BaTiO3 heterostructure

Solid State Communications 151 (2011) 1659–1661

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Effect of preferred orientation on magnetoelectric properties of multiferroic La0.7 Sr0.3 MnO3 /BaTiO3 heterostructure T.X. Li a , M. Zhang a,∗ , Z. Hu a , K.S. Li b , D.B. Yu b , H. Yan a a

College of Materials Science and Engineering. Beijing University of Technology, Beijing, 100124, China

b

National Engineering Research Central for Rare Earth Materials, General Research Institute for Nonferrous Metals, and the Grirem Advanced Materials Co. Ltd., Beijing 100088, China

article

info

Article history: Received 4 May 2011 Received in revised form 21 July 2011 Accepted 4 August 2011 by J. Fontcuberta Available online 19 August 2011 Keywords: A. Multiferroic materials A. Ferroelectric_ferromagnetic composite films B. Pulsed laser deposition D. Magnetoelectric effect

abstract The epitaxial La0.7 Sr0.3 MnO3 /BaTiO3 bilayer heterostructures were deposited on LaAlO3 (001) and (110) substrates by pulsed laser deposition. The inherent ferromagnetic, ferroelectric properties and strong magnetoelectric (ME) effect at room temperature were approved, which correlated to the preferred orientation of the films. Both heterostructures showed similar frequency-dependent ME behavior in 0.1 kHz–100 kHz, the ME voltage coefficients were around 140 mV/cm Oe and 104.8 mV/cm Oe at 1 kHz for (001) and (110) oriented bilayers, respectively. This was at least one order of magnitude higher than previously reported results of the related heterostructures, which is mainly ascribed to the lower dielectric constant of BTO film. © 2011 Elsevier Ltd. All rights reserved.

1. Introduction Multiferroic magnetoelectrics are materials of which both ferromagnetism and ferroelectricity occur simultaneously in a certain temperature range. Coexistence of ferromagnetic and ferroelectric subsystems in the materials offers a possibility to obtain the magnetoelectric (ME) effect, which is defined as a magnetic field-induced polarization or an electric field-induced magnetization. It has recently attracted scientific and technological interests due to its potential applications in sensors, actuators, multiple state memory elements, magnetic read/electric write hard disks, etc [1,2]. However, very few single-phase multiferroic magnetoelectrics exist in nature because the ferroelectric and ferromagnetic properties are often incompatible [3]. Moreover, its working temperature is usually much lower than room temperature and its ME effect is too weak for device applications. Therefore, artificial ME composites consisting of ferroelectric and ferromagnetic materials have been developed [4,5]. A strong ME effect can be realized in two-phase or multiphase composites, of which the mechanical deformation of the magnetostrictive phase results in polarization of the piezoelectric phase, and vice versa. Multilayers of such composites are especially promising

∗

Corresponding author. Tel.: +86 010 67392445; fax: +86 010 67392445. E-mail address: [email protected] (M. Zhang).

0038-1098/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2011.08.008

candidates for applications because of their low leakage current and superior poling properties. Furthermore, thin-film type multilayer composites present flexibility and more degrees of freedom to tune ME properties, i.e., preferred orientation, film thickness, and lattice strain, etc. [6,7]. Additionally, these composites provide us with some unique advantages for studying the ME physical mechanisms in nanoscale as well as the potential applications in advanced magnetic and electronic devices. La0.7 Sr0.3 MnO3 /BaTiO3 (LSMO/BTO) bilayers were deposited on LaAlO3 (LAO) (001) and (110) substrates. LSMO film was grown not only as a ferromagnetic-layer element but also as the bottom electrode for BTO film. Recently, it has been reported that the (110) oriented La1−x Aex MnO3 films (Ae = Ca, Sr) present enhanced magnetic properties compared to (001) one [8,9]. Thus, the performance could be improved by using (110) oriented La1−x Aex MnO3 film in the ME composites. The structure, magnetic and electric properties, and the ME effect were investigated. It was found that the ferroelectricity, ferromagnetism and ME effect strongly correlate to the film’s orientation. 2. Experiment LSMO/BTO bilayers were grown on LAO (001) and (110) substrates (10 mm × 5 mm × 0.5 mm) by pulsed laser deposition. The stoichiometric La0.7 Sr0.3 MnO3 and BaTiO3 ceramic target were

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T.X. Li et al. / Solid State Communications 151 (2011) 1659–1661

Fig. 1. (a) θ − 2θ XRD pattern of LSMO/BTO bilayers, (b) and (c) show the typical cross section SEM of (001) and (110) oriented samples, respectively.

Fig. 2. (a) Present P–E hysteresis loops, and (b) show the frequency dependence of εr and tan δ for (001) and (110) oriented LSMO/BTO bilayers.

ablated using a KrF excimer laser at a wavelength of 248 nm with 1.5 J/cm2 and 5 Hz. During deposition, the substrate temperature of 700 °C and a flowing 10 Pa oxygen pressure were maintained. After deposition, the bilayers were annealed at 800 °C for 30 min at 1000 Pa oxygen pressure. X-ray diffraction measurements (Bruker D8 advance) were employed to probe the structure. Scanning electron microscopy (Hitachi S4800) was utilized to observe the microstructure and film thickness. Magnetic properties were measured by a physical property measurement system (Quantum Design PPMS-9). The Ag electrodes with 3 mm × 1 mm area were deposited for electrical measurement by sputtering. Dielectric properties were measured by using an impedance analyzer (Agilent E4294A). The ferroelectric loops were recorded with a ferroelectric analyzer (aixACCT TF2000). For ME measurement, the samples were placed in a Helmholtz coil that generates an AC magnetic field, and a DC bias magnetic field was superimposed on the AC magnetic field in parallel. The induced voltage signal was collected by a lock-in amplifier (Stanford SR 830). 3. Results and discussion XRD patterns of the bilayers are shown in Fig. 1(a). Only (00l) and (110) diffraction peaks of LSMO and BTO are observed along with corresponding diffraction of LAO substrate, indicating that LSMO and BTO films are grown epitaxially on LAO. Fig. 1(b) and (c) show the cross section images of (001) and (110) oriented samples measured by the scanning electron microscopy (SEM), respectively. They both display clear interfaces between LSMO and BTO. The thicknesses are about 310 nm and 220 nm for LSMO and BTO in (110) oriented bilayer, respectively, which is slightly thinner than those of the (001) sample. They are around 330 and 240 nm. As shown in Fig. 2(a), the evident ferroelectric hysteresis loop demonstrates that the saturation polarization Ps , remnant polarization Pr and coercive field EC for (001) sample are 31.5 µC/cm2 , 7.6 µC/cm2 and 49.7 kV/cm, respectively, which are comparable to those BTO films deposited by pulsed laser deposition [10]. The ferroelectric polarization of (110) oriented

Fig. 3. (a) Shows M–H curves at 300 K, and (b) presents M–T curves for (001) and (110) oriented LSMO/BTO bilayers, The inset of (a) shows the close look of coercive field.

sample is suppressed a little bit. In detail, smaller saturated polarization and remnant polarization of 25.1 µC/cm2 and 6.4 µC/cm2 , respectively, and a larger electrical coercive field of 66.5 kV/cm are approved. Apparently, for tetragonal BaTiO3 the projection of the polarization along [110] crystallographic directions is smaller than that along [001]. Thus, the ferroelectric properties of (001) oriented LSMO/BTO is stronger than the (110) one. Fig. 2(b) gives the relative dielectric constant (εr ) and loss factor (tan δ ) as a function of frequency. Both the bilayers present a similar frequency-dependent dielectric behavior in 0.1–100 kHz. εr are around 263 and 297, and tan δ only are around 0.013 and 0.021 at 1 kHz for (001) and (110) oriented samples, respectively. They are much lower than that of bulk materials (εr > 1000), which most probably is due to the formation of low-dielectricconstant interface layers of BTO/LSMO and the structure defect originating from the lattice mismatch. The values of εr are similar to the previous reports [11,12]. Magnetic properties are mainly dominated by the LSMO layer, the ferromagnetic hysteresis loops at 300 K are displayed in Fig. 3(a). In these measurements, magnetic field is aligned parallel to film surface. The curves show typical characteristics of soft ferromagnet with low coercive field (Hc = 30 Oe, 70 Oe for (110) and (001) samples, respectively). The saturated magnetization is around 247 emu/cm3 and 190 emu/cm3 for (110) and (001) heterostructures, respectively, which is consistent with previous reports [13]. Fig. 3(b) presents the temperature-dependent magnetization measured at a field of 50 Oe (the magnetic field parallels to film surface). The curves exhibit a clear paramagnetic–ferromagnetic phase transition, and the Curie temperature of (110) sample around 332 K is a little bit higher than that of the (001) one (around 317 K). Furthermore, the magnetization of the (110) sample is also higher than that of the (001) one throughout the whole temperature range, indicating that the LSMO film could be magnetized more easily along the (110) direction than the (001) direction. For ME measurements, an AC magnetic field δ H (10 Oe, 0.1–100 kHz) was superimposed on a DC bias magnetic field Hbias (2500 Oe). The DC and AC magnetic fields were arranged to be coaxial for all measurements, and the induced voltage signal δ V from the BTO layer and the phase difference (1θ ) were collected by a lock-in amplifier. The ME voltage coefficient αE is defined by αE = δ E /δ H = δ V /(t × δ H ), where t is the thickness of BTO layer. Based on the theory of the ME effect for bilayer composites [14–17], αE can be theoretically calculated by: αE = =

E3 H1 p

−kν(1 − ν)(m q11 + m q12 )p d31 ε33 (m s11 + m s12 )kν + p ε33 (p s11 + p s12 )(1 − ν) − 2kp d231 (1 − ν)

(1)

where the superscript m and p represent the ferromagnetic and ferroelectric constituent; the sij , dki and qki are the effective compliance, piezoelectric and piezomagnetic coefficients, respectively;

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Fig. 4. (a) Shows theoretical expectation of αE as a function of εr , (b) presents the frequency dependence of αE and 1θ .

the ε33 denotes the effective dielectric constant; especially, ν = p ν/( m ν + p ν), here p ν and m ν are the volume fraction of piezoelectric and magnetostrictive phase; and k is the interface coupling parameter and reads: k = (p Si − p Si0 )/(m Si − m Si0 )

(2)

where Si0 is strain tensor components with no mechanical coupling between layers. The parameter k depends on the interface quality and is a kind of measure of mechanical coupling between the piezoelectric and magnetostrictive layers. For an ideal interface that k = 1, all the strain components induced by the magnetostrictive layer transfer to the piezoelectric layer. For the case of k = 0, the mechanical coupling is absent. Fig. 4(a) indicates the theoretical expectation of αE with the change of εr . Here the parameters sij , dki , and qki derive from Ref. [17]. The volume fraction ν = dBTO /(dBTO + dLSMO ) are 0.585 and 0.579 for (110) and (001) heterostructures, where dBTO and dLSMO are the thickness of corresponding film, respectively. As shown in Fig. 4(a), αE increases with decreasing εr and especially it changes sharply as εr is lower than 500, i.e., the values of αE for εr being around 200 is one order of magnitude higher than that for εr > 1000. Fig. 4(b) represents the measured values of αE and the phase angle 1θ as a function of the frequency (0.1–100 kHz). αE for the (001) oriented heterostructure is higher than that of the (110) one within the whole frequency range. The induced voltages at 1 kHz are around 46.2 and 32.5 µV for (001) and (110) samples, and the corresponding αE is around 140 and 104.8 mV/cm Oe, respectively. These measured values of αE are at least one order of magnitude larger than the previous results, 30 mV/cm Oe, reported by Ref. [14] and 4.2 mV/cm Oe from Ref. [18]. In the LSMO/BTO heterostructure, the strain induced by the LSMO film could transfer to the BTO film through the interface. According to the equation in Ref. [19], p αE = km 33 k31



m

µ33 /p ε33

× (dLSMO + dBTO )

√ v(1 − v) m s33 p s11 v m s33 + (1 − v) p s11

Fig. 5. Theoretical expectation of αE as a function of k, and the larger symbols at the corresponding curves represent the measured value of αE .

Fig. 5 indicates the theoretical expectation of αE as a function of k according to the Eq. (1). The measured values of αE are also indicated in the corresponding curve by large symbols. The values of k are around 0.68 and 0.51 for (001) and (110) oriented samples, respectively. It seems that some interface coupling for both cases exists, and the coupling in (001) oriented bilayers is a little bit stronger than that in the (110) one. 4. Conclusions To summarize, LSMO/BTO bilayers were epitaxially grown on LAO (001) and (110) substrates. The ferroelectric, ferromagnetic properties and magnetoelectric effects were investigated. The ferromagnetic and ferroelectric properties depended on the preferred orientations. For (110) oriented bilayer, the ferroelectric polarization is suppressed, which had the lower remnant polarization and larger electrical coercive field, but its saturated magnetization and ferromagnetic Curie temperature is higher than the (001) one. They both show similar frequency-dependent ME behavior in 0.1–100 kHz. The value of αE for the (001) oriented heterostructure is higher than that of the (110) one, and they are at least one order of magnitude higher than previously reported similar structures due to a rather low dielectric constant of BTO film. Acknowledgment This work is supported by National Natural Science Foundation of China (NSFC) (Grant No. 51002013 and 11174021). References [1] [2] [3] [4] [5]

(3)

αE is directly proportional to the electromechanical coupling factor (kP31 ) in the piezoelectric layer. In addition, kP31 is inversely proportional to the square root of εr [19]. Based on the above analysis, αE increases with decreasing εr . Hence, the large αE mainly results from the rather small εr of the BTO film in this work. Furthermore, as is shown in Fig. 2(b), εr decreases with increasing frequency, which results in the kP31 of BTO film increasing. As a result, αE increases with increasing frequency. The phase angle 1θ , indicating the phase difference between induced voltage signal and the AC magnetic field signal, decreases with increasing frequency from Fig. 4(b), which reflects that the lag time between the magnetoelastic and electromechanical coupling becomes short with increasing frequency. As a result, ME coupling grows efficiently.

[6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]

W. Eerenstein, N.D. Mathur, J.F. Scott, Nature 442 (2006) 759–765. N.A. Spaldin, M. Fiebig, Science 309 (2005) 391–392. N.A. Hill, J. Phys. Chem. B 104 (2000) 6694–6709. C.W. Nan, M.I. Bichurin, S.X. Dong, D. Viehland, G. Srinivasan, J. Appl. Phys. 103 (2008) 031101. W. Eerenstein, M. Wiora, J.L. Prieto, J.F. Scott, N.D. Mathur, Nat. Mater. 6 (2007) 348–351. Y. Zhang, C.Y. Deng, J. Ma, Y.H. Lin, C.W. Nan, Appl. Phys. Lett. 92 (2008) 062911. J.P. Zhou, H.C. He, Z. Shi, C.W. Nan, Appl. Phys. Lett. 88 (2006) 013111. M. Minohara, Y. Furukawa, R. Yasuhara, H. Kumigashira, M. Oshima, Appl. Phys. Lett. 94 (2009) 242106. L.M. Berndt, V. Balbarin, Y. Suzuki, Appl. Phys. Lett. 77 (2000) 2903–2905. H.F. Cheng, M.H. Yeh, K.S. Liu, I.N. Lin, Japan. J. Appl. Phys. Part 1 32 (1993) 5656–5660. B.D. Qu, M. Evstigneev, D.J. Johnson, R.H. Prince, Appl. Phys. Lett. 72 (1998) 1394–1396. M.H. Yeh, Y.C. Liu, K.S. Liu, I.N. Lin, J.Y.M. Lee, H.F. Cheng, J. Appl. Phys. 74 (1993) 2143–2145. A.M. Haghiri-Gosnet, J. Wolfman, B. Mercey, C. Simon, P. Lecoeur, M. Korzenski, M. Hervieu, R. Desfeux, G. Baldinozzi, J. Appl. Phys. 88 (2000) 4257–4264. G. Srinivasan, E.T. Rasmussen, B.J. Levin, R. Hayes, Phys. Rev. B 65 (2002) 134402. M. Avellaneda, G. Harshe, J. Intell. Mater. Syst. Struct. 5 (1994) 501–513. V. Petrov, G. Srinivasan, O.V. Ryabkov, S.V. Averkin, M.I. Bichurin, Solid State Commun. 144 (2007) 50–53. M.I. Bichurin, V.M. Petrov, G. Srinivasan, Phys. Rev. B 68 (2003) 054402. Y.G. Ma, W.N. Cheng, M. Ning, C.K. Ong, Appl. Phys. Lett. 90 (2007) 152911. B. Leixiang, W. Yumei, L. Ping, G. Qiuling, Z. Yong, Y. Miao, IEEE Sens. J. 9 (2009) 1620.