SrTiO3 Superlattices

In fact, a spatially resolved electron energy loss spectroscopy (EELS) map has been demonstrated to provide chemical and bonding information of a numb...
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J. Phys. Chem. C 2010, 114, 13068–13070

Interface Structures of La0.67Sr0.33MnO3/SrTiO3 Superlattices Studied by TEM and EELS Ming He† and Z. H. Zhang*,‡ Department of Physics, Liaoning Key Materials Laboratory for Railway, School of Materials Science and Engineering, Dalian Jiaotong UniVersity, Dalian 116028, China ReceiVed: May 2, 2010; ReVised Manuscript ReceiVed: June 12, 2010

The layered structures of La0.67Sr0.33MnO3/SrTiO3 superlattices have been investigated by energy filtered transmission electron microscopy (EFTEM) and spatially resolved electron energy loss spectroscopy (EELS). A general picture of the elemental distribution of the chemically modulated layers was given by the EFTEM images. Chemical shift in both Ti-L2,3 and Mn-L2,3 edges, together with an appearance of distinctive shoulders in both Mn-L3 and Mn-L2 edges at the interfacial region were observed. These electronic structure changes suggest a stronger crystal field and/or a partial loss of oxygen at the interfaces, accounting for the observed oscillatory exchange coupling and enhanced magnetoresistance in such superlattices. 1. Introduction The artificial superlattice structure composed of alternating heteroepitaxial perovskite oxide layers has attracted enormous attention because of the intriguing physics involved in such a system, such as low-field magnetoresistance,1 hall effect,2 oscillatory exchange coupling,3 interfacial exchange bias,4 etc. The periodic stacking of different perovskite oxide layers makes it possible to control the interlayer coupling between the neighboring ferromagnetic layers.1,2 Among various material systems, the La0.67Sr0.33MnO3/SrTiO3 superlattice is one of the most studied systems. The ferromagnetic strontium-doped lanthanum manganese oxide (La0.67Sr0.33MnO3, LSMO) belongs to the family of half-metallic manganese oxides that exhibit very large (colossal) magnetoresistance.5 Strontium titanate (SrTiO3, STO) was chosen as the spacer material due to its similar structure to the manganites and the close lattice match between the two.6 As multiple interfaces exist in such a superlattice system, the carrier confinement effect, specific magnetic interaction, and some lattice strain effect at the interfacial region play important roles in determining the physical properties of the superlattices.7 Therefore, detailed information on the interface structure is essential to the understanding of many novel physical phenomena observed in such superlattices. Transmission electron microscopy (TEM) is an effective way of investigating superlattices with a high spatial resolution8 and energy loss near edge spectroscopy (ELNES) gives information on the density of unoccupied states above the Fermi level, which allows one to obtain insight into the electronic structure of the materials.9 When ELNES is performed in a line scan mode using a scanning transmission electron microscope, spatially resolved electronic structure information of the superlattices can be obtained. In fact, a spatially resolved electron energy loss spectroscopy (EELS) map has been demonstrated to provide chemical and bonding information of a number of material systems with atomic resolution.10–12 For example, Muller et al. have carried out EELS mapping of the LSMO/STO multilayers using a fifth-order aberration-corrected scanning transmission * To whom correspondence should be addressed. E-mail: zhzhang@ djtu.edu.cn. † Department of Physics, Dalian Jiaotong University. ‡ Liaoning Key Materials Laboratory for Railway, School of Materials Science and Engineering, Dalian Jiaotong University.

electron microscope (STEM), and found an asymmetry between the chemical intermixing on the Mn-Ti and La-Sr sublattice.12 Although the chemical composition/bonding change across the LSMO/STO interface has been disclosed in the literature,13 a complete understanding of the interfacial electronic structure of the LSMO/STO superlattice, which information can be obtained by the line scan ELNES study of different compositional elements (e.g., Ti, Mn, and O) across the superlattice interfaces, is not available. In this work, the interfaces of La0.67Sr0.33MnO3/SrTiO3 superlattices have been carefully studied. The elemental distribution of the chemically modulated layers was disclosed by the energy filtered TEM (EFTEM) images taken at specific energy losses of the corresponding compositional elements. In particular, the Ti-L2,3, Mn-L2,3, and O-K ELNES obtained in the line scan mode of STEM provide the fingerprints of the interfacial electronic structures of the superlattices, based on which the interfacial structure and its effect on the physical properties observed in the same superlattices are discussed. 2. Experimental Details The LSMO/STO superlattices were grown on (001) STO single crystal substrates in oxygen environment (40 Pa) at 780 °C by pulsed laser deposition. Postannealing of the samples was carried out in situ in oxygen (0.8 atm) at 700 °C for 2 h to avoid the oxygen deficiency.14,15 The cross-sectional TEM specimens were prepared by using the conventional method, i.e., first by cutting, gluing, and mechanically polishing, followed by Ar-ion milling until perforation occurred. The microstructures of the samples were examined with various TEM (Tecnai F20 ST)-related techniques. Conventional EELS mapping was carried out with the software provided by the Gatan imaging filtering system attached to the same microscope with an exposure time of 30 s. Elemental maps are calculated by subtracting the nonspecific background from the element-specific signal using the standard three-window method.9 The STEMEELS were performed in the line-scan mode and the electron beam is parallel to the superlattice interfaces during the EELS data acquisition. The energy resolution of the spectra is ∼0.85 eV. A relatively small convergence semiangle (∼10 mrad) and a relatively large collection semiangle (∼20 mrad) have been used, corresponding to the experimental configuration11 in favor of the local approximation.16

10.1021/jp1039892  2010 American Chemical Society Published on Web 07/12/2010

La0.67Sr0.33MnO3/SrTiO3 Superlattices

Figure 1. (a) Zero loss EFTEM image of the LSMO/STO superlattice; (b) HRTEM image of the LSMO/STO superlattice; and (c-f) Ti, La, O, and Mn elemental maps.

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Figure 2. STEM-EELS line-scan results of five consecutive superlattice periods. The inset shows the abundance profiles of the compositional elements.

3. Results and Discussion A cross-sectional low-magnification energy filtered image of the superlattice taken at zero energy loss is shown in Figure 1a. Flat and well-defined interfaces can be observed between the successive LSMO and STO layers in such an image. Light and dark contrast is observed along the film stacking direction, which mainly originates from the difference in the scattering factor of different compositional elements, suggesting that the bright thinner bands are the STO layers and the dark thicker ones LSMO. The modulation periods of the superlattice can be estimated as ∼4 nm for STO layers and ∼11 nm for LSMO layers, which is consistent with the nominal values as expected from the deposition rate.14,15 In the high-resolution TEM image shown in Figure 1b, the epitaxy relationship between the two materials across the interface is clearly observed, without any secondary phase or amorphous layer presenting at the interfacial region. The corresponding element maps of Figure 1a are acquired by using the La-M edge at ∼832 eV (Figure 1c), the Ti-L edge at ∼456 eV (Figure 1d), the Mn-L edge at ∼640 eV (Figure 1e), and the O-K edge at ∼532 eV (Figure 1f). La and Mn elements are found to have similar spatial distributions, while the situation for Ti is complementary to them. O is found to be more evenly distributed through the whole superlattice, as it presents in both LSMO and STO layers. While the O contents are expected to be the same in both layers, the more intense signal observed in the STO layers is caused by the preedge background resulting from the tail of the Ti L edge in such layers during the mapping process, when a three-window method9 is employed. Such spatial distributions of the compositional elements agree well with the nominal composition of the specimen (LSMO/STO). The spatial distributions of the compositional elements are further confirmed by EELS spectra recorded in the line-scan mode across several consecutive superlattice interfaces. With an electron probe size fixed at 0.4 nm, the scanning length was set at ∼92 nm and a total set of 200 spectra were acquired containing information on the LSMO layers, the STO layers, and the LSMO/STO interfaces. Every third spectrum was selected from the data set and plotted in Figure 2, and an obvious

Figure 3. (a-c) The energy-loss spectra of Ti-L2,3, Mn-L2,3, and O-K edges taken across the LSMO/STO/LSMO interfaces.

modulation on the chemical composition can be seen in the superlattice system. From the EELS spectra series (200 spectra), the abundance profiles of the compositional elements are plotted based on the background subtracted intensities of the energy loss edges of the specific elements (inset of Figure 2). To study the evolution of the core-loss EELS edges in detail, the ELNES spectra were acquired with finer step size at ∼0.2 nm. Three edges including the Ti-L2,3 (455-475 eV), Mn-L2,3 (640-660 eV), and O-K (530-550 eV) edges are examined, from which detailed electronic structure evolutions are observed from one perovskite oxide layer to another. All interfaces along the superlattice growth direction have been investigated and similar results were obtained, which fully supports the results in the tunnel junctions.13 Panels a-c of Figure 3 show the typical evolution of the spectra features across two consecutive interfaces, i.e., LSMO/STO/LSMO. Since the signal of the MnL2,3 edges drops to noise level and the fine structures of both Ti-L2,3 and O-K edges are rather similar in the interior of the homogeneous STO layer, eight spectra between spectrum “6” and “7” were omitted in Figure 3. While the spatial resolution remains at the subnanometer range due to the hardware

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limitations, it can be estimated that the thickness of the interior homogeneous STO layer is =2.0 nm, i.e., with transition region =1 nm thick existing at each interface of the superlattice film. Figure 3a shows the ELNES of the Ti-L2,3 edge across the interfaces. In general, the L3-L2 spin-orbit splitting in all spectra is ∼5.4 eV, being consistent with those reported in the literature.17 Both L3 and L2 peaks are split into two peaks with a separation of ∼2.1 eV, which is explained by the crystal field splitting due to the octahedral symmetry at the Ti sites (the Ti atoms are surrounded by six O atoms in a perovskite structure), i.e., the degenerated unoccupied 3d state in the material conduction band is split into a lower energy 2t2g and a higher energy 3eg molecular orbital level.17,18 When the electron probe accessed the interfaces, the energy positions of Ti-L2,3 edges shifted toward lower-energy losses, which suggests partial reduction of Ti (from Ti4+ in STO to lower valence states) and thus possibly O loss at the interfacial region. The Mn-L2,3 edges taken at the corresponding regions are plotted in Figure 3b. Similarly, the L3 and L2 spin-orbit splitting is ∼11 eV for all spectra acquired, and it is in agreement with the literatures results of La0.67Sr0.33MnO3 characterized with X-ray absorption spectroscopy (XAS).19 A similar trend of peak shifting is observed (compared to that of Ti-L2,3 edge) when the electron probe moves across the interfaces;the energy positions of Mn-L2,3 edges red-shifted. The weak chemical shift is also attributed to a local under-stoichiometry of oxygen around Mn, which would reduce the charge transfer from the Mn cation toward its oxygen anion neighbors, consequently slightly reducing the local valence state of Mn. Distinctive shoulders of both L3 and L2 peaks dominated by multiplet effects are observed (as marked by arrows in Figure 3b), which are intrinsic for these manganites and have been observed in the XAS measurements of the Mn 2p states in single phase LSMO as a function of the Sr concentration.19 The more obvious shoulder observed in the L3 and L2 edges of Mn at the interfacial regions of LSMO/STO suggests a stronger crystal field at the interface, which is the origin of the interesting physics phenomena observed in the same superlattice system, such as the exchange bias effect, the enhanced magnetoresistance, as well as the oscillatory exchange coupling.14,15 Unlike Ti and Mn, oxygen presents in both the LSMO and the STO layer. Figure 3c shows the O-K ELNES to across the interfaces. The fine structure of the O-K edge reflects the unoccupied oxygen p state. Intra-atomic multiplet effects are negligible, and this kind of spectrum is usually interpreted based on partial density-of-states calculations.8 Three peaks can be distinguished in the O-K edges and are attributed to the excitations from the O 1s states to the partially filled O 2p state hybridized with Mn (Ti) 3d (peak a), La 5d or Sr 4d (peak b), and Mn (Ti) 4sp states (peak c).8 When the electron probe approaches the interfaces, the energy position of peak a shifts toward lower energy loss, while peak b shows little change. According to the reference spectrum of the O-K edge in La1-xSrxMnO3 (x ) 0.33 here) and SrTiO3,8 the energy position changes for peak a of the O-K edge describe a chemical environment evolution from La0.67Sr0.33MnO3 and SrTiO3, rather than having any other indications.

He and Zhang 4. Conclusion In conclusion, the interfacial electronic structures in LSMO/ STO superlattice are studied by TEM-related techniques. A general picture of the elemental distribution of the chemically modulated layers was given by the EFTEM images. Welldefined interfaces between LSMO and STO layers and uniform bilayer modulation are observed in the superlattice. Detailed understanding of the interfacial electronic structure of such superlattice systems is obtained based on ELNES of Ti, Mn, and O taken across multiple interfaces. In particular, the chemical shifts observed in both Ti and Mn edges in the interfacial region indicate the decreasing of cation valences, likely due to the presence of oxygen vacancies. The more resolved shoulder of both L2 and L3 edges of Mn detected in the interfacial region suggest increased crystal field strength in such regions, giving a plausible explanation to the observed interesting physical properties, such as the oscillatory exchange couplingandenhancedmagnetoresistance,ofthesamesuperlattices. Acknowledgment. This work was sponsored by the National Natural Science Foundation of China under Grant No. 50902014. References and Notes (1) Kwon, C.; Kim, K.-C.; Robson, M. C.; Gu, J. Y.; Rajeswari, M.; Venkatesan, T.; Ramesh, R. J. Appl. Phys. 1997, 81, 4950–4952. (2) Wang, L. M. Phys. ReV. Lett. 2006, 96, 077203(1-4). (3) Nikolaev, K. R.; Dobin, A. Yu.; Krivorotov, I. N.; Cooley, W. K.; Bhattacharya, A.; Kobrinskii, A. L.; Glazman, L. I.; Wentzovitch, R. M.; Dahlberg, E. D.; Goldman, A. M. Phys. ReV. Lett. 2000, 85, 3728–3731. (4) Moutis, N.; Christides, C.; Panagiotopoulos, I.; Niarchos, D. Phys. ReV. B 2001, 64, 094429(1-10). (5) Zhang, Y. B.; Li, S.; Sun, C. Q.; Gao, W. Mater. Sci. Eng., B 2003, 98, 54–59. (6) Zhao, K.; Jin, K. J.; Lu, H. B.; He, M.; Huang, Y. H.; Yang, G. Z.; Zhang, J. D. Appl. Phys. Lett. 2008, 93, 252110(1-3). (7) Izumi, M.; Murakami, Y.; Konishi, Y.; Mananko, T.; Kawasaki, M.; Tokura, Y. Phys. ReV. B 1999, 60, 1211–1215. (8) Pailloux, F.; Imhoff, D.; Sikora, T.; Barthe´le´my, A.; Maurice, J. L.; Contour, J. P.; Colliex, C.; Fert, A. Phys. ReV. B 2002, 66, 014417(1-9). (9) Egerton, R. F. Electron Energy Loss Spectroscopy in the Electron Microscope; Plenum Press: New York, 1986. (10) Batson, P. E. Nature 1993, 366, 727–728. (11) Kimoto, K.; Asaka, T.; Nagai, T.; Saito, M.; Matsui, Y.; Ishizuka, K. Nature 2007, 450, 702–704. (12) Muller, D. A.; Fitting Kourkoutis, L.; Murfitt, M.; Song, J. H.; Hwang, H. Y.; Silcox, J.; Dellby, N.; Krivanek, O. L. Science 2008, 319, 1073–1076. (13) Samet, L.; Imhoff, D.; Maurice, J. L.; Contour, J. P.; Gloter, A.; Manobi, T.; Fert, A.; Colliex, C. Eur. Phys. J. B 2003, 34, 179–192. (14) Zhu, S. J.; Yuan, J.; Zhu, B. Y.; Zhang, F. C.; Xu, B.; Cao, L. X.; Qiu, X. G.; Zhao, B. R.; Zhang, P. X. Appl. Phys. Lett. 2007, 90, 112502(13). (15) Zhu, S. J.; Zhao, B. R.; Zhu, B. Y.; Xu, B.; Cao, L. X.; Qiu, X. G. Appl. Phys. Lett. 2007, 91, 012505(1-3). (16) Oxley, M. P.; Cosgriff, E. C.; Allen, L. J. Phys. ReV. Lett. 2005, 94, 203906(1-4). (17) Leapman, R. D.; Grunes, L. A.; Fejes, P. L. Phys. ReV. B. 1982, 26, 614–635. (18) Ikarashi, N.; Hosoi, N. J. Appl. Phys. 1999, 85, 7874–7878. (19) Abbate, M.; de Groot, F. M. F.; Fuggle, J. C.; Fujimori, A.; Strebel, O.; Lopez, F.; Domke, M.; Kaindl, G.; Sawatzky, G. A.; Takano, M.; Takeda, Y.; Eisaki, H.; Uchida, S. Phys. ReV. B 1992, 46, 4511–4519.

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