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LCMO bilayer heterostructures
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Orientation-modulated multiferroic properties of BCZT/LCMO bilayer heterostructures Mingzhe Hua, Songbin Lib, Chuanbin Wangb,∗ a b
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: Orientation BCZT/LCMO heterostructure Multiferroic properties Magnetoelectric properties
In this paper, orientation-modulated multiferroic properties of Ba0.85Ca0.15Zr0.1Ti0.9O3/La0.67Ca0.33MnO3 (BCZT/LCMO) bilayer heterostructures grown on (001), (110), and (111)-oriented Nb-doped SrTiO3 singlecrystal substrates are investigated. Epitaxially grown heterostructures with sharp and clear interfaces are verified by TEM analysis. Results show the strong orientation dependence of multiferroic properties for the heterostructures. The optimal ferroelectric and dielectric properties lie in the (001) direction, whereas the improved saturation magnetization and piezoelectric response are achieved in (111)-oriented heterostructure. The maximum magnetoelectric coupling is obtained in (111)-oriented heterostructure with a αE31 value of 207 mV/ cm·Oe.
1. Introduction Magnetoelectric (ME) multiferroics are those materials possessing simultaneous ferroelectric ordering and ferromagnetic ordering along with the coupling between them, which have attracted ever-increasing interest owing to the great prospect for application in high sensitivity transducers, multiple-state memories and so on [1–5]. According to the phase constituents, these materials can be classified as single-phase ME materials where both ferroelectricity and ferromagnetism occur in one single phase, and artificially multi-phase ME composites comprising ferroelectric and ferromagnetic phases connected in a certain configuration. However, very little single-phase ME materials have been discovered due to the incompatible between the ferroelectric and ferromagnetic orderings [6,7]. Furthermore, the observed ME coupling effect in these single-phase ME materials is very small or only exists at far below room temperature, restricting their practical applications [8,9]. Consequently, the artificially multi-phase ME composites have been developed, which can exhibit room temperature ME coefficient greater than that of the single-phase ME materials [10,11]. The coupling mechanisms for multi-phase ME composites are closely associated with the cross-coupling between the ferromagnetic-ferroelectric interfaces and ferroic odering [12,13]. In recent past, as the devices design trends to miniaturization and multifunctionality, the research emphasis of multi-phase multiferroic composites has been converged on film-based ME composites [14,15].
∗
As compared with the ME bulk composites, the ME composite thin films possess more freedom and flexibility, i.e., film thickness, epitaxial strain and substrate orientation, to design and optimize their ME behaviors [16–19]. In addition, the ME composite thin films can facilitate the understanding of physical mechanism associated with ME coupling in atomic scale and enable the design of novel phase structures for application in integrated magnetic and electric devices. In this paper, the ferroelectric component of the bilayer heterostructure is Ba0.85Ca0.15Zr0.9Ti0.1O3 (BCZT), known as one of the most promising lead-free ferroelectric materials exhibiting piezoelectric properties equivalent to the lead-based toxic piezoelectric materials [20]. Furthermore, perovskite lanthanum manganite La0.67Ca0.33MnO3 (LCMO) displays relatively large magnetostrictive coefficient and similar lattice parameters to BCZT piezoelectric material [21–23], and is therefore selected as the ferromagnetic phase of the bilayer heterostructure. In previous works [24,25], it has been found that BaTiO3based thin film with different epitaxial orientations represents discrepant electrical properties, including piezoelectric constant and saturation polarization, owing to the orientation engineered relative aligned crystallites and spontaneous polarization vector. In another paper, Pei et al. [26] reported (111)-oriented BaTiO3/La0.67Sr0.33MnO3 epitaxial thin film displayed stronger ME effect than the (001)-oriented one because of the increased shared oxygen atoms at the interface. It seems like that different crystallographic orientations could significantly affect the electrical polarization and the ME coupling of the
Corresponding author. E-mail address: [email protected] (C. Wang).
https://doi.org/10.1016/j.ceramint.2019.10.051 Received 5 August 2019; Received in revised form 14 September 2019; Accepted 5 October 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Mingzhe Hu, Songbin Li and Chuanbin Wang, Ceramics International, https://doi.org/10.1016/j.ceramint.2019.10.051
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multiferroic heterostructure. As far as we know, there is no work has been conducted on orientation-modulated multiferroic properties of BCZT/LCMO bilayer heterostructures. Therefore, the artificially multiferroic bilayer heterostructures built by combining the ferroelectric phase BCZT and ferromagnetic phase LCMO, were deposited on (001), (110), and (111)-oriented Nb-doped SrTiO3 single-crystal substrates, to discuss the multiferroic behaviors of heterostructures in relation with the crystallographic orientations.
Table 1 The structure parameters of the LCMO and BCZT layers for the (001), (110), and (111)-oriented multiferroic BCZT/LCMO heterostructures. Orientations
a (Å) c (Å) c/a εa (%) εc (%)
2. Experiment Epitaxial Ba0.85Ca0.15Zr0.1Ti0.9O3/La0.67Ca0.33MnO3 (BCZT/LCMO) bilayer heterostructures with bottom LCMO layer and top BCZT layer, were grown on (001), (110), and (111)-oriented Nb-doped SrTiO3 (Nb:STO) single-crystal substrates, using pulsed laser deposition technique with a laser beam of 248 nm wavelength from a KrF excimer laser. The LCMO and BCZT films were deposited at 650 °C, with laser energy (frequency ~ 10 Hz) of 300 mJ and 350 mJ and oxygen pressure of 15 Pa and 20 Pa, respectively. The crystalline and cross-sectional structure of the bilayer heterostructures were observed by using a PANalytical Empyrean four-circle diffraction system and transmission electron microscopy, respectively. The room temperature ferroelectric hysteresis loops were recorded by using a ferroelectric analyzer with frequency fixed at 1 kHz. An impedance analyzer was utilized to determine the room temperature dielectric properties. The piezoresponse force microscopy was used to observe the local piezoelectric hysteresis loops. Ferromagnetic hysteresis loops were recorded 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.
(001)
(110)
(111)
LCMO
BCZT
LCMO
BCZT
LCMO
BCZT
3.901 3.835 0.983 1.11 −0.60
4.002 4.048 1.011 −0.13 0.57
3.900 3.839 0.984 1.09 −0.49
3.997 4.050 1.013 −0.25 0.62
3.898 3.847 0.987 1.04 −0.29
3.991 4.052 1.015 −0.40 0.67
Table 1, determined from the corresponding RSM peaks. It can be found that the lattice parameter a of LCMO layer and BCZT layer both decreases while the lattice parameter c of LCMO layer and BCZT both increases with the heterostructure orientation changing from (001) to (111), resulting in the increase in c/a ratio. In addition, the in-plane (εa) and out-of-plane (εc) strains of the bilayer heterostructures are followed by εa = a-a0/a0 and εc = c-c0/c0, respectively. Where a0, c0 are the LCMO (a0 = c0 = 3.858 Å) [27] or BCZT (a0 = 4.007 Å, c0 = 4.025 Å) [20] bulk values; a, c are the lattice parameters of the BCZT film or LCMO film. The calculated εa and εc values of the LCMO and BCZT layers of the heterostructures with different orientations are collected in Table 1. Results showed that the LCMO layer is suffered from an inplane tensile stain and out-of-plane compress stain while the BCZT layer is subjected to an in-plane compress stain and out-of-plane tensile stain for all heterostructures. As the heterostructure orientation varying from (001) to (111), the in-plane tensile stain in LCMO layer decreases while the in-plane compress stain in BCZT layer increases. TEM analysis was performed to investigate the cross-sectional microstructure at the LCMO-Nb:STO and BCZT-LCMO interfaces. The HRTEM images and corresponding SAED patterns of the LCMO-Nb:STO and BCZT-LCMO interfaces for multiferroic BCZT/LCMO heterostructures with various orientations are shown in Fig. 2 (a)-(f). The HRTEM images clearly disclose the sharp and well-defined LCMONb:STO and BCZT-LCMO interfaces for all heterostructures, and the BCZT layer, LCMO layer and Nb:STO substrate are hetero-epitaxial arrangement well with each other. In addition, for all heterostructures, the diffraction spots of the Nb:STO substrate and LCMO layer are fully overlapped in the same order while those of the LCMO and BCZT layers are almost coincident. The ordered SAED diffraction spots reveal the crystallization of good quality for all heterostructures, which further proof the well epitaxial relationship between LCMO layer, BCZT layer and Nb:STO substrates.
3. Results and discussion 3.1. Structure Fig. 1 (a)-(c) show the XRD diffraction peaks of the BCZT/LCMO bilayer heterostructures grown on (001), (110), and (111)-oriented Nb:STO substrates, respectively. It is obvious that all heterostructures display single-crystal structure with preferred orientation along the substrate reflections. The insets in Fig. 1 (a)–(c) present the reciprocal space mapping (RSM) around the asymmetric (103), (103), and (112) reflections for the (001), (110), and (111)-oriented BCZT/LCMO heterostructures, respectively. It is obvious that the RSM peaks of LCMO layers for all heterostructures are aligned with its substrate peaks while those of 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 layer and BCZT layer for the bilayer heterostructures with various orientations are listed in
3.2. Electrical properties Fig. 3 presents the ferroelectric hysteresis loops of (001), (110), and
Fig. 1. XRD θ-2θ diffraction peaks of the multiferroic BCZT/LCMO heterostructures grown on the (a) (001), (b) (110), and (c) (111)-oriented Nb:STO. The insets are the corresponding RSM figures. 2
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Fig. 2. High-resolution TEM (HRTEM) images and corresponding selected area electron diffraction (SAED) pattern of the LCMO-Nb:STO and BCZT-LCMO interfaces for the heterostructures grown on the (a) and (b) (001), (c) and (d) (110), and (e) and (f) (111)-oriented Nb:STO substrates, respectively.
5.0 μC/cm2 while the coercive field (EC) are 194.2 kV/cm, 163.8 kV/cm and 146.6 kV/cm for the (001), (110), and (111)-oriented BCZT/LCMO heterostructures, respectively. The ferroelectric performance of the heterostructures is in descending order of (001) > (110) > (111) orientations, which has also been found in the previous works [24,26]. The ferroelectricity of the BCZT/LCMO bilayer heterostructures derived from the ferroelectric BCZT phase that is known as a principally tetragonal-like perovskite structure at room temperature whose spontaneous polarization vector lies in the [001] crystallization direction. As a result, the (001)-oriented heterostructure exhibits the largest Pr value, followed by the (110)-oriented heterostructure, and the (111)-oriented heterostructure shows the minimum value. This is because the projection of the [001] spontaneous polarization direction along the heterostructure orientation reduces from the (001) to (111). Fig. 4 exhibits the frequency dependence of εr and tan δ for the (001), (110), and (111)-oriented heterostructures. It is obvious that εr of all heterostructures tends to decrease with higher frequencies, while tanδ reduces firstly then increases with increasing frequencies. The frequency dispersion dielectric behaviors of the different oriented heterostructures are similar to those found in BCZT thin films [28,29]. Furthermore, the value of εr at 1 kHz are approximately 935, 667 and 615, and the value of tanδ are 0.2, 0.13 and 0.07 for (001), (110), and (111)-oriented heterostructures, respectively. The anisotropic dielectric behaviors in the different oriented BCZT/LCMO heterostructures may be ascribed to the different amplitude of the polarization along the heterostructure orientation or the varying degree of residual strain [30].
Fig. 3. P-E hysteresis loops of the (001), (110), and (111)-oriented multiferroic BCZT/LCMO heterostructures. The inset table presents the values of Pr and Ec.
(111)-oriented multiferroic BCZT/LCMO heterostructures. The wellshaped ferroelectric hysteresis loops demonstrate the ferroelectric nature of all bilayer heterostructures. As can be seen from the inset table, the remnant polarization (Pr) are 11.7 μC/cm2, 7.9 μC/cm2 and 3
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Fig. 6. Magnetic hysteresis (M-H) loops of the (001), (110), and (111)-oriented multiferroic BCZT/LCMO heterostructures.
Fig. 4. Frequency dependence of dielectric constant (εr) and dielectric loss (tan δ) for the (001), (110), and (111)-oriented multiferroic BCZT/LCMO heterostructures.
between the heterostructure orientation and [001] spontaneous polarization direction in (111)-oriented heterostructure has the maximum piezoelectric response.
The local PFM hysteresis loops of amplitude and phase for the (001), (110) and (111)-oriented multiferroic BCZT/LCMO heterostructures are shown in Fig. 5 (a)-(c), respectively. It can be observed that the butterfly loops are well-shaped and the piezoresponse phases are almost 0–180° reversibly switchable, confirming the good switching behavior for all bilayer heterostructures. Besides, the asymmetry behavior observed in the PFM hysteresis loops should be resulted from the electrode self-poling effect caused built-in electric field [31]. The calculated effective piezoelectric coefficient d33 value reveals that the (111)-oriented BCZT/LCMO heterostructure shows the maximum value of 68 p.m./V, while the (001), and (110)-oriented heterostructures exhibits the relatively smaller values of 45 p.m./V and 56 p.m./V, respectively. The piezoelectric performance of the heterostructures is in ascending order of (001)＜(110)＜(111), which is in sharp contrast to its ferroelectric behaviors that show the opposite relation order. As compared with the (001), and (110)-oriented heterostructures, the above-mentioned larger c/a value can be considered as one of the reasons for the enhanced piezoelectric property in the (111)-oriented heterostructure due to the larger tetragonal lattice distortion in BCZT layer [24]. Another reason for the piezoelectric anisotropy of the heterostructures may be explicated by the model of engineered domain configuration [32] for the BCZT layer with tetragonal distortion, which reveals that the piezoelectric constant is larger along the non-polar direction due to the easy tilt of the polar vector by the electric field. Once a (001) direction electric field is applied to the heterostructures, the polar vectors would rotate towards the direction of the applied field, leading to improved tetragonal lattice distortion, and therefore the enhanced piezoelectric property [25]. As a result, the biggest angle relationship
3.3. Ferromagnetic properties M-H loops of the multiferroic BCZT/LCMO bilayer heterostructures with various orientations are shown in Fig. 6, which were measured with the direction of applied magnetic field parallel to the heterostructure surface (in-plane). The well-shaped M-H loops demonstrate the ferromagnetic nature of the different oriented multiferroic BCZT/ LCMO heterostructures. It is obvious that the saturated magnetization (MS) of the heterostructures increases from (001)-oriented heterostructure to (111)-oriented heterostructure. This trend is like those reported earlier for BaTiO3/La0.67Sr0.33MnO3 heterostructures [26], which is closely related to the strain-imposed MnO6 octahedra distortion in manganite thin films. In most cases, the in-plane tensile strain decreases the Mn–O bond angle which will enhance the electronphonon interaction and weaken the electronic hopping amplitude, and in turn suppress ferromagnetism. As mentioned above, the LCMO layers of the heterostructures are subjected to an in-plane tensile strain which decreases from (001)-oriented heterostructure to (111)-oriented heterostructure. As a consequence, the (111)-oriented heterostructure exhibits the larger MS compared with the other two orientations. 3.4. Magnetoelectric properties The room temperature transverse ME coefficient (αE31) coefficients as a function of magnetic field for the (001), (110), and (111)-oriented
Fig. 5. Local PFM hysteresis loops of amplitude and phase for the (a) (001), (b) (110), and (c) (111)-oriented multiferroic BCZT/LCMO heterostructures. 4
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dependence on the crystallographic orientations. Results showed that the biggest ferroelectric and dielectric responses were obtained in (001)-oriented heterostructure, while the maximum saturation magnetization and piezoelectric constant were achieved in (111)-oriented heterostructure. Furthermore, the (111)-oriented heterostructure has the largest magnetoelectric coefficient with αE31 of 207 mV/cm·Oe. Our results suggest that the crystallographic orientation is a significant factor in modulating the multiferroic properties of the ferroelectric/ ferromagnetic heterostructures. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (51972252, 51567017), the Natural Science Foundation of Hubei Province (2016CFA006), the National Science Foundation of Guizhou Minzu University and the Fundamental Research Funds for the Central Universities (WUT: 2019Ⅲ029). References Fig. 7. Transverse ME coefficient (αE31) coefficients as a function of magnetic field for the (001), (110), and (111)-oriented multiferroic BCZT/LCMO heterostructures.
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multiferroic BCZT/LCMO heterostructures are displayed in Fig. 7, which were performed with the applied magnetic field parallel to the heterostructure surface. Notably, the value of αE31 increases monotonically with the increase of magnetic field for the (001), (110), and (111)-oriented heterostructures, and then get saturated until the magnetic field reaches about 6000 Oe. Furthermore, the achieved maximum values of αE31 of the (001), (110), and (111)-oriented heterostructures are 153 mV/cm·Oe, 180 mV/cm·Oe and 207 mV/cm·Oe, respectively. These values of αE31 are comparable to the reported ME values in La0.67Sr0.33MnO3/PbZr0.52Ti0.48O3 [33] heterostructures, and larger than most of the lead-free systems such as Bi3.15Nd0.85Ti3O12/ La0.7Ca0.3MnO3 [34] and BaTiO3/La0.7Sr0.3MnO3 [35] heterostructures. On the other hand, the (111)-oriented heterostructure displays the optimal ME effect compared with the (001), and (110)-oriented heterostructures, whereas the (110)-oriented heterostructure exhibits a better ME effect than the (001)-oriented heterostructure. There may be two reasons for the enhanced ME effect in (111)-oriented heterostructure. One is owing to the largest piezoelectric coefficient in the (111)-oriented heterostructure, which would improve the ME response due to the coupling effect of piezoelectricity and magnetostriction [36]. The other is the increased shared oxygen atoms at the BCZT-LCMO interface in the (111)-oriented heterostructure [26,37]. The increase in shared oxygen atoms will lead to efficient stress transfer which should in favor of a stronger ME coupling. For (001)-oriented heterostructure, there are only one oxygen atoms shared at the BCZT-LCMO interface per unit cell, whereas there are two and three oxygen atoms for the (110), and (111)-oriented heterostructures, respectively. Accordingly, (111)-oriented heterostructure shows the largest ME coupling response among all heterostructures.
4. Conclusions In summary, multiferroic heterostructures composed of the bottom LCMO layer and the top BCZT layer were deposited on (001), (110), and (111) Nb:STO single-crystal substrates by using pulsed laser deposition technique. XRD and TEM analysis verified that all heterostructures exhibited single-crystal structure and were epitaxially grown on the Nb:STO substrate with sharp and well-defined interfaces. The ferromagnetic and ferroelectric hysteresis loops were investigated, revealing the multiferroic nature of the heterostructures. It was found that the multiferroic properties of the heterostructures displayed strong 5
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Orientation-modulated multiferroic properties of BCZT/LCMO bilayer heterostructures Mingzhe Hua, Songbin Lib, Chuanbin Wangb,∗ a b
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: Orientation BCZT/LCMO heterostructure Multiferroic properties Magnetoelectric properties
In this paper, orientation-modulated multiferroic properties of Ba0.85Ca0.15Zr0.1Ti0.9O3/La0.67Ca0.33MnO3 (BCZT/LCMO) bilayer heterostructures grown on (001), (110), and (111)-oriented Nb-doped SrTiO3 singlecrystal substrates are investigated. Epitaxially grown heterostructures with sharp and clear interfaces are verified by TEM analysis. Results show the strong orientation dependence of multiferroic properties for the heterostructures. The optimal ferroelectric and dielectric properties lie in the (001) direction, whereas the improved saturation magnetization and piezoelectric response are achieved in (111)-oriented heterostructure. The maximum magnetoelectric coupling is obtained in (111)-oriented heterostructure with a αE31 value of 207 mV/ cm·Oe.
1. Introduction Magnetoelectric (ME) multiferroics are those materials possessing simultaneous ferroelectric ordering and ferromagnetic ordering along with the coupling between them, which have attracted ever-increasing interest owing to the great prospect for application in high sensitivity transducers, multiple-state memories and so on [1–5]. According to the phase constituents, these materials can be classified as single-phase ME materials where both ferroelectricity and ferromagnetism occur in one single phase, and artificially multi-phase ME composites comprising ferroelectric and ferromagnetic phases connected in a certain configuration. However, very little single-phase ME materials have been discovered due to the incompatible between the ferroelectric and ferromagnetic orderings [6,7]. Furthermore, the observed ME coupling effect in these single-phase ME materials is very small or only exists at far below room temperature, restricting their practical applications [8,9]. Consequently, the artificially multi-phase ME composites have been developed, which can exhibit room temperature ME coefficient greater than that of the single-phase ME materials [10,11]. The coupling mechanisms for multi-phase ME composites are closely associated with the cross-coupling between the ferromagnetic-ferroelectric interfaces and ferroic odering [12,13]. In recent past, as the devices design trends to miniaturization and multifunctionality, the research emphasis of multi-phase multiferroic composites has been converged on film-based ME composites [14,15].
∗
As compared with the ME bulk composites, the ME composite thin films possess more freedom and flexibility, i.e., film thickness, epitaxial strain and substrate orientation, to design and optimize their ME behaviors [16–19]. In addition, the ME composite thin films can facilitate the understanding of physical mechanism associated with ME coupling in atomic scale and enable the design of novel phase structures for application in integrated magnetic and electric devices. In this paper, the ferroelectric component of the bilayer heterostructure is Ba0.85Ca0.15Zr0.9Ti0.1O3 (BCZT), known as one of the most promising lead-free ferroelectric materials exhibiting piezoelectric properties equivalent to the lead-based toxic piezoelectric materials [20]. Furthermore, perovskite lanthanum manganite La0.67Ca0.33MnO3 (LCMO) displays relatively large magnetostrictive coefficient and similar lattice parameters to BCZT piezoelectric material [21–23], and is therefore selected as the ferromagnetic phase of the bilayer heterostructure. In previous works [24,25], it has been found that BaTiO3based thin film with different epitaxial orientations represents discrepant electrical properties, including piezoelectric constant and saturation polarization, owing to the orientation engineered relative aligned crystallites and spontaneous polarization vector. In another paper, Pei et al. [26] reported (111)-oriented BaTiO3/La0.67Sr0.33MnO3 epitaxial thin film displayed stronger ME effect than the (001)-oriented one because of the increased shared oxygen atoms at the interface. It seems like that different crystallographic orientations could significantly affect the electrical polarization and the ME coupling of the
Corresponding author. E-mail address: [email protected] (C. Wang).
https://doi.org/10.1016/j.ceramint.2019.10.051 Received 5 August 2019; Received in revised form 14 September 2019; Accepted 5 October 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Mingzhe Hu, Songbin Li and Chuanbin Wang, Ceramics International, https://doi.org/10.1016/j.ceramint.2019.10.051
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multiferroic heterostructure. As far as we know, there is no work has been conducted on orientation-modulated multiferroic properties of BCZT/LCMO bilayer heterostructures. Therefore, the artificially multiferroic bilayer heterostructures built by combining the ferroelectric phase BCZT and ferromagnetic phase LCMO, were deposited on (001), (110), and (111)-oriented Nb-doped SrTiO3 single-crystal substrates, to discuss the multiferroic behaviors of heterostructures in relation with the crystallographic orientations.
Table 1 The structure parameters of the LCMO and BCZT layers for the (001), (110), and (111)-oriented multiferroic BCZT/LCMO heterostructures. Orientations
a (Å) c (Å) c/a εa (%) εc (%)
2. Experiment Epitaxial Ba0.85Ca0.15Zr0.1Ti0.9O3/La0.67Ca0.33MnO3 (BCZT/LCMO) bilayer heterostructures with bottom LCMO layer and top BCZT layer, were grown on (001), (110), and (111)-oriented Nb-doped SrTiO3 (Nb:STO) single-crystal substrates, using pulsed laser deposition technique with a laser beam of 248 nm wavelength from a KrF excimer laser. The LCMO and BCZT films were deposited at 650 °C, with laser energy (frequency ~ 10 Hz) of 300 mJ and 350 mJ and oxygen pressure of 15 Pa and 20 Pa, respectively. The crystalline and cross-sectional structure of the bilayer heterostructures were observed by using a PANalytical Empyrean four-circle diffraction system and transmission electron microscopy, respectively. The room temperature ferroelectric hysteresis loops were recorded by using a ferroelectric analyzer with frequency fixed at 1 kHz. An impedance analyzer was utilized to determine the room temperature dielectric properties. The piezoresponse force microscopy was used to observe the local piezoelectric hysteresis loops. Ferromagnetic hysteresis loops were recorded 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.
(001)
(110)
(111)
LCMO
BCZT
LCMO
BCZT
LCMO
BCZT
3.901 3.835 0.983 1.11 −0.60
4.002 4.048 1.011 −0.13 0.57
3.900 3.839 0.984 1.09 −0.49
3.997 4.050 1.013 −0.25 0.62
3.898 3.847 0.987 1.04 −0.29
3.991 4.052 1.015 −0.40 0.67
Table 1, determined from the corresponding RSM peaks. It can be found that the lattice parameter a of LCMO layer and BCZT layer both decreases while the lattice parameter c of LCMO layer and BCZT both increases with the heterostructure orientation changing from (001) to (111), resulting in the increase in c/a ratio. In addition, the in-plane (εa) and out-of-plane (εc) strains of the bilayer heterostructures are followed by εa = a-a0/a0 and εc = c-c0/c0, respectively. Where a0, c0 are the LCMO (a0 = c0 = 3.858 Å) [27] or BCZT (a0 = 4.007 Å, c0 = 4.025 Å) [20] bulk values; a, c are the lattice parameters of the BCZT film or LCMO film. The calculated εa and εc values of the LCMO and BCZT layers of the heterostructures with different orientations are collected in Table 1. Results showed that the LCMO layer is suffered from an inplane tensile stain and out-of-plane compress stain while the BCZT layer is subjected to an in-plane compress stain and out-of-plane tensile stain for all heterostructures. As the heterostructure orientation varying from (001) to (111), the in-plane tensile stain in LCMO layer decreases while the in-plane compress stain in BCZT layer increases. TEM analysis was performed to investigate the cross-sectional microstructure at the LCMO-Nb:STO and BCZT-LCMO interfaces. The HRTEM images and corresponding SAED patterns of the LCMO-Nb:STO and BCZT-LCMO interfaces for multiferroic BCZT/LCMO heterostructures with various orientations are shown in Fig. 2 (a)-(f). The HRTEM images clearly disclose the sharp and well-defined LCMONb:STO and BCZT-LCMO interfaces for all heterostructures, and the BCZT layer, LCMO layer and Nb:STO substrate are hetero-epitaxial arrangement well with each other. In addition, for all heterostructures, the diffraction spots of the Nb:STO substrate and LCMO layer are fully overlapped in the same order while those of the LCMO and BCZT layers are almost coincident. The ordered SAED diffraction spots reveal the crystallization of good quality for all heterostructures, which further proof the well epitaxial relationship between LCMO layer, BCZT layer and Nb:STO substrates.
3. Results and discussion 3.1. Structure Fig. 1 (a)-(c) show the XRD diffraction peaks of the BCZT/LCMO bilayer heterostructures grown on (001), (110), and (111)-oriented Nb:STO substrates, respectively. It is obvious that all heterostructures display single-crystal structure with preferred orientation along the substrate reflections. The insets in Fig. 1 (a)–(c) present the reciprocal space mapping (RSM) around the asymmetric (103), (103), and (112) reflections for the (001), (110), and (111)-oriented BCZT/LCMO heterostructures, respectively. It is obvious that the RSM peaks of LCMO layers for all heterostructures are aligned with its substrate peaks while those of 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 layer and BCZT layer for the bilayer heterostructures with various orientations are listed in
3.2. Electrical properties Fig. 3 presents the ferroelectric hysteresis loops of (001), (110), and
Fig. 1. XRD θ-2θ diffraction peaks of the multiferroic BCZT/LCMO heterostructures grown on the (a) (001), (b) (110), and (c) (111)-oriented Nb:STO. The insets are the corresponding RSM figures. 2
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Fig. 2. High-resolution TEM (HRTEM) images and corresponding selected area electron diffraction (SAED) pattern of the LCMO-Nb:STO and BCZT-LCMO interfaces for the heterostructures grown on the (a) and (b) (001), (c) and (d) (110), and (e) and (f) (111)-oriented Nb:STO substrates, respectively.
5.0 μC/cm2 while the coercive field (EC) are 194.2 kV/cm, 163.8 kV/cm and 146.6 kV/cm for the (001), (110), and (111)-oriented BCZT/LCMO heterostructures, respectively. The ferroelectric performance of the heterostructures is in descending order of (001) > (110) > (111) orientations, which has also been found in the previous works [24,26]. The ferroelectricity of the BCZT/LCMO bilayer heterostructures derived from the ferroelectric BCZT phase that is known as a principally tetragonal-like perovskite structure at room temperature whose spontaneous polarization vector lies in the [001] crystallization direction. As a result, the (001)-oriented heterostructure exhibits the largest Pr value, followed by the (110)-oriented heterostructure, and the (111)-oriented heterostructure shows the minimum value. This is because the projection of the [001] spontaneous polarization direction along the heterostructure orientation reduces from the (001) to (111). Fig. 4 exhibits the frequency dependence of εr and tan δ for the (001), (110), and (111)-oriented heterostructures. It is obvious that εr of all heterostructures tends to decrease with higher frequencies, while tanδ reduces firstly then increases with increasing frequencies. The frequency dispersion dielectric behaviors of the different oriented heterostructures are similar to those found in BCZT thin films [28,29]. Furthermore, the value of εr at 1 kHz are approximately 935, 667 and 615, and the value of tanδ are 0.2, 0.13 and 0.07 for (001), (110), and (111)-oriented heterostructures, respectively. The anisotropic dielectric behaviors in the different oriented BCZT/LCMO heterostructures may be ascribed to the different amplitude of the polarization along the heterostructure orientation or the varying degree of residual strain [30].
Fig. 3. P-E hysteresis loops of the (001), (110), and (111)-oriented multiferroic BCZT/LCMO heterostructures. The inset table presents the values of Pr and Ec.
(111)-oriented multiferroic BCZT/LCMO heterostructures. The wellshaped ferroelectric hysteresis loops demonstrate the ferroelectric nature of all bilayer heterostructures. As can be seen from the inset table, the remnant polarization (Pr) are 11.7 μC/cm2, 7.9 μC/cm2 and 3
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Fig. 6. Magnetic hysteresis (M-H) loops of the (001), (110), and (111)-oriented multiferroic BCZT/LCMO heterostructures.
Fig. 4. Frequency dependence of dielectric constant (εr) and dielectric loss (tan δ) for the (001), (110), and (111)-oriented multiferroic BCZT/LCMO heterostructures.
between the heterostructure orientation and [001] spontaneous polarization direction in (111)-oriented heterostructure has the maximum piezoelectric response.
The local PFM hysteresis loops of amplitude and phase for the (001), (110) and (111)-oriented multiferroic BCZT/LCMO heterostructures are shown in Fig. 5 (a)-(c), respectively. It can be observed that the butterfly loops are well-shaped and the piezoresponse phases are almost 0–180° reversibly switchable, confirming the good switching behavior for all bilayer heterostructures. Besides, the asymmetry behavior observed in the PFM hysteresis loops should be resulted from the electrode self-poling effect caused built-in electric field [31]. The calculated effective piezoelectric coefficient d33 value reveals that the (111)-oriented BCZT/LCMO heterostructure shows the maximum value of 68 p.m./V, while the (001), and (110)-oriented heterostructures exhibits the relatively smaller values of 45 p.m./V and 56 p.m./V, respectively. The piezoelectric performance of the heterostructures is in ascending order of (001)＜(110)＜(111), which is in sharp contrast to its ferroelectric behaviors that show the opposite relation order. As compared with the (001), and (110)-oriented heterostructures, the above-mentioned larger c/a value can be considered as one of the reasons for the enhanced piezoelectric property in the (111)-oriented heterostructure due to the larger tetragonal lattice distortion in BCZT layer [24]. Another reason for the piezoelectric anisotropy of the heterostructures may be explicated by the model of engineered domain configuration [32] for the BCZT layer with tetragonal distortion, which reveals that the piezoelectric constant is larger along the non-polar direction due to the easy tilt of the polar vector by the electric field. Once a (001) direction electric field is applied to the heterostructures, the polar vectors would rotate towards the direction of the applied field, leading to improved tetragonal lattice distortion, and therefore the enhanced piezoelectric property [25]. As a result, the biggest angle relationship
3.3. Ferromagnetic properties M-H loops of the multiferroic BCZT/LCMO bilayer heterostructures with various orientations are shown in Fig. 6, which were measured with the direction of applied magnetic field parallel to the heterostructure surface (in-plane). The well-shaped M-H loops demonstrate the ferromagnetic nature of the different oriented multiferroic BCZT/ LCMO heterostructures. It is obvious that the saturated magnetization (MS) of the heterostructures increases from (001)-oriented heterostructure to (111)-oriented heterostructure. This trend is like those reported earlier for BaTiO3/La0.67Sr0.33MnO3 heterostructures [26], which is closely related to the strain-imposed MnO6 octahedra distortion in manganite thin films. In most cases, the in-plane tensile strain decreases the Mn–O bond angle which will enhance the electronphonon interaction and weaken the electronic hopping amplitude, and in turn suppress ferromagnetism. As mentioned above, the LCMO layers of the heterostructures are subjected to an in-plane tensile strain which decreases from (001)-oriented heterostructure to (111)-oriented heterostructure. As a consequence, the (111)-oriented heterostructure exhibits the larger MS compared with the other two orientations. 3.4. Magnetoelectric properties The room temperature transverse ME coefficient (αE31) coefficients as a function of magnetic field for the (001), (110), and (111)-oriented
Fig. 5. Local PFM hysteresis loops of amplitude and phase for the (a) (001), (b) (110), and (c) (111)-oriented multiferroic BCZT/LCMO heterostructures. 4
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dependence on the crystallographic orientations. Results showed that the biggest ferroelectric and dielectric responses were obtained in (001)-oriented heterostructure, while the maximum saturation magnetization and piezoelectric constant were achieved in (111)-oriented heterostructure. Furthermore, the (111)-oriented heterostructure has the largest magnetoelectric coefficient with αE31 of 207 mV/cm·Oe. Our results suggest that the crystallographic orientation is a significant factor in modulating the multiferroic properties of the ferroelectric/ ferromagnetic heterostructures. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (51972252, 51567017), the Natural Science Foundation of Hubei Province (2016CFA006), the National Science Foundation of Guizhou Minzu University and the Fundamental Research Funds for the Central Universities (WUT: 2019Ⅲ029). References Fig. 7. Transverse ME coefficient (αE31) coefficients as a function of magnetic field for the (001), (110), and (111)-oriented multiferroic BCZT/LCMO heterostructures.
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multiferroic BCZT/LCMO heterostructures are displayed in Fig. 7, which were performed with the applied magnetic field parallel to the heterostructure surface. Notably, the value of αE31 increases monotonically with the increase of magnetic field for the (001), (110), and (111)-oriented heterostructures, and then get saturated until the magnetic field reaches about 6000 Oe. Furthermore, the achieved maximum values of αE31 of the (001), (110), and (111)-oriented heterostructures are 153 mV/cm·Oe, 180 mV/cm·Oe and 207 mV/cm·Oe, respectively. These values of αE31 are comparable to the reported ME values in La0.67Sr0.33MnO3/PbZr0.52Ti0.48O3 [33] heterostructures, and larger than most of the lead-free systems such as Bi3.15Nd0.85Ti3O12/ La0.7Ca0.3MnO3 [34] and BaTiO3/La0.7Sr0.3MnO3 [35] heterostructures. On the other hand, the (111)-oriented heterostructure displays the optimal ME effect compared with the (001), and (110)-oriented heterostructures, whereas the (110)-oriented heterostructure exhibits a better ME effect than the (001)-oriented heterostructure. There may be two reasons for the enhanced ME effect in (111)-oriented heterostructure. One is owing to the largest piezoelectric coefficient in the (111)-oriented heterostructure, which would improve the ME response due to the coupling effect of piezoelectricity and magnetostriction [36]. The other is the increased shared oxygen atoms at the BCZT-LCMO interface in the (111)-oriented heterostructure [26,37]. The increase in shared oxygen atoms will lead to efficient stress transfer which should in favor of a stronger ME coupling. For (001)-oriented heterostructure, there are only one oxygen atoms shared at the BCZT-LCMO interface per unit cell, whereas there are two and three oxygen atoms for the (110), and (111)-oriented heterostructures, respectively. Accordingly, (111)-oriented heterostructure shows the largest ME coupling response among all heterostructures.
4. Conclusions In summary, multiferroic heterostructures composed of the bottom LCMO layer and the top BCZT layer were deposited on (001), (110), and (111) Nb:STO single-crystal substrates by using pulsed laser deposition technique. XRD and TEM analysis verified that all heterostructures exhibited single-crystal structure and were epitaxially grown on the Nb:STO substrate with sharp and well-defined interfaces. The ferromagnetic and ferroelectric hysteresis loops were investigated, revealing the multiferroic nature of the heterostructures. It was found that the multiferroic properties of the heterostructures displayed strong 5
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