Interface-Induced Enhancement of Ferromagnetism in Insulating

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Interface-Induced Enhancement of Ferromagnetism in Insulating LaMnO3 Ultrathin Films Liang Wu, Changjian Li, Mingfeng Chen, Yujun Zhang, Kun Han, Shengwei Zeng, Xin Liu, Ji Ma, Chen Liu, Jiahui Chen, Jinxing Zhang, * Ariando, T. Venkatesan, Stephen J. Pennycook, John Michael David Coey, Lei Shen, Jing Ma, X Renshaw WANG, and Ce-Wen Nan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b15364 • Publication Date (Web): 04 Dec 2017 Downloaded from http://pubs.acs.org on December 6, 2017

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ACS Applied Materials & Interfaces

Interface-Induced Enhancement of Ferromagnetism in Insulating LaMnO3 Ultrathin Films Liang Wu, Changjian Li, Mingfeng Chen, Yujun Zhang, Kun Han, Shengwei Zeng, Xin Liu, Ji Ma, Chen Liu, Jiahui Chen, Jinxing Zhang, Ariando, T. Venky Venkatesan, Stephen J. Pennycook, J. M. D. Coey, Lei Shen, Jing Ma*, X. Renshaw Wang* and Ce-Wen Nan* L. Wu, M. Chen, Y. Zhang, J. Ma, C. Liu, J. Chen, Prof. Jing Ma, Prof. C.-W. Nan School of Materials Science and Engineering, State Key Lab of New Ceramics and Fine Processing, Tsinghua University, Beijing 100084, China E-mail: [email protected], [email protected] C. Li, Prof. Stephen J. Pennycook, Prof. T. Venky Venkatesan Department of Materials Science and Engineering, National University of Singapore, Singapore 117575, Singapore K. Han, S. Zeng, Prof. Ariando, Prof. T. Venky Venkatesan NUSNNI-NanoCore, National University of Singapore, Singapore 117411, Singapore Department of Physics, National University of Singapore, Singapore 117542, Singapore Prof X. Renshaw Wang E-mail: [email protected] School of Physical and Mathematical Sciences & School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 637371, Singapore X. Liu, Prof. J. Zhang Department of Physics, Beijing Normal University, Beijing 100875, China Prof. J. M. D. Coey School of Physics, Trinity College, Dublin 2, Ireland Faculty of Materials Science and Engineering, Beihang University, Beijing, 100191, China Prof. L. Shen Department of Mechanical Engineering, Engineering Science Programme, Faculty of Engineering, National University of Singapore, Singapore 117575, Singapore

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Abstract: Engineering ferromagnetism, by modulating its magnitude or anisotropy, is an important topic in the field of magnetism and spintronics. Among different types of magnetic materials, ferroamgnetic insulators, in which magnetic moment unusually coexists with localized electrons, are of particular interest. Here, we report a remarkable interfacial enhancement of the ferromagnetism by adding one unit-cell LaAlO3 adjacent to an insulating LaMnO3 ultrathin film. The enhancement of ferromagnetism is explained in terms of charge transfer at the interface, as evidenced by X-ray absorption spectroscopy and ab initio calculations. This study demonstrates an effective and dramatic approach to modulate the functionality of ferromagnetic insulators, contributing to the arsenal of engineering techniques for future spintronics. Key words Ferromagnetic insulators; LaMnO3; ultrathin film; ferromagnetism enhancement; interface

Introduction Ferromagnetic insulators (FMIs), which integrate ferromagnetic and insulating properties together in a single material, have drawn ever-increasing attention in the fields of materials science and spintronics; and the FMIs provide a platform to investigate, manipulate and potentially utilize the unusual coexistence of spins and localized electrons. For instance, FMIs are the parent materials of multiferroics and magnetic topological insulators. On the other hand, for the applications in spintronics, the insulating property in FMIs can provide distinct advantages and enable unique device architectures. When working as spin filters, the FMI is considered as a better option to realize low dissipation Josephson junctions 2 ACS Paragon Plus Environment

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intrinsically1-2 and one of the highly efficient materials to inject spin current into a semiconductor3-4. Moreover, unique device architectures have been developed based on the FMIs, including spin-current generator based on the longitudinal spin-Seebeck effect5, proximity-effect-based devices6-9 with no additional conducting channel introduced, and magneto-optical devices which could require transparency.10-11 Hence, engineering FMIs is unequivocally important to the advances in the fundamental understanding and spintronic applications. Traditionally, the study of FMIs has focused on europium chalcogenides and yttrium iron garnet based heterostructures4, 12 owing to the extreme high spin-filtering efficiency in europium chalcogenides and ultralow damping in yttrium iron garnet. With the rapid development of perovskite-based spintronics, perovskite FMIs recently become a natural choice due to their chemical and structural compatibility with other functional perovskites.13 However, only a few perovskites have been known as FMIs, and their performance in spintronics is not satisfactory so far.3, 14-17 Taking into account the scarcity, new perovskite FMIs with robust ferromagnetism (FM) and high Curie temperature are of strong demand. In addition, the surfaces and interfaces of FMIs are also critical, as the interface effect is generally responsible for the emergence of non-ferromagnetic dead layer, which is thought to be inimical to the device applications.18 Hence, seeking a way to achieve a robust ferromagnetism in perovskite FMI heterostructures is of pronounced significance for device performance. Recently, a polar discontinuity interface of LaMnO3 (LMO)/SrTiO3 (STO) has been designed to trigger ferromagnetism in LMO thin film.19 In the present work, we validated the FMI behavior in LMO thin films down to a nanometer scale and demonstrated



that the FM can be significantly enhanced through capping or buffering even one unit cell (uc) LaAlO3 (LAO). In this study, atomically thin LMO films were epitaxially grown on (001)-orientated STO substrates. The thickness of LMO was varied from 4 to 20 uc (see Experimental section and Supplementary information). The interface between LAO and LMO is atomically sharp, as supported by the results of high-resolution scanning transmission electron microscopy (HR-STEM). Figure 1a shows both the HR-STEM annular bright field (ABF) and high-angle annular dark-field (HAADF) images of a typical 3 uc LAO/ 6 uc LMO/ STO heterostructure. The epitaxial property among LMO, LAO and STO was guaranteed by their similar bulk lattice parameters, which are 3.95 Å, 3.79 Å and 3.905 Å, respectively. More structural characterization can be found in the Supplementary Information. Bulk LMO is a Mott insulator and its insulating property remains in thin film form,20 as shown in the resistivity measurement at macroscale of a representative 6 uc LMO/STO in Figure 1b. To investigate the interface effect on the electrical property of LMO film, LAO was capped on top of LMO film with thicknesses varied from 1 to 6 uc. Figure 1b shows that the resistivity of both bare LMO and 3 uc LAO capped 6 uc LMO films are above 1 kΩ·m, exhibiting an insulating behavior within the limits of resistance measurements. The LMO with the LAO capping layer possesses a slightly larger resistivity than that of bare LMO, demonstrating that the capping of LAO sustains the insulating nature of LMO even the enhancement of thee resistivity is not clear yet. We further investigate the homogeneity of the insulating property of LMO via the electrostatic force microscopy (EFM) with a lateral resolution of dozens of nanometers.21 When patterning a film using EFM, one can expect that the EFM signal of a


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uniform conductor should not show any apparent contrast due to the screening effect, while an insulator surface could show some contrast if the ability of holding charges are not uniform, although the contrast can also be influenced by oxygen migration, redox processes, etc.22 In this study, periodically rectangular patterns were written on the surfaces of 5 and 20 uc LMO using a Ti/Ir-coated tip with biases of +4 and -4 V, respectively. Subsequently, the patterns were read using the tip with a smaller bias voltage of +3 V. In Figure 1c,d, the dark regions correspond to a positive potential (positive surface charge) induced by applying a positive bias of 4 V, and the bright correspond to negative potential due to the negative bias. Within the spatial resolution of our EFM, the uniformity of the same poling voltage ranges confirms the homogeneity of the insulating behavior in the LMO films both below (5 uc) and above (20 uc) the critical thickness of 6 uc for ferromagnetism.19 This validates the FMI nature down to nanometer scale in the LMO film. Bulk LMO possesses a canted antiferromagnetism as a consequence of the anisotropic Dzyaloshinskii-Moriya interaction. Compared to its electrical property, the magnetic properties of LMO are more complicated due to its sensitivity to oxygen content, strain and interface effects. Such sensitivity has been utilized to manipulate the magnetic properties artificially by choosing substrates wisely and controlling growth conditions systematically. Figure 2a shows that the ferromagnetism emerges in the LMO films grown on STO when the thickness is above 5 uc, which is consistent with the 6 uc critical thickness for ferromagnetism reported recently.19 As shown in Figure 2b, the magnetic moment in LMO increases as the thickness increases, with the magnetic moment of LMO about 1 µB/Mn in a 7 uc LMO. The interface effect was investigated by capping a 6 uc LAO layer on top of



LMO/STO samples (see Figure 2b). The magnetic moment in 6 uc LAO capped 7 uc LMO unexpectedly exceeds 2 µB/Mn, which is about twice of that in bare LMO of the same thickness. It is of particular interest to note that the enhancement of magnetization of these LMO thin films is realized by capping non-magnetic LAO. Furthermore, the capping of LAO reduces the critical thickness of ferromagnetism in LMO/STO heterostructures from 6 to 4 uc. The thickness diminution is beneficial for spintronic device applications, especially for using the FMI as a spin-filtering barrier. In such devices, ferromagnetism must be retained to allow substantial tunneling probability, where the barrier thickness is usually below 3 nm. We further investigated the relation between magnetic moment of LMO and capping thickness of LAO. The M-H loops of the 5 uc LMO films with the LAO capping layers of various thicknesses at 10 K are shown in Figure 2c, and the saturated magnetic moments per Mn atom are summarized in Figure 2d. The results demonstrate that capping only one monolayer LAO can significantly enhance the magnetic moment of the 5 uc LMO films, restoring the ferromagnetism in LMO films below the critical thickness. More strikingly, such an enhancement of magnetization is almost independent of the LAO capping thickness, i.e., the thicker LAO layer does not produce a more pronounced effect. This independence of the capping thickness also confirms that this is an interface-enhanced FM effect. Considering the mechanism of such interface effect, there are generally three possibilities, i.e., dangling bonds23 at the surface of LMO, strain imposed from the substrate19 or buffering layer,24 and charge transfer25 between LAO and LMO. In order to disentangle these effects, we further designed new heterostructures with the LAO position


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symmetric to the previous one. In the new heterostructures, the LAO layer was shifted from the capping location on the surface to a buffering location between the LMO film and the STO substrate by modifying deposition sequence. Figure 3a shows the M-H loops of these LAO buffered 6 uc LMO. A remarkable enhancement of magnetization occurs by buffering with 1 uc LAO as well, which is similar to the ferromagnetism enhancement effect by capping with 1 uc LAO (Figure 2d). Moreover, buffering LAO can also reduce the critical thickness of ferromagnetic LMO on STO substrate from 6 to 4 uc, as shown in Figure 3c, d. These results indicate that the dangling bonds, such as surface hydrate layer, are unlikely the mechanism, since the LAO buffering layers do not influence the surface state of the LMO while the LAO capping layers do. Unlike capping LAO, the magnetization of LMO decreases as the thickness of the buffering LAO increases (Figure 3b). A 6 uc buffer layer largely suppresses the FM in LMO, which resembles the FM suppression effect when growing LMO on LAO substrate.19 The strain relaxation should account for the magnetism suppression in this work with thick buffer layers. As reported in 26, the strain in LAO grown on STO substrate could relax at even 3 uc LAO, which would affect the strain condition in LMO when the thickness of the LAO buffering layers is greater than 3 uc. However, in the LAO/LMO/STO heterostructures, as LMO was firstly grown on STO substrate, the capping LAO would have little effect on the strain condition of LMO. This is consistent with the insensitivity of the LMO magnetism to the thickness of the capping layer. In addition, it is worth noting that additional magnetization may emerge at the LAO/STO interface when the LAO layer exceeds 3 uc.27 However, the magnetic moment at the LAO/STO interface (i.e., less than 1 emu/cc


27

) is


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almost negligible compared to the magnetic moment in the LMO here (more than 100 emu/cc). Since the strain effect could only explain the thickness dependent magnetization, and the compensation of dangling bond effect are ruled out, the charge transfer effect is most likely the origin of the observed FM enhancement. The charge transfer was directly verified by measuring the L-edge of Mn using X-ray absorption spectroscopy (XAS), which is highly sensitive to the Mn valence state28. As shown in Figure 4a, Mn2+ signals were detected in all bare LMO films as indicated by a distinct shoulder of Mn2+ peaks. The origin of the Mn2+, possibly due to the electronic reconstruction19 or oxygen vacancies,29 requires further study and beyond the scope of this work. Of interest, by capping a LAO layer, the intensity of Mn2+ peak was reduced (indicated by the black dashed lines in Figure 4a), and the Mn3+-related main peak (indicated by the red dashed lines in Figure 4a) was slightly shifted to higher energy. According to the previous studies,25, 30 the blue-shift of the main peak is considered as the evidence of the emergence of Mn4+, and thus the shift of the main peak in Figure 4a indicates that the LAO capping layer is capable of converting some Mn2+ into Mn4+ and can act as an electron acceptor (hole donator). For a more quantitative analysis, the Mn L3 shows a linear shift to higher energies with increasing Mn valence state from Mn3+ to Mn4+

31

. Hence, the Mn

valence can be estimated by the relative shift of Mn L3. Due to the limit of the energy resolution of our XAS facility, which is 0.1 eV here, we can estimate the relative shift of Mn L3 is about 0.2 to 0.3 eV, which corresponds a 13 to 20% of Mn4+ in the capped LMO. Hence, it can be concluded that the LAO capping layer increases the oxidation state of Mn, and eventually enhances the FM in LMO by strengthening the double exchange effect. Moreover,


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further increasing the thickness of the LAO capping layer does not produce any detectable influence on the degree of the shift of the main peak, as shown in Figure 4b, which is analogous to the capping-thickness-independent magnetic moment in Figure 2b. Furthermore, Figure 4c shows a comparison on the XAS results of bare, capped and buffered LMO samples. The capping and buffering LAO produces the similar effect on raising the Mn oxidation state and on enhancing the FM in LMO. By linking the magnetic properties and XAS measurements, we confirm that the FM enhancement through the adjacent LAO layer is a result of increasing Mn oxidation state by the pumping electrons from LMO to LAO. In order to understand the reason for the changes of Mn valence and the nature of the LMO/LAO interface, we performed ab initio calculations (see Experimental section). The formation energy calculations show that the formation energy of La vacancy (VLa) and Al vacancy (VAl) in LAO are 1.6 and 3.0 eV, respectively. Therefore, the formation energy of VLa is 1.4 eV lower than that of VAl, hence La vacancies are more favorable in LAO, and can act as an electron acceptor under appropriate circumstance. Some experimental results reported by different groups

32-34

also demonstrated the La deficiency state in LAO films

grown by PLD under similar growth conditions to our experiment. At the LMO/LAO interface, as shown in the calculated energy band diagram (Figure 5a), the defect state of La vacancy is 0.3 eV below the valance band maximum of LMO, so the energy difference initiates the electrons transfer from the narrow band-gap LMO to the wide band-gap LAO. Such transfer then leaves holes in the LMO layer and results in the increase of Mn valence.

Conclusion



To summarize, we demonstrate a simple but effective way to enhance the FM in homogeneously insulating LMO films through capping or buffering an LAO layer as thin as a single unit-cell. Supported by the magnetic and XAS measurements and ab initio calculations, the physical nature of the magnetic enhancement was identified as the raising of the Mn valence induced by the adjacent LAO layer. The remarkable monolayer sensitivity provides not only insights into the delicate spin-charge interaction at oxide interfaces, but also demonstrates a design principle, which could empower future spintronic devices at the atomic level.

Experimental Procedures Sample fabrication: Both LMO and LAO thin films were prepared by pulsed laser deposition on TiO2-terminated 5×5 mm2 and 0.5 mm-thick STO (001) substrates from polycrystalline stoichiometric LMO and single crystalline LAO targets, respectively. A growth temperature of 800°C, oxygen background pressure of 10 mTorr, an excimer laser with a wavelength of 248 nm, repetition rate of 1 Hz, and energy density of 1.8 J cm-2 were used. After deposition, all samples were cooled down to room temperature at a rate of 10°C min-1 in an oxygen environment the same as the deposition pressure. Both LMO and LAO growth followed the layer-by-layer growth mode and were monitored by in-situ reflection high energy electron diffraction (RHEED, Figure S1). Consequently, all samples display atomically flat surfaces (Figure S3a-c). Detailed structural characterization can be found in the Supplementary Information. STEM: High-resolution STEM-HAADF/ABF imaging and electron diffraction were


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performed using the JEOL-ARM200F microscope equipped with ASCOR aberration corrector and cold-field emission gun and operated at 200 kV. The cross-section TEM samples were prepared by focused ion beam (FIB). The HAADF and ABF images were acquired with condensor aperture of 30 mrad and collection angle of 68-280 mrad for HAADF and 7-14 mrad for ABF images, respectively. Images were filtered by radial Wiener filters. Electrical properties measurement: The electrical resistivity of LMO films were measured by physical property measurement system (PPMS, PPMS-9, Quantum Design) by four probe technique in a Van der Pauw geometry. The electrical contact to the LMO films were formed using a wire bonding machine (Westbond Inc). In the bonding process, Al wires, which were used in this study, contact both thin films by penetrating the whole film. Its insulating property was verified by electrical electrostatic force microscopy (EFM, MFP-3D-Infinity, Asylum Research). Magnetic properties measurement: The magnetic properties were measured using superconducting quantum interference device (SQUID, Quantum Design) magnetometry, with an external magnetic field applied along to the in-plane STO [001]. Magnetization versus applied fields ranging from −1 to +1 Tesla was measured at 10 K. All MH data were normalized by subtracting the temperature-independent diamagnetic signals of STO substrates. This extra weak ferromagnetic signals from STO substrate35 is considered as backgrounds for calculating the magnetic moment per Mn atom. Here, the diamagnetic signal and saturation ferromagnetic signal of a 5×5×0.5 mm substrate at 10 K are around 5.5×10-5 emu/Tesla and 3×10-6 emu.



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XAS measurement: A series of Mn L-edge X-ray absorption spectroscopy measurements were performed at Beamline BL08U1A of the Shanghai Synchrotron Radiation Facility (SSRF) in total electron yield (TEY) mode at room temperature. All the raw XAS data were normalized to range from 0 to 1 by a linear scaling transform. Ab initio calculation: All calculations are done using the Vienna ab-initio Simulation Package

(VASP)

code.36

Projector

augmented

wave

method

within

the

Perdew-Burke-Ernzerhof (PBE) flavor of generalized gradient approximation (GGA) was used for both LMO and LAO. We used the rotationally invariant coulomb-corrected local spin density approximation (LSDA+U) method with the experimentally deduced on-site coulomb energy37 U=3.5 eV for the Mn d electrons. In the band alignment, the difference κ method38 was used to correct the electron affinity as the GGA method underestimates the band gap of LAO even considering the LSDA+U38 or hybrid functional correction.39 The work functions were calculated for (001) BO2-terminated surface of LAO and LMO using the hybrid functional of Heyd, Scuseria and Ernzerhof (HSE)40 within density functional theory (DFT) with Hartree-Fock exchange fractions obtained from Ref

41

, which can yield

correct bulk electronic properties for LAO and LMO. The orthorhombic LMO (pnma) and rhombohedral LAO (R3c) were used in our calculations. Energy cutoff of 500 eV and k-point mesh grid of 7×7×5 for bulk and 3×3×1 for interface were used. The ions are relaxed until the Hellmann-Feynman forces on each atom are less than 0.01 eV/Å. Note that the LaO layers in LAO and LMO are different because of their different bulk structures. In their interfacial supercell (Figure 5b), one can see a distinguished difference between the LaO layer of LAO and the LaO layer of LMO. Hence, we conclude that the La vacancy in our


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model is in LAO, not LMO.

Supporting Information Detailed growth of LaMnO3 and LaAlO3 films monitering by RHEED, and structure characterization by High-resolution X-ray diffraction (HR-XRD), as well as topography characterization by atomic force microscopy (AFM) can be found in the Supporting Information. Author contribution Liang Wu and Changjian Li contributed equally to this work. Acknowledgements This work was supported by NSF of China (Nos. 51332001 and 11234005), the National Basic Research Program of China (No. 2016YFA0300103), the Elite Nanyagn Assistance Professorship grant from Nanyang Technological University and Academic Research Fund Tier 1 from Singapore Ministry of Education, and the Lee Kuan Yew Postdoctoral Fellowship through MOE Tier 1 (Grant R-284-000-158-114). L.W. is also supported by Tsinghua Fudaoyuan Research Fund from Tsinghua University. J.M.D.C. acknowledges supports from China 1000 Foreign Talents Program. We are also grateful to the discussion and assistance provided by staffs in Beamline BL08U1A of Shanghai Synchrotron Radiation Facility and Dr. Yang Ping from Singapore Synchrotron Light Source. References (1) Kawabata, S.; Asano, Y.; Tanaka, Y.; Golubov, A. A.; Kashiwaya, S. Josephson Pi State in a Ferromagnetic Insulator. Phys. Rev. Lett. 2010, 104 (11), 117002. 13 ACS Paragon Plus Environment


(2) Senapati, K.; Blamire, M. G.; Barber, Z. H. Spin-Filter Josephson Junctions. Nat. Mater. 2011, 10 (11), 849-52. (3) Bibes, M.; Villegas, J. E.; Barthélémy, A. Ultrathin Oxide Films and Interfaces for Electronics and Spintronics. Adv. Phys. 2011, 60 (1), 5-84. (4) Moodera, J. S.; Santos, T. S.; Nagahama, T. The Phenomena of Spin-Filter Tunnelling. J. Phys. Condens. Matter. 2007, 19 (16), 165202. (5) Uchida, K.-i.; Adachi, H.; Ota, T.; Nakayama, H.; Maekawa, S.; Saitoh, E. Observation of Longitudinal Spin-Seebeck Effect in Magnetic Insulators. Appl. Phys. Lett. 2010, 97 (17), 172505. (6) Katmis, F.; Lauter, V.; Nogueira, F. S.; Assaf, B. A.; Jamer, M. E.; Wei, P.; Satpati, B.; Freeland, J. W.; Eremin, I.; Heiman, D.; Jarillo-Herrero, P.; Moodera, J. S. A High-Temperature Ferromagnetic Topological Insulating Phase by Proximity Coupling. Nature 2016, 533 (7604), 513-6. (7) Huang, S. Y.; Fan, X.; Qu, D.; Chen, Y. P.; Wang, W. G.; Wu, J.; Chen, T. Y.; Xiao, J. Q.; Chien, C. L. Transport Magnetic Proximity Effects in Platinum. Phys. Rev. Lett. 2012, 109 (10), 107204. (8) Swartz, A. G.; Odenthal, P. M.; Hao, Y.; Ruoff, R. S.; Kawakami, R. K. Integration of the Ferromagnetic Insulator EuO onto Graphene. Acs Nano 2012, 6 (11), 10063-9. (9) Lang, M. R.; Montazeri, M.; Onbasli, M. C.; Kou, X. F.; Fan, Y. B.; Upadhyaya, P.; Yao, K. Y.; Liu, F.; Jiang, Y.; Jiang, W. J.; Wong, K. L.; Yu, G. Q.; Tang, J. S.; Nie, T. X.; He, L.; Schwartz, R. N.; Wang, Y.; Ross, C. A.; Wang, K. L. Proximity Induced High-Temperature Magnetic Order in Topological Insulator - Ferrimagnetic Insulator Heterostructure. Nano Lett. 2014, 14 (6), 3459-3465. (10) Kobayashi, N.; Masumoto, H.; Takahashi, S.; Maekawa, S. Optically Transparent Ferromagnetic Nanogranular Films with Tunable Transmittance. Sci. Rep. 2016, 6, 34227. (11) Sato, K.; Katayama-Yoshida, H. Material Design for Transparent Ferromagnets with Zno-Based Magnetic Semiconductors. Jpn. J. Appl. Phys. 2000, 39 (6B), L555-L558. (12) Wu, M.; Hoffmann, A. Recent Advances in Magnetic Insulators-from Spintronics to Microwave Applications, Academic Press: 2013; Vol. 64. (13) Suwardi, A.; Prasad, B.; Lee, S.; Choi, E. M.; Lu, P.; Zhang, W.; Li, L.; Blamire, M.; Jia, Q.; Wang, H.; Yao, K.; MacManus-Driscoll, J. L. Turning Antiferromagnetic Sm0.34Sr0.66MnO3 into a 140 K Ferromagnet Using a Nanocomposite Strain Tuning Approach. Nanoscale 2016, 8 (15), 8083-90. (14) Gajek, M.; Bibes, M.; Barthélémy, A.; Bouzehouane, K.; Fusil, S.; Varela, M.; Fontcuberta, J.; Fert, A. Spin Filtering through Ferromagneticbimno3tunnel Barriers. Phys. Rev. B 2005, 72 (2), 020406(R). (15) Gajek, M.; Bibes, M.; Varela, M.; Fontcuberta, J.; Herranz, G.; Fusil, S.; Bouzehouane, K.; Barthélémy, A.; Fert, A. La2∕3Sr1∕3MnO3–La0.1Bi0.9MnO3 Heterostructures for Spin Filtering. J. Appl. Phys. 2006, 99 (8), 08E504. (16) Prasad, B.; Egilmez, M.; Schoofs, F.; Fix, T.; Vickers, M. E.; Zhang, W.; Jian, J.; Wang, H.; Blamire, M. G. Nanopillar Spin Filter Tunnel Junctions with Manganite Barriers. Nano Lett. 2014, 14 (5), 2789-93. (17) Prasad, B.; Zhang, W.; Jian, J.; Wang, H.; Blamire, M. G. Strongly Bias-Dependent Tunnel Magnetoresistance in Manganite Spin Filter Tunnel Junctions. Adv. Mater. 2015, 27 14 ACS Paragon Plus Environment

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Figure 1. Structural and insulating properties of both bare and LAO-capped LMO thin films. (a) HR-STEM ABF and HAADF images of a 3 uc LAO/ 6 uc LMO/ STO. (b) A log–log plot of temperature dependent DC resistivity of both bare and 3 uc LAO capped 6 uc LMO films. (c,d) EFM images of 5 and 20 uc LMO films after poling.



Figure 2. LAO capping effect on FM of LMO. (a) M-H loops of bare LMO and 6 uc LAO capped LMO films in different thickness. (b) Summary of the 6 uc LAO capping effect as a function of LMO thickness. (c) M-H loops of 5 uc LMO with LAO capping layers in different thickness. (d) Summary of the capping effect as a function of LAO thickness. All the magnetic moment measurements were conducted at 10 K.


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Figure 3. LAO buffering effect on FM of LMO. (a) M-H loops of 6 uc LMO with LAO buffering layers in different thickness. (b) Summary of the buffering effect on the magnetic moment of LMO as a function of LAO thickness. (c) M-H loops of LMO in different thickness with a 1 uc LAO buffering layer. (d) Summary of the buffering effect by 1 uc LAO as a function of LMO thickness. All the magnetic moment measurements were conducted at 10 K.



Figure 4. Mn L-edge spectra of LMO. (a) XAS of bare LMO and 6 uc LAO capped LMO heterostructures with different LMO thicknesses. (b) XAS of LAO capped 5 uc LMO. The reduction of Mn2+ peak and shift of Mn3+ peak were detected after capping. (c) XAS of bare, capped and buffered 6 uc LMO. LAO capping and buffering play a similar effect in reducing Mn2+ peak and shifting Mn3+ peak. The black and red dashed lines indicate the Mn2+ and Mn3+ peaks, respectively.


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Figure 5. (a) Band alignment of LMO and LAO with respect to the vacuum energy level. The Fermi level is set to the top of the valence band. CB and VB denote the valence band and conduction band, and Vo, VLa and VAl denote the vacancy bands of O, La and Al, respectively. (b) Charge transfer at the LAO/LMO interface with a La vacancy in LAO. Yellow color indicates an increasing of electrons, while blue color indicates a depletion of electrons in the region. Using the Bader analysis of electrons, the amount of charge transferred is around 0.5e/1.3 nm2 with a 5.5% atomic La vacancy concentration.



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Interface-Induced Enhancement of Ferromagnetism in Insulating

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