Spatial control of functional properties via octahedral modulations in complex oxide superlattices

Effects of electric field and light on resistivity switching of Eu0.7Sr0.3MnO3 thin films

Based on the excellent piezoelectric properties of 0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 (PMN-PT) single crystals, a hole-doped manganite film/PMN-PT heterostructure has been constructed to achieve electric-field and light co-control of physical properties. Here, we report the resistivity switching behavior of Eu0.7Sr0.3MnO3/PMN-PT(111) multiferroic heterostructures under different in-plane reading currents, temperatures, light stimuli and electric fields, and discuss the underlying coupling mechanisms of resistivity change. The transition from the electric-field induced lattice strain effect to polarization current effect can be controlled effectively by decreasing the in-plane reading current at room temperature. With the decrease of temperature, the interfacial charge effect dominates over the lattice strain effect due to the reduced charge carrier density. In addition, light stimulus can lead to the delocalization of eg carriers, and thus enhance the lattice strain effect and suppress the interfacial charge effect. This work helps to understand essential physics of magnetoelectric coupling and also provides a potential method to realize energy-efficient multi-field control of manganite thin films.

Emergent and robust ferromagnetic-insulating state in highly strained ferroelastic LaCoO3 thin films

Transition metal oxides are promising candidates for the next generation of spintronic devices due to their fascinating properties that can be effectively engineered by strain, defects, and microstructure. An excellent example can be found in ferroelastic LaCoO₃ with paramagnetism in bulk. In contrast, unexpected ferromagnetism is observed in tensile-strained LaCoO₃ films, however, its origin remains controversial. Here we simultaneously reveal the formation of ordered oxygen vacancies and previously unreported long-range suppression of CoO₆ octahedral rotations throughout LaCoO₃ films. Supported by density functional theory calculations, we find that the strong modification of Co 3d-O 2p hybridization associated with the increase of both Co-O-Co bond angle and Co-O bond length weakens the crystal-field splitting and facilitates an ordered high-spin state of Co ions, inducing an emergent ferromagnetic-insulating state. Our work provides unique insights into underlying mechanisms driving the ferromagnetic-insulating state in tensile-strained ferroelastic LaCoO₃ films while suggesting potential applications toward low-power spintronic devices.

Dimensionality control of magnetic coupling at interfaces of cuprate-manganite superlattices

The dimensionality of the crystal structure plays a vital role in artificial heterostructures composed of different transition metal oxides. Nonlinear layer-thickness dependence of the exchange bias effect was observed in high-quality SrCuO₂/La_0.7Sr_0.3MnO (LSMO) superlattices induced in the present work by dimensional evolution. In the SCO(n)/LSMO(8) superlattices with thickness below the critical value (5 u.c.), the exchange bias effect decreased and the saturated magnetization increased with increase in SCO thickness. By contrast, the exchange bias effect increased and the saturated magnetization decreased in S(n)L(8) superlattices with thickness above the critical value. This is because the lattice SCO material underwent a breathing-like structural transformation from the planar to a chain-like structure. The results indicate the interfacial superexchange coupling mainly present in the chain-like S(n)L(8) superlattices through X-ray absorption spectroscopy and first principles calculations. This superexchange coupling generated a weak localized magnetic moment to pin the adjacent ferromagnetic layer. However, in the thicker S(n)L(8) superlattices, evolution of magnetic properties was induced by the long-range antiferromagnetic order in the planar SCO layer. Our findings demonstrate that the dimensionality driven structural variation is an effective method to manipulate the electronic reconstruction and the associated physical properties, paving a pathway for the advancement of strongly correlated materials.

Dimensional Control of Octahedral Tilt in SrRuO3 via Infinite-Layered Oxides

Manipulation of octahedral distortion at atomic scale is an effective means to tune the ground states of functional oxides. Previous work demonstrates that strain and film thickness are variable parameters to modify the octahedral parameters. However, selective control of bonding geometry by structural propagation from adjacent layers is rarely studied. Here we propose a new route to tune the ferromagnetism in SrRuO₃ (SRO) ultrathin layers by oxygen coordination of adjacent SrCuO₂ (SCO) layers. The infinite-layered CuO₂ exhibits a structural transformation from "planar-type" to "chain-type" with reduced film thickness. Two orientations dramatically modify the polyhedral connectivity at the interface, thus altering the octahedral distortion of SRO. The local structural variation changes the spin state of Ru and orbital hybridization strength, leading to a significant change in the magnetoresistance and anomalous Hall resistivity. These findings could launch investigations into adaptive control of functionalities in quantum oxide heterostructures using oxygen coordination.

Exotic Long-Range Surface Reconstruction on La0.7Sr0.3MnO3 Thin Films

Due to an extremely diverse phase space, La_1-xSr_xMnO₃, as with other manganites, offers a wide range of tunability and applications including colossal magnetoresistance and use as spin-polarized electrodes. Here, we study an unprecedented, exotic surface reconstruction (6 × 6) in La_1-xSr_xMnO₃ (x = 0.3) observed via low-energy electron diffraction (LEED). Scanning tunneling microscopy (STM) shows the surface is relatively flat, with unit-cell step heights, and X-ray photoelectron spectroscopy (XPS) reveals a strong degree of Sr segregation at the surface. By combining electron diffraction and first-principles computations, we propose that the long-range surface reconstruction consists of a Sr-segregated surface with La (6 × 6) ordering. This study expands our understanding of manganite systems and underscores their ability to form interesting surface reconstructions, driven largely by cation segregation that can potentially be controlled for tuning surface ordering.

Spatially Controlled Octahedral Rotations and Metal-Insulator Transitions in Nickelate Superlattices

The properties of correlated oxides can be manipulated by forming short-period superlattices since the layer thicknesses are comparable with the typical length scales of the involved correlations and interface effects. Herein, we studied the metal-insulator transitions (MITs) in tetragonal NdNiO₃/SrTiO₃ superlattices by controlling the NdNiO₃ layer thickness, n in the unit cell, spanning the length scale of the interfacial octahedral coupling. Scanning transmission electron microscopy reveals a crossover from a modulated octahedral superstructure at n = 8 to a uniform nontilt pattern at n = 4, accompanied by a drastically weakened insulating ground state. Upon further reducing n the predominant dimensionality effect continuously raises the MIT temperature, while leaving the antiferromagnetic transition temperature unaltered down to n = 2. Remarkably, the MIT can be enhanced by imposing a sufficiently large strain even with strongly suppressed octahedral rotations. Our results demonstrate the relevance for the control of oxide functionalities at reduced dimensions.

Propagation Control of Octahedral Tilt in SrRuO3 via Artificial Heterostructuring

Bonding geometry engineering of metal-oxygen octahedra is a facile way of tailoring various functional properties of transition metal oxides. Several approaches, including epitaxial strain, thickness, and stoichiometry control, have been proposed to efficiently tune the rotation and tilt of the octahedra, but these approaches are inevitably accompanied by unnecessary structural modifications such as changes in thin-film lattice parameters. In this study, a method to selectively engineer the octahedral bonding geometries is proposed, while maintaining other parameters that might implicitly influence the functional properties. A concept of octahedral tilt propagation engineering is developed using atomically designed SrRuO3/SrTiO3 (SRO/STO) superlattices. In particular, the propagation of RuO6 octahedral tilt within the SRO layers having identical thicknesses is systematically controlled by varying the thickness of adjacent STO layers. This leads to a substantial modification in the electromagnetic properties of the SRO layer, significantly enhancing the magnetic moment of Ru. This approach provides a method to selectively manipulate the bonding geometry of strongly correlated oxides, thereby enabling a better understanding and greater controllability of their functional properties.

Interface Engineered Room-Temperature Ferromagnetic Insulating State in Ultrathin Manganite Films

Ultrathin epitaxial films of ferromagnetic insulators (FMIs) with Curie temperatures near room temperature are critically needed for use in dissipationless quantum computation and spintronic devices. However, such materials are extremely rare. Here, a room-temperature FMI is achieved in ultrathin La0.9Ba0.1MnO3 films grown on SrTiO3 substrates via an interface proximity effect. Detailed scanning transmission electron microscopy images clearly demonstrate that MnO6 octahedral rotations in La0.9Ba0.1MnO3 close to the interface are strongly suppressed. As determined from in situ X-ray photoemission spectroscopy, O K-edge X-ray absorption spectroscopy, and density functional theory, the realization of the FMI state arises from a reduction of Mn eg bandwidth caused by the quenched MnO6 octahedral rotations. The emerging FMI state in La0.9Ba0.1MnO3 together with necessary coherent interface achieved with the perovskite substrate gives very high potential for future high performance electronic devices.

Nanoscale magnetic and charge anisotropies at manganite interfaces

Strong correlated manganites are still under intense research owing to their complex phase diagrams in terms of Sr-doping and their sensitivity to intrinsic and extrinsic structural deformations. Here, we performed X-ray absorption spectroscopy measurements of manganite bilayers to explore the effects that a local Sr-doping gradient produce on the charge and antiferromagnetic anisotropies. In order to gradually tune the Sr-doping level along the axis perpendicular to the samples we have grown a series of bilayers with different thicknesses of low-doped manganites (from 0 nm to 6 nm) deposited over a La_0.7Sr_0.3MnO₃ metallic layer. This strategy permitted us to resolve with high accuracy the thickness region where the charge and spin anisotropies vary and the critical thickness t_c over which the out of plane orbital asymmetry does not have any further modifications. We found that the antiferromagnetic spin axis points preferentially out of the sample plane regardless the capping layer thickness. However, it tilts partially into the sample plane far from this critical thickness, owing to the combined contributions of the external structural strain and electron doping. Furthermore, we found that the doping level of the capping layer strongly affects the critical thickness, giving clear evidence of the influence exerted by the electron doping on the orbital and magnetic configurations. These anisotropic changes induce subtle modifications on the domain reorientation of La_0.7Sr_0.3MnO₃, as evidenced from the magnetic hysteresis cycles.

Nanoscale variation of antiferromagnetic and charge anisotropies has been found at manganite interfaces with an artificially created Sr-doping.

Interfacial Engineering of Ferromagnetism in Epitaxial Manganite/Ruthenate Superlattices via Interlayer Chemical Doping

Interfacial charge transfer and structural proximity effects are the two essential routes to trigger and tune numerous functionalities of perovskite oxide heterostructures. However, the cooperation and competition of these two interfacial effects in one epitaxial system have not been fully understood. Herein, we fabricate a series of La_0.67Ca_0.33MnO₃/CaRuO₃ superlattices and introduce various chemical doping in the nonmagnetic CaRuO₃ interlayers. We found that Ti, Sr, and La doping in the CaRuO₃ layer can effectively tune the interfacial charge transfer and octahedral rotation, thus modulating the ferromagnetism of the superlattices. Specifically, the B-site Ti doping depletes the Ru 4d band and suppresses the interfacial charge transfer, leading to a decay of ferromagnetic Curie temperature ( T_C). In contrast, the A-site Sr doping maintains a sizable charge transfer and meanwhile suppresses the octahedral rotation, which facilitates ferromagnetism and significantly enhances the T_C up to 291 K. The La doping turns out to localize the itinerant electrons in the CaRuO₃ layer, which suppresses both the interfacial charge transfer and ferromagnetism. The observed intriguing interfacial engineering of magnetism would pave a new way to understand the collective effects of interfacial charge transfer and structural proximity on the physical properties of oxide heterostructures.

Dynamic Field Modulation of the Octahedral Framework in Metal Oxide Heterostructures

Control over the oxygen octahedral framework is widely recognized as key to the design of functional properties in perovskite oxide heterostructures. Although the oxygen octahedral framework can be manipulated during synthesis, the as-grown oxygen octahedra generally remain fixed, preventing the development of adaptive behavior in electronic and ionotronic systems. Here, it is demonstrated that the oxygen octahedral framework can be dynamically and reversibly manipulated by an electric field through the coupling with oxygen vacancies. Studying model WO₃ heterostructures during ionic liquid gating with a combination of in situ X-ray scattering and spectroscopy, it is shown that large changes in electronic properties can arise due to the increased flexibility of the octahedral network at high vacancy concentrations. The results describe a generic framework for the construction of dynamic systems and devices with an array of field-tunable properties.

Interface Engineering and Emergent Phenomena in Oxide Heterostructures

Complex oxide interfaces have mesmerized the scientific community in the last decade due to the possibility of creating tunable novel multifunctionalities, which are possible owing to the strong interaction among charge, spin, orbital, and structural degrees of freedom. Artificial interfacial modifications, which include defects, formal polarization, structural symmetry breaking, and interlayer interaction, have led to novel properties in various complex oxide heterostructures. These emergent phenomena not only serve as a platform for investigating strong electronic correlations in low-dimensional systems but also provide potentials for exploring next-generation electronic devices with high functionality. Herein, some recently developed strategies in engineering functional oxide interfaces and their emergent properties are reviewed.

Effect of (1 1 1)-oriented strain on the structure and magnetic properties of La0.7Sr0.3MnO3 thin films

Using strain, i.e. subtle changes in lattice constant in a thin film induced by the underlying substrate, opens up intriguing new ways to control material properties. We present a study of the effects of strain on structural and ferromagnetic properties of (1 1 1)_pc-oriented La_0.7Sr_0.3MnO₃ epitaxial thin films grown on NdGaO₃, SrTiO₃, and DyScO₃ substrates. (The subscript pc denotes the pseudo-cubic symmetry.) The results show that La_0.7Sr_0.3MnO₃ assumes a monoclinic unit cell on NdGaO₃ and DyScO₃ and a rhombohedral unit cell on SrTiO₃. For La_0.7Sr_0.3MnO₃ on NdGaO₃ and DyScO₃ a uniaxial magnetic anisotropy is found, while La_0.7Sr_0.3MnO₃ on SrTiO₃ is magnetically isotropic. The Néel model is used to explain the anisotropy of the thin films on NdGaO₃ and SrTiO₃, however, for La_0.7Sr_0.3MnO₃ on DyScO₃ the effect of octahedral rotations needs to be included through the single ion model. Through examination of the Curie temperature of the strained films we suggest that (1 1 1)-strain has a different effect on the Jahn-Teller splitting of e _g and t _2g electron levels than what is seen in (0 0 1)_pc-oriented La_0.7Sr_0.3MnO₃ thin films.

Structural "δ Doping" to Control Local Magnetization in Isovalent Oxide Heterostructures

Modulation and δ-doping strategies, in which atomically thin layers of charged dopants are precisely deposited within a heterostructure, have played enabling roles in the discovery of new physical behavior in electronic materials. Here, we demonstrate a purely structural "δ-doping" strategy in complex oxide heterostructures, in which atomically thin manganite layers are inserted into an isovalent manganite host, thereby modifying the local rotations of corner-connected MnO_{6} octahedra. Combining scanning transmission electron microscopy, polarized neutron reflectometry, and density functional theory, we reveal how local magnetic exchange interactions are enhanced within the spatially confined regions of suppressed octahedral rotations. The combined experimental and theoretical results illustrate the potential to utilize noncharge-based approaches to "doping" in order to enhance or suppress functional properties within spatially confined regions of oxide heterostructures.

Melting of Oxygen Vacancy Order at Oxide-Heterostructure Interface

Modifications in oxygen coordination environments in heterostructures consisting of dissimilar oxides often emerge and lead to unusual properties of the constituent materials. Although lots of attention has been paid to slight modifications in the rigid oxygen octahedra of perovskite-based heterointerfaces, revealing the modification behaviors of the oxygen coordination environments in the heterostructures containing oxides with oxygen vacancies have been challenging. Here, we show that a significant modification in the oxygen coordination environments-melting of oxygen vacancy order-is induced at the heterointerface between SrFeO_2.5 (SFO) and DyScO₃ (DSO). When an oxygen-deficient perovskite (brownmillerite structure) SrFeO_2.5 film grows epitaxially on a perovskite DyScO₃ substrate, both FeO₆ octahedra and FeO₄ tetrahedra in the (101)-oriented SrFeO_2.5 thin film connect to ScO₆ octahedra in DyScO₃. As a consequence of accommodating a structural mismatch, the alternately ordered arrangement of oxygen vacancies is significantly disturbed and reconstructed in the 2 nm thick heterointerface region. The stabilized heterointerface structure consists of Fe³⁺ octahedra with an oxygen vacancy disorder. The melting of the oxygen vacancy order, which in bulk SrFeO_2.5 occurs at 1103 K, is induced at the present heterointerface at ambient temperatures.

Interface-induced multiferroism by design in complex oxide superlattices

Interfaces between materials present unique opportunities for the discovery of intriguing quantum phenomena. Here, we explore the possibility that, in the case of superlattices, if one of the layers is made ultrathin, unexpected properties can be induced between the two bracketing interfaces. We pursue this objective by combining advanced growth and characterization techniques with theoretical calculations. Using prototype La_2/3Sr_1/3MnO₃ (LSMO)/BaTiO₃ (BTO) superlattices, we observe a structural evolution in the LSMO layers as a function of thickness. Atomic-resolution EM and spectroscopy reveal an unusual polar structure phase in ultrathin LSMO at a critical thickness caused by interfacing with the adjacent BTO layers, which is confirmed by first principles calculations. Most important is the fact that this polar phase is accompanied by reemergent ferromagnetism, making this system a potential candidate for ultrathin ferroelectrics with ferromagnetic ordering. Monte Carlo simulations illustrate the important role of spin-lattice coupling in LSMO. These results open up a conceptually intriguing recipe for developing functional ultrathin materials via interface-induced spin-lattice coupling.

Conductivity and Local Structure of LaNiO3 Thin Films

A marked conductivity enhancement is reported in 6-11 unit cell LaNiO₃ thin films. A maximal conductivity is also observed in ab initio calculations for films of the same thickness. In agreement with results from state of the art scanning transmission electron microscopy, the calculations also reveal a differentiated film structure comprising characteristic surface, interior, and heterointerface structures. Based on this observation, a three-element parallel conductor model is considered and leads to the conclusion that the conductivity enhancement for films of 6-11 unit cells, stems from the onset of intercompetition between the three local structures in the film depth.

Interface-Induced Phenomena in Magnetism

This article reviews static and dynamic interfacial effects in magnetism, focusing on interfacially-driven magnetic effects and phenomena associated with spin-orbit coupling and intrinsic symmetry breaking at interfaces. It provides a historical background and literature survey, but focuses on recent progress, identifying the most exciting new scientific results and pointing to promising future research directions. It starts with an introduction and overview of how basic magnetic properties are affected by interfaces, then turns to a discussion of charge and spin transport through and near interfaces and how these can be used to control the properties of the magnetic layer. Important concepts include spin accumulation, spin currents, spin transfer torque, and spin pumping. An overview is provided to the current state of knowledge and existing review literature on interfacial effects such as exchange bias, exchange spring magnets, spin Hall effect, oxide heterostructures, and topological insulators. The article highlights recent discoveries of interface-induced magnetism and non-collinear spin textures, non-linear dynamics including spin torque transfer and magnetization reversal induced by interfaces, and interfacial effects in ultrafast magnetization processes.

Interfacial Symmetry Control of Emergent Ferromagnetism at the Nanoscale

The emergence of complex new ground states at interfaces has been identified as one of the most promising routes to highly tunable nanoscale materials. Despite recent progress, isolating and controlling the underlying mechanisms behind these emergent properties remains among the most challenging materials physics problems to date. In particular, generating ferromagnetism localized at the interface of two nonferromagnetic materials is of fundamental and technological interest. Moreover, the ability to turn the ferromagnetism on and off would shed light on the origin of such emergent phenomena and is promising for spintronic applications. We demonstrate that ferromagnetism confined within one unit cell at the interface of CaRuO3 and CaMnO3 can be switched on and off by changing the symmetry of the oxygen octahedra connectivity at the boundary. Interfaces that are symmetry-matched across the boundary exhibit interfacial CaMnO3 ferromagnetism while the ferromagnetism at symmetry-mismatched interfaces is suppressed. We attribute the suppression of ferromagnetic order to a reduction in charge transfer at symmetry-mismatched interfaces, where frustrated bonding weakens the orbital overlap. Thus, interfacial symmetry is a new route to control emergent ferromagnetism in materials such as CaMnO3 that exhibit antiferromagnetism in bulk form.

Epitaxial-strain-induced polar-to-nonpolar transitions in layered oxides

Epitaxial strain can induce collective phenomena and new functionalities in complex oxide thin films. Strong coupling between strain and polar lattice modes can stabilize new ferroelectric phases from nonpolar dielectrics or enhance electric polarizations and Curie temperatures. Recently, strain has also been exploited to induce novel metal-insulator transitions and magnetic reconstructions through its coupling to nonpolar modes, including rotations of BO6 transition-metal octahedra. Although large strains are thought to induce ferroelectricity, here we demonstrate a polar-to-nonpolar transition in (001) films of layered A3B2O7 hybrid-improper ferroelectrics with experimentally accessible biaxial strains. We show the origin of the transition originates from the interplay of trilinear-related lattice mode interactions active in the layered oxides, and those interactions are directly strain tunable. Our results call for a careful re-examination of the role of strain-polarization coupling in ferroelectric films with nontrivial anharmonicities and offer a route to search for new functionalities in layered oxides.

Tuning magnetic anisotropy by interfacially engineering the oxygen coordination environment in a transition metal oxide

Strong correlations between electrons, spins and lattices--stemming from strong hybridization between transition metal d and oxygen p orbitals--are responsible for the functional properties of transition metal oxides. Artificial oxide heterostructures with chemically abrupt interfaces provide a platform for engineering bonding geometries that lead to emergent phenomena. Here we demonstrate the control of the oxygen coordination environment of the perovskite, SrRuO3, by heterostructuring it with Ca0.5Sr0.5TiO3 (0-4 monolayers thick) grown on a GdScO3 substrate. We found that a Ru-O-Ti bond angle of the SrRuO3 /Ca0.5Sr0.5TiO3 interface can be engineered by layer-by-layer control of the Ca0.5Sr0.5TiO3 layer thickness, and that the engineered Ru-O-Ti bond angle not only stabilizes a Ru-O-Ru bond angle never seen in bulk SrRuO3, but also tunes the magnetic anisotropy in the entire SrRuO3 layer. The results demonstrate that interface engineering of the oxygen coordination environment allows one to control additional degrees of freedom in functional oxide heterostructures.

Towards 3D Mapping of BO6 Octahedron Rotations at Perovskite Heterointerfaces, Unit Cell by Unit Cell

The rich functionalities in the ABO3 perovskite oxides originate, at least in part, from the ability of the corner-connected BO6 octahedral network to host a large variety of cations through distortions and rotations. Characterizing these rotations, which have significant impact on both fundamental aspects of materials behavior and possible applications, remains a major challenge at heterointerfaces. In this work, we have developed a unique method to investigate BO6 rotation patterns in complex oxides ABO3 with unit cell resolution at heterointerfaces, where novel properties often emerge. Our method involves column shape analysis in ABF-STEM images of the ABO3 heterointerfaces taken in specific orientations. The rotating phase of BO6 octahedra can be identified for all three spatial dimensions without the need of case-by-case simulation. In several common rotation systems, quantitative measurements of all three rotation angles are now possible. Using this method, we examined interfaces between perovskites with distinct tilt systems as well as interfaces between tilted and untilted perovskites, identifying an unusual coupling behavior at the CaTiO3/LSAT interface. We believe this method will significantly improve our knowledge of complex oxide heterointerfaces.