imposed strain engineering of multiferroic properties in Ba0.85Ca0.15Zr0.9Ti0.1O3/La0.67Ca0.33MnO3 (BCZT/LCMO) bilayer heterostructures, which were grown on LAO, LSAT and STO three different single-crystalline substrates with (0 0 1) orientation. 2. Experimental BCZT/LCMO bilayer heterostructures with bottom LCMO layer (~50 nm) and top BCZT layer (~100 nm), were epitaxial grown on (0 0 1) LaAlO3 (LAO, 3.788 Å), (0 0 1) (LaAlO3)0.3(Sr2AlTaO6)0.7 (LSAT, 3.868 Å) and (0 0 1) SrTiO3 (STO, 3.905 Å) single-crystalline substrates, using pulsed laser deposition technique with a laser beam of 248 nm wavelength from a KrF excimer laser. During deposition, the substrate temperature of 650 ℃ and laser frequency of 10 Hz were maintained. The laser energy and oxygen pressure of the LCMO layer were 350 mJ and 20 Pa, respectively, while those of the BCZT layer were 300 mJ and 15 Pa. The crystal structure and reciprocal space mapping (RSM) measurements of the bilayer heterostructures were performed by using a PANalytical Empyrean four-circle diffraction system. The film surface morphology and corresponding piezoresponse force microscope (PFM) were recorded by using atomic force microscope (AFM, Dimension Icon Atomic Force Microscope, Bruker). The amplitude and phase of the outof-plane piezoresponse were obtained from the AFM vertical domains signal. The cantilever used was Pt/Ir coated SCM-PIT with a stiffness value of 28 N/m. An impedance analyzer was utilized to test the dielectric properties at room temperature. The room temperature ferroelectric hysteresis loops were measured by using a ferroelectric analyzer with frequency fixed at 1 kHz. Ferromagnetic hysteresis loops were measured at 10 K by using a physical property measurement system. The dc magnetic field dependence of ME coefficient αE31 was recorded by using a Quantum Design ME measurement system.
3.2. Electrical properties 3. Results and discussion The frequency dependent dielectric constant (εr) for the BCZT/ LCMO bilayer heterostructures grown on LAO, LSAT and STO substrates measured at room temperature are displayed in Fig. 3. As can be observed that εr decreases monotonically with the increase in frequency for all bilayer heterostructures. At low frequencies, the εr decreases sharply with a slight increase in measuring frequency, which may be caused by the charge polarization resulted from the inhomogeneous dielectric structure and interfacial polarization [21]. The εr decreases sharply higher values of the measuring frequency should be resulted from the lack of ability of dipoles to follow the fast-changing applied electric field at higher frequency, similar phenomenon has also been found in BCZT/CoFe2O4 bilayer film [22]. Moreover, the εr values at 1 kHz are 581.2, 697.0 and 932.9 for the BCZT/LCMO/LAO, BCZT/ LCMO/LSAT and BCZT/LCMO/STO bilayer heterostructures, respectively. The order of the εr values for the bilayer heterostructures is contrary to the order of the in-plane compressive strain in BCZT layers, revealing that the diminution of in-plane compressive strain in the BCZT layers results in the enhancement of εr. Some related reports [23–25] have pointed out that the two-dimensional compression with the direction applied normal to the axis of polarization could lead to a reduction in the dielectric constant of ferroelectric materials. This is
3.1. Structure analysis Fig. 1(a)–(c) present the X-ray diffraction (XRD) patterns of the BCZT/LCMO bilayer heterostructures deposited on LAO, LSAT and STO substrates, respectively. Only exists the (0 0 l) diffraction peaks belonged to the bilayer heterostructures and corresponding substrates indicate that all bilayer heterostructures display single-crystal structure with preferred orientation along the substrate reflections. Fig. 1(d) displays the rocking curves of the LCMO layer and BCZT layer for the BCZT/LCMO bilayer heterostructures deposited on different substrates. The obtained full width at half-maximum (FWHM) of the bilayer heterostructures is collected in Table 1. The FWHM values of the LCMO layers are 0.15°, 0.07° and 0.11° while those of the BCZT layers are 0.33°, 0.17° and 0.13° for the bilayer heterostructures deposited on LAO, LSAT and STO substrates, respectively. For LCMO layer, the minimum FWHM value of the BCZT/LCMO/LSAT bilayer heterostructure suggests the best crystallinity of the LCMO layer, attributing to the lowest lattice mismatch between the LSAT substrate and LCMO layer. For BCZT layer, the increased FWHM value indicates that the crystallinity of the BCZT layer is getting better when the substrate is 2
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Fig. 1. XRD θ–2θ scans and corresponding XRD RSMs of the BCZT/LCMO bilayer heterostructures grown on (a) LAO, (b) LSAT and (c) STO substrates. (d) XRD rocking curves of the LCMO and BCZT layers for BCZT/LCMO bilayer heterostructures grown on different substrates.
largest remnant polarization (Pr) at 1000 kV/cm of the BCZT/LCMO/ LAO, BCZT/LCMO/LSAT and BCZT/LCMO/STO bilayer heterostructures are 7.9μC/cm2, 16.6 μC/cm2 and 25.1 μC/cm2, respectively, and their corresponding electric coercive field (EC) are 148.2 kV/cm, 183.9 kV/cm and 364.2 kV/cm. The Pr of the bilayer heterostructures is in ascending order of LAO < LSAT < STO, which should be closely related to the improving domain wall motion that could be affected by several factors (e.g., grain size, clamping effect) [26,27]. As described above, the BCZT layer in all bilayer heterostructures shows similar grain sizes but different substrate-imposed in-plane compressive strains. Previous works [28,29] have found that polarization switching and the domain wall motion would be hampered by the pinning effect resulted from the substrate-imposed strain. As a result, the BCZT/LCMO/STO bilayer heterostructure exhibits the largest Pr value due to the reduction of the domain wall pinning. The polarization switching behaviors of the BCZT/LCMO bilayer heterostructures deposited on LAO, LSAT and STO substrates were illustrated in Fig. 5(a)–(c). The out-of-plane PFM images of all samples show a box-in-box switching with a positive + 8 V (1 μm × 1 μm) and negative −8 V (3 μm × 3 μm) probe bias switching. The uniform yellow tone of the PFM images reveals that the out-of-plane polarization states point upwards in all bilayer heterostructures, and therefore the purple tone represents the polarization states point downwards. The writing box-in-box patterns using opposite tip bias demonstrates that
Table 1 Structure parameters and strains of the BCZT/LCMO bilayer heterostructures deposited on LAO, LSAT and STO substrates. Samples
FWMH/° a/Å c/Å c/a εa/% εc/%
BCZT/LCMO/LAO
BCZT/LCMO/LSAT
BCZT/LCMO/STO
LCMO
BCZT
LCMO
BCZT
LCMO
BCZT
0.15 3.790 3.911 1.032 −1.76 1.37
0.33 3.996 4.048 1.013 −0.28 0.57
0.07 3.866 3.853 0.997 0.21 −0.13
0.17 4.002 4.044 1.010 −0.13 0.47
0.11 3.904 3.831 0.981 1.19 −0.70
0.13 4.004 4.039 1.009 −0.08 0.35
because the strain in the ferroelectric thin films can modify the ionic positions and vibrations, which are coupled to the polarization mechanism in the ferroelectric materials. The polarization versus electric field (P-E) hysteresis loops of the BCZT/LCMO bilayer heterostructures deposited on LAO, LSAT and STO substrates are displayed in Fig. 4(a)–(c), respectively, measured at different electric field amplitude but fixed at 1 kHz. The P-E hysteresis loops of all bilayer heterostructures are gradually getting saturated and fully developed as the applied electric field increases, which show the typical ferroelectric hysteresis loops, confirming the ferroelectric nature of all samples at room temperature. Additionally, the determined 3
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Fig. 2. AFM images of the BCZT/LCMO bilayer heterostructures grown on (a) LAO, (b) LSAT and (c) STO substrates.
downward polarization switching, and thereby hinders upward polarization rotation. In addition, the effective piezoelectric coefficient d33 determined from the amplitude-voltage butterfly loops are 29.8 ± 5 pm/V, 36.9 ± 5 pm/V and 42.2 ± 5 pm/V for the BCZT/ LCMO/LAO, BCZT/LCMO/LSAT and BCZT/LCMO/STO bilayer heterostructures, respectively. The piezoelectric performance of the bilayer heterostructures, is in ascending order of LAO < LSAT < STO, which is analogous to its ferroelectric behaviors. This is because of the minimum compressive strain in the BCZT layer of the BCZT/LCMO/STO bilayer heterostructure, which show improved domain wall motion compared to the other two samples.
3.3. Ferromagnetic and magnetoelectric properties Fig. 7 presents the magnetic hysteresis (M−H) loops of the BCZT/ LCMO bilayer heterostructures deposited on LAO, LSAT and STO substrates. The well-shaped M−H loops confirm the ferromagnetic nature of all bilayer heterostructures. It can be found that the saturated magnetization (MS) of the bilayer heterostructures is in descending order of LAO > LSAT > STO, while the magnetic coercive field (MC) is in descending order of LAO > STO > LSAT. Since the BCZT layer is a ferroelectric phase, the ferromagnetic properties of the bilayer heterostructures should stem from the ferromagnetic LCMO layer, indicating that the substrate-imposed strain has an impact on the ferromagnetic properties as a result of the strain-induced distortion of MnO6 octahedra in the LCMO layer. In most cases, the in-plane compressive strain increases the Mn-O bond angle which would improve the electronic hopping amplitude and in turn enhance ferromagnetism, while the tensile strain is reversed [32,33]. As mentioned in the XRD analysis, the LCMO layer of BCZT/LCMO/LSAT and BCZT/LCMO/STO bilayer heterostructures suffered form an in-plane tensile strain while that of the BCZT/LCMO/LAO bilayer heterostructure is under an in-plane compressive strain. As a result, the BCZT/LCMO/LAO bilayer heterostructure shows the largest MS, while the BCZT/LCMO/STO bilayer heterostructure exhibits the minimum MS. It should also be noted that the increasing in lattice distortion in LCMO layer could cause stronger
Fig. 3. Frequency dependent dielectric constant of the BCZT/LCMO bilayer heterostructures grown on LAO, LSAT and STO substrates.
the out-of-plane polarization of all bilayer heterostructures is electrically switchable. Furthermore, the contrast between outside unpoled region and the positively poled region is minimal in the BCZT/LCMO/ STO bilayer heterostructure, suggesting that the domains are primarily aligned upwards at their virgin states. Fig. 6(a)–(c) display the local PFM hysteresis loops of amplitude and phase for the BCZT/LCMO bilayer heterostructures grown on LAO, LSAT and STO substrates, respectively. It is obvious that the butterfly loops are well-shaped and the piezoresponse phases are almost 0-180° reversibly switchable, revealing the good switching behavior for all bilayer heterostructures. Notably, the amplitude-voltage butterfly loops with varying degrees of asymmetry behavior could be observed in all bilayer heterostructures, which has also been reported in the previous works [30,31]. This phenomenon should be ascribed to the electrode self-poling effect caused built-in electric field, which in turn improves
Fig. 4. P-E hysteresis loops of the BCZT/LCMO bilayer heterostructures grown on (a) LAO, (b) LSAT and (c) STO substrates. 4
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Fig. 5. Out-of-plane PFM images of the BCZT/LCMO bilayer heterostructures grown on (a) LAO, (b) LSAT and (c) STO substrates.
pinning centers for the magnetic domains, and thus result in a reinforcement in the coercivity [34]. As can be deduced from the Table 1, the LCMO layer of the BCZT/LCMO/LAO bilayer heterostructure is subjected to the largest residual strain, that is, the largest lattice distortion. Therefore, the BCZT/LCMO/LAO bilayer heterostructure displays the maximum MC. The magnetic field as a function of the transverse ME coefficient (αE31) for the BCZT/LCMO bilayer heterostructures deposited on LAO, LSAT and STO substrates are displayed in Fig. 8, which were performed at room temperature with the applied magnetic field parallel to the heterostructure surface. Results show that the αE31 value increases monotonically with increasing applied magnetic field, and then get saturated once at a certain magnetic field. The certain magnetic field at saturation is approximately 7000 Oe, 4000 Oe and 6000 Oe for the BCZT/LCMO/LAO, BCZT/LCMO/LSAT and BCZT/LCMO/STO bilayer heterostructures, respectively. This should be explained by the free magnetostriction of the LCMO layer limited by the substrate-imposed clamping effect. The greater the substrate-imposed strain in the LCMO layer is, the larger the applied magnetic field is required to get saturated. Consequently, the BCZT/LCMO/LAO bilayer heterostructure is in need of the largest applied magnetic field. In addition, the maximum values of αE31 are 26.1, 40.4 and 61.2 mV/cm·Oe for the BCZT/LCMO/ LAO, BCZT/LCMO/LSAT and BCZT/LCMO/STO bilayer heterostructures, respectively. As we all known, the ME coupling effect is a cross-coupling effect between the piezoelectricity of ferroelectric phase and magnetostriction of ferromagnetic phase, which is very sensitive to the strain state of the BCZT and LCMO layers imposed by the substrate [35,36]. As a result, the BCZT/LCMO/LAO bilayer heterostructure shows the smallest αE31 value due to the largest substrate-imposed strain in LCMO layer and the minimum piezoelectric coefficient, while the BCZT/LCMO/STO bilayer heterostructure exhibits the largest αE31 value attributed to the strongest piezoelectric effect.
Fig. 7. M−H loops of the BCZT/LCMO bilayer heterostructures deposited on LAO, LSAT and STO substrates. The inset displays the partially enlarged loops at low field region.
4. Conclusions In summary, epitaxial bilayer heterostructures of BCZT/LCMO were grown on various single-crystalline substrates by using pulsed laser deposition technique. The XRD and RSM analysis verified that all bilayer heterostructures were epitaxial grown on substrates with varying strain states. Results showed that the substrate-imposed strain can modulate the polarization switching and domain wall motion, and thus modified the electrical, ferromagnetic as well as ME properties of the bilayer heterostructures. The optimal electrical properties were achieved in the BCZT/LCMO bilayer heterostructure deposited on STO substrate while the greatest saturated magnetization was obtained in the BCZT/LCMO bilayer heterostructure grown on LAO substrate. Furthermore, the BCZT/LCMO bilayer heterostructure deposited on
Fig. 6. Local amplitude-voltage butterfly loops and phase-voltage hysteresis loops of the BCZT/LCMO bilayer heterostructures grown on (a) LAO, (b) LSAT and (c) STO substrates. 5
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Fig. 8. Magnetic field as a function of the αE31 coefficients of the BCZT/LCMO bilayer heterostructures grown on (a) LAO, (b) LSAT and (c) STO substrates.
STO substrate displays the largest magnetoelectric coefficient with αE31 of 61.2 mV/cm·Oe. These findings reveal that substrate-imposed strain engineering in ME heterostructure can turn the multiferroic properties toward to application for the new type multiferroic functional devices. CRediT authorship contribution statement Mingzhe Hu: Conceptualization, Methodology, Writing - original draft. Songbin Li: Data curation, Formal analysis. Chuanbin Wang: Funding acquisition, Writing - review & editing. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement This work was financially supported by the National Natural Science Foundation of China (51972252, 51567017), 111 Project (B13035), the National Science Foundation of Guizhou Minzu University (GZMU [2019]YB20) and the Fundamental Research Funds for the Central Universities (WUT: 2019III029). References [1] L.W. Martin, R. Ramesh, Multiferroic and magnetoelectric heterostructures, Acta Mater. 60 (6–7) (2012) 2449–2470. [2] J. Ma, J.M. Hu, Z. Li, C.W. Nan, Recent progress in multiferroic magnetoelectric composites: from bulk to thin films, Adv. Mater. 42 (18) (2011) 1062–1087. [3] M. Naveed-Ul-Haq, V.V. Shvartsman, H. Trivedi, S. Salamon, S. Webers, H. Wende, U. Hagemann, J. Schröder, D.C. Lupascu, Strong converse magnetoelectric effect in (Ba, Ca)(Zr, Ti)O3-NiFe2O4 multiferroics: a relationship between phase-connectivity and interface coupling, Acta Mater. 144 (2018) 305–313. [4] X. Yang, Z. Zhou, T. Nan, Y. Gao, G.M. Yang, M. Liu, N.X. Sun, Recent advances in multiferroic oxide heterostructures and devices, J. Mater. Chem. C 4 (2) (2016) 234–243. [5] M. Fiebig, T. Lottermoser, D. Meier, M. Trassin, The evolution of multiferroics, Nat. Rev. Mater. 4 (2) (2019) 146. [6] R. Ramesh, N.A. Spaldin, Multiferroics: progress and prospects in thin films, Nat. Mater. 6 (1) (2007) 21–29. [7] G. Schileo, C. Pascual-Gonzalez, M. Alguero, I.M. Reaney, P. Postolache, L. Mitoseriu, K. Reichmann, M. Venet, A. Feteira, Multiferroic and magnetoelectric properties of Pb0.99[Zr0.45Ti0.47(Ni1/3Sb2/3)0.08]O3-CoFe2O4 multilayer composites fabricated by tape casting, J. Eur. Ceram. Soc. 38 (4) (2018) 1473–1478. [8] G. Zhang, S. Dong, Z. Yan, Y. Guo, Q. Zhang, S. Yunoki, E. Dagotto, J.M. Liu, Multiferroic properties of CaMn7O12, Phys. Rev. B 84 (17) (2011) 174413. [9] X.X. Shi, X.Q. Liu, X.M. Chen, Readdressing of magnetoelectric effect in bulk BiFeO3, Adv. Funct. Mater. 27 (12) (2017) 1604037.
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