Orientation-Dependent Optical Magnetoelectric Effect in Patterned

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Orientation-dependent Optical Magneto-electric Effect in patterned BaTiO/La Sr MnO Heterostructures 3

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Huanyu Pei, Yunjie Zhang, Shujin Guo, Lixia Ren, Hong Yan, Bingcheng Luo, Chang-Le Chen, and Kexin Jin ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b10566 • Publication Date (Web): 17 Aug 2018 Downloaded from http://pubs.acs.org on August 22, 2018

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ACS Applied Materials & Interfaces

Orientation-dependent Optical Magneto-electric Effect in patterned BaTiO3/La0.67Sr0.33MnO3 Heterostructures Huanyu Pei, Yunjie Zhang, Shujin Guo, Lixia Ren, Hong Yan, Bingcheng Luo, Changle Chen,* and Kexin Jin* Shaanxi Key Laboratory of Condensed Matter Structures and Properties, Northwestern Polytechnical University, Xi’an 710072, China

ABSTRACT The optical magneto-electric effect has been widely investigated, but how to obtain the large and tunable optical magneto-electric effect at room temperature is still a big challenge. We here design ferroelectric/ferromagnetic heterostructures with various orientations, which are composed of titanate BaTiO3 and manganese oxide La0.67Sr0.33MnO3. This artificial bilayer structure presents room-temperature ferroelectric and ferromagnetic properties. After patterning a 4 µm grating structure on the bilayer thin film, the optical magneto-electric effect for near-infrared light is investigated systematically through the Bragg diffraction method. The relative change of diffracted light intensity of the order n=1 has a strong dependence on the magnetization and polarization of the thin films, whether the superlattice is irradiated in reflection or transmission geometries. For (100) and (111)-oriented samples, both show the room-temperature optical magneto-electric effect, while the (111)-oriented thin film has a stronger optical magneto-electric effect. These results pave the way for designing next-generation optical magneto-electric devices based on ferroelectric/ferromagnetic structure.

KEYWORDS: Optical Magneto-electric Effect, Heterostructure, Multiferroic, Perovskite, Grating INTRODUCTION Multiferroics are a kind of materials with a coexistence of magnetic and ferroelectric order, which have a unique advantage for the control of magnetism by electric fields or of ferroelectricity by magnetic fields.1-2 However, such materials with single phase are rare in nature, because the ferroelectric order and magnetic order need different broken inversion symmetries.3-4 In recent years, the studies of multiferroics have already been the general focus of the material field and 1

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neighbouring research areas. Multiferroic materials are so intriguing for two reasons.5-7 On the one hand, it is very hopeful that obtaining gigantic magneto-electric (ME) coupling in multiferroics, because of the coexistence of magnetic and ferroelectric states. On the other hand, because multiferroic material can provide an additional degree of freedom, some novel features are worth expecting.8-9 Generally, the strength of magneto-electric coupling is an important index to evaluate the pros and cons of multiferroic materials.10 The optical magneto-electric effect (OME), which is seen as an expansion of the ME effect, is an unconventional electromagnetic responses with employing the ME coupling.11 In multiferroic materials, the OME effect can be characterized by a toroidal moment T, which can be defined as T

∝

P×M,12 where P and M are the spontaneous

electric polarization and magnetization, respectively. When the propagation direction of diffracted light is parallel or antiparallel to the toroidal moment, the OME effect generally can be observed. So far the OME effects in multiferroic materials have been reported in some literatures.13-15 However, the effects are too small, or only can be observed at low temperature. All the work have their own limitations and cannot meet the needs of applications. Therefore, how to achieve the enhanced OME effect at room temperature is still a big challenge. Moreover, since the OME effect is dependent on extrinsic ME coupling, exploring ways to improve the ME coupling at room temperature has become the most important issue. In view of this, we attempt to obtain room-temperature OME effect by customizing a ferroelectric/ferromagnetic composite structure. In this article, the ferroelectric material, used in the heterostructure, is BaTiO3 (BTO), which is one of the most studied ferroelectric materials due to its simplicity and various electronic applications such as piezoelectric transducers and multilayer capacitors.16-18 Otherwise, BTO is a non-centrosymmetric tetragonal phase (P4mm) instead of centrosymmetric cubic phase at room temperature, which gives rise to spontaneous polarizations.19 These spontaneous polarizations can be switched by applying an external electric field, which is in accordance with the requirements for any ferroelectric. The ferromagnetic layer in our study is perovskite manganite La0.67Sr0.33MnO3 (LSMO), which shows a large variety of magnetic and electronic properties such as ferromagnetism, anti-ferromagnetism, charge and orbital ordering, magnetic-field driven metal-insulator transition.20 LSMO is a distorted perovskite with a lattice parameter and unit cell angle of 0.387 nm and 90.26° respectively.21-22 Additionally, when doping concentration x is in the

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range of 0.3 ≤ x ≤ 0.35, LSMO is only metallic and ferromagnetic with the maximum Curie transition temperature Tc.23 By combining a ferroelectric material BTO and a ferromagnetic material LSMO, an artificial composite multiferroic material could be built. The interaction between the ferroelectric layer and ferromagnetic layer can lead to an extrinsic ME coupling, which may further influence the OME property. In this article, the (100) and (111)-oriented bilayer BTO/LSMO films are fabricated respectively. Comparing to the (100)-oriented thin film, the position of oxygen atoms in (111)-oriented BTO may have slight displacements due to the piezoelectric properties. Such displacements may be transferred to the LSMO layer and alter the magnetic properties via strain-coupling to the BTO layer. Otherwise, the (100)-oriented thin film has more shared oxygen atoms at the interface, heralding a stronger coupling. On this basis, the ferroelectric and ferromagnetic properties of them are investigated to confirm the coexistence of ferroelectric and ferromagnetic states. Then, the OME effects in the BTO/LSMO thin films for near-infrared light of 808 nm are observed in temperature range of 10-300 K by patterning grating structures and applying the Bragg diffraction method.

EXPERIMENTAL PROCEDURES Both BTO and LSMO targets are prepared by the standard solid-state reaction technique. The bilayer BTO/LSMO heterostructures are successively fabricated on the (100) and (111)-oriented (La0.3Sr0.7)(Al0.65Ta0.35)O3 (LSAT) single crystal substrates with the area of 5×10 mm2 by using a laser molecular-beam epitaxy (L-MBE) deposition method. The frequency of a KrF excimer laser with a wavelength of 248 nm is maintained at 1 Hz focused on the BTO and LSMO polycrystal targets. The specular-spot intensity of reflection high-energy electron diffraction (RHEED) technology was used to monitor the film deposition process for obtaining the atomic-layer growth. After the deposition, the films are naturally cooled to room temperature. The total thickness of thin film is 95 nm confirmed by the X-ray reflection. The crystallographic structure of BTO/LSMO is characterized by X-ray diffractometer (XRD) using Cu Kα radiation. The surface of the heterostructure is acquired by using an atomic force microscope (AFM) (MFP-3D, Asylum Research). For electrical measurements, the square Pt electrodes with an area of 0.5×0.5 mm2 are grown on the top of BTO layer by magnetron sputtering technique. Furthermore, the polarization 3



versus electric field (P-E) hysteresis loops are measured by the modified Sawyer-Tower circuit (Precision LC, Radiant) at room temperature, and the magnetization versus temperature (M-T) curves of BTO/LSMO thin films are measured by a Superconducting Quantum Interference Device (MPMS-XL-7) in the range of 10-300 K. The measurements of OME effect are carried out in warming processes, and each temperature point is held for 5 minute to allow enough thermal relaxation to reduce the experimental error.

RESULTS AND DISCUSSION The BTO layer and LSMO layer are sequentially grown on the (100) and (111)-oriented LSAT substrate, respectively. Figure 1(a) and 1(c) are the θ-2θ XRD patterns of the BTO/LSMO thin films with different orientation, clearly suggesting that both the films have single-phase perovskite structure without impurity or secondary phase. According to the XRD pattern of (100)-oriented sample, the lattice constants of LSAT, LSMO and BTO are 0.388 nm, 0.391 nm and 0.395 nm, respectively. The lattice mismatch between LSMO and BTO is 1%. However, the lattice mismatch of (111)-oriented sample is 3%, which is much larger than that of (100)-oriented sample. Figure 1(b) and 1(d) present the surface topographies of the (100) and (111)-oriented thin films respectively. It demonstrates that both the films exhibit well-grown grains with a relatively uniform distribution, and the corresponding root-mean-square roughness are determined to be 0.68 nm ± 0.05 nm and 1.38 nm ± 0.05 nm for the samples with (100) and (111)-orientations, respectively. Since the coexistence of ferroelectric and ferromagnetic states is prerequisite for the OME effect, the magnetic and ferroelectric properties of the samples are investigated firstly. Figure S1 presents the magnetic hysteresis loops of (100) and (111)-oriented films at room temperature, which indicates the room-temperature ferromagnetism of samples. Figure 2(a) shows the magnetizations of the (100) and (111)-oriented BTO/LSMO films as a function of temperature. The magnetization decreases with the increasing temperature, but the transition critical temperatures for the films are not observed in the measured temperature region. It is reasonably expected that both the (100) and (111)-oriented BTO/LSMO films are ferromagnetic at room temperature. Similar phenomena are observed in other literatures.24,25 Notably, the magnetism of the (111)-oriented film is larger than that of the (100)-oriented sample, which can be contributed to the change of Mn-O bond angle and 4


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length induced by epitaxial strain.26 Otherwise, the macroscopic polarization hysteresis loops of thin films with and without an external magnetic field at room temperature are also measured with a frequency of 1 kHz, as presented in Figure 2(b). Figure 2(c) is a configuration of experimental set-up. Both the (100) and (111)-oriented films are ferroelectric at room temperature, while the (100)-oriented sample has a better ferroelectric performance, which is consistent with previous reports.27 After applying a magnetic field of 5 kOe, the remnant polarizations (Pr) for (100)-oriented film changes from 4.27 µC/cm2 to 4.38 µC/cm2, while Pr for (111)-oriented sample changes from 1.73 µC/cm2 to 2.11 µC/cm2. Obviously, the (111)-oriented sample has a stronger dependence on magnetic field, heralding a stronger ME coupling. Generally, the ME effect in ferromagnetic/ferroelectric heterostructure can be explained as the product of magnetorestrictive effect and the piezoelectric effect.28 Therefore, the ME coupling in BTO/LSMO heterostructure is considered to be strained-mediated. An external magnetic field would induce strain in the ferromagnetic layer, which can be transferred to the ferroelectric layer. Therefore, it will bring changes of electric polarization as a result of the piezoelectric effect. Based on the above-mentioned results, we can confirm that both the films are multiferroic at room temperature, and thus the subsequent OME effect is worth expecting. Next, a grating structure with an area of 4×4 mm2 is patterned on the thin film along the c axis by conventional photolithography to detect the OME effect. Figure 3(a) is the photograph of the grating structure patterned on the (100)-oriented film by applying an optical microscope (AXIO imager. A1M). The distinct stripes of light and shade can be seen clearly, heralding that the well grating structure is achieved. The grating period d and the width of the film are 4 µm and 2 µm, respectively. The thicknesses of BTO layer and LSMO layer are determined to be 66 nm and 29 nm respectively, as shown in Figure S2. Therefore, the depth of the groove is 103 nm, which is enough to deplete the region of the film, as shown in Figure 3(b). Subsequently, a Bragg diffraction technique in transmission and reflection geometries is used to collect the signal of the OME effect. Owing to the transparency of the LSAT substrate at near-infrared light, a continuous wave (CW) laser diode (808 nm) is used as the light source in our experiments. Then, a beam of polarized light irradiates the area of the grating structure on (100)-oriented thin film at a normal incidence. Correspondingly, the bright Bragg diffraction spots can be observed in transmission geometry, as displayed in the inset of Figure 3(c). The Bragg 5



diffraction spots comply with the Bragg diffraction formula given by d·sinθ = n·λ, where θ and λ are the n-order diffraction angle and the wavelength of polarized light, respectively. The Bragg diffraction intensity In of the order n = ±1 is measured with silicon photodiodes. Here, the relative change △I of In is defined as

∆I / I n =

I n (+ H ) − I n (− H ) I n ( 0) ,

where In(+H), In(-H), and In(0) are the diffraction light intensities under positive, negative, and zero magnetic fields, respectively. Figure 3(c) presents the corresponding △I/In of the diffraction light in (100)-oriented sample as a function of the diffraction angle θ at room temperature. The three distinct peaks correspond to the three Bragg diffraction spots, and the intensities of them decrease with the increasing n. The oscillation of △I/In can be attributed to the nth order Fourier component of the grating structure.29 In order to make sure that the phenomenon we observed are the OME effect, △I/In of the (100)-oriented thin film as a function of light-polarization angle α is investigated. Figure 4(a) and Figure 4(b) display the schematic diagrams when the sample is rotated by 0° and 180°, respectively. As shown in Figure 4(c), the values of △I/In change little with the increasing α, whether the sample is rotated by 180° or not, clearly suggesting that the effect is independent of α. Thus the magneto-optical Kerr effect in our sample is not obvious and can be neglected. Meanwhile, the sign of △I/In changes from positive to negative when the sample is rotated by 180°, but the absolute values are nearly the same. The reversal of the substrate will bring the change in the direction of electric polarization P, which further leads to the reversal of T mentioned above. It means that the in-plane projection of detected reflected light is opposite to the direction of T relatively. Therefore, these results clearly show that △I/In of our sample is dependent on P. Next, it is certain that the dependence of △I/In on the magnetization M should be carried out. Figure 4(d) presents the temperature-dependence of △I/In at different applied magnetic fields. It is clear that the effect increases with the increasing magnetic field, giving us a straightforward fact

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that △I/In is dependent on M. Based on the above analysis, it is undisputed that the observed effect is the OME effect, which relies on P and M. In order to understand the effect further, △I/In of BTO/LSMO thin films with different orientation as a function of temperature is investigated, as displayed in Figure 5(a). It presents two important features. On the one hand, for (100) and (111)-oriented BTO/LSMO thin films, △I/In decreases with the increasing temperature, which is similar to other literatures.30 With increasing the temperature, the thermal effect exacerbates, which leads to the decrease in P and M.31 Moreover, the distance between the adjacent atoms will increase with the increasing temperature, meaning the decrease of exchange interaction associated with M. It should be noticed that both samples can display the OME effects in temperature range of 10-300 K. The value of △I/In at room temperature can reach 1.5%. The results are in line with the expectation, because of the room-temperature ferroelectric and ferromagnetic performances of thin films shown above. On the other hand, the (111)-oriented BTO/LSMO thin film has a better OME effect compared to the (100)-oriented sample. Generally, the ferroelectric and ferromagnetic properties of bilayer BTO/LSMO films are closely related to the oxygen atoms at the interface.32 For a (100)-oriented thin film, only one oxygen atom is shared at the interface per unit cell, as illustrated in Figure 5(b). However, it is different from the case of the (111)-oriented thin film, where three oxygen atoms are shared per unit cell, as shown in Figure 5(c). The increase in shared oxygen atoms can bring a stronger ME coupling, which further leads to a stronger T relied on P and M. It is reasonable that the (111)-oriented thin film has a stronger OME effect. Otherwise, it should be mentioned that the sign of △I/In = -1 is the opposite of △I/In=1, which can be contributed to the fact that the in-plane projection of diffracted light is parallel or anti-parallel to the direction of T.

CONCLUSIONS In summary, we have successfully fabricated the ferroelectric/ferromagnetic composite hetero-structure composed of BaTiO3 and La0.67Sr0.33MnO3 on the (100) and (111) LSAT single crystal substrate by using a laser molecular-beam epitaxy technology. The P-E hysteresis loops and magnetization curves of thin films are investigated, suggesting that both (100) and

7



(111)-oriented samples have ferroelectric and ferromagnetic properties at room temperature. Then, a grating structure with d = 4 µm is patterned on the thin film, and a Bragg diffraction technology is applied to detect the OME effect, yielding the relative change of diffracted light intensity. Both the (100) and (111)-oriented BTO/LSMO thin films present the room-temperature OME effect, which is in line with our expectations. Otherwise, the (111)-oriented thin film has a stronger OME effect compared to the (100)-oriented thin film, which is attributed to the fact that the (111)-oriented thin film has a stronger ME coupling. Our work achieves the room-temperature OME effect, paving a way to the designing of novel OME devices based on ferroelectric/ferromagnetic heterostructure.

ASSOCIATED CONTENT Supporting Information Magnetic hysteresis loops of (100) and (111)-oriented BTO/LSMO heterostructures at room temperature; XRR patterns of momolayer BTO and LSMO films grown on (100)-oriented LSAT substrates.

AUTHOR INFORMATION Corresponding Author *Changle Chen

Tel.: +86 13193308139. E-mail: [email protected].

*Kexin Jin

Tel.: +86 13088952782. E-mail: [email protected].

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant No. 61471301).

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Figure captions Figure 1 X-ray diffraction patterns for (a) (100) and (c) (111) BTO/LSMO thin films, and surface morphology of (b) (100) and (d) (111) BTO/LSMO thin films. Figure 2 Proofs of multiferrocity in BTO/LSMO composite films. (a) Magnetization of 11



BTO/LSMO thin films with various orientations as a function of temperature. (b) P-E hysteresis loops of BTO/LSMO thin films with various orientations measured with a frequency of 1 kHz at room temperature. (c) Schematic view of the electrodes fabricated on thin films. Figure 3 Details of the grating structure patterned on (100)-oriented BTO/LSMO thin film. (a) The optical microscopy image of the grating structure with a grating period d 4 µm. (b) AFM profile of the BTO/LSMO thin film. (c) △I/In versus diffraction angle θ in reflection geometry measured at room temperature. The inset is the Bragg spots of n up to 3. Figure 4 Evidences of the OME effect. (a) Schematics of Bragg diffractions from the grating of the thin film in transmission and reflection geometries. (b) Schematics of Bragg diffractions when the sample was rotated by 180°. (c) △I/In=1 versus light polarization angle α, measured at room temperature. (d) Temperature-dependence of △I/In=1 at different magnetic fields. Figure 5 (a) Orientation-dependence of △I/In=±1 of the BTO/LSMO thin film. Schematic diagrams of interface region for (b) (100) and (c) (111)-oriented BTO/LSMO thin film. The BTO layer is at the top while the LSMO layer is at the bottom. One oxygen atom is shared per unit cell across the interface for the (100)-oriented structure, while three oxygen atoms are shared at (111) interfaces.

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Figure 5

Table of Contents Graphic

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Orientation-Dependent Optical Magnetoelectric Effect in Patterned

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