Oxygen Vacancy Ordering Modulation of Magnetic Anisotropy in

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Functional Inorganic Materials and Devices

Oxygen Vacancy Ordering Modulation of Magnetic Anisotropy in Strained LaCoO3-x Thin Films Ningbin Zhang, Yinlian Zhu, Da Li, DeSheng Pan, Yunlong Tang, Mengjiao Han, Jinyuan Ma, Bo Wu, Zhidong Zhang, and Xiuliang Ma ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b13674 • Publication Date (Web): 19 Oct 2018 Downloaded from http://pubs.acs.org on October 20, 2018

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ACS Applied Materials & Interfaces

Oxygen Vacancy Ordering Modulation of Magnetic Anisotropy in Strained LaCoO3-x Thin Films Ningbin Zhang†,‡, Yinlian Zhu†*, Da Li†, Desheng Pan†,‡, Yunlong Tang†, Mengjiao Han†, Jinyuan Ma†,∥, Bo Wu†,‡, Zhidong Zhang† and Xiuliang Ma†,∥

†Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Wenhua Road 72, 110016 Shenyang, China ‡School of Material Science and Engineering, University of Science and Technology of China, Hefei 230026, China ∥State Key Lab of Advanced Processing and Recycling on Non-ferrous Metals, Lanzhou University of Technology, Langongping Road 287, 730050 Lanzhou, China

*Correspondence authors: E-mail address: [email protected] (Y.L. Zhu)

KEYWORDS: LaCoO3-x thin film, oxygen vacancy, strain, anisotropy field, pulsed laser deposition, transmission electron microscopy.

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ABSTRACT. Oxygen vacancy configurations and concentration are coupled with the magnetic, electronic and transport properties of perovskite oxides, manipulating the physical properties by tuning vacancy structures of thin films is crucial for applications of many functional devices. In this study, we report a direct atomic resolution observation of preferred orientation of vacancy ordering structure in the epitaxial LaCoO3-x (LCO) thin films under various strains from large compressive to large tensile strain utilizing scanning transmission electron microscopy (STEM). Under compressive strains, the oxygen vacancy ordering prefers to along the planes parallel to the heterointerface. Changing the strains from compressive to tensile, the oxygen vacancy planes turn to be perpendicular to the heterointerface. Aberration-corrected STEM images, electron diffractions, X-ray diffraction (XRD) combined with X-ray photoelectron spectroscopy (XPS) demonstrate that with increasing misfit strains, the vacancy concentration increases, and vacancy distribution is more ordered and homogeneous. The temperature dependent magnetization curves show the Curie temperature increases displaying a positive correlation with the misfit strains. With changing the strain from compressive to tensile, anisotropy fields vary and show large values under tensile strains. It is proposed that oxygen vacancy concentration and preferred ordering planes are responsible for the enhanced magnetic properties of LCO films. Our results have realized a controllable preparation of oxygen vacancy ordering structures via strains, and thus provide an effective method to regulate and optimize the physical properties such as magnetic properties by strain engineering.


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1. INTRODUCTION Defects are very common and inevitable in crystal. Oxygen vacancy is a kind of such defects in transition metal oxides, and it has an important influence on the performance of electronic devices.1 For example, the operation of solid oxide fuel cells and gas sensors is determined by oxygen chemical potentials and dynamics.2,3 Functionality of memristors is also affected by oxygen vacancy mobility.4 More importantly, new functionalities such as field effect transistors may be created through surface ionic phenomena.5 Fatigue and other processes of high-k dielectrics and ferroelectrics are believed to be related to oxygen vacancies as well.6-10 Moreover, in the past several years, there has been an increasing interest in the indispensable role of oxygen vacancy in physics. A number of recent studies focus on the effect of oxygen vacancy on magnetic, electronic, transport and optical properties of transition metal oxides.11-16 Manipulation of oxygen vacancy concentration is also critical for metal-insulator transition, ferroelectric polarity, high-Tc superconductors, colossal magnetoresistance, catalysis, and energy storage etc..17-19 To tune oxygen vacancy structure and physical properties of materials, there are many parameters that can be utilized. For instance, oxygen partial pressure and temperature can easily change oxygen stoichiometry of transition metal oxides.20,21 With increasing film thickness, magnetization of the film can be effectively modulated.22,23 It was reported that the oxygen vacancy plane orientation was altered when changing the substrate orientation.24,25 It is proposed that there is a close relationship between doping content and saturation magnetization of La1xSrxCoO3

films.26,27 Unfortunately oxygen partial pressure and temperature are passively relied

on the environment, which severely limits the widespread application of oxygen vacancy in


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functional devices. Whereas strain is illustrated to be another powerful tool to affect structures and properties of materials.28-31 Previous studies have shown that oxygen vacancies are extensively present in perovskite manganite, cobaltite, etc.32-35 Among these materials, LaCoO3 has a simple structure and valence state, and there is no element doping, which make it convenient to study the effect of strain on both structure and physical properties alone. LaCoO3 also has many unusual physical properties, such as low temperature ferromagnetic properties of films, giant magnetoresistance, high temperature conductivity, excellent chemical stability, and photocatalysis of nanoparticles.36-38 Since these studies are mainly on macroscopic properties, the influence of especially compressive strains on the oxygen vacancy ordering

misfit strains

and associated magnetic

properties in LCO thin films remains elusive. As a result, it is necessary to have an in-depth understanding of the behavior of oxygen vacancy comprehensively, and to do more to unravel the mystery of perovskite films. In this study, we report the preferred orientation of oxygen vacancy ordering structure and the relevant magnetic properties of LCO thin films tailored by misfit strains from both compressive and tensile. Cross-sectional high-angle annular dark field (HAADF) images, electron diffraction patterns, XRD combined with XPS exhibit the variation tendency of both oxygen vacancy distribution and concentration with changing misfit strains. Direct mapping of vacancy structure coupled with the measurement of temperature dependent magnetization curves and magnetic hysteresis processes exemplify that structures and magnetic properties are tunable by strain engineering, opening a pathway to tailor and control functionalities of perovskite materials. 2. EXPERIMENTAL SECTION


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The reciprocal space mappings were performed using a high-resolution X-ray diffractometer (BRUKER, D8 Advance). A series of LCO thin films were grown on YAO, LAO, NGO, STO, DSO, and KTO substrates by pulsed laser deposition system with a 248 nm KrF excimer laser. Before film growth, these substrates were heated up to 850 oC with a heating rate of 25 oC/min and stayed for 5 min to clean the surface and then cooled down to 700 oC with a cooling rate of 15 oC/min, The LCO target was pre-ablated for 15 min to clean the oxidized surface. During the growing process of these LCO films, an oxygen pressure of 100 mTorr, a laser energy density of 380 mJcm-2 and a repetition rate of 6 Hz were kept. After deposition, the films were kept at 700 oC for 5 min and then cooled down to room temperature with a cooling rate of 5 oC min-1 in an oxygen pressure of 200 Torr. Cross-sectional and plane-view specimens were prepared by traditional slicing, grinding, dimpling and finally ion milling with a 4.5 kV voltage by using Gatan 691 precision ion polishing system, different from cross-sectional specimens, the plane-view specimens were milled only from the substrate side to protect the film from damaging. The final ion milling was set less than 20 min with voltage less than 0.6 kV to reduce amorphous layer produced by ion beam damage. A FEI Tecnai G2 F30 transmission electron microscope was used for selected area electron diffraction. The HAADF/ABF images in STEM mode were acquired using a Titan G2 60-300 kV microscope with a high-brightness field-emission gun and double aberration correctors from CEOS, the operating voltage is 300kV, the beam convergence angle was set at 25 mrad and the collection angle is 50 mrad - 250 mrad. The X-ray photoelectron spectroscopy measurements were conducted on an


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ESCALAB250 Scientific instrument. The etching rate is ~0.1 nm/s taking Ta2O5 as measuring standard, and the etching time is 230 s. A superconducting quantum interference device (SQUID) system was used to measure the magnetic properties of the films. The temperature range for measuring temperature dependence magnetization curves was 10 K~150 K, an external field of H = 200 Oe was applied parallel to the film surface. In-plane and out-of-plane magnetic hysteresis process was measured at T = 10 K, for the in-plane (out-of-plane) magnetic hysteresis process, the magnetic field was applied parallel (perpendicular) to the film surface, and the field range was -2 T~2 T. 3. RESULTS AND DISCUSSION For comparison, we grew LCO films with the same thickness of 30 nm on YAlO3 (001)o (YAO), LaAlO3 (001)pc (LAO), NdGaO3 (110)o (NGO), SrTiO3 (001) (STO), DyScO3 (110)o (DSO) and KTaO3 (001) (KTO) substrates, respectively. All the substrates are insulated, and the films were grown with the same growth conditions.


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Figure 1. (a) X-ray diffraction studies of LCO films under different strains. The asterisks denote pseudo cubic 001 and 002 peaks of the substrates, the red and blue arrows indicate the peaks of the LCO films under different strains. (b), (c) Off-axis reciprocal space mapping of the pseudo cubic 103-diffraction of LCO films under compressive and tensile strains, respectively. Low magnification HAADF images of 30 nm thick LCO films grown on YAO (d), LAO (e), NGO (f), STO (g), DSO (h), and KTO (i) substrates, respectively. Each interface is marked with a pair of white arrows. Misfit strain values are indicated in the images.


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Figure 1a is the XRD result showing macroscopic structure changes of LCO films under different strains. No extra peaks are identified indicating that pure LCO films are obtained. The small full width at half maximum indicates that the films have great crystal quality. Under compressive strains, LCO 002pc peaks lie on the left side of 002pc peaks of the substrates; while under tensile strains, the peaks turn to be at the right side of 002pc peaks of the substrates. It is noted that there is a small peak present for the film under -3.02% strain as denoted with a blue arrow. The corresponding lattice parameter is calculated approximately 3.71 Å, implying that the reduction of out-of-plane lattice parameter is present in this film. To further explore the strain states in the films, high resolution RSM measurements are conducted. Figure 1b and 1c are RSM results for LCO/YAO and LCO/STO systems, respectively. The horizontal shift between YAO 103pc and LCO 103pc indicates the strain is partly released; while the very small horizontal shift between STO 103 and LCO 103pc indicates this film is nearly fully strained. The elongation of LCO 103pc (Figure 1c) implies that the lattice spacing of the film is gradually variant in the inplane direction, which is verified by the following HAADF imaging. To directly visualize the structure changes under different strains, we carried out scanning transmission electron microscopic imaging (Figure 1d-1i). In the low magnification images, we can see that each interface is flat and sharp as denoted by a pair of white arrows. In Figure 1d, high density of stripe contrast parallel to the interface is discernable, implying an ordering distribution in LCO thin films grown on YAO substrates with large compressive strains. Decreasing compressive strain to -0.45%, the density of stripes in the LCO films grown on LAO substrates reduces. With changing the strain from compressive to tensile, the stripe distribution varies dramatically. It is noted that the line directions are normal to the interfaces in LCO films on NGO, STO, DSO and KTO substrates with progressively increased tensile strains as shown in


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Figure 1f-1i, respectively. High density is especially observed in Figure 1g, indicating a critical strain value in LCO films to dominate the stripe density. As reported previously, these stripe-like contrasts in these films are believed to be a typical lattice expansion induced by oxygen vacancy.39-42 From Figure 1d-1i, we can conclude that varying the misfit strains from compressive to tensile, both oxygen vacancy density and preferred orientations change accordingly. To better figure out the fine structure of oxygen vacancy ordering, high resolution HAADF-STEM images were performed on various thick LCO thin films, which are shown in Figure 2.

Figure 2. Atomic resolution HAADF images of the LCO films grown on YAO (a), LAO (b), NGO (c), STO (d), DSO (e), and KTO (f), respectively. The interfaces are marked with a pair of white arrows. (a) Images along [110]pc direction, (b), (c), (d), (e) and (f) are images along [100]pc


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direction. The inset image is a magnification of the red rectangle in each image, the red arrows denote the ordering direction. It is seen that the oxygen vacancies features a well-arranged stacking (Figure 2). The stacking period of the LCO/YAO system (Figure 2a) is 3 unit-cell as evidenced by the inset, and oxygen concentration near the interface and surface is remarkably lower than that in the middle part of the film. In LCO/LAO systems (Figure 2b), same as the compressively strained LCO/YAO system, the preferred stacking orientation of oxygen vacancy keeps to be parallel to the interface. It is worthy of note that though the period of dark stripes keeps to be around 3 unitcell as indicated in the inset, the distribution is more or less inhomogeneous . It is also noted that the oxygen vacancy concentration in LCO/LAO is slightly lower than that in LCO/YAO systems. In LCO/NGO systems with the tensile strain of 1.47% (Figure 2c), different from films under compressive strains, the preferred stacking orientation of oxygen vacancy turns to be perpendicular to the interface, and it is also observed that the vacancy tends to stably exist inside of the film rather at the surface of the film. For LCO/STO systems with slightly increased tensile strains (Figure 2d), the arrangement of the oxygen vacancy ordering is more homogenous and the density increases. The period of ordering structure is measured to be also about 3 unit-cell as revealed in the inset. Figure 2e and 2f display the HAADF images of LCO films with large tensile strains of 3.84% and 4.84%, respectively. It is of interest to notice that the density of oxygen vacancy ordering structure tends to decrease, although the preferred orientation is still along the direction normal to the interface. To qualitatively understand the oxygen vacancy ordering distribution, we extract the structural information based on the HAADF images in Figure 2 and the details are shown in Figure 3.


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Figure 3. Atomic spacing changes along the out-of-plane and in-plane directions of the LCO films grown on YAO (a), LAO (b), NGO (c), STO (d), DSO (e), and KTO (f), respectively. The B site HAADF intensity change is shown as well. The vertical red arrows denote the oxygen vacancy sites. For (a) and (b), extractions were conducted over transversal numbered atomic rows of the white rectangle areas shown in Figure 2a and 2b, respectively. For (c), (d), (e) and


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(f), averaged information over vertical numbered atomic rows of the white rectangle areas in Figure 2c, 2d, 2e and 2f were collected. The error bars show the standard deviation with respect to the averages for each atomic layer in the white rectangles of the images. Two dimensional lattice spacing mapping display the arrangements of oxygen vacancy in the films grown on YAO (g), LAO (h), NGO (i), STO (j), DSO (k) and KTO (l), respectively. (g, h) are out-of-plane lattice spacing mappings, and (i-l) are in-plane lattice spacing mappings. The information in (g-l) is extracted from the orange rectangles in Figure 2a-f, respectively. Figure 3a-3f show interatomic distance changes in the LCO films along the out-of-plane (solid circle) and in-plane (solid square) directions extracted from the white rectangle areas in Figure 2. The HAADF intensity of the B-site cations is shown as well. From the statistical analysis of the lattice spacings of the LCO/YAO systems (Figure 3a), it is seen that the in-plane lattice spacing remains nearly constant while the out-of-plane lattice spacing fluctuates mainly around 3.6-3.7 Å from the interface to the film surface with the peaks of about 4.4 Å at the vacancy sites; it then falls to around 3.6 Å in the next unit cells where oxygen is stoichiometric. For the statistical analysis of B-site intensity, a sharp change near the vacancy is seen as indicated with the vertical red arrows. In the LCO/LAO systems, the compressive strain is smaller than in the LCO film grown on YAO. In Figure 3b, it is seen that the in-plane lattice spacing keeps constant and equal to the substrate lattice parameter of 3.79 Å, while the out-ofplane lattice spacing changes from the interface to the film surface. It remains approximately 3.9 Å near the interface, rises sharply to about 4.2 Å along oxygen vacancy sites, and then drops to around 3.8 - 3.9 Å in the next unit cells. Figure 3c shows the lattice spacing mapping of the LCO/NGO system. Different from the films applied with compressive strain, it is seen that the out-of-plane lattice spacings remain at 3.86 Å, while the in-plane lattice spacings change


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periodically. They expand to values larger than 4 Å when oxygen vacancies emerge, and then fall down to about 3.86 Å when oxygen vacancy sites disappear. In Figure 3d, the out-of-plane lattice spacings of the LCO/STO thin film systems keep constant with the value around 3.905 Å, the in-plane lattice spacings change periodically from 3.7 Å to 4.3 Å. When the tensile strain increases to 3.84%, as shown in Figure 3e, it is seen that the periodic change of the in-plane lattice spacing is not as remarkable as that in the LCO/STO. Further increasing the tensile strain to 4.84% (Figure 3f), the in-plane lattice spacing varies from 3.6 Å to about 4.4 Å for most case, and the distribution is not uniform. To further analyze the two dimensional distribution of oxygen vacancy ordering structure, two dimensional lattice mappings corresponding to Figure 2 are performed and shown in Figure 3g-3l. Figure 3g is the two dimensional lattice mapping for the LCO/YAO systems. It is noted that oxygen vacancies are distributed periodically. In 2012, A. Y. Borisevich et al demonstrated that there is a linear relationship between the vacancy concentration and the lattice spacing.43 According to this theory, from Figure 3g we can conclude that the vacancy concentration is very large, so that the oxygen vacancy in this thin film is not segregated locally, but homogenously distributed over the whole film. At the same time, the oxygen vacancies present the characteristic of well-ordered stacking. While in Figure 3h where the strain is very small (-0.45%) for the LCO/LAO systems, the oxygen vacancy concentration in this thin film is not as high as that in films grown on YAO, and the distribution of oxygen vacancy is not uniform. With a small tensile strain in LCO/NGO, Figure 3i shows that the oxygen vacancy concentration in the film is not large, and the distribution of oxygen vacancy is relatively heterogeneous. For the LCO/STO systems with a large tensile strain shown in Figure 3j, we can see the oxygen vacancy concentration turns to be high again, and the distribution of oxygen vacancy becomes


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homogeneous. But further increasing the strains as shown in Figure 3k and 3l, it is observed that the distribution of oxygen vacancy is non uniform. In the meantime, the oxygen vacancy concentration becomes lower than that in the LCO/STO film systems. In order to further figure out the film structure and oxygen vacancy distribution, we performed electron diffraction experiment in the plan-view direction of these films, as shown in Figure 4.

Figure 4. Plan-view selected area electron diffraction (SAED) patterns of the LaCoO3 films grown on YAO (a), LAO (b), NGO (c), STO (d), DSO (e), and KTO (f) substrates. The white arrows denote superlattice reflections due to oxygen vacancy ordering. In the LCO films grown on YAO, no extra diffraction spots resulting from periodical lattice expansion and reduction are identified along [001]pc zone axis (Figure 4a), demonstrating that the oxygen vacancy planes are stacked along out-of-plane direction rather than in-plane


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direction. Similarly, the plan-view diffraction pattern of the LCO film grown on LAO (Figure 4b) shows that diffraction spots caused by oxygen vacancy ordering cannot be seen along [001]pc zone axis as well. For the LCO films on NGO substrate with 1.47% tensile strain, the diffraction pattern (Figure 4c) shows faint superlattice reflections along [100]pc direction as marked with the white arrows, which demonstrates that oxygen vacancy stacking prefers to be along [100]pc direction mainly. For the LCO films on STO with the 2.63% tensile strain, the diffraction pattern (Figure 4d) shows obviously additional reflections along both [100]pc and [010]pc directions due to oxygen vacancy ordering distributions, as indicated by the white arrows. Moreover, it is noted that the intensity of the additional reflections is much brighter than that in Figure 4c. The strong super diffractions demonstrate that both the oxygen vacancy content and degree of ordering are tremendous. The diffraction pattern of the LCO films on DSO with the tensile strain of 3.84% (Figure 4e) shows additional reflections along [100]pc and [010]pc directions as well; nevertheless, the intensity of the additional reflections is relatively weak compared with that in Figure 4d. Since the misfit strain is large, the formation of interfacial dislocations may partly relieve the large misfit strains, and thus oxygen vacancy concentration may decrease. When the tensile strain reaches 4.84%, the largest value in the present study, the extra spots caused by oxygen vacancy ordering is too faint to be distinguished, implying that both oxygen vacancy content and the ordering distribution are low. In Figure 4, it is concluded that varying the misfit strains from compressive to tensile, extra spots resulting from oxygen vacancy ordering can be identified and the intensity discloses the oxygen vacancy concentrations and ordering distributions. There is a critical strain value with which both the content and ordering degree show optimal behavior, i.e. 2.63% for the LCO/STO thin film system. To further impose a better


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and clear demonstration of oxygen vacancies and variation tendency of oxygen vacancy concentration, we perform the ABF and XPS studies, as shown in Figure 5.

Figure 5. (a) ABF-STEM imaging of oxygen vacancy in the LCO film. (b) Intensity line profiles corresponding to the red lines in (a). (c) XPS of the LCO films under different strains. The ABF image is inverted in contrast and is taken from the LCO/YAO systems. The red arrows denote oxygen vacancy layers in the film. The red hollow circles denote the oxygen deficient columns, red solid circles indicates the non-oxygen deficient columns, yellow solid circles denote the Co columns, blue solid circles labels the La columns. In the ABF image (Figure 5a), the oxygen deficient and non-oxygen deficient columns can be directly seen as denoted by the red hollow circles and red solid circles, respectively. To further illustrate the presence of oxygen vacancy, intensity line profiles from the red lines in Fig. 5a is shown (Fig. 5b), and a sharp reduction of oxygen contrast is observed in profile 1 compared with profile 2. To further verify the oxygen content change in the LCO films with strain, XPS measurements are conducted (Fig. 5c). The O1s spectra are deconvoluted into two peaks corresponding to non-oxygen deficient lattice oxide (main peaks Oi) and oxygen deficient lattice oxide (satellite peaks Oii). An increase of Oii/Oi ratio reflect the increase of oxygen vacancies.44 The calculated Oii/Oi ratios are approximately 0.148 (± 0.02), 0.060 (± 0.02), 0.066 (± 0.03),


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0.146 (± 0.03), 0.11 (± 0.02), 0.048 (± 0.02) for the LCO films under -3.02%, -0.45%, 1.47%, 2.63%, 3.84% and 4.84% strain, respectively, which confirmed the results of Figure 2, 3 and 4, namely, with compressive strain increasing, the percentage of oxygen vacancies increases, and it is the same for tensile strain conditions. Furthermore, when tensile strain is larger than a critical strain, the percentage of oxygen vacancies is reduced.

Figure 6. In-plane and out-of-plane magnetic hysteresis loops measured at T = 10 K for the LCO films grown on YAO (a), LAO (b), STO (c) and KTO (d), respectively. (e) Temperature dependence of field-cooled magnetization of LCO films under different misfit strains measured in an applied field of H = 200 Oe. (f)

Strain dependence of Curie temperature (Tc) and

anisotropy field of the LCO films grown on the different substrates. The hysteresis loops were normalized to the saturation magnetization Ms. The blue dotted lines and arrows denote the anisotropy field values of the films with different oxygen vacancy structure. M0 is the initial magnetization before Curie transition of the films grown on the different substrates. The Curie


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temperatures Tc was obtained by extrapolation methods from 6e, the anisotropic fields of the films under different strains were extracted from Figure 6a, b, c and d, respectively. To explore whether different oxygen vacancy structure would be correlated with physical properties, we carried out magnetic property measurements (Figure 6). Comparing the normalized out-of-plane and in-plane magnetic hysteresis loops in each image (Figure 6a-6d), obvious different magnetization processes are observed in these LCO films, originating from the shape anisotropy due to the planar geometry of the thin layers.45 For all the samples, the shape anisotropy makes the spins tend to align along in-plane direction of the LCO films, leading to that the [010]pc direction in the films becomes the easy magnetization axis and the out-of-plane direction is the hard magnetization axis. The anisotropy fields of the LCO films under -3.02%, -0.45%, 2.63% and 4.84% strains obtained from the crossover points of inplane and out-of-plane magnetization processes in Figure 6a-6d are 12 kOe, 12 kOe, 16 kOe and 14 kOe, respectively, suggesting that tensile strains can distinctly enhance the anisotropy fields of the films in comparison with the compressive strains. The anisotropy field of the LCO film varies with the substrate due to different oxygen vacancy content and the ordering distribution in the film, as shown in Figure 6f. And then the Curie temperature (Tc) of the LCO films is determined by the point of intersection of the two tangents around the inflection point of the M/M0-T curves (Figure 6e). It is obvious that the Curie temperature is varied with the strain state. Under the compressive strain of 3.02% with the YAO substrate, the Tc is 74 K, while the Tc becomes 29 K as the compressive strains decreases to 0.45%. However, the Curie temperatures of the LCO films increase to 79 K and 81 K, respectively, if the 2.63% and 4.84% tensile strains were loaded on the LCO films. In contrast with the relative low Tc of 29 K for the LCO film with a 0.45% strain value, the LCO films under larger strains show higher Tc values, and it is noted


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that the Curie temperatures are quite close to each other. Figure 6f clearly reveals the variations of magnetic properties with changing the misfit strains. Meanwhile, Curie temperature seems to saturate around 80 K when the strain is large enough. Previously, D. Fuchs et al reported that Curie temperature would saturate at Tc = 85 K in LCO systems,30 which is in accord with the present study. Combining reciprocal space mapping and XRD patterns, cross-sectional real-space images and lattice analysis, plan-view reciprocal space electron diffractions, XPS and magnetic property measurements, we find that compressive and tensile strains would induce different oxygen vacancy structure, that is to say, the oxygen vacancies tend to stack along different directions. This is understood by the fact that oxygen vacancy ordering leads to lattice expansion along stacking direction, and oxygen vacancy tends to stack along the direction with elongated lattice spacings. As a result, tensile strain is in favor of oxygen vacancy stacking along in-plane direction, while compressive strain, along out-of-plane direction. With strain increasing, oxygen vacancy concentration, ordering degree and Curie temperature increase accordingly, and there is a critical strain value for optimal vacancy content, ordering degree, Curie temperature and anisotropy field. Previously, B. Yildiz et al used first-principles calculations and proposed that a positive correlation is found between strain and oxygen vacancy concentration.46 As to the Curie temperature, with oxygen vacancy concentration increasing, the total spin increases, and thus Curie temperature increases. When tensile strain is larger than 3%, though oxygen vacancy concentration tends to saturate, the strain makes Co changing from low-spin state to intermediate-spin state, which may contribute to the enhancement of the total spin and consequently Curie temperature increases.46 For anisotropy field, the change of out-of-plane lattice parameter induced by oxygen vacancy may lead to structural (oxygen octahedra)


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distortions and then produces magnetic state modifications as reported previously.45 It is the octahedral rotation modification induced by oxygen vacancy that would affect the anisotropy field, and finally lead to the similar anisotropy fields in the LCO/YAO and LCO/LAO systems, because the vacancy concentration is low in both cases. Both shape anisotropy and structural (octahedral distortion created by oxygen vacancy) anisotropy induced by tensile strain increase the anisotropy field in the LCO/STO. In the LCO/KTO system, the strain is relaxed, which reduced the anisotropy field. It is suggested that Curie temperature and anisotropy field are closely associated with oxygen vacancy concentration and structure via strain modulations. 4. CONCLUSIONS In summary, we demonstrate that the LCO epitaxial films grown on different substrates present regular changes of oxygen vacancy preferred orientation, distribution, concentration, and homogeneity under different compressive and tensile strains. Under compressive strains, the oxygen vacancy ordering takes place along the plane parallel to the interface; under tensile strains, the stacking turns to be along in-plane plane. The oxygen vacancy concentration and distribution vary with strains. The Curie temperature and anisotropy field show close relationships with the oxygen vacancy concentration and distribution influenced by the strains imposed on the films. These studies may shed some light on further understanding the behaviors of oxygen vacancies and association with the magnetic properties via strain modulations, which would be helpful for monitoring the oxygen vacancy and magnetic property via strain engineering. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]


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ORCID Yinlian Zhu: 0000-0002-0356-3306 Author Contributions X.L.M. and Y.L.Z. conceived the project of film characterization in oxides by using aberrationcorrected STEM. N.B.Z., Y.L.Z. and X.L.M. designed the experiments. N.B.Z. performed the thin-film growth and STEM observations. D.L., D.S.P. and Z.D.Z. participated in the magnetic property measurement. Y.L.T., M.J.H., J.Y.M. and B.W. participated in the thin-film growth and STEM imaging. All authors contributed to the discussions and manuscript preparation. Funding Sources This work is supported by the National Natural Science Foundation of China (No. 51571197, 51501194, 51671194, 51401212 and 51521091), National Basic Research Program of China (2014CB921002), and the Key Research Program of Frontier Sciences CAS (QYZDJ-SSWJSC010). Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (No. 51571197, 51231007, 51501194 and 51671194), National Basic Research Program of China (2014CB921002), and the Key Research Program of Frontier Sciences CAS (QYZDJ-SSWJSC010). Y. L. T. acknowledges the IMR SYNL-T.S. Kê Research Fellowship and the Youth Innovation Promotion Association CAS (No. 2016177). We are grateful to Mr. B. Wu and Mr.


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Oxygen Vacancy Ordering Modulation of Magnetic Anisotropy in

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