Magnetic Mesocrystal-Assisted Magnetoresistance in Manganite

Oct 14, 2014 - Mesocrystal, a new class of crystals as compared to conventional and well-known single crystals and polycrystalline systems, has captur...
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Magnetic Mesocrystal-Assisted Magnetoresistance in Manganite Jan-Chi Yang,†,¶ Qing He,*,‡,¶ Yuan-Min Zhu,§ Jheng-Cyuan Lin,† Heng-Jui Liu,† Ying-Hui Hsieh,† Ping-Chun Wu,† Yen-Lin Chen,∥ Shang-Fan Lee,∥ Yi-Ying Chin,⊥ Hong-Ji Lin,⊥ Chien-Te Chen,⊥ Qian Zhan,§ Elke Arenholz,# and Ying-Hao Chu*,†,∥,∇ †

Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu 300, Taiwan Department of Physics, Durham University, Durham DH1 3LE, United Kingdom § School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China ∥ Institute of Physics, Academia Sinica, Taipei 155, Taiwan ⊥ National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan # Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States ∇ Department of Electrophysics, National Chiao Tung University, Hsinchu 300, Taiwan ‡

S Supporting Information *

ABSTRACT: Mesocrystal, a new class of crystals as compared to conventional and well-known single crystals and polycrystalline systems, has captured significant attention in the past decade. Recent studies have been focused on the advance of synthesis mechanisms as well as the potential on device applications. In order to create further opportunities upon functional mesocrystals, we fabricated a self-assembled nanocomposite composed of magnetic CoFe2O4 mesocrystal in Sr-doped manganites. This combination exhibits intriguing structural and magnetic tunabilities. Furthermore, the antiferromagnetic coupling of the mesocrystal and matrix has induced an additional magnetic perturbation to spin-polarized electrons, resulting in a significantly enhanced magnetoresistance in the nanocomposite. Our work demonstrates a new thought toward the enhancement of intrinsic functionalities assisted by mesocrystals and advanced design of novel mesocrystal-embedded nanocomposites. KEYWORDS: mesocrystal, self-assembled, nanocomposite, colossal magnetoresistance, complex oxide, nanocrystal

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and advanced modulation of functionalities driven or assisted by mesocrystals, we have fabricated a self-assembled mesocrystal-embedded nanocomposite. In this work, we have modulated the colossal magnetoresistance (CMR) effect of Sr-doped manganite by embedding self-assembled ferrimagnetic mesocrystals in it. In this system, the CMR properties can be elegantly tuned and significantly enhanced via inherent magnetic and structural coupling within the nanocomposite. Our work paves a route to modify magnetoresistance (MR) by changing structural and magnetic environments and demonstrates a new thought to create further opportunities driven by functional mesocrystals. Figure 1 illustrates a nanocomposite consisting of functional mesocrystal of ferrimagnetic CoFe2O4 (CFO) spinel embedded in a ferromagnetic CMR matrix. For this study, we chose perovskite La0.7Sr0.3MnO3 (LSMO) as the fundamental parent/ matrix material, which exhibits typical CMR behavior around

esocrystals, that is, ordered three-dimensional nanocrystal superstructures with novel properties and distinct functionalities, have been the focus in the fields of solid state chemistry and condensed matter physics over the past decade due to their exciting potential for advanced applications.1,2 The goal of the mesocrystal research has been to reach superior performance for novel catalytic, electronic, optical, drug delivery, and reaction precursor applications.3,4 Due to their structural nature, the mesoscopically structured crystals can offer unique properties and functionalities that are distinct from bulk biominerals or functional materials. Typical example of mesocrystals can be found in various materials as diverse as metal oxide,5−9 complex oxide,10,11 fluoride,12 phosphate,13 II/ IV semiconductor,14,15 organic molecular systems,16,17 and so forth. Considerable efforts have been made to develop synthesis mechanisms and to acquire new members of the mesocrystal family as well as tailoring their tantalizing functionalities. However, although most of the studies hitherto have addressed the advances and potential applications of mesocrystals, rarely has the attention been placed on new possibilities triggered by the elegant combination of mesocrystals and functional materials. In order to further explore intriguing properties © XXXX American Chemical Society

Received: May 22, 2014 Revised: October 7, 2014

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Figure 1. Illustration of mesocrystal-embedded nanocomposite, composed of magnetic spinel-phase mesocrystal and perovskite-phase manganite matrix.

Figure 2. Structure characterizations on mesocrystal-embedded nanocomposite. (a) X-ray diffraction θ−2θ scan of annealed CFO−LSMO nanocomposite showing only (00l) type peaks, corresponding to LSMO, CFO, and STO substrate. (b) A 4-fold symmetry and epitaxial correlations between two constituents and single crystal substrate revealed by phi-scans. (c) Surface morphology probed by AFM. (d) Cross-sectional TEM image of annealed CFO−LSMO nanocomposite revealing the embedded CFO mesocrystal in the LSMO matrix. The diffraction pattern along [110] is shown in the inset, where the diffraction spots of CFO and LSMO are indicated by red and yellow arrows, respectively.

room temperature.18 Unlike conventional strategies for mesocrystal syntheses, which is usually achieved by chemical−solution−based methods and subsequent controlled growth for highly ordered crystalline structure,19 we synthesized mesocrystal-embedded composites by pulsed laser deposition using a dual-target setup.20 The composites were

deposited on single crystal SrTiO3 (STO) substrates by repeating hundreds of CFO−LSMO pulse alternate cycles. Each cycle is well controlled so that neither CFO nor LSMO deposited more than one unit cell to avoid the formation of multilayered structure. Taking advantage of the dual-target technique, the nanostructure compositions can be controlled B

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Figure 3. Magnetic mesocrystal assisted magnetoresistance effects. Temperature-dependent transport measurements with various applied magnetic field performed on (a) pure LSMO thin film and (b) as-grown and (c) annealed CFO−LSMO nanocomposites grown on STO substrates (upper panels). The CMR changes at 1 and 8 T are extracted and shown in the lower panels, in which the CMR changes are defined as (ρH − ρo/ρH) × 100 (%).

uniformly distributed in the atomically flat surface, consisting of a periodic arrangement, as shown in figure S1 of the Supporting Information. Through recovery, recrystallization, and grain growth steps during the annealing process, self-assembled mesocrystal develop to accommodate local strains and structural difference. For the annealed nanocomposite sample, the AFM result indicates that CFO forms mesocrystal features of 20 to 50 nm diameter embedded homogeneously in the LSMO matrix, as shown in Figure 2c. The mesocrystal features of the embedded CFO are resolved by cross-sectional TEM, as shown in Figure 2d. TEM image of as-grown sample shows that the CFO nanocrystals form in “bamboo”-like columnar structures vertically aligned in LSMO matrix. Further details of as-grown CFO nanocrystals are referred in Supporting Information Figure S2. The structure nature of the bamboo-like heteroepitaxial CFO can be understood as results of the growth dynamic, immiscibility, and large lattice misfit between CFO and LSMO, where similar structure can be found in BaZrO3−YBa2Cu3O7−x systems.21,22 In contrast to the nanorod-type microstructure in as-grown CFO−LSMO nanocomposites, the annealed samples possess octahedra-like mesocrystal features. The high-resolution crosssectional TEM image shown in Figure 2d indicates that the interface between CFO and LSMO are formed into {111}-type facets, shaping CFO as the octahedra-like nanocrystals within the nanocomposite. This result is attributed to the minimization of elastic energy and surface energy terms in CFO and LSMO, as a compromise of preferred {111}-plane family in spinel and preferred {100}-plane family in perovskite materials.23 At the top of the annealed nanocomposite, the CFO mesocrystal forms a pyramid with characteristic facets, which the facets are 54.7° with respect to (001) LSMO, consisting with the {111}-type plane feature. The high quality

simply by altering deposition pulse ratio of the materials in each cycle, instead of changing multiple targets with different constituents. The composites were grown in a dynamic oxygen pressure of 100 mTorr and at substrate temperature of 750 °C. The pulse ratio was set at 20 vs 80 pulses per cycle with a repetition rate of 10 Hz for CFO and LSMO, respectively, and the total repetitive period was set to be ∼200 cycles. The end composition was estimated to be 1:4−1:5 in molar ratio via Xray reflectivity and scanning electron microscopy measurements based on single CFO and LSMO layers grown under the same growth condition (not shown here). To derive the wellorganized mesocrystal with clear facets and fewer structure defects, postannealing process was applied at 1100 °C for 12 h at 1 atm in air. A combination of X-ray diffraction (XRD), atomic force microscopy (AFM), and transmission electron microscopy (TEM) has been employed to reveal the mesocrystal features as well as the structural correlation in the LSMO matrix. Figure 2a shows the θ−2θ scan of CFO−LSMO composite, where the LSMO (00l)- and CFO (00l)-oriented peaks can be clearly observed. This indicates that the system spontaneously separated into two phases during growth. The c axis lattice parameters of the CFO and LSMO are calculated as 8.39 and 3.88 Å, respectively, suggesting the 3-dimensional strain is fully relaxed during the annealing process. In addition, the in-plane phi-scans shown in Figure 2b reveal 4-fold symmetry and the cubic-to-cubic crystalline correlation of both constituents and single crystal STO substrate, suggesting a structure-coupled nature. To make the structural natures more accessible, we’ve chosen the nanocomposite with higher CFO ratio for further AFM and TEM analyses. Surface morphologies of as-grown and annealed composites were probed by AFM. With regard to the as-grown nanocomposite, in which ∼5 nm nanodots are C

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Figure 4. XAS-based studies on the magnetism of CFO−LSMO nanocomposite. XAS and XMCD spectra with respect to Co L2,3 (a) and Mn L2,3 edges (b). XAS spectra (blue curves) are normalized in order to acquire XMCD spectra (red curves) taken with opposite polarizations. (c) XMCD hysteresis loops of Co2+ and Mn3+ in annealed nanocomposite. The Co2+ and Mn3+ ions possess the same coercivity (∼50 Oe) and antiparallel spin moments, exhibiting the inherent magnetic coupling in the CFO−LSMO nanocomposite. (d) Schematic of the antiferromagnetic coupling between CFO and LSMO, where the blue arrows represent the magnetic moment of CFO mesocrystal, and the green arrow stands for that of LSMO matrix.

a system, the larger the low-field MR can be achieved. However, in our study, we have observed a further enhancement of MR effects in both low- and high-field on annealed samples, in which the MR changes have increased to −30% and −250% for 1 and 8 T magnetic fields, respectively, as shown in Figure 3c. During the annealing process, the high temperature environment effectively reduces the phase boundaries and defects in the materials. Therefore, we would have expected a decrease in low-field MR based on the spin-polarized tunneling model. Moreover, the effective increase in spin-polarized tunneling barrier driven by the structural disorder cannot qualitatively explained the enhanced high-field MR observed in both of the as-grown and annealed mesocrystal-embedded CMR systems. These results imply that additional disorder caused by the magnetic modulation in the mesocrystal might play an important role driving the significant enhancement of MR in the system. Because the two constituents composing the mesocrystal nanocomposite are basically magnetic materials with partially filled 3d orbitals, magnetic interactions between the two materials should be taken into consideration. To determine the magnetic order as well as the magnetic coupling of the embedded mesocrystal and the matrix, X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) measurements have been conducted at Ledges of all magnetic elements (Co, Mn, and Fe) in our system. These techniques offer element specific information on valence states and spin order as well as orbital contributions. The XAS spectra were recorded in total electron yield mode at Advanced Light Source, Berkeley as well as at National Synchrotron Radiation Research Center, Taiwan. The XAS and XMCD

epitaxial growth of mesocrystal-embedded nanocomposite can also be identified via district selected diffraction patterns, as shown in the inset of Figure 2d. After building up the structural configuration, new functionalities of such mesocrystal-embedded nanocomposites can be further explored. Systematic magnetotransport measurements have been applied to reveal the mesocrystal-assisted MR change in the CMR LSMO matrix. Temperature-dependent resistivity as a function of applied magnetic field of pure LSMO thin film, as-grown and annealed CFO−LSMO nanocomposites are shown in the upper panel of Figure 3. The corresponding CMR, defined as (ρH − ρo/ρH) × 100%, at 1 and 8 T external magnetic fields are extracted and shown in the lower panel of Figure 3. Pure LSMO thin film was deposited on (001)-oriented STO as the reference system. The CMR values of the strained LSMO with ∼60 nm in thickness are −10% and −68% at 1 and 8 T applied fields, respectively, which are close to the results of previous studies.24 However, it is striking to observe that both the MR at low and high magnetic field are significantly increased for the as-gown CFO−LSMO nanocomposite, where the MR reaches approximately −20% at 1 T field and −175% at 8 T field. Previous studies have indicated that structural disorder (mainly grain boundaries and phase boundaries) forming in columnar vertical-aligned nanostructures would lead to the enhancement of low-field MR.25,26 The additional energy barrier of the spin-polarized tunneling process, resulting from a decoupling of the neighboring ferromagnetic grains, is mainly attributed to the enhanced low-field MR phenomenon.27−29 Based on such a scenario, the more grain boundaries or structural/spin disorder is present in D

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Figure 5. Correlation between magnetic disorder and local conduction of mesocrystal-embedded nanocomposite. (a) Topography image of CFO− LSMO probed by photoemission electron microscopy (PEEM), where the dark and bright contrasts stand for the mesocrystal and matrix, respectively. XMCD−PEEM images of Mn3+ (b) and Co2+ (c) in the same area of panel a. The selected red, green, and blue circles are marked to present the same mesocrystal site in topography and XMCD−PEEM images. In XMCD−PEEM images, the observed opposite contrasts represent for the opposite spin directions. (d) Schematic illustrating the spin disorders around the mesocrystal, where the light blue and green arrows represent for the spin moments near the phase boundaries. (e) CAFM study on the CFO−LSMO nanocomposite, where the topography is shown in the upper panel, whereas the current mapping is presented in the lower panel. The CAFM image taken a 1.1 V bias shows remarkably higher conduction in LSMO matrix, as shown in bright color. On the contrary, the insulating areas are presented by dark contrasts. The insulating CFO nanocrystals are surrounded by additional insulating rings/regions, revealing the additional insulating region induced by the mesocrystal. The cross-sectional height− current profile of a selected nanocrystal is shown in the inset.

spectra of Co and Mn L-edges of an annealed sample are depicted in blue and red curves and shown in Figure 4a and b, respectively. The spectra of Fe3+ are not presented here because the total moment contributed from Fe3+ ions is negligible owing to the fact that the spin coupling between tetrahedral and octahedral sites of CFO is antiferromagnetic. As a result, Co and Mn are the main elements that drive the emergent ferromagnetism in CFO mesocrystal and LSMO matrix, respectively. For a standard CFO spinel single crystal (an typical inverse spinel arrangement), the Co2+ ions sit in octahedral sites while Fe3+ ions occupy octahedral and tetrahedral sites with equal population. However, our simulation result based on XMCD measurements (Figure 4a) has suggested the Co2+ occupancy between octahedral and tetrahedral sites to be ∼1:1, that is, more tetrahedral sites of our spinel-phased nanocrystals are occupied by Co. We attributed this occupancy change to a complex combination of strain effect and interspecies exchange while composing the nanocomposite. From the absorption spectra shown in Figure 4b, the valence states of Mn indicate a combination of Mn2+, Mn3+, and Mn4+, where Mn2+ is not expected in divalent ion doped manganite. However, the Mn2+ peak observed at ∼640 eV in the nanocomposite increases with the X-ray exposure suggesting it is created in a X-ray stimulated process in the surface near sample region. Attention should be

paid to the relative alignment of Co and Mn spin moments. In the XMCD spectra, Mn3+ and Mn4+ show positive XMCD features at the L3 edge, whereas Co2+ exhibits an XMCD peak with opposite sign. This is indicative of an antiparallel alignment of the Co and Mn moments, that is, the majority spins of the CFO mesocrystal and LSMO CMR matrix are exhibiting antiferromagnetic coupling with each other. Figure 4c shows the XMCD magnetic hysteresis loops for the corresponding magnetic species. The curves of Figure 4c are effective magnetic M−H loops obtained at the Co and Mn L3 edges for element specific magnetic information. The XMCD signal of Co and Mn always show opposite sign through the magnetic measurement. Furthermore, it is worth to note that both Co and Mn share the same coercivity in the system, suggesting that the mesocrystal and the LSMO are strongly magnetically coupled with each other. Based on the information, the schematic of the magnetic coupling between the CFO mesocrystal and LSMO matrix is illustrated in Figure 4d, where the Co and Mn ions are revealed to have antiferromagnetic coupling in the CFO-embedded nanocomposite. The antiferromagnetic coupling may play a key role to induce additional magnetic disorder states in the CMR material before applying magnetic fields. The coupling may effectively increases the resistance of CFO−LSMO nanocomposite in the ground state (at zero magnetic field), as E

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shown in Figure 3b and c. Once the magnetic field is high enough to suppress the magnetic disorder states caused by CFO mesocrystal, the spin-polarized electrons of LSMO are able to hop with significantly higher possibility, leading to extraordinary high MR changes (both low- and high-field MR). Photoemission electron microscopy (PEEM) and conductive atomic force microscopy (CAFM) have been used to gain further insight into the correlation between the local magnetic disorder and conduction environment. Spatially resolved PEEM images were obtained using both left- and right-circularly polarized (LCP and RCP, respectively) X-rays at the Advanced Light Source, Berkeley. Figure 5a is the PEEM image obtained by LCP, showing magnetic contrast superimposed with topographic contrast (main contribution). The darker areas in this image represent the positions of the CFO nanocrystals. Figure 5b and c are the XMCD−PEEM images of Mn3+ and Co2+ ions, respectively. The image contrast of XMCD−PEEM is an effective map of the in-plane local magnetization vectors, where regions that have their magnetic moments aligned parallel to the X-ray wave vector appear in bright contrast, whereas those that are antiparallel show dark contrast. Please note that only specific elements with obvious ferromagnetism will be observed in the XMCD−PEEM image. From the PEEM image of Mn, we conclude that some Mn ions are included in the CFO nanocrystals. By comparing the Mn XMCD−PEEM image and the topography image of the same area (Figure 5a), we find that not only the Mn ions in the mesocrystal are magnetized but in the matrix area surrounding the mesocrystal, the Mn ions also have magnetic moment pointing to the same direction. Namely, the magnetization of the Mn ions in the matrix area surrounding the mesocrystal is pointing to the same direction as that of Mn ions in the mesocrystal. In addition, where Mn has bright contrast, Co always shows dark contrast, suggesting that magnetic moments of the two elements are antiparallel to each other. This result is in good agreement with aforementioned XMCD spectra study. Furthermore, an environment with more magnetic disorder around the mesocrystal can be virtualized by comparing the XMCD− PEEM images and the topography image. The key information gained from our PEEM study is that the antiferromagnetic coupling occurs close to CFO mesocrystal would result in a disordered environment that interrupts spin-polarized electron hopping in the LSMO matrix. Based on these results, a further schematic of the magnetically coupled CFO−LSMO nanocomposite is illustrated in Figure 5d, in which each CFO mesocrystal is surrounded with pronounced magnetic disorders, showing antiparallel magnetizations between the mesocrystal and matrix. To integrate this finding with the local conduction of mesocrystal-embedded CMR matrix, CAFM measurements were performed on the annealed sample. The CAFM was conducted with 1 V tip bias, as shown in Figure 5e for the topography and the corresponding CAFM image. In the CAFM image, the bright contrast represents the conducting regime, where the dark contrast indicates region that are insulating. By comparing the topography and CAFM, it is easy to see the LSMO matrix shown in the region with much higher conduction, whereas the CFO nanocrystals are presented in area that are much less conducting. This result is reasonable taking the conduction of individual species into account (conducting LSMO and semiconducting/insulating CFO). However, additional insulating “rings/regions” are found surrounding each CFO mesocrystal upon closer inspection of

the CAFM image. These rings extend the low conducting regions from individual nanocrystals to the CMR matrix, bringing additional conduction perturbations to the hopping electrons. Furthermore, this finding of the low conduction ringshaped area is in nice agreement with the location and size of the magnetic disorder regions found in XMCD−PEEM study. This result offers a promising result suggesting that the antiferromagnetic coupling feature of CFO mesocrystal and LSMO matrix has an important role in driving the anomalous magnetoresistance behaviors. To validate our hypothesis on the magnetic-perturbationdriven feature, a NiFe2O4 mesocrystal-embedded system was carried out as a reference system. In such a reference system, the Ni and Mn elements show ferromagnetic coupling nature, as shown in Supporting Information. Similar work has been used to explore the magnetic interaction and local conduction as well as the MR behavior (Supporting Information Figure S2). On the basis of the same scenario, the ferromagneticcoupled environment will not hinder the transport of spinpolarized electrons. Our result shown in Supporting Information indicates that the ferromagnetic coupling between NFO mesocrystal and LSMO matrix has shown a significantly smaller MR changes compared with CFO mesocrystal systems, which provides a promising evidence to support the role of magnetic perturbation modulated by embedded mesocrystals. In summary, we have synthesized a new type of complex oxide nanocomposite with CFO spinel-structured mesocrystal embedded in a perovskite-structured CMR matrix. Heteroepitaxy and high structural correlation between CFO and LSMO have been observed via a combination of XRD, AFM, and TEM. The as-grown nanocomposite possesses a “bamboo”type vertical aligned feature, whereas the annealed nanocomposite shows a dispersed octahedral-mesocrystal characteristic. Significant low- and high-field MR enhancement has been observed in both systems. Our XAS-based studies have revealed the antiferromagnetic coupling nature between two main elements (Co and Mn) that drive the ferromagnetism in the system. A combination of PEEM and CAFM measurements has pointed out an additional magnetic disorder state induced by CFO mesocrystal as the core factor to derive extraordinary MR effects. Such a mesocrystal-embedded nanocomposite has successfully demonstrated a new possibility to modulate the CMR effects via tuning the extrinsic magnetic coupling between different materials. Our work demonstrates potential applications of the oxide nanocomposites and provides a new thought to enhance the intrinsic functionalities via self-assembled mesocrystals.

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ASSOCIATED CONTENT

S Supporting Information *

Structural natures of as-grown CoFe2O4−La0.7Sr0.3MnO3 nanocomposite are developed in Section I (Figure S1). Magnetotransport and magnetism in NiFe2O4−La0.7Sr0.3MnO3 nanocomposite (Figure S2) are detailed in the Section II. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Address: Room 235, Rochester Building, Science Site, South Road, Durham, DH1 3LE, United Kingdom. Phone: +44-191-334-3812. F

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*E-mail: [email protected]. Address: Room 709, Engineering Building VI, 1001 University Road, Hsinchu 30010, Taiwan. Phone: +886-972-781-386

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Author Contributions ¶

These authors contributed equally in this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

Our work is supported by Ministry of Science and Technology, R.O.C. (MOST 103-2119-M-009-003-MY3), Center for Interdisciplinary Science of National Chiao Tung University, Ministry of Education, Taiwan (MOE-ATU 101W961), and National Natural Science Foundation of China with Grant Nos. 50971015 and 51371031.

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