Control of octahedral tilts and magnetic properties of perovskite oxide heterostructures by substrate symmetry

Reformation of La0.7Sr0.3MnO3 properties by using ZnO in La0.7Sr0.3MnO3-ZnO heterostructures grown on (001) oriented Si

Study of the available density of states (DOS) close-to-zero bias for conduction in strongly correlated electron systems, such as half-metallic La0.7Sr0.3MnO3 (LSMO) and its heterostructures, is important for fundamental and application reasons. As the DOS is proportional to the differential conductance (dI/dV), the dI/dV of a 120 Å LSMO film and its reformation in LSMO/ZnO heterostructures was investigated for different ZnO thicknesses. Unlike in conventional metals, the dI/dV of LSMO exhibits a power-law dependent zero-bias anomaly, i.e., dI/dV ∝ Vm (m ∼ 1) near zero bias in the ferromagnetic metallic state at 10 K. The growth of ZnO on LSMO reforms the linear dI/dVvs. V of LSMO near zero bias to non-linear. The exponent 'm' becomes ∼0.5 for a higher ZnO thickness, revealing increased electron-electron interactions and suppression of Kondo-like, double and superexchange interactions, which are responsible for the depression of the DOS of LSMO near zero bias. In a magnetically disordered state, i.e., around the Curie temperature, ZnO reforms the linear V-shaped dI/dV vs. V of LSMO to parabolic U-shaped dI/dVvs.V and controls the electron concentrations in the t2g-orbitals of Mn realized from the DOS simulations. Additionally, ZnO introduces a peak in the dI/dV vs. V due to Fowler-Nordheim tunnelling, and the peak voltage can be tuned by varying the ZnO thickness or temperature from 300 K to 360 K. Such functions of ZnO yield major perspectives for novel applications in thin-film-based devices.

Significance of Engineering the MnO6 Octahedral Units to Promote the Oxygen Reduction Reaction of Perovskite Oxides

The electronic structure and geometric configuration of catalysts play a crucial role to design novel perovskite-type catalysts for oxygen reduction reaction (ORR). Nowadays, many studies are more concerned with the influence of electronic structure and ignore the geometric effect, which plays a nonnegligible role in enhancing catalytic performances. Herein, this work regulates the MnO6 octahedral tilting degree of LaMnO3 by modulating the concentration of Y3+ , excluding the electronic effect from the valence state of manganese. Plotting the MnO6 octahedral tilting degree as a function of concentration of Y3+ produces a volcano-shaped plot. The octahedral tilting can reduce the Mn-O covalency, generating more highly active Mn3+ and oxygen vacancies during ORR process. The specific activity has a positive correlation with octahedral tilting degree. Meanwhile, the octahedral tilting stabilizes Mn-O interactions during ORR process and promote stability. Based on experimental results and DFT calculations, octahedral tilting alters the rate-determining step (RDS) and decrease the energy barrier. Subsequent extended experiment confirms that octahedral tilting is the key factor to affect the catalytic performances.

Pyrovskite: A software package for the high-throughput construction, analysis, and featurization of two- and three-dimensional perovskite systems

The increased computational and experimental interest in perovskite systems comprising novel phases and reduced dimensionality has greatly expanded the search space for this class of materials. In similar fields, unified frameworks exist for the procedural generation and subsequent analysis of these complex condensed matter systems. Given the relatively recent rise in popularity of these novel perovskite phases, such a framework is yet to be created. In this work, we introduce Pyrovskite, an open source software package, to aid in both the high-throughput and fine-grained generation, simulation, and subsequent analysis of this expanded family of perovskite systems. Additionally, we introduce a new descriptor for octahedral distortions in systems, including, but not limited to, perovskites. This descriptor quantifies diagonal displacements of the B-site cation in a BX6 octahedral coordination environment, which has been shown to contribute to increased Rashba-Dresselhaus splitting in perovskite systems.

Atomic scale interfacial magnetism and origin of metal-insulator transition in (LaNiO[Formula: see text])[Formula: see text]/(CaMnO[Formula: see text])[Formula: see text] superlattices: a first principles study

Interfacial magnetism and metal-insulator transition at LaNiO[Formula: see text]-based oxide interfaces have triggered intense research efforts, because of the possible implications in future heterostructure device design and engineering. Experimental observation lack in some points a support from an atomistic view. In an effort to fill such gap, we hereby investigate the structural, electronic, and magnetic properties of (LaNiO[Formula: see text])[Formula: see text]/(CaMnO[Formula: see text])[Formula: see text] superlattices with varying LaNiO[Formula: see text] thickness (n) using density functional theory including a Hubbard-type effective on-site Coulomb term. We successfully capture and explain the metal-insulator transition and interfacial magnetic properties, such as magnetic alignments and induced Ni magnetic moments which were recently observed experimentally in nickelate-based heterostructures. In the superlattices modeled in our study, an insulating state is found for n=1 and a metallic character for n=2, 4, with major contribution from Ni and Mn 3d states. The insulating character originates from the disorder effect induced by sudden environment change for the octahedra at the interface, and associated to localized electronic states; on the other hand, for larger n, less localized interfacial states and increased polarity of the LaNiO[Formula: see text] layers contribute to metallicity. We discuss how the interplay between double and super-exchange interaction via complex structural and charge redistributions results in interfacial magnetism. While (LaNiO[Formula: see text])[Formula: see text]/(CaMnO[Formula: see text])[Formula: see text] superlattices are chosen as prototype and for their experimental feasibility, our approach is generally applicable to understand the intricate roles of interfacial states and exchange mechanism between magnetic ions towards the overall response of a magnetic interface or superlattice.

Study on the decisive factor for metal-insulator transitions in a LaVO3 Mott-Hubbard insulator

The decisive physical parameters on electrical conduction in a LaVO₃ Mott-Hubbard system are systematically investigated by analyzing pure, Ca-, and Sr-doped samples. The Rietveld refinement of the X-ray diffraction data indicates that a drastic change occurs along the c-axis to reduce the octahedral tilt thereby relaxing the distortion for the doped compounds, in contrast to an insignificant change in the in-plane distortion. From electrical, optical, and photoemission measurements, both Ca and Sr-doping in LaVO₃ induce insulator to metal transitions under a similar hole carrier concentration as suppressing the Mott-gap excitation. Fitting results on temperature-dependent resistivity based on various conduction models indicate that the most localized conduction behavior takes place for the highly distorted pure LaVO₃, while disordered Fermi liquid behavior starts to appear for moderately distorted Ca-doped LaVO₃. The least distorted Sr-doped LaVO₃ exhibits fully delocalized conduction governed by a non-Fermi-liquid-like behavior in the whole temperature range. Our analysis indicates that the difference in the transport mechanism arises from the differing degree of hybridization of the V 3d and O 2p states in the pure and doped systems, strongly associated with the structural distortion.

Chiral assemblies of pinwheel superlattices on substrates

The unique topology and physics of chiral superlattices make their self-assembly from nanoparticles highly sought after yet challenging in regard to (meta)materials^1-3. Here we show that tetrahedral gold nanoparticles can transform from a perovskite-like, low-density phase with corner-to-corner connections into pinwheel assemblies with corner-to-edge connections and denser packing. Whereas corner-sharing assemblies are achiral, pinwheel superlattices become strongly mirror asymmetric on solid substrates as demonstrated by chirality measures. Liquid-phase transmission electron microscopy and computational models show that van der Waals and electrostatic interactions between nanoparticles control thermodynamic equilibrium. Variable corner-to-edge connections among tetrahedra enable fine-tuning of chirality. The domains of the bilayer superlattices show strong chiroptical activity as identified by photon-induced near-field electron microscopy and finite-difference time-domain simulations. The simplicity and versatility of substrate-supported chiral superlattices facilitate the manufacture of metastructured coatings with unusual optical, mechanical and electronic characteristics.

Interfacial charge transfer induced antiferromagnetic metals and magnetic phase transition in (CrO2)m/(TaO2)nsuperlattices

As a class of remarkable spintronic materials, intrinsic antiferromagnetic (AFM) metals are rare. The exploration and investigation of AFM metals are still in its infancy. Based on first-principles calculations, the interface-induced magnetic phenomena in the (CrO₂)_m/(TaO₂)_nsuperlattices are investigated, and a new series of AFM metals is predicted. Under different ratios ofm:nwith varying valence states of Cr, the (CrO₂)_m/(TaO₂)_nsuperlattices exhibit three different phases, including the AFM metal, the AFM semiconductor, and the ferromagnetic (FM) metal. In the AFM semiconducting phases, theintra-CrO₂-monolayer magnetic exchange interaction is systematically discussed, corresponding tom = 1 orm = 2. Both the localization of the Cr 3 dorbitals and the crystal-field splitting are crucial for magnetic ordering in super-exchange interactions. Based on the analyses of the AFM semiconducting phases withm = 1 andm = 2, the mechanisms of AFM metallic phases with radios ofm:n<1/2and1/2<m:n<1/1are discussed in detail. Additionally, the AFM metallic superlattices can be tuned into a FM metallic phase by applying strain in thec-direction, such as a compression of 7% in the (CrO₂)₁/(TaO₂)₃superlattice, and a tensile strain of 7% in the (CrO₂)₂/(TaO₂)₃superlattice. The phase diagram of the (CrO₂)_m/(TaO₂)_nsuperlattices is obtained as a function of the layer thickness. This work provides new insights about realizing and manipulating AFM metals in artificial superlattices or heterostructures in experiments.

Topological Hall effect in SrRuO3thin films and heterostructures

Transition metal oxides hold a wide spectrum of fascinating properties endowed by the strong electron correlations. In 4dand 5doxides, exotic phases can be realized with the involvement of strong spin-orbit coupling (SOC), such as unconventional magnetism and topological superconductivity. Recently, topological Hall effects (THEs) and magnetic skyrmions have been uncovered in SrRuO₃thin films and heterostructures, where the presence of SOC and inversion symmetry breaking at the interface are believed to play a key role. Realization of magnetic skyrmions in oxides not only offers a platform to study topological physics with correlated electrons, but also opens up new possibilities for magnetic oxides using in the low-power spintronic devices. In this review, we discuss recent observations of THE and skyrmions in the SRO film interfaced with various materials, with a focus on the electric tuning of THE. We conclude with a discussion on the directions of future research in this field.

Emergent interface vibrational structure of oxide superlattices

As the length scales of materials decrease, the heterogeneities associated with interfaces become almost as important as the surrounding materials. This has led to extensive studies of emergent electronic and magnetic interface properties in superlattices^1–9. However, the interfacial vibrations that affect the phonon-mediated properties, such as thermal conductivity^10,11, are measured using macroscopic techniques that lack spatial resolution. Although it is accepted that intrinsic phonons change near boundaries^12,13, the physical mechanisms and length scales through which interfacial effects influence materials remain unclear. Here we demonstrate the localized vibrational response of interfaces in strontium titanate–calcium titanate superlattices by combining advanced scanning transmission electron microscopy imaging and spectroscopy, density functional theory calculations and ultrafast optical spectroscopy. Structurally diffuse interfaces that bridge the bounding materials are observed and this local structure creates phonon modes that determine the global response of the superlattice once the spacing of the interfaces approaches the phonon spatial extent. Our results provide direct visualization of the progression of the local atomic structure and interface vibrations as they come to determine the vibrational response of an entire superlattice. Direct observation of such local atomic and vibrational phenomena demonstrates that their spatial extent needs to be quantified to understand macroscopic behaviour. Tailoring interfaces, and knowing their local vibrational response, provides a means of pursuing designer solids with emergent infrared and thermal responses.

The vibrational states emerging at the interface in oxide superlattices are characterized theoretically and at atomic resolution, showing the impact of material length scales on structure and vibrational response.

Symmetry mismatch controlled ferroelastic domain ordering and the functional properties of manganite films on cubic miscut substrates

We have studied the magnetotransport properties and strain release mechanisms in ferroelastic La0.9Sr0.1MnO3 (LSMO) epitaxial thin films on SrTiO3 (STO)(001) substrates with different miscut angles. The substrate miscut angle plays a critical role in releasing shear strain and has a huge impact on the properties of the films. The strain relaxes by monoclinic distortion for films on low miscut substrates and for higher miscut substrates, the strain relaxation causes the formation of periodic twin domains with larger periodicities. We observe that the Curie temperature (TC) decreases systematically, and magnetoresistance (MR) increases with increasing the miscut angle. Such changes in the magnetic and transport properties could be due to the increased density of phase boundaries (PBs) with the increase of miscut angle. This work provides a way to tailor film microstructures and subsequent functional properties of other complex oxide films on miscut substrates with symmetry mismatch.

High-sensitivity of initial SrO growth on the residual resistivity in epitaxial thin films of SrRuO[Formula: see text] on SrTiO[Formula: see text] (001)

The growth of SrRuO[Formula: see text] (SRO) thin film with high-crystallinity and low residual resistivity (RR) is essential to explore its intrinsic properties. Here, utilizing the adsorption-controlled growth technique, the growth condition of initial SrO layer on TiO[Formula: see text]-terminated SrTiO[Formula: see text] (STO) (001) substrate was found to be crucial for achieving a low RR in the resulting SRO film grown afterward. The optimized initial SrO layer shows a c(2 [Formula: see text] 2) superstructure that was characterized by electron diffraction, and a series of SRO films with different thicknesses (ts) were then grown. The resulting SRO films exhibit excellent crystallinity with orthorhombic-phase down to [Formula: see text] 4.3 nm, which was confirmed by high resolution X-ray measurements. From X-ray azimuthal scan across SRO orthorhombic (02 ± 1) reflections, we uncover four structural domains with a dominant domain of orthorhombic SRO [001] along cubic STO [010] direction. The dominant domain population depends on t, STO miscut angle ([Formula: see text]), and miscut direction ([Formula: see text]), giving a volume fraction of about 92 [Formula: see text] for [Formula: see text] 26.6 nm and [Formula: see text] (0.14[Formula: see text], 5[Formula: see text]). On the other hand, metallic and ferromagnetic properties were well preserved down to t [Formula: see text] 1.2 nm. Residual resistivity ratio (RRR = [Formula: see text]/[Formula: see text]) reduces from 77.1 for t [Formula: see text] 28.5 nm to 2.5 for t [Formula: see text] 1.2 nm, while [Formula: see text] increases from 2.5 [Formula: see text]cm for t [Formula: see text] 28.5 nm to 131.0 [Formula: see text]cm for t [Formula: see text] 1.2 nm. The ferromagnetic onset temperature ([Formula: see text]) of around 151 K remains nearly unchanged down to t [Formula: see text] 9.0 nm and decreases to 90 K for t [Formula: see text] 1.2 nm. Our finding thus provides a practical guideline to achieve high crystallinity and low RR in ultra-thin SRO films by simply adjusting the growth of initial SrO layer.

Emergent Magnetic Phenomenon with Unconventional Structure in Epitaxial Manganate Thin Films

A variety of emergent phenomena are enabled by interface engineering in the complex oxides heterostructures. While extensive attention is attracted to LaMnO3 (LMO) thin films for observing the control of functionalities at its interface with substrate, the nature of the magnetic phases in the thin film is, however, controversial. Here, it is reported that the ferromagnetism in two and five unit cells thick LMO films epitaxially deposited on (001)-SrTiO3 substrates, a ferromagnetic/ferromagnetic coupling in eight and ten unit cells ones, and a striking ferromagnetic/antiferromagnetic pinning effect with apparent positive exchange bias in 15 and 20 unit cells ones are observed. This novel phenomenon in both 15 and 20 unit cells films indicates a coexistence of three magnetic orderings in a single LMO film. The high-resolution scanning transmission electron microscopy suggests a P21/n to Pbnm symmetry transition from interface to surface, with the spatial stratification of MnO6 octahedral morphology, corresponding to different magnetic orderings. These results can shed some new lights on manipulating the functionality of oxides by interface engineering.

Exploring order parameters and dynamic processes in disordered systems via variational autoencoders

We suggest and implement an approach for the bottom-up description of systems undergoing large-scale structural changes and chemical transformations from dynamic atomically resolved imaging data, where only partial or uncertain data on atomic positions are available. This approach is predicated on the synergy of two concepts, the parsimony of physical descriptors and general rotational invariance of noncrystalline solids, and is implemented using a rotationally invariant extension of the variational autoencoder applied to semantically segmented atom-resolved data seeking the most effective reduced representation for the system that still contains the maximum amount of original information. This approach allowed us to explore the dynamic evolution of electron beam-induced processes in a silicon-doped graphene system, but it can be also applied for a much broader range of atomic scale and mesoscopic phenomena to introduce the bottom-up order parameters and explore their dynamics with time and in response to external stimuli.

Progress and Perspectives on Aurivillius-Type Layered Ferroelectric Oxides in Binary Bi4Ti3O12-BiFeO3 System for Multifunctional Applications

Driven by potentially photo-electro-magnetic functionality, Bi-containing Aurivillius-type oxides of binary Bi4Ti3O12-BiFeO3 system with a general formula of Bin+1Fen−3Ti3O3n+3, typically in a naturally layered perovskite-related structure, have attracted increasing research interest, especially in the last twenty years. Benefiting from highly structural tolerance and simultaneous electric dipole and magnetic ordering at room temperature, these Aurivillius-phase oxides as potentially single-phase and room-temperature multiferroic materials can accommodate many different cations and exhibit a rich spectrum of properties. In this review, firstly, we discussed the characteristics of Aurivillius-phase layered structure and recent progress in the field of synthesis of such materials with various architectures. Secondly, we summarized recent strategies to improve ferroelectric and magnetic properties, consisting of chemical modification, interface engineering, oxyhalide derivatives and morphology controlling. Thirdly, we highlighted some research hotspots on magnetoelectric effect, catalytic activity, microwave absorption, and photovoltaic effect for promising applications. Finally, we provided an updated overview on the understanding and also highlighting of the existing issues that hinder further development of the multifunctional Bin+1Fen−3Ti3O3n+3 materials.

Engineered Layer-Stacked Interfaces Inside Aurivillius-Type Layered Oxides Enables Superior Ferroelectric Property

Layer engineering with different layer numbers inside Aurivillius-type layered structure, similar to interface engineering in heterojunctions or superlattices, can give rise to excellent physical properties due to the correlated layer-stacked interfaces of two different layer phases with different strain states. In this work, using the solid-state reactions from Aurivillius-type Bi3TiNbO9 (2-layer) and Bi4Ti3O12 (3-layer) ferroelectric powder mixtures, single-phase compound of Bi7Ti4NbO21 with an intergrowth structure of 2-layer and 3-layer perovskite slabs sandwiched between the Bi-O layers was synthesized and the effects of this layer-engineered strategy on the structure, Raman-vibration and ferroelectric properties were systematically investigated. The mostly-ordered intergrowth phase was observed clearly by utilizing X-ray diffraction and advanced electron micro-techniques. Uniformly dispersions and collaborative vibrations of Ti and Nb ions in the layer-engineered Bi7Ti4NbO21 were demonstrated. Remarkably, dielectric and ferroelectric properties were also recorded and an enhanced ferroelectric response was found in the layer-engineered mixed-layer sample with high ferroelectric Curie temperature, compared with the homogeneous 2-layer and 3-layer samples. Analyses of the Raman spectra and atomic structures confirmed that the performance improvement of the layer-engineered sample is intrinsic to the correlated layer-stacked interfaces inside the Aurivillius-type layered oxides, arising from strain-induced lattice distortions at the interfaces.

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.

First-principles calculations of oxygen octahedral distortions in LaAlO3/SrTiO3(001) superlattices

The size, shape and connectivity of oxide octahedra are essential for understanding and controlling the emergent functional properties of ABO₃ perovskites. Using first-principles calculations, we systematically studied the oxygen octahedral rotation and deformation in LaAlO₃/SrTiO₃(001) superlattices. Superlattices with electron- or hole-doped interfaces, or both, are compared. The results showed that there are at least three different types of oxygen octahedral distortions in these superlattices, which is more than what had previously been reported in the literature. We demonstrate that interfacial oxygen octahedral coupling and hole-doping, in addition to epitaxial strain, are the key factors underlying the formation of multiple types of oxygen octahedral rotations in these systems. We confirm that oxygen octahedral rotations and deformations play an essential role in insulator-metal transitions. Furthermore, octahedral distortion leads to ferroelectricity like dipole formation with the polarization vector always pointing to the positively charged interfaces.

Polarization Screening Mechanisms at La0.7Sr0.3MnO3-PbTiO3 Interfaces

The structural, electronic, and magnetic properties of interfaces between epitaxial La_0.7Sr_0.3MnO₃ and PbTiO₃ have been explored via atomic resolution transmission electron microscopy of a functional multiferroic tunnel junction. Measurements of the polar displacements and octahedral tilting show the competition between the two distortions at the interface and demonstrate strong dependence on the polarization orientation. The density functional theory provides information on the electronic and magnetic properties, where the interface termination plays a crucial role in the screening mechanisms.

Effects of the Heterointerface on the Growth Characteristics of a Brownmillerite SrFeO2.5 Thin Film Grown on SrRuO3 and SrTiO3 Perovskites

Manipulation of the heterointerfacial structure and/or chemistry of transition metal oxides is of great interest for the development of novel properties. However, few studies have focused on heterointerfacial effects on the growth characteristics of oxide thin films, although such interfacial engineering is crucial to determine the growth dynamics and physical properties of oxide heterostructures. Herein, we show that heterointerfacial effects play key roles in determining the growth process of oxide thin films by overcoming the simple epitaxial strain energy. Brownmillerite (SrFeO_2.5; BM-SFO) thin films are epitaxially grown along the b-axis on both SrTiO₃(001) and SrRuO₃/SrTiO₃(001) substrates, whereas growth along the a-axis is expected from conventional epitaxial strain effects originating from lattice mismatch with the substrates. Scanning transmission electron microscopy measurements and first principles calculations reveal that these peculiar growth characteristics of BM-SFO thin films originate from the heterointerfacial effects governed by their distinct interfacial structures. These include octahedral connectivity between dissimilar oxides containing different chemical species and a peculiar transition layer for BM-SFO/SrRuO₃/SrTiO₃(001) and BM-SFO/SrTiO₃(001) heterostructures, respectively. These effects enable subtle control of the growth process of oxide thin films and could facilitate the fabrication of novel functional devices.

Interfacial Control of Ferromagnetism in Ultrathin SrRuO3 Films Sandwiched between Ferroelectric BaTiO3 Layers

Interfaces between materials provide an intellectually rich arena for fundamental scientific discovery and device design. However, the frustration of magnetization and conductivity of perovskite oxide films under reduced dimensionality is detrimental to their device performance, preventing their active low-dimensional application. Herein, by inserting the ultrathin 4d ferromagnetic SrRuO₃ layer between ferroelectric BaTiO₃ layers to form a sandwich heterostructure, we observe enhanced physical properties in ultrathin SrRuO₃ films, including longitudinal conductivity, Curie temperature, and saturated magnetic moment. Especially, the saturated magnetization can be enhanced to ∼3.12 μ_B/Ru in ultrathin BaTiO₃/SrRuO₃/BaTiO₃ trilayers, which is beyond the theoretical limit of bulk value (2 μ_B/Ru). This observation is attributed to the synergistic ferroelectric proximity effect (SFPE) at upper and lower BaTiO₃/SrRuO₃ heterointerfaces, as revealed by the high-resolution lattice structure analysis. This SFPE in dual-ferroelectric interface cooperatively induces ferroelectric-like lattice distortions in RuO₆ oxygen octahedra and subsequent spin-state crossover in SrRuO₃, which in turn accounts for the observed enhanced magnetization. Besides the fundamental significance of interface-induced spin-lattice coupling, our findings also provide a viable route to the electrical control of magnetic ordering, taking a step toward low-power applications in all-oxide spintronics.

Correlation between structural phase transition and surface chemical properties of thin film SrRuO3/SrTiO3 (001)

The correlation between the structural phase transition (SPT) and oxygen vacancy in SrRuO₃ (SRO) thin films was investigated by in situ X-ray diffraction (XRD) and ambient pressure X-ray photoelectron spectroscopy (AP-XPS). In situ XRD shows that the SPT occurs from a monoclinic SRO phase to a tetragonal SRO phase near ∼200 °C, regardless of the pressure environment. On the other hand, significant core level shifts in both the Ru and Sr photoemission spectra are found under ultrahigh vacuum, but not under the oxygen pressure environment. The directions and behavior of the core level shift of Ru and Sr are attributed to the formation of oxygen vacancy across the SPT temperature of SRO. The analysis of in situ XRD and AP-XPS results provides an evidence for the formation of metastable surface oxide possibly due to the migration of internal oxygen atoms across the SPT temperature, indicating the close relationship between oxygen vacancy and SPT in SRO thin films.

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.

Designing Optimal Perovskite Structure for High Ionic Conduction

Solid-oxide fuel/electrolyzer cells are limited by a dearth of electrolyte materials with low ohmic loss and an incomplete understanding of the structure-property relationships that would enable the rational design of better materials. Here, using epitaxial thin-film growth, synchrotron radiation, impedance spectroscopy, and density-functional theory, the impact of structural parameters (i.e., unit-cell volume and octahedral rotations) on ionic conductivity is delineated in La_0.9 Sr_0.1 Ga_0.95 Mg_0.05 O_{3- δ} . As compared to the zero-strain state, compressive strain reduces the unit-cell volume while maintaining large octahedral rotations, resulting in a strong reduction of ionic conductivity, while tensile strain increases the unit-cell volume while quenching octahedral rotations, resulting in a negligible effect on the ionic conductivity. Calculations reveal that larger unit-cell volumes and octahedral rotations decrease migration barriers and create low-energy migration pathways, respectively. The desired combination of large unit-cell volume and octahedral rotations is normally contraindicated, but through the creation of superlattice structures both expanded unit-cell volume and large octahedral rotations are experimentally realized, which result in an enhancement of the ionic conductivity. All told, the potential to tune ionic conductivity with structure alone by a factor of ≈2.5 at around 600 °C is observed, which sheds new light on the rational design of ion-conducting perovskite electrolytes.

Depth-Resolved Modulation of Metal-Oxygen Hybridization and Orbital Polarization across Correlated Oxide Interfaces

Interface-induced modifications of the electronic, magnetic, and lattice degrees of freedom drive an array of novel physical properties in oxide heterostructures. Here, large changes in metal-oxygen band hybridization, as measured in the oxygen ligand hole density, are induced as a result of interfacing two isovalent correlated oxides. Using resonant X-ray reflectivity, a superlattice of SrFeO₃ and CaFeO₃ is shown to exhibit an electronic character that spatially evolves from strongly O-like in SrFeO₃ to strongly Fe-like in CaFeO₃ . This alternating degree of Fe electronic character is correlated with a modulation of an Fe 3d orbital polarization, giving rise to an orbital superstructure. At the SrFeO₃ /CaFeO₃ interfaces, the ligand hole density and orbital polarization reconstruct in a single unit cell of CaFeO₃ , demonstrating how the mismatch in these electronic parameters is accommodated at the interface. These results provide new insight into how the orbital character of electrons is altered by correlated oxide interfaces and lays out a broadly applicable approach for depth-resolving band hybridization.

The growth and improved magnetoelectric response of strain-modified Aurivillius SrBi4.25La0.75Ti4FeO18 thin films

In this study, we grew 5-layered SrBi4.25La0.75Ti4FeO18 (SBLFT) polycrystalline thin films (80-330 nm thick) via pulsed-laser deposition to study their ferroelectric and magnetoelectric response. Structural/microstructural analysis confirmed the formation of orthorhombic SBLFT with good crystallinity and randomly oriented Aurivillius phases. Detailed scanning transmission electron microscopy analysis of 120 nm film revealed a predominantly five-layered structure with the coexistence of four-layer stacking. Such stacking defects are found to be pertinent to the high structural flexibility of Bi-rich Aurivillius phases, alleviated by lattice strain. Raman spectral features at ambient temperatures depict the signature of the orthorhombic-tetragonal phase transition. SBLFT films have a strong ferroelectric nature (remanent polarization 2Pr of 35 μC cm-2) with a fatigue endurance up to 1010 cycles and strongly improved, switchable magnetization as opposed to its antiferromagnetic bulk counterpart. The scaling behavior of dynamic hysteresis reveals that ferroelectric domain reversal has good stability and low energy consumption. We observed the presence of SBLFT nanoregions (1-5 nm), distributed across the film, with Bi and Fe-rich compositions and oxygen vacancies that contribute to the weak ferromagnetic behavior mediated by the Dzyaloshinskii-Moriya interactions. Subtle changes in the structural strain and lattice distortions of thin films with varied thicknesses led to distinct ferroic properties. Stronger ferroelectric polarization of 80 nm and 120 nm films compared to that of thicker ones can be due to structural strain and the possible rearrangement of BO6 octahedra. The observation of the improved magnetoelectric coefficient of 50 mV cm-1 Oe-1 for 120 nm film, as compared to that of several Aurivillius oxides, indicates that the structural strain modification in SBLFT is beneficial for the fatigue-free magnetic field switching of ferroelectric polarization. The structural strain of the unit cell as well as the presence of Bi- and ferromagnetic Fe-rich nanoregions was found to be responsible for the improved multiferroic behaviour of the SBLFT films.

Tuning the magnetism of two-dimensional hematene by ferroelectric polarization

Magnetism in two-dimensional (2D) materials, that is, a 2D version of the magnetism of three-dimensional bulk materials, and the associated novel physics have recently been the focus of many spintronics researchers. Here we investigate the manipulation of 2D magnetism at the interfaces of ferromagnetic/ferroelectric hematene/BaTiO₃(001) heterostructures (HSs) fabricated via a precisely chosen sequence. By introducing four types of interfaces of 2D hematene and three-dimensional BaTiO₃ that induce different oxygen environments, the control of magnetism is directly demonstrated from first-principles. An obvious 2D electron gas originates from the Fe-3d and O-2p hybridization; the electron gas is sensitive to the interfacial atomic displacements. Robust control of both the direction and magnitude of the net magnetization has been realized for an Fe/TiO₂ terminated bilayer HS. The electron occupancies of the d_xy and d_xz orbitals and changes to the Fe-O bond play a key role in determining the magnetism of our systems. Our work not only demonstrates the technique's potential for manipulating magnetism in 2D hematene, but also sheds light on the underlying mechanism and the fundamental properties of hematene HSs.

Measuring non-equilibrium dynamics in complex solids with ultrashort X-ray pulses

Strong interactions between electrons give rise to the complexity of quantum materials, which exhibit exotic functional properties and extreme susceptibility to external perturbations. A growing research trend involves the study of these materials away from equilibrium, especially in cases in which the stimulation with optical pulses can coherently enhance cooperative orders. Time-resolved X-ray probes are integral to this type of research, as they can be used to track atomic and electronic structures as they evolve on ultrafast timescales. Here, we review a series of recent experiments where femtosecond X-ray diffraction was used to measure dynamics of complex solids. This article is part of the theme issue 'Measurement of ultrafast electronic and structural dynamics with X-rays'.

Enhanced magnetism in lightly doped manganite heterostructures: strain or stoichiometry?

Lattice mismatch induced epitaxial strain has been widely used to tune functional properties in complex oxide heterostructures. Apart from the epitaxial strain, a large lattice mismatch also produces other effects including modulations in microstructure and stoichiometry. However, it is challenging to distinguish the impact of these effects from the strain contribution to thin film properties. Here, we use La0.9Sr0.1MnO3 (LSMO), a lightly doped manganite close to the vertical phase boundary, as a model system to demonstrate that both epitaxial strain and cation stoichiometry induced by strain relaxation contribute to functionality tuning. The thinner LSMO films are metallic with a greatly enhanced TC which is 97 K higher than the bulk value. Such anomalies in TC and transport cannot be fully explained by the epitaxial strain alone. Detailed microstructure analysis indicates La deficiency in thinner films and twin domain formation in thicker films. Our results have revealed that both epitaxial strain and strain relaxation induced stoichiometry/microstructure modulations contribute to the modified functional properties in lightly doped manganite perovskite thin films.

Robust manipulation of magnetism in LaAO3/BaTiO3 (A = Fe, Mn and Cr) superstructures by ferroelectric polarization

Robust control of magnetism is both fundamentally and practically meaningful and highly desirable, although it remains a big challenge. In this work, perovskite oxide superstructures LaFeO₃/BaTiO₃ (LFO/BTO), LaMnO₃/BaTiO₃ (LMO/BTO) and LaCrO₃/BaTiO₃ (LCO/BTO) (001) are designed to facilitate tuning of magnetism by the electric field from ferroelectric polarization, and are systemically investigated via first-principles calculations. The results show that the magnetic ordering, conductivity and exchange interactions can be controlled simultaneously or individually by the reorientation of the ferroelectric polarization of BTO in these designed superstructures. Self-consistent calculations within the generalized gradient approximation plus on-site Coulomb correction did not produce distinct rotations of oxygen octahedra, but there were obvious changes in bond length between oxygen and the cations. These changes cause tilting of the oxygen octahedra and lead to spin, orbital and bond reconstruction at the interface, which is the structural basis responsible for the manipulation. With the G-type antiferromagnetic (G-AFM) ordering unchanged for both ±P cases, a metal-insulator transition can be observed in the LFO/BTO superstructure, which is controlled by the LFO thin film. The LMO/BTO system has A-type antiferromagnetic (A-AFM) ordering with metallic behavior in the +P case, while it shifts to a half-metallic ferromagnetic ordering when the direction of the polarization is switched. LCO/BTO exhibits C-type antiferromagnetic (C-AFM) and G-AFM orders in the +P and -P cases, respectively. The three purpose-designed superstructures with robust intrinsic magnetoelectric coupling are a particularly interesting model system that can provide guidance for the development of this field for future applications.

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.

An algebraic approach to cooperative rotations in networks of interconnected rigid units

Crystalline solids consisting of three-dimensional networks of interconnected rigid units are ubiquitous amongst functional materials. In many cases, application-critical properties are sensitive to rigid-unit rotations at low temperature, high pressure or specific stoichiometry. The shared atoms that connect rigid units impose severe constraints on any rotational degrees of freedom, which must then be cooperative throughout the entire network. Successful efforts to identify cooperative-rotational rigid-unit modes (RUMs) in crystals have employed split-atom harmonic potentials, exhaustive testing of the rotational symmetry modes allowed by group representation theory, and even simple geometric considerations. This article presents a purely algebraic approach to RUM identification wherein the conditions of connectedness are used to construct a linear system of equations in the rotational symmetry-mode amplitudes.

Surface-screening mechanisms in ferroelectric thin films and their effect on polarization dynamics and domain structures

For over 70 years, ferroelectric materials have been one of the central research topics for condensed matter physics and material science, an interest driven both by fundamental science and applications. However, ferroelectric surfaces, the key component of ferroelectric films and nanostructures, still present a significant theoretical and even conceptual challenge. Indeed, stability of ferroelectric phase per se necessitates screening of polarization charge. At surfaces, this can lead to coupling between ferroelectric and semiconducting properties of material, or with surface (electro) chemistry, going well beyond classical models applicable for ferroelectric interfaces. In this review, we summarize recent studies of surface-screening phenomena in ferroelectrics. We provide a brief overview of the historical understanding of the physics of ferroelectric surfaces, and existing theoretical models that both introduce screening mechanisms and explore the relationship between screening and relevant aspects of ferroelectric functionalities starting from phase stability itself. Given that the majority of ferroelectrics exist in multiple-domain states, we focus on local studies of screening phenomena using scanning probe microscopy techniques. We discuss recent studies of static and dynamic phenomena on ferroelectric surfaces, as well as phenomena observed under lateral transport, light, chemical, and pressure stimuli. We also note that the need for ionic screening renders polarization switching a coupled physical-electrochemical process and discuss the non-trivial phenomena such as chaotic behavior during domain switching that stem from this.

Atomically Resolved Electronic States and Correlated Magnetic Order at Termination Engineered Complex Oxide Heterointerfaces

We map electronic states, band gaps, and interface-bound charges at termination-engineered BiFeO₃/La_0.7Sr_0.3MnO₃ interfaces using atomically resolved cross-sectional scanning tunneling microscopy. We identify a delicate interplay of different correlated physical effects and relate these to the ferroelectric and magnetic interface properties tuned by engineering the atomic layer stacking sequence at the interfaces. This study highlights the importance of a direct atomically resolved access to electronic interface states for understanding the intriguing interface properties in complex oxides.

Nanoscale Structural Modulation and Low-temperature Magnetic Response in Mixed-layer Aurivillius-type Oxides

Nanoscale structural modulation with different layer numbers in layer-structured complex oxides of the binary Bi₄Ti₃O₁₂-BiFeO₃ system can give rise to intriguing phenomena and extraordinary properties, originating from the correlated interfaces of two different phases with different strain states. In this work, we studied the nanoscale structural modulation induced by Co-substitution in the Aurivillius-type oxide of Bi₁₁Fe₃Ti₆O₃₃ with a unique and naturally occurred mixed-layer structure. Nanoscale structural evolution via doping occurred from the phase-modulated structure composed of 4- and 5-layer phases to a homogeneous 4-layer structure was clearly observed utilizing x-ray diffraction and electron micro-techniques. Significantly, magnetic response for the samples under various temperatures was recorded and larger magnetic coercive fields (e.g. H_c ∼ 10 kOe at 50 K) were found in the phase-modulated samples. Analyses of the x-ray absorption spectra and magnetic response confirmed that the low-temperature magnetic behaviour should be intrinsic to the phase-modulated structure inside the structural transformation region, mainly arising from structural distortions at the correlated interfaces.

Degradable rhenium trioxide nanocubes with high localized surface plasmon resonance absorbance like gold for photothermal theranostics

The applications of inorganic theranostic agents in clinical trials are generally limited to their innate non-biodegradability and potential long-term biotoxicity. To address this problem, herein via a straightforward and tailored space-confined on-substrate route, we obtained rhenium trioxide (ReO₃) nanocubes (NCs) that display a good biocompatibility and biosafety. Importantly, their aqueous dispersion has high localized surface plasmon resonance (LSPR) absorbance in near-infrared (NIR) region different from previous report, which possibly associates with the charge transfer and structural distortion in hydrogen rhenium bronze (H_xReO₃), as well as ReO₃'s cubic shape. Such a high LSPR absorbance in the NIR region endows them with photoacoustic (PA)/infrared (IR) thermal imaging, and high photothermal conversion efficiency (∼57.0%) for efficient ablation of cancer cells. Also, ReO₃ NCs show X-ray computed tomography (CT) imaging derived from the high-Z element Re. More attractively, those ReO₃ NCs, with pH-dependent oxidized degradation behaviors, are revealed to be relatively stable in hypoxic and weakly acidic microenvironment of tumor for imaging and treatment whilst degradable in normal physiological environments of organs to enable effective clearance. In spite of their degradability, ReO₃ NCs still possess tumor targeting capabilities. We thus develop a simple but powerful, safe and biodegradable inorganic theranostic platform to achieve PA/CT/IR imaging-guided cancer photothermal therapy (PTT) for improved therapeutic efficacy and decreased toxic side effects.

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.

Localized Control of Curie Temperature in Perovskite Oxide Film by Capping-Layer-Induced Octahedral Distortion

With reduced dimensionality, it is often easier to modify the properties of ultrathin films than their bulk counterparts. Strain engineering, usually achieved by choosing appropriate substrates, has been proven effective in controlling the properties of perovskite oxide films. An emerging alternative route for developing new multifunctional perovskite is by modification of the oxygen octahedral structure. Here we report the control of structural oxygen octahedral rotation in ultrathin perovskite SrRuO_{3} films by the deposition of a SrTiO_{3} capping layer, which can be lithographically patterned to achieve local control. Using a scanning Sagnac magnetic microscope, we show an increase in the Curie temperature of SrRuO_{3} due to the suppression octahedral rotations revealed by the synchrotron x-ray diffraction. This capping-layer-based technique may open new possibilities for developing functional oxide materials.

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.

Photostriction of strontium ruthenate

Transition metal oxides with a perovskite crystal structure exhibit a variety of physical properties associated with the lattice. Among these materials, strontium ruthenate (SrRuO₃) displays unusually strong coupling of charge, spin and lattice degrees of freedom that can give rise to the photostriction, that is, changes in the dimensions of material due to the absorption of light. In this study, we observe a photon-induced strain as high as 1.12% in single domain SrRuO₃, which we attribute to a nonequilibrium of phonons that are a result of the strong interaction between the crystalline lattice and electrons excited by light. In addition, these light-induced changes in the SrRuO₃ lattice affect its electrical resistance. The observation of both photostriction and photoresistance in SrRuO₃ suggests the possibility of utilizing the mechanical and optical functionalities of the material for next-generation optoelectronics, such as remote switches, light-controlled elastic micromotors, microactuators and other optomechanical systems.

Light-induced deformation known as photostriction could be used for green energy devices but in most materials the effect is too small to be of practical use. Here, Wei et al. study the photostriction of strontium ruthenate and find photon-induced strain efficiencies of more than one percent.

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.

Structure of epitaxial SrIrO3 perovskite studied by interference between X-ray waves diffracted by the substrate and the thin film

A high-pressure metastable orthorhombic phase of SrIrO3 perovskite has been epitaxially stabilized on several substrates (DyScO3, GdScO3, NdScO3 and SrTiO3) in the form of thin monocrystalline layers with (110) surface orientation. The unit-cell parameters of the pseudomorphic thin SrIrO3 layers depend on the biaxial strain imposed by the various substrates due to the different lattice mismatches of the particular substrate and the bulk orthorhombic SrIrO3 structure. Using X-ray diffractometry, it is shown that both compressive and tensile strain increase the lattice parameters a and b, while the angle γ scales with the applied strain, being smaller or larger than 90° for compressive or tensile strain, respectively, resulting in a small monoclinic distortion of the layer unit cell. Owing to the similarity of the substrate and layer lattices, the diffraction signals from the two structures overlap partially, which complicates structure determination by standard refinement methods using measured integrated intensities. The measured signal is composed of two interfering components corresponding to the waves diffracted by the substrate and by the layer, where the first component is calculated exactly using the known substrate structure, while the second one is determined by the unknown unit-cell parameters of the layer. The unit-cell parameters were refined in order to fit the experimental data with the simulation. The fractional coordinates of the atoms in the unit cell resulting from the fit are similar to those in the bulk structure.

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.