Article

Oxygen Vacancy Ordering Modulation of Magnetic Anisotropy in Strained LaCoO3-x Thin Films

Authors:
  • Institute of Metal Research, Chinese Academy of Sciences, shenyang
  • Institute of Metal Reseach, Chinese Academy of Sciences, Shenyang, China
To read the full-text of this research, you can request a copy directly from the authors.

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.

No full-text available

Request Full-text Paper PDF

To read the full-text of this research,
you can request a copy directly from the authors.

... 19,24,25 When relatively thick films are under a compressive (tensile) strain, these superstructures are expected to be arranged parallel (perpendicular) to the interface, as shown in a study for LaCoO 3−x films. 26 For thin, tensile-strained films where the surface energy becomes dominant compared to the bulk energy, the effect is seen, in order to minimize the surface energy, the perpendicular arranged oxygen defects become parallel. 27 While for doping ratios x ≤ 0.1, there are reports of some vacancy ordering under certain conditions, 28 other results indicate that they are in most cases not ordered in superstructures. ...
... In the literature, dark stripes in HAADF images of perovskites have been described as structural modifications due to the local accumulation of oxygen vacancies. 19,20,26,32,58 Geng et al. were able to show directly with EELS for an undoped BFO film that the dark stripes are indeed oxygen-deficient. They could confirm this by etching combined with XPS and the fact that films grown with less oxygen partial pressure showed longer dark stripes in the cross-sectional samples. ...
... 19 Earlier studies on oxygen vacancies in epitaxially grown LaCoO 3−δ films suggest that under compressive strain from the substrate, out-of-plane oxygen defects appear and under tensile strain in-plane defects occur. 19,26 Here, the oxygendeficient defects appear simultaneously in both directions. We expected to see a difference and preference between in-plane and out-of-plane line defects either concerning the amount of oxygen vacancies δ or concerning their exact arrangement. ...
Article
Full-text available
The interaction of oxygen vacancies and ferroelectric domain walls is of great scientific interest because it leads to different domain-structure behaviors. Here, we use high-resolution scanning transmission electron microscopy to study the ferroelectric domain structure and oxygen-vacancy ordering in a compressively strained Bi0.9Ca0.1FeO3−δ thin film. It was found that atomic plates, in which agglomerated oxygen vacancies are ordered, appear without any periodicity between the plates in out-of-plane and in-plane orientation. The oxygen non-stoichiometry with δ ≈ 1 in FeO2−δ planes is identical in both orientations and shows no preference. Within the plates, the oxygen vacancies form 1D channels in a pseudocubic [010] direction with a high number of vacancies that alternate with oxygen columns with few vacancies. These plates of oxygen vacancies always coincide with charged domain walls in a tail-to-tail configuration. Defects such as ordered oxygen vacancies are thereby known to lead to a pinning effect of the ferroelectric domain walls (causing application-critical aspects, such as fatigue mechanisms and countering of retention failure) and to have a critical influence on the domain-wall conductivity. Thus, intentional oxygen vacancy defect engineering could be useful for the design of multiferroic devices with advanced functionality.
... Another intriguing property is that the magnetic easy-axes are fixed to the in-plane direction regardless of the direction of epitaxial strain 14,18,37 This phenomenon also suggests that the ferromagnetism is governed by a specific atomistic origin, not by the global strain, in epitaxially strained LCOs. ...
... Since the latter unit has La-La distances similar to the lattice parameter of bulk LCO, we denoted the corresponding phase as the bulk-like phase. The inhomogeneous occurrence of dark stripes in HAADF STEM images 19,37 implies the coexistence of the twin-wall superlattice and bulk-like phases within the film interior. ...
... Previously, it was shown that the twin walls were propagated in a different direction in tensile-and compressive-strained LCO films, forming vertical and horizontal twin-wall superlattices, respectively. 19,30,32,37 Importantly, in both strain cases, we found that the elongated bonds in c-units were always aligned along the in-plane direction (see Fig. S7 in Supplementary Information). 37 This observation may explain the universal in-plane magnetic easy-axis in both compressive-and tensile-strained LCO films with the stripe domains aligned horizontally and vertically, respectively. ...
Preprint
The origin of strain-induced ferromagnetism, which is robust regardless of the type and degree of strain in LaCoO3 (LCO) thin films, is enigmatic despite intensive research efforts over the past decade. Here, by combining scanning transmission electron microscopy with ab initio density functional theory plus U calculations, we report that the ferromagnetism does not emerge directly from the strain itself, but rather from the creation of compressed structural units within ferroelastically formed twin-wall domains. The compressed structural units are magnetically active with the rocksalt-type high-spin/low-spin order. Our study highlights that the ferroelastic nature of ferromagnetic structural units is important for understanding the intriguing ferromagnetic properties in LCO thin films.
... 40 Secondly, earlier work shows that the stoichiometric LCO3 epitaxial films exhibit an unconventional strain relaxation behavior, resulting in the stripe-like domain patterns due to the formation of OVCs. [41][42][43][44] The pseudocubic (pc) lattice constant of bulk LCO3 is apc = 3.81 Å (Figure 1a). Single-crystalline SrTiO3 (a = 3.905 Å) and LaAlO3 (apc = 3.79 Å) substrates may introduce −0.5% (compressive) and +2.5% (tensile) strain to the as-grown LCO3 films, respectively. ...
... 41 As shown in Figure 1b, tensile strain would favor the creation of ordered ̈ in the films along the out-of-plane direction because the ̈-related chemical expansion can reduce the lattice misfit strain. [41][42][43][44] On the contrary, compressive strain leads to in-plane ̈ orderings, which is parallel to the interface due to the concomitant lattice elongation along the out-ofplane direction ( Figure 1c). ...
Preprint
Full-text available
Orientation control of oxygen vacancy channel (OVC) is a highly desirable for tailoring oxygen diffusion as it serves fast transport channel in ion conductors, which is widespread exploited in solid-state fuel cells, catalysts, and ion-batteries. Direct observation of oxygen-ions hopping towards preferential vacant sites is a key to clarifying migration pathways. Here we report the anisotropic oxygen-ion migration mediated by strain in ultrathin cobaltites via in-situ thermal activation in an atomic-resolved transmission electron microscopy. Oxygen migration pathways are constructed on the basis of the atomic structure during the OVC switching, which is manifested as the vertical-to-horizontal OVC switching under tensile strain, but the horizontal-to-diagonal switching under compression. We evaluate the topotactic structural changes to OVC, determine the crucial role of tolerance factor for OVC stability and establish the strain-dependent phase diagram. Our work provides a practical guide for engineering OVC orientation that is applicable ionic-oxide electronics.
... Although ferromagnetism appears in LCO thin films, its exact origin of it remains unclear. Previously, some studies demonstrated that epitaxial strain induced by substrates plays an important role in the formation of ferromagnetic ordering in LCO thin films [10,11], while others proposed that oxygen vacancy formation was crucial for the occurrence of ferromagnetism [12,13]. Therefore, there are controversies in theory about the mechanism of ferromagnetism in LCO films. ...
Article
Full-text available
The perovskite LaCoO3 and La0.8Sr0.2CoO3 thin films were synthesized successfully by a polymer-assisted deposition method. The structural characterization and thermal decomposition measurements indicate that appropriately increasing the annealing temperature is helpful in improving the film crystallinity. Compared with LaCoO3 films, the Curie temperature is enhanced to a higher temperature in La0.8Sr0.2CoO3 films, which is attributed to the changes in both the valence state and spin state induced by the chemical doping. In addition, a broad transition temperature region is observed in La0.8Sr0.2CoO3 films, revealing the existence of inhomogeneous ground states in this system.
... The tensile strain reduces the bond angle Co-O (β ) and increases the length of the Co-O-Co bond (d), which is conducive to increasing the bandwidth of the energy band (e g ) and reducing the crystal field splitting effect (∆ CF ), hence showing an advantage in occupying the IS state and the HS state. [29] The reduction in FM response in thick films on LSAT and STO is expected due to structural relaxation. On the other hand, 15 nm PV-LCO film strained on LAO exhibits hardly any ferromagnetism, thus suggesting that compressive strain does not give rise to ferromagnetism. ...
Article
Full-text available
The origin of ferromagnetism in epitaxial strained LaCoO 3-x films has been a long controversial hitherto. Here, we investigated the magnetic behavior of a series of oxygen vacancies ordered LaCoO 3-x films on different substrates. Obvious ferromagnetism was observed in perovskite LaCoO 3 /LSAT and LaCoO 3 /STO films, while LaCoO 3 /LAO films show weak ferromagnetism behavior. Meanwhile, LaCoO 2.67 films exhibit antiferromagnetic behavior. An unexpected low-temperature ferromagnetism phenomenon with a Curie temperature of ~83 K and a saturation magnetization of ~1.2 μ B /Co was discovered in15-nm-thick LaCoO 2.5 /LSAT thin films, which is probably related to the interface CoO 6 octahedral rotation pattern change. Meanwhile, the observed ferromagnetism gradually disappears as the thickness of the film increases, indicating a tensile strain relaxation. Analysis suggests that the rotation and rhombohedral distortion of the CoO 6 octahedron weakened the crystal field splitting and promoted the generation of the ordered high-spin state of Co ²⁺ , thus super-exchange effect between Co ²⁺ (HS), Co ²⁺ (LS), and Co ²⁺ (HS) produced a low-temperature ferromagnetic behavior. However, compressive-strained LaCoO 2.5 film on LaAlO 3 substrate shows normal anti-ferromagnetism behavior. These results demonstrate both oxygen vacancies and tensile strain are correlated with the emergent magnetic properties in epitaxial LaCoO 3-x films and provides a new perspective to regulate the magnetic properties of transition oxide thin films.
... 7,8 In thin films, the OVCs can be formed either parallel or perpendicular to the substrate surface and thereby strongly influence the properties of the films. The OVC orientation can be affected by applying strain in thin films by suitable choice of the substrate on which the film is deposited, 21,22,26,31 by the deposition conditions of the film growth, 28−30 or by using an appropriate capping layer. 20 However, all these methods are passive and cannot be employed to control the OVCs in films after they have been formed. ...
Article
Full-text available
Oxygen defects and their atomic arrangements play a significant role in the physical properties of many transition metal oxides. The exemplary perovskite SrCoO3-δ (P-SCO) is metallic and ferromagnetic. However, its daughter phase, the brownmillerite SrCoO2.5 (BM-SCO), is insulating and an antiferromagnet. Moreover, BM-SCO exhibits oxygen vacancy channels (OVCs) that in thin films can be oriented either horizontally (H-SCO) or vertically (V-SCO) to the film's surface. To date, the orientation of these OVCs has been manipulated by control of the thin film deposition parameters or by using a substrate-induced strain. Here, we present a method to electrically control the OVC ordering in thin layers via ionic liquid gating (ILG). We show that H-SCO (antiferromagnetic insulator, AFI) can be converted to P-SCO (ferromagnetic metal, FM) and subsequently to V-SCO (AFI) by the insertion and subtraction of oxygen throughout thick films via ILG. Moreover, these processes are independent of substrate-induced strain which favors formation of H-SCO in the as-deposited film. The electric-field control of the OVC channels is a path toward the creation of oxitronic devices.
... After annealed 2 h in vacuum, the dark stripes in every third Co (001) plane with a diffraction vector of 1/3 (0, 0, 1) appear in the film, as labeled by small yellow arrows in Fig. 1b. This phase is recognized as LaCoO 2.67 (LCO 2.67 ) or La 3 Co 3 O 8 24 , which is in agreement with previous reports that indicated that compressive-strained LCO 2.67 favored in-plane Vo stripes 25 . We continuously annealed the sample in vacuum for 4 h. ...
Article
Full-text available
Dedicated control of oxygen vacancies is an important route to functionalizing complex oxide films. It is well-known that tensile strain significantly lowers the oxygen vacancy formation energy, whereas compressive strain plays a minor role. Thus, atomic reconstruction by extracting oxygen from a compressive-strained film is challenging. Here we report an unexpected LaCoO 2.5 phase with a zigzag-like oxygen vacancy ordering through annealing a compressive-strained LaCoO 3 in vacuum. The synergetic tilt and distortion of CoO 5 square pyramids with large La and Co shifts are quantified using scanning transmission electron microscopy. The large in-plane expansion of CoO 5 square pyramids weaken the crystal field splitting and facilitated the ordered high-spin state of Co ²⁺ , which produces an insulating ferromagnetic state with a Curie temperature of ~284 K and a saturation magnetization of ~0.25 μ B /Co. These results demonstrate that extracting targeted oxygen from a compressive-strained oxide provides an opportunity for creating unexpected crystal structures and novel functionalities.
... This phase is recognized as LaCoO2.67 (LCO2.67) or La3Co3O8 24 , which is in agreement with previous reports that indicated that compressive-strained LCO2.67 favored in-plane Vo stripes 25 . ...
Preprint
Full-text available
Dedicated control of oxygen vacancies is an important route to functionalizing complex oxide films. It is well-known that tensile strain significantly lowers the oxygen vacancy formation energy, whereas compressive strain plays a minor role. Thus, atomically reconstruction by extracting oxygen from a compressive-strained film is challenging. Here we report an unexpected LaCoO2.5 phase with a zigzag-like oxygen vacancy ordering through annealing a compressive-strained LaCoO3 in vacuum. The synergetic tilt and distortion of CoO5 square pyramids with large La and Co shifts are quantified using scanning transmission electron microscopy. The large in-plane expansion of CoO5 square pyramids weaken the crystal-field splitting and facilitated the ordered high-spin state of Co2+, which produces an insulating ferromagnetic state with a Curie temperature of ~284 K and a saturation magnetization of ~0.25 {\mu}B/Co. These results demonstrate that extracting targeted oxygen from a compressive-strained oxide provides an opportunity for creating unexpected crystal structures and novel functionalities.
... High enough concentration of the oxygen vacancies in perovskites might result in the doubling of the unit cell to form so-called Brownmillerite phase 12 . Such superstructure results in periodic bright and dark stripes on the high resolution scanning transmission electron microscopy (STEM) images [13][14][15] and typically it is not observed in Sr-doped LSCO at Sr substitution below 50% [16][17][18][19][20] . Recent computational studies on related LaCoO 3 21 also revealed that changes of the lattice strain can strongly influence the mobility of oxygen in certain directions. ...
Article
Full-text available
The possibility to control oxygen transport in one of the most promising solid oxide fuel cell cathode materials, La 0.6 Sr 0.4 CoO 3−δ, by controlling lattice strain raises questions regarding the contribution of atomic scale effects. Here, high-resolution transmission electron microscopy revealed the different atomic structures in La 0.6 Sr 0.4 CoO 3−δ thin films grown under tensile and compressive strain conditions. The atomic structure of the tensile-strained film indicated significant local concentration of the oxygen vacancies, with the average value of the oxygen non-stoichiometry being much larger than for the compressive-strained film. In addition to the vacancy concentration differences that are measured by isotope exchange depth profiling, significant vacancy ordering was found in tensile-strained films. This understanding might be useful for tuning the atomic structure of La 0.6 Sr 0.4 CoO 3−δ thin films to optimize cathode performance.
Article
Charged domain walls in ferroelectric materials exhibit nontrivial electronic and transport properties, which have promising application in many electronic devices, such as high-density nonvolatile memories. Taking advantage of oxygen vacancy engineering, charged domain walls are introduced into the PbTiO3/LaCoO2.5 superlattice system, and the quasi-two-dimensional electron gas is formed near the domain walls, featuring an intrinsic n-type conductive characteristic. The charged domain walls can be erased by eliminating oxygen vacancies in the LaCoO2.5, resulting in an insulating state. The proposed scheme of writing and erasing of charged domain walls may facilitate the development of domain wall based devices.
Article
The formation of oxygen vacancies in heteroepitaxial LaCoO3 thin films deposited on different substrates was investigated by using electron beam irradiation in atomic-scale scanning transmission electron microscopy (STEM). As the electron beam irradiation intensified, distinctive dark stripe patterns were identified in high-angle annular dark-field STEM images, demonstrating the formation and subsequent ordering of oxygen vacancies. A comprehensive quantitative analysis of the lattice parameter changes verified the significant expansion of unit cells associated with the presence of oxygen vacancies. In particular, a uniform distribution of these expanded unit cells was observed in the films under large tensile strain. These experimental findings emphasize the significant role of strain in generating oxygen vacancies in perovskite oxide materials.
Article
Environmental problems, especially increasing atmospheric pollution, are urgently prompting new strategies to assist in sustainable development. Many studies have demonstrated that oxygen vacancies (OVs) play an important role in the degradation of gaseous pollutants by modulating the surface and electronic structure of catalysts. To clarify the oxygen vacancy effect, this paper reviews recent research advances in improving the performance of catalysts for the degradation of gaseous pollutants based on controlling OVs. The constitutive relationship between OVs and performance in the degradation of typical gaseous pollutants is elucidated. Various approaches for generating and characterizing OVs in catalysts are also summarized. Finally, some perspectives on the challenges and future opportunities for the development of oxygen-deficient catalysts are discussed. This review is significant for the study of the reaction mechanism of OVs with gaseous pollutants and the design of efficient catalysts.
Article
Full-text available
Ferromagnetic insulators play a crucial role in the development of low‐dissipation quantum magnetic devices for spintronics. Epitaxial LaCoO3 thin film is a prominent ferromagnetic insulator, in which the robust ferromagnetic ordering emerges owing to epitaxial strain. Whereas it is evident that strong spin‐lattice coupling induces ferromagnetism, the reported ferromagnetic properties of epitaxially strained LaCoO3 thin films are highly consistent. For example, even under largely modulated degree of strain, the reported Curie temperatures of epitaxially strained LaCoO3 thin films lie in a narrow range of 80–85 K, without much deviation. In this study, substantial enhancement (≈18%) in the Curie temperature of epitaxial LaCoO3 thin films is demonstrated via crystallographic orientation dependence. By changing the crystallographic orientation of the films from (111) to (110), the crystal‐field energy is reduced and the charge transfer between the Co and O orbitals is enhanced. These modifications lead to a considerable enhancement of the ferromagnetic properties (including the Curie temperature and magnetization), despite the identical nominal degree of epitaxial strain. The findings of this study provide insights into facile tunability of ferromagnetic properties via structural symmetry control in LaCoO3. Tailoring ferromagnetism via crystallographic orientation control is demonstrated. Crystal field energy is an essential component to determine the spin state of LaCoO3, which can be fine‐tuned via microscopic crystal structure. In addition, the charge transfer between Co 3d and O 2p orbitals is modulated, leading to systematic change of ferromagnetic properties, consistently captured by both experiment and theoretical calculation.
Article
Controlling crystal structure is one of the most crucial steps for tuning the physical properties in layered perovskite oxides. The growth of mosaic-patterned double perovskite PrBaCo2O5+δ (PBCO) thin films was achieved to study the growth dynamics and the effect on the magnetic anisotropy (MA). High-resolution X-ray diffraction (XRD) and scanning transmission electron microscopy (STEM) studies indicate the crystal structures of PBCO thin films are highly oriented with three variants, i.e., <100>(013) grains tilted around ±18° away from [00l] direction along a or b axis, <110>(001) domains rotated 45° along c axis, and common cube-on-cube <100>(001) epitaxy. The MA transformation to isotropic and an anomalous anisotropic magnetoresistance (AMR) were observed in the films with the mosaic-patterned nanostructures. These evidences not only highlight the correlation between nanostructures and physical properties in perovskite systems, but also lay out the new avenues to manipulate the MA through controlling the preferred oriented nanostructures, which paves the way to create future nanoelectronics and spintronic devices via nanostructure design.
Thesis
Full-text available
BiFeO₃ is one of the most promising functional materials on perovskite basis. It is well known for its multiferroic properties, which it even maintains at temperatures far above room temperature, in the form of coupling between its ferroelectric polarization and antiferromagnetic ordering. With the successful demonstration of a direct switching of the antiferromagnetic domains via an electric field, research interest in BiFeO₃ has enormously increased in the past years because it offers intriguing application prospects. Besides the magnetoelectric properties, BiFeO₃ is also piezoelectric, shows a photovoltaic effect, and is highly birefringent. Additionally, it does not use a toxic and environmentally problematic component like Pb-based perovskites. Possible applications include smaller, faster, and more energy-efficient digital storage since changing magnetic orientation via the application of an electric field and the magnetoelectric coupling needs much less energy than to have electric current create a magnetic field to rewrite magnetic domains. With an increasing demand for digital data storage, this could be valuable to reduce environmental impact by reduced energy consumption. Other interesting applications include spin valves, spintronic devices, sensors, and optoelectronic devices. Doping and strain engineering of BiFeO₃ can be used to tune and change the material properties for applications. For example, the antiferromagnetic behavior can be changed to a ferromagnetic one. To understand effects in the material at an atomic scale, advanced transmission electron microscopy is one of the best methods since it delivers not just structural but also elemental and chemical information. This is combined in this work with density functional theory calculations, which showed substantial synergy effects. Scanning electron microscopy, X-Ray diffraction, X-ray photoelectron spectroscopy, and atomic force microscopy were used for additional characterization of the BiFeO₃ thin films. This dissertation focuses on thin films with Ca as a dopant or co-dopant. Specifically, it contains 3 studies focused on different aspects: • A new finding of segregation of Ca dopant toward the in-plane compressively strained interface between the Ca and Mn co-doped bismuth ferrite film and a strontium titanate substrate. The Ca segregation triggers atomic and electronic structure changes at the interface. The strain at the interface is reduced according to the Ca concentration gradient. Variations in the interplanar spacing and oxygen vacancies are introduced. The observed segregation behavior is confirmed with density functional theory calculations. • A study on the interaction of oxygen vacancies and ferroelectric domain walls on the case of a Ca doped BiFeO₃ film. The results revealed that the oxygen vacancies agglomerate in plates which simultaneously represent negatively charged domain walls in a tail-to-tail configuration. The plates appear without any periodicity between each other in out-of-plane as well as in-plane direction. Within the plates, the oxygen vacancies form 1D channels in pseudocubic [010] direction with one site containing lots of vacancies and the adjacent ones on both sites few. Interestingly, no variety between the characteristics of out-of-plane and in-plane plates could be found. Charged defects such as oxygen vacancies are known for their application-decisive pinning effect on domain walls, which on the one hand leads to fatigue mechanisms but on the other hand also counteracts retention failure. Charged defects also strongly influence domain wall conductivity. • The discovery of a significantly higher Ca solubility in BiFeO₃ than in the secondary Bi₂O₃ phase. The solubility behavior is confirmed and expanded on with density functional theory calculations. Bi₂O₃ can be used to evoke the super-tetragonal phase in BiFeO₃, which has very interesting material properties, without the need of a substrate that imposes very strong compressive strain. Using this effect for the fabrication of functional devices, the different Ca solubility is critical since it could increase the dopant level in the BiFeO₃ matrix.
Article
The formation of the exchange interaction between HS Co3+ ions, which are excited in LaCoO3, is studied within the multielectron approach. Two main contributions appear to be antiferromagnetic (AFM) and ferromagnetic (FM). When the ground state is LS, the total interaction is AFM. The crossover to the HS state may result in the FM ordering. The mean-field magnetic phase diagrams on the plane spin gap-temperature have been calculated without and with spin-orbital interaction in the HS term. Without spin-orbital interaction the reentrant magnetic order is possible. The spin-orbital coupling removes the reentrant phase transition and stabilizes the LS state. For known from experimental data values of the spin gap and exchange interaction, the ideal stoichiometric LaCoO3 is very close to the LS–HS crossover and magnetic ordering border. The violations of local coordination and symmetry of the Co3+-oxygen complexes that take place in the intergrain boundaries, at the surface of single crystals, and in the thin films on the strained substrate, may result in the formation of the HS state and FM order for such materials.
Article
Apart from the impact of different substrates on epitaxial thin films due to lattice constant mismatches, the properties of epitaxial thin films are also influenced by the same substrates with different crystal orientations. Here, the (111)‐oriented SrTiO3 (STO) substrates was used to fabricate the LaCoO3 (LCO) epitaxial thin films rather than the more common (001)‐oriented STO substrates. It undergoes one paramagnetic‐ferromagnetic phase transition around 50 K. This phase transition is similar to the previously observed for (001)‐LCO/STO, but the Curie temperature (T c ) is significantly lower than the previously reported 80 K. To investigate the decrease of T c and gain further insight into the phase transition, the critical behavior analysis of thin film was performed. The critical exponents were determined to be β = 1.591(3) and γ = 0.937(1) by reconstructing a modified Arrott plot, and the reliability of these exponents was confirmed by the Widom scaling relation. Using the renormalization group theory approach, we discover that the film has a long‐range magnetic interaction with space dimensionality d = 2 and spin dimensionality n = 1, and the magnetic exchange interaction decays J (r) ∼ r−2.4473. This article is protected by copyright. All rights reserved.
Article
The structure, surface topography and magnetic properties of LaCoO3 (LCO) thin films deposited epitaxially on the SrTiO3 substrates have been investigated in detail. The LCO thin films show a typical ferromagnetic-paramagnetic (FM-PM) phase transition at 78 K. Based on the measurements of isothermal magnetization around the Curie point, the magnetic entropy change (ΔSM) of LCO thin films under various applied magnetic fields were obtained. By utilizing scaling theory, all of the ΔSM curves can be re-scaled, confirming that the FM-PM phase transition is second-order magnetic phase transition. Moreover, the magnetic entropy change -ΔSM features a maximum around TC, whereas, power law fitting of -ΔSMmax with H gives n = 0.9704, which is obviously deviating from the standard value n = 2/3 for the Landau mean field model. This indicates that the mean field theory cannot be used to explain the critical behavior of LCO thin films which implies that the critical behavior of LCO thin films may involve complex magnetic interactions. These interactions are correlated with a long-term puzzling finding in this system why its Curie point is always around 80 K regardless of the different fabrication methods and conditions used in the film growth.
Article
The origin of strain-induced ferromagnetism, which is robust regardless of the type and degree of strain in LaCoO3 (LCO) thin films, is enigmatic despite intensive research efforts over the past decade. Here, by combining scanning transmission electron microscopy with ab initio density functional theory plus U calculations, we report that the ferromagnetism does not emerge directly from the strain itself but rather from the creation of compressed structural units within ferroelastically formed twin-wall domains. The compressed structural units are magnetically active with the rocksalt-type high-spin/low-spin order. Our study highlights that the ferroelastic nature of ferromagnetic structural units is important for understanding the intriguing ferromagnetic properties in LCO thin films.
Article
The physical properties of perovskite oxides are very sensitive to oxygen vacancy structures, and an unambiguous acquaintance of vacancy structures of thin films is the precondition for applications of many functional devices. Here, we report critical thickness control over oxygen vacancy ordering structure in epitaxial LaCoO3-x (LCO) thin films characterized by atomic resolution Scanning transmission electron microscopy (STEM). Lattice mappings extracted from Cs-corrected STEM images demonstrate that vacancy ordering planes are vertically distributed in LCO films grown on SrTiO3 substrates with the thickness above 5 nm, transversally distributed when the thickness is below 5 nm and distributed in a mixed manner when the thickness is 5 nm. Similar phenomenon also happens in LCO/NdGaO3 film systems with the critical thickness of around 13 nm due to the reduced tensile strain imposed by the substrate. The thickness effect phenomenon is collaborated by first-principles calculations and the formation mechanism is discussed from the point of view of strain relaxation. These findings demonstrate that the thickness can exquisitely control the periodic modulations of vacancy ordering structure in LCO thin films, suggesting an effective pathway of thickness controlling and enhancing physical properties of functional oxide films.
Article
Domain walls in ferroelectric materials attract great interest since they can possess fascinating functionalities. Therefore, it is very important to modulate domain structures. Our recent experiments showed that oxygen vacancy plates could induce charged domain walls with different types. However, the detailed transition behavior between different charged domain walls was not explored. In this work, systematical phase field simulations were performed to reveal the evolution of domain structures with the size and charge density of the oxygen vacancy plate. These results could provide a route to build complex patterns of charged domain walls.
Article
Oxygen vacancy is one of the pivotal factors for tuning/creating various oxide properties. Understanding the behavior of oxygen vacancies is of paramount importance. In this study, we identify a metastable oxygen vacancy ordering state other than the well-known Magnéli phases in TiO2 crystals from both experimental and theoretical studies. The oxygen vacancy ordering is found to be a zigzag chain along the [0 0 1] direction in the (1 1 0) plane occurring in a wide temperature range of 200–500 °C. This metastable ordering state leads to a first-order phase transition accompanied by significant enhancement of dielectric permittivity and a memristive effect featuring a low driving electric field. Our results can improve oxide properties by engineering oxygen vacancies.
Article
Thin films have attracted much interest because they often have novel properties different from those of their bulk counterparts. In this work, we tune two metastable states in three kinds of lanthanum cobalt oxide thin films by electron beam irradiation and record their dynamic transition process in situ in a transmission electron microscope. The lanthanum cobalt oxide thin films exhibit a homogeneous microstructure in the initial state and then transfer to a stripelike superstructure with 3a0 periodicity (a0 is the perovskite lattice parameter), further developing into a superstructure with 2a0 periodicity in dark stripes (brownmillerite structure). To explore the inherent energy discrepancy within the two metastable states, we perform first-principles calculations on a LaCoO3-δ (0 ≤ δ ≤ 0.5) thin film system by geometry optimization. The calculation results suggest that the forming energy of the 3a0 periodicity stripelike structure is a little lower than that of the 2a0 periodicity in the LaCoO3-δ thin film. Our work explains why the two stripelike structures coexist in lanthanum cobalt oxide thin films and extends prospective applications related to oxygen vacancies in thin films.
Article
Oxygen non-stoichiometry plays a critical role in determining the physical and chemical functionalities of oxide materials. For wide spread applications involving oxygen transport and exchange with the environment, fast, inexpensive and reversible control of oxygen deficiency is highly desired. This article illustrates voltage-controlled oxygen non-stoichiometry in SrCoO3-δ (SCO) thin films, in which the oxygen deficiency () can be tuned between 0 and 0.5 within tens of seconds by a small applied voltage (< 1.7 V). Correspondingly, its magnetism as well as the electrical and optical properties can be tuned accordingly from one end to the other, making it a good candidate for a number of commercial applications, such as oxygen capacitors, catalysts and smart windows etc. This approach can be used as an effective method in imaging the phase diagrams of transition metal oxides, such as ternary ABO3-δ (A = Ln, Ca, Sr, Bi; B = Cr, Mn, Co, Fe, Ni), or binary TiOx, WOx, VOx and NiOx etc., paving an avenue to the searching for novel properties in redox materials.
Article
Full-text available
Significance Ferromagnetic insulators are highly needed as the necessary components in developing next-generation dissipationless quantum-spintronic devices. Such materials are rare, and those high symmetric ones without chemical doping available so far only work below 16 K. Here we demonstrate a tensile-strained LaCoO 3 film to be a strain-induced high-temperature ferromagnetic insulator. Both experiments and first-principles calculations demonstrated that the tensile-strain–supported ferromagnetism reaches its strongest when the composition is nearly stoichiometric. It disappears when the Co ²⁺ defect concentration reaches around 10%. The discovery represents a chance for the availability of such materials, a high operation temperature, and a high epitaxial integration potential for making future devices.
Article
Full-text available
LaCoO3 (LCO) has attracted much attention due to the unique magnetic transition and spin transition of Co³⁺ ions. Epitaxial LCO film exhibits an unexpected ferromagnetism, in contrast to the non-magnetism of bulk LCO. An in-depth study on the property of strained LCO film is of great importance. We have fabricated 30 nm LCO films on various substrates and studied the magnetic and transport properties of films in different strain states (compressed strain for LCO/LaAlO3, tensile strain for LCO/(LaAlO3)0.3(Sr2TaAlO6)0.35, SrTiO3). The in-plane tensiled LCO films exhibit ferromagnetic ground state at 5K and magnetic transition with TC around 85K, while compressed LCO/LaAlO3 film has a negligibly small moment signal. Our results reveal that in-plane tensile strain and tetragonal distortion are much more favorable for stabilizing the FM order in LCO films.
Article
Full-text available
Controlling magnetic anisotropy is an important objective towards engineering novel magnetic device concepts in oxide electronics. In thin film manganites, magnetic anisotropy is weak and it is primarily determined by the substrate, through induced structural distortions resulting from epitaxial mismatch strain. On the other hand, in cobaltites, with a stronger spin orbit interaction, magnetic anisotropy is typically much stronger. In this paper, we show that interfacing La0.7Sr0.3MnO3 (LSMO) with an ultrathin LaCoO3 (LCO) layer drastically modifies the magnetic anisotropy of the manganite, making it independent of the substrate and closer to the magnetic isotropy characterizing its rhombohedral structure. Ferromagnetic resonance measurements evidence a tendency of manganite magnetic moments to point out-of-plane suggesting non collinear magnetic interactions at the interface. These results may be of interest for the design of oxide interfaces with tailored magnetic structures for new oxide devices.
Article
Full-text available
Perpendicular magnetization and precise control over the magnetic easy axis in magnetic thin film is necessary for a variety of applications, particularly in magnetic recording media. A strong (111) orientation is successfully achieved in the CoFe2O4 (CFO) thin film at relatively low substrate temperature of 100 °C, whereas the (311)-preferred randomly oriented CFO is prepared at room temperature by the DC magnetron sputtering technique. The oxygen-deficient porous CFO film after post-annealing gives rise to compressive strain perpendicular to the film surface, which induces large perpendicular coercivity. We observe the coercivity of 11.3 kOe in the 40-nm CFO thin film, which is the highest perpendicular coercivity ever achieved on an amorphous SiO2/Si substrate. The present approach can guide the systematic tuning of the magnetic easy axis and coercivity in the desired direction with respect to crystal orientation in the nanoscale regime. Importantly, this can be achieved on virtually any type of substrate.
Article
Full-text available
High quality epitaxial SrCoOx (oxygen deficient SrCoO3) thin films were grown on (110) DyScO3 substrates by pulsed laser deposition.The disappearance of half order peaks in X-ray diffraction as well as the XAS at the O K-edge indicates an oxygen content of x ≈ 2.8 in the thin films. Magnetization measurements reveal that the specific substrate strain suppresses the ferromagnetism found in the corresponding bulk material and the emergence of an antiferromagnetic-type spin correlation as predicted by theoretical calculations. Our work demonstrates that the magnetism can be tuned by in-plane strain in SrCoOx thin films.
Article
Full-text available
Spin state controlling has always been a focus of intensive studies due to its importance for novel effect exploration and information technology. Complex oxides with competitive mechanisms are suitable objects of study for this purpose due to their susceptibility to external stimuli. Perovskite cobaltate La1-xSrxCoO3 is one of such oxides. Combined effects of lattice strains and hole-doping have been studied for the LSCO films with 0 ≤ x ≤ 0.5. It is found that the lattice strain, either tensile or compressive, destabilizes the ferromagnetic (FM) state of the epitaxial films, leading to a nonmagnetic state that extensively exists in a doping window embedding deep into the range of the FM phase in bulk counterparts. Density functional theory calculations reveal a distinct spin state transition due to the combined effects of lattice distortion and hole-doping, explaining the unique magnetic behaviors of LSCO.
Article
Full-text available
The atomic-scale structural and electric parameters of the 90° domain-walls in tetragonal ferroelectrics are of technological importance for exploring the ferroelectric switching behaviors and various domain-wall-related novel functions. We have grown epitaxial PbTiO3/SrTiO3 multilayer films in which the electric dipoles at 90° domain-walls of ferroelectric PbTiO3 are characterized by means of aberration-corrected scanning transmission electron microscopy. Besides the well-accepted head-to-tail 90° uncharged domain-walls, we have identified not only head-to-head positively charged but also tail-to-tail negatively charged domain-walls. The widths, polarization distributions, and strains across these charged domain-walls are mapped quantitatively at atomic scale, where remarkable difference between these domain-walls is presented. This study is expected to provide fundamental information for understanding numerous novel domain-wall phenomena in ferroelectrics.
Article
Full-text available
The properties of complex oxide films depend sensitively on epitaxial strain. This strain affects bond lengths and angles, and defect types and densities, thus impacting physical properties. In this work we perform detailed characterization of depth-dependent strain in epitaxial La0.5Sr0.5CoO3-δ (LSCO) films on SrTiO3(001), SrTiO3(110), and LaAlO3(001) substrates, combining high resolution x-ray diffraction and scanning transmission electron microscopy, in addition to geometric phase analysis. We elucidate a fundamental link between strain state and O vacancy ordering in LSCO films, where lattice mismatch and crystallographic orientation can be used to manipulate the modulation vector of the long-range vacancy order, thus providing a new approach to tailor the properties of such films.
Article
Full-text available
Oxygen vacancy distributions and dynamics directly control the operation of solid-oxide fuel cells and are intrinsically coupled with magnetic, electronic and transport properties of oxides. For understanding the atomistic mechanisms involved during operation of the cell it is highly desirable to know the distribution of vacancies on the unit-cell scale. Here, we develop an approach for direct mapping of oxygen vacancy concentrations based on local lattice parameter measurements by scanning transmission electron microscopy. The concept of chemical expansivity is demonstrated to be applicable on the subunit-cell level: local stoichiometry variations produce local lattice expansion that can be quantified. This approach was successfully applied to lanthanum strontium cobaltite thin films epitaxially grown on substrates of different symmetry, where polarized neutron reflectometry revealed a strong difference in magnetic properties. The different vacancy content found in the two films suggests the change in oxygen chemical potential as a source of distinct magnetic properties, opening pathways for structural tuning of the vacancy concentrations and their gradients.
Article
Full-text available
The polar interface between LaAlO3_{3} and SrTiO3_{3} has shown promise as a field effect transistor, with reduced (nanoscale) feature sizes and potentially added functionality over conventional semiconductor systems. However, the mobility of the interfacial two-dimensional electron gas (2DEG) is lower than desirable. Therefore to progress, the highly debated origin of the 2DEG must be understood. Here we present a case for surface redox reactions as the origin of the 2DEG, in particular surface O vacancies, using a model supported by first principles calculations that describes the redox formation. In agreement with recent spectroscopic and transport measurements, we predict a stabilization of such redox processes (and hence Ti 3d occupation) with film thickness beyond a critical value, which can be smaller than the critical thickness for 2D electronic conduction, since the surface defects generate trapping potentials that will affect the interface electron mobility. Several other recent experimental results, such as lack of core level broadening and shifts, find natural explanation. Pristine systems will likely require changed growth conditions or modified materials with a higher vacancy free energy.
Article
Full-text available
A nanoscale phase is known to coincide with colossal magnetoresistance (CMR) in manganites, but its volume fraction is believed to be too small to affect CMR. Here we provide scanning-electron-nanodiffraction images of nanoclusters as they form and evolve with temperature in La(1-x)Ca(x)MnO(3), x = 0.45. They are not doping inhomogeneities, and their structure is that of the bulk compound at x = 0.60, which at low temperatures is insulating. Their volume fraction peaks at the CMR critical temperature and is estimated to be 22% at finite magnetic fields. In view of the known dependence of the nanoscale phase on magnetic fields, such a volume fraction can make a significant contribution to the CMR peak.
Article
Full-text available
According to recent experiments and predictions, the orientation of the polarization at the surface of a ferroelectric material can affect its surface chemistry. Here we demonstrate the converse effect: the chemical environment can control the polarization orientation in a ferroelectric film. In situ synchrotron x-ray scattering measurements show that high or low oxygen partial pressure induces outward or inward polarization, respectively, in an ultrathin PbTiO3 film. Ab initio calculations provide insight into surface structure changes observed during chemical switching.
Article
Full-text available
The perovskite LaCoO3 evolves from a nonmagnetic Mott insulator to a spin cluster ferromagnet (FM) with the substitution of Sr2+ for La3+ in La1-xSrxCoO3. The clusters increase in size and number with x and the charge percolation through the clusters leads to a metallic state. Using elastic neutron scattering on La1-xSrxCoO3 single crystals, we show that an incommensurate spin superstructure coexists with the FM spin clusters. The incommensurability increases continuously with x, with the intensity rising in the insulating phase and dropping in the metallic phase as it directly competes with the commensurate FM, itinerant clusters. The spin incommensurability arises from local order of Co3+-Co4+ clusters but no long-range static or dynamic spin stripes develop. The coexistence and competition of the two magnetic phases explain the residual resistivity at low temperatures in samples with metalliclike transport.
Cover Page
The recent research advances regarding materials and strategies for low-temperature solid-oxide fuel cells (LT-SOFCs) are summarized by Wei Zhou, Zhonghua Zhu, and co-workers in article number 1700132 with a focus on new materials, structures, and techniques for LT-SOFCs operating at around 500 °C. These recent developments highlight the need for efficient materials to generate high electrochemical performance at lower temperatures, thus promoting the large-scale application of LT-SOFCs.
Article
Electrical control of oxygen off-stoichiometry of transition-metal oxides at room temperature is a desired strategy to simultaneously switch the electrical conductance and magnetism of the device. Although the use of the electrochemical redox reaction of transition-metal oxides is the most reasonable way to achieve the aforementioned switch, such a device has not been realized because of the lack of a leakage-free liquid electrolyte. Here, we demonstrate an electromagnetic device that can reversibly switch a transition-metal oxide from an insulator/non-magnet to a metal/magnet (Tc=275 K) using a newly developed 'leakage-free electrolyte', incorporated in an amorphous NaTaO3 nanopillar array film. Reversible switching occurs electrically, obeying Faraday's laws of electrolysis, under a DC voltage of +(-)3 V within 2-3 s at RT. The present electromagnetic device does not have the drawback of liquid leakage, and the leakage-free electrolyte provides a novel design concept for practical electromagnetic devices using transition-metal oxides.
Article
Engineering defects and strains in oxides provides a promising route for the quest of thin film materials with coexisting ferroic orders, multiferroics, with efficient magnetoelectric coupling at room temperature. Precise control of the strain gradient would enable custom tailoring of the multiferroic properties, but presently remains challenging. Here we explore the existence of a polar-graded state in epitaxially-strained antiferromagnetic SrMnO3 thin films, whose polar nature was predicted theoretically, and recently demonstrated experimentally. By means of aberration-corrected scanning transmission electron microscopy we map the polar rotation of the ferroelectric polarization at atomic resolution, both far from and near the domain walls and find flexoelectricity resulting from vertical strain gradients. The origin of this particular strain state is a gradual distribution of oxygen vacancies across the film thickness, according to electron energy loss spectroscopy. Herein we present a chemistry-mediated route to induce polar rotations in oxygen-deficient multiferroic films, resulting in flexoelectric polar rotations and with potentially enhanced piezoelectricity.
Article
The ability to manipulate oxygen anion defects rather than metal cations in complex oxides can facilitate creating new functionalities critical for emerging energy and device technologies. However, the difficulty in activating oxygen at reduced temperatures hinders the deliberate control of important defects, oxygen vacancies. Here, strontium cobaltite (SrCoOx) is used to demonstrate that epitaxial strain is a powerful tool for manipulating the oxygen vacancy concentration even under highly oxidizing environments and at annealing temperatures as low as 300 °C. By applying a small biaxial tensile strain (2%), the oxygen activation energy barrier decreases by ≈30%, resulting in a tunable oxygen deficient steady-state under conditions that would normally fully oxidize unstrained cobaltite. These strain-induced changes in oxygen stoichiometry drive the cobaltite from a ferromagnetic metal towards an antiferromagnetic insulator. The ability to decouple the oxygen vacancy concentration from its typical dependence on the operational environment is useful for effectively designing oxides materials with a specific oxygen stoichiometry.
Article
Using the technique of aberration-corrected scanning transmission electron microscopy, we performed a systematic analysis for the atomic lattice of the strained La1−xSrxCoO3 (0 ≤ x ≤ 0.1) epitaxialfilms, which have drawn a great attention in recent years because of their anomalous magnetism. Superstructures characterized by dark stripes are observed in the lattice image, evolving with combined Sr-doping and lattice strains. Fascinatingly, we found a close relation between the proportion of the Co ions in dark stripes and the saturation magnetization of the film: the latter grows linearly with the former. This result implies that the magnetism could be exclusively ascribed to the Co ions in dark stripes.
Article
The energetics at oxide semiconductor/La1−xSrxCoO3 heterojunctions, including the respective alignment of the valence and conduction bands, govern charge transfer and have to be determined for the design of future La1−xSrxCoO3-based devices. In this letter, the electronic and atomic structures of epitaxial La1−xSrxCoO3 on Nb-doped strontium titanate are revealed by scanning transmission electron microscopy, electron energy loss spectroscopy, and in situ x-ray and ultra violet photoelectron spectroscopies. For LaCoO3, a valence band (VB) offset of 2.8 ± 0.1 eV is deduced. The large offset is attributed to the orbital contributions of the Co 3d states to the VB maximum of the LaCoO3 thin films, with no evidence of interface dipole contributions. The sensitivity of the valence band orbital character to spin state ordering and oxygen vacancies is assessed using density functional theory.
Article
The recent interest in building a decentralized, hydrogen-based energy economy has refocused attention on the solid oxide fuel cell (SOFC) as potential source of efficient, environmentally friendly, fuel-versatile electric power. This review covers the advances made to date in the understanding of SOFC cathodes. Focus is on how new approaches have been used by workers to better understand cathode mechanisms and how these mechanisms relate to materials properties and microstructure.
Article
The development of interface-based magnetoelectric devices necessitates an understanding of polarization-mediated electronic phenomena and atomistic polarization screening mechanisms. In this work, the LSMO/BFO interface is studied on a single unit-cell level through a combination of direct order parameter mapping by scanning transmission electron microscopy and electron energy-loss spectroscopy. We demonstrate an unexpected ~5% lattice expansion for regions with negative polarization charge, with a concurrent anomalous decrease of the Mn valence and change in oxygen K-edge intensity. We interpret this behaviour as direct evidence for screening by oxygen vacancies. The vacancies are predominantly accumulated at the second atomic layer of BFO, reflecting the difference of ionic conductivity between the components. This vacancy exclusion from the interface leads to the formation of a tail-to-tail domain wall. At the same time, purely electronic screening is realized for positive polarization charge, with insignificant changes in lattice and electronic properties. These results underline the non-trivial role of electrochemical phenomena in determining the functional properties of oxide interfaces. Furthermore, these behaviours suggest that vacancy dynamics and exclusion play major roles in determining interface functionality in oxide multilayers, providing clear implications for novel functionalities in potential electronic devices.
Article
The origin of ferromagnetism in strained epitaxial LaCoO3 films has been a long-standing mystery. Here, we combine atomically resolved Z-contrast imaging, electron-energy-loss spectroscopy, and density-functional calculations to demonstrate that, in epitaxial LaCoO3 films, oxygen-vacancy superstructures release strain, control the film's electronic properties, and produce the observed ferromagnetism via the excess electrons in the Co d states. Although oxygen vacancies typically dope a material n-type, we find that ordered vacancies induce Peierls-like minigaps which, combined with strain relaxation, trigger a nonlinear rupture of the energy bands, resulting in insulating behavior.
Article
A strategy to enhance the catalytic activity at the surface of an oxide thin film is unveiled through epitaxial orientation control of the surface oxygen vacancy concentration. By tuning the direction of the oxygen vacancy channels (OVCs) in the brownmillerite SrCoO2.5 , a 100-fold improvement in the oxygen reduction kinetics is realized in an epitaxial thin film that has the OVCs open to the surface.
Article
Pulsed laser epitaxy of brownmillerite SrCoO2.5 thin films and their phase transformation to the perovskite SrCoO3-δ are investigated. While the direct growth of the fully oxidized perovskite films is found to be an arduous task, filling some of oxygen vacancies into SrCoO2.5 by topotactic oxidation accompanies systematic evolution of electronic, magnetic, and thermoelectric properties, useful for many information and energy technologies.
Article
LaCoO3 films grown epitaxially on oriented (LaAlO3)0.3(Sr2AlTaO6)0.7 substrates by pulsed laser deposition exhibit ferromagnetic ordering below a critical temperature, Tc , of 85K . Polycrystalline films of LaCoO3 prepared in the same way did not show ferromagnetic order down to T&ap;5K , and their temperature dependent susceptibility was identical to that of bulk LaCoO3 . The ferromagnetism in epitaxial films is not simply a property of the surface region, rather it extends over the complete film thickness, as shown by the linear increase of the saturated magnetic moment with increasing film thickness. We discuss this surprising result in terms of epitaxial tensile strain via the properly chosen substrate inducing ferromagnetic order.
Article
Atomic resolution Z-contrast imaging and electron energy-loss spectroscopy (EELS) were used to study the defect structure of oxygen deficient strontium cobaltite (SrCoO3−δ) after repeated oxidation/reduction cycles at elevated temperatures. Ordered microdomains were found in the reduced sample. Z-contrast imaging showed ordering in alternate CoO2 planes, characteristic for a brownmillerite-type vacancy ordered structure. EELS showed that the average valence state of cobalt had decreased after testing. In the tested sample, cobalt was found in +3 and +2 valence states, respectively. In addition, cobalt oxide precipitates were found at grain boundaries and in triple pockets in this sample, indicating a cobalt deficiency of the grains. The mechanisms for the formation of the observed microstructure are discussed.
Article
Ferromagnetic order can be induced in LaCoO3 (LCO) thin films by epitaxial strain. Here, we show that the magnetic properties can be ``tuned'' by epitaxial strain imposed on LCO thin films by the epitaxial growth on various substrate materials, i.e., (001) oriented SrLaAlO4 , LaAlO3 , SrLaGaO4 , (LaAlO3)0.3(Sr2AlTaO6)0.7 , and SrTiO3 . The lattice mismatch at room temperature of the in-plane lattice parameters between the substrate, as , and bulk LCO, ab , ranges from -1.31% to +2.63% . Single-phase, oriented LCO thin films were grown by pulsed laser deposition on all these substrates. Due to the difference of the thermal-expansion coefficients between LCO and the substrates, the films experience an additional tensile strain of about +0.3% during the cooling process after the deposition at Ts=650°C . The film lattice parameters display an elastic behavior, i.e., an increase of the in-plane film lattice parameter with increasing as . From the ratio between the out-of-plane and in-plane strain, we obtain a Poisson ratio of nu&ap;1/3 . All films show a ferromagnetic transition as determined from magnetization measurements. The magnetization increases strongly with increasing tensile strain, whereas the transition temperature TC after a rapid initial rise appears to saturate at TC&ap;85K above a=3.86Å . The effective magnetic moment mueff in the paramagnetic state increases almost linearly as a function of the mean lattice parameter , indicating an enhanced population of higher spin states, i.e., intermediate- or high-spin states. The experimental results are discussed in terms of a decrease of the octahedral-site rotation with increasing tensile strain.
Article
Planar strain effects on oxygen-vacancy formation and oxygen adsorption on LaCoO3 are shown to manifest through competing mechanisms. Through first-principles calculations, we demonstrate that these unit processes are facilitated by elastic stretching. On the other hand, spin-state transitions and Co-O bond exchange hinder these processes by trapping the lattice oxygen with increasing tensile strain. A transition from chemisorption to physisorption of the oxygen molecule is identified at high strains. Insights on charge-density profiles, density of electronic states, and stress thresholds suggest the possibility of tuning strain-mediated reactivity in LaCoO3 and related perovskite oxides.
Article
The Curie temperature Tc of ferromagnetic La0.7A0.3CoO3 (A=Ca, Sr, Ba) thin films was studied as a function of the film thickness d for 400 nm>d⩾2.6 nm. A finite-size shift of the critical temperature relative to the bulk value, 1−Tc(d)∕Tc(∞)∝d−λ, with a constant critical shift exponent of λ≈1 over the entire thickness range was observed. This value of λ is rather unusual for thick films (d>100 nm) and normally expected for two-dimensional magnetic structures (i.e., ultrathin magnetic films with a thickness of a few monolayers) or spin-glass systems. Measurements of the magnetoresistivity reveal spin-dependent variable range hopping in the ultrathin films, which is strongly suppressed with increasing film thickness by percolation. The occurrence of charge-carrier localization and spin-misorientation hints to an exchange interaction between isovalent Co ions and thus to a phase-separation of ferromagnetic and antiferromagnetic regions, which may be the reason for the observed unusual critical shift exponent λ.
Article
Capacitance−voltage characteristics of high quality Pt Schottky diodes fabricated on oxygen-vacancy-doped SrTiO3 single crystals were used to obtain the oxygen vacancy profiles within one microns of the Pt interface. Computer simulations based on solving the drift-diffusion equations for electrons and ionized vacancies were performed to understand the experimentally observed oxygen vacancy profile’s time-evolution at room temperature and 0 V applied bias. Building upon this understanding, the diode’s room temperature profile evolution under −35 V applied bias was analyzed to yield a vacancy mobility value of 1.5 × 10−13 cm2/V·s at an electric field of 500 kV/cm. This mobility is 8 orders of magnitude too low to produce nanosecond resistance switching in thin film devices. The applicability of the results to oxygen-migration-based resistance switching is discussed relative to recent observations and modeling.
Article
High-resolution transmission electron microscopy (HRTEM), electron energy-loss spectroscopy (EELS), and electron diffraction were used to study the microstructure of epitaxial La0.5Sr0.5CoO3−∂ thin films grown on (001) oriented LaAlO3 substrates. Films were characterized before and after annealing treatments under different oxygen partial pressures. EELS shows that annealing reduces the valence state of cobalt due to loss of oxygen. HRTEM image simulations show that the superstructure contrast observed in HRTEM can be explained by shifts of cations in planes containing ordered oxygen vacancies. The as-deposited film showed weak, short-range ordering of oxygen vacancies within nanometer-sized domains. The annealed film showed long-range order and a strong anisotropy in ordering, with the oxygen-deficient planes aligned parallel to the film/substrate interface. We propose that the anisotropy in ordering is a mechanism of stress relief in these films. Implications of the observed microstructure on the oxygen transport and surface oxygen exchange properties are discussed. © 2001 American Institute of Physics.
Article
Complex cobalt oxides known as cobaltites are reviewed, including LnCoO3-based perovskite-structured rare-earth cobaltites (where Ln is lanthanum or a lanthanide), quasi-two-dimensional and quasi-one-dimensional cobaltites of the types LnCo2O5+δ, La2CoO4, and Ca3Co2O8, and NaxCoO2 yH2O superconducting compounds. Key experimental and theoretical results are presented, with emphasis on the interplay between charge, spin, and orbital degrees of freedom. Two problems of specific relevance to cobaltites — the spin state instability of Co3+ ions in LnCoO3, and the nature of superconductivity in NaxCoO2 yH2O — are also given significant attention.
Article
Epitaxial strain imposed in complex oxide thin films by heteroepitaxy is recognized as a powerful tool for identifying new properties and exploring the vast potential of materials performance. A particular example is LaCoO(3), a zero spin, nonmagnetic material in the bulk, whose strong ferromagnetism in a thin film remains enigmatic despite a decade of intense research. Here, we use scanning transmission electron microscopy complemented by X-ray and optical spectroscopy to study LaCoO(3) epitaxial thin films under different strain states. We observed an unconventional strain relaxation behavior resulting in stripe-like, lattice modulated patterns, which did not involve uncontrolled misfit dislocations or other defects. The modulation entails the formation of ferromagnetically ordered sheets comprising intermediate or high spin Co(3+), thus offering an unambiguous description for the exotic magnetism found in epitaxially strained LaCoO(3) films. This observation provides a novel route to tailoring the electronic and magnetic properties of functional oxide heterostructures.
Article
A ceramic fuel cell in an all solid-state energy conversion device that produces electricity by electrochemically combining fuel and oxidant gases across an ionic conducting oxide. Current ceramic fuel cells use an oxygen-ion conductor or a proton conductor as the electrolyte and operate at high temperatures (>600°C). Ceramic fuel cells, commonly referred to as solid-oxide fuel cells (SOFCs), are presently under development for a variety of power generation applications. This paper reviews the science and technology of ceramic fuel cells and discusses the critical issues posed by the development of this type of fuel cell. The emphasis is given to the discussion of component materials (especially, ZrO2 electrolyte, nickel/ZrO2 cermet anode, LaMnO3 cathode, and LaCrO3 interconnect), gas reactions at the electrodes, stack designs, and processing techniques used in the fabrication of required ceramic structures.
Article
This work, based on the experimental and theoretical research performed by the authors during last three decades, presents an overview of phase and defect thermodynamics, electronic transport properties, and the stability of cobaltite-based mixed conductors that are promising for electrode and membrane applications. Attention is centered on (1) the phase equilibria in La–Me–Co–T–O (where Me=Ca, Sr, Ba and T=Mn, Fe, Ni, Cu) systems and crystal structure of the complex oxides formed in these systems, thermodynamic stability and the homogeneity ranges of solid solutions; (2) the defect structure of the oxygen-deficient undoped and acceptor- or/and donor-doped lanthanum cobaltites; and (3) their conductivity and Seebeck coefficient as functions of temperature and oxygen partial pressure. The relationships between the peculiarities of the defect structure and the transport properties of the lanthanum cobaltites with different dopant natures are analyzed.
Article
Electrochromes are materials that have the ability to reversibly change from one colour state to another with the application of an electric field. Electrochromic colouration efficiency is typically large in organic materials that are not very stable chemically. Here we show that inorganic Bi(0.9)Ca(0.1)FeO(3-0.05) thin films exhibit a prominent electrochromic effect arising from an intrinsic mechanism due to the melting of oxygen-vacancy ordering and the associated redistribution of carriers. We use a combination of optical characterization techniques in conjunction with high-resolution transmission electron microscopy and first-principles theory. The absorption change and colouration efficiency at the band edge (blue-cyan region) are 4.8×10(6) m(-1) and 190 cm(2) C(-1), respectively, which are the highest reported values for inorganic electrochromes, even exceeding values of some organic materials.
Article
We investigate oxygen vacancy ordering in epitaxial ( La 0.5 Sr 0.5) CoO 3-∂ thin films grown by sputter deposition on (001) LaAlO 3 and (001) SrTiO 3. After annealing at 500 ° C under oxygen partial pressures greater than those used during deposition, films transform to a long-range oxygen vacancy ordered structure with orthorhombic symmetry. Observed orientation variants of the oxygen vacancy ordered structures are different for the two substrates. We discuss the relationship between film stress due to lattice and thermal mismatch with the substrate, and vacancy ordering. © 2003 American Institute of Physics.
Article
Various classes of resistive random access memory (RRAM) classes involving chemical effects related to redox processes in the MIM cell are presented. The electrochemical metallization (ECM) cells systems involve Ag and Cu as electrochemically active metals and phase separated amorphous selenides and sulfides and various oxides, acting as solid electrolytes. The polarity of the bipolar switching cycle is determined by many factors such as the work function and the oxygen affinity of the electrode metals. The SET and RESET switching of an individual filament at the surface of a SrTiO3 single crystal shows that the resistance of the filament can be switched between an ONand OFFstate by a voltage applied to the tip of the LC-AFM. The dissolution of cylindric metal filaments studied by electrothermal simulation is described by a thermally activated process of the filament surface.
Article
First-principles calculations are presented for the epitaxial-strain dependence of the ground-state phase stability of perovskite SrCoO(3). Through the combination of the large spin-phonon coupling with polarization-strain coupling and the coupling of the band gap to the polar distortion, both tensile and compressive epitaxial strain are seen to drive the bulk ferromagnetic-metallic (FM-M) phase to antiferromagnetic-insulating-ferroelectric (AFM-I-FE) phases, the latter having unusually low elastic energy. For compressive strain, there is a single coupled magnetic-ferroelectric metal-insulator transition. At this phase boundary, cross responses to applied electric and magnetic fields and stresses are expected. In particular, a magnetic field or compressive uniaxial stress applied to the AFM-FE(z) phase could induce an insulator-metal transition, and an electric field applied to the FM-M phase could induce a metal-insulator transition.
Article
A new method for creating oxygen vacancy ordered (brownmillerite) phases in epitaxial manganite thin films is demonstrated. The method involves depositing an oxygen- deficient complex oxide film on top of a stoichiometric manganite film. Once the getter layer exceeds a critical thickness, a phase transition to an oxygen vacancy ordered superlattice occurs in the manganite film.
Article
Certain cobalt oxides are known to exhibit ordered Co spin states, as determined from macroscopic techniques. Here we report real-space atomic-resolution imaging of Co spin-state ordering in nanopockets of La(0.5)Sr(0.5)CoO(3-δ) thin films. Unlike the bulk material, where no Co spin-state ordering is found, thin films present a strain-induced domain structure due to oxygen vacancy ordering, inside of which some nanometer sized domains show high-spin Co ions in the planes containing O vacancies and low-spin Co ions in the stoichiometric planes. First-principles calculations provide support for this interpretation.
Article
Oxygen-ion conduction in transition-metal oxides is exploited in, for example, electrolytes in solid-oxide fuel cells and oxygen-separation membranes, which currently work at high temperatures. Conduction at low temperature is a key to developing further utilization, and an understanding of the structures that enable conduction is also important to gain insight into oxygen-diffusion pathways. Here we report the structural changes observed when single-crystalline, epitaxial CaFeO₂.₅ thin films were changed into CaFeO₂ by low-temperature reductions with CaH₂. During the reduction process from the brownmillerite CaFeO₂.₅ into the infinite-layer structure of CaFeO₂, some of the oxygen atoms are released from and others are rearranged within the perovskite-structure framework. We evaluated these changes and the reaction time they required, and found two oxygen diffusion pathways and the related kinetics at low temperature. The results demonstrate that oxygen diffusion in the brownmillerite is highly anisotropic, significantly higher along the lateral direction of the tetrahedral and octahedral layers.
Article
Thermodynamic theory is developed for the ferroelectric phase transition of an ultrathin film in equilibrium with a chemical environment that supplies ionic species to compensate its surface. Equations of state and free energy expressions are developed based on Landau-Ginzburg-Devonshire theory, using electrochemical equilibria to provide ionic compensation boundary conditions. Calculations are presented for a monodomain PbTiO3_3 (001) film coherently strained to SrTiO3_3 with its exposed surface and its electronically conducting bottom electrode in equilibrium with a controlled oxygen partial pressure. The stability and metastability boundaries of phases of different polarization are determined as a function of temperature, oxygen partial pressure, and film thickness. Phase diagrams showing polarization and internal electric field are presented. At temperatures below a thickness-dependent Curie point, high or low oxygen partial pressure stabilizes positive or negative polarization, respectively. Results are compared to the standard cases of electronic compensation controlled by either an applied voltage or charge across two electrodes. Ionic surface compensation through chemical equilibrium with an environment introduces new features into the phase diagram. In ultrathin films, a stable non-polar phase can occur between the positive and negative polar phases when varying the external chemical potential at fixed temperature, under conditions where charged surface species are not present in sufficient concentration to stabilize a polar phase.
Article
Die aktive Zutat: Epitaktische dünne Filme aus La0.8Sr0.2CoO3−δ (LSC) wurden auf (001)-orientierten Y2O3-stabilisierten Zr2O3(YSZ)-Einkristallen mit einer Pufferschicht aus Gadolinium-dotiertem CeO2 (GDC) hergestellt (siehe Bild). Die epitaktischen LSC-Filme zeigen eine bessere Sauerstoffreduktionskinetik als massives LSC. Die verbesserte Aktivität wird zum Teil der höheren Stöchiometrieabweichung des Sauerstoffs zugeschrieben.
Article
The reduction in switchable polarization of ferroelectric thin films due to electrical stress (polarization fatigue) is a major problem in ferroelectric nonvolatile memories. There is a large body of available experimental data and a number of existing models which address this issue, however the origin of this phenomena is still not properly understood. This work synthesizes the current experimental data, models, and approaches in order to draw conclusions on the relative importance of different macro- and microscopic scenarios of fatigue. Special attention is paid to the role of oxygen vacancy migration and electron injection into the film and it is concluded that the latter plays the predominant role. Experiments and problems for theoretical investigations, which can contribute to the further elucidation of polarization fatigue mechanisms in ferroelectric thin films, are suggested. (C) 2001 American Institute of Physics.
Article
Oxygen vacancies are known to dominate the overall electrical behavior of perovskite oxides, which are used in many applications. Although theories have been developed to explain the effect of these vacancies and the defect chemistry of perovskites, there has yet to be incontrovertible evidence of the fundamental origins of the structure-property relationship. However, recently developed technologies in scanning transmission electron microscopy, such as Z-contrast imaging and EELS combined with in-situ heating experiments, provide a new opportunity to address vacancy characteristics and defect chemistry on the basic atomic level. In this paper we discuss the practical aspects of these techniques and demonstrate their application to the characterization of defect chemistry and vacancies in ordered micro-domains, at domain boundaries and at grain boundaries.