Dimensionality Controlled Octahedral Symmetry-Mismatch and Functionalities in Epitaxial LaCoO₃/SrTiO₃ Heterostructures

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.

Interface-related phenomena in epitaxial complex oxide ferroics across different thin film platforms: opportunities and challenges

Interfaces in complex oxides give rise to fascinating new physical phenomena arising from the interconnected spin, lattice, charge and orbital degrees of freedom. Most commonly, interfaces are engineered in epitaxial superlattice films. Of growing interest also are epitaxial vertically aligned nanocomposite films where interfaces form by self-assembly. These two thin film forms offer different capabilities for materials tuning and have been explored largely separately from one another. Ferroics (ferroelectric, ferromagnetic, multiferroic) are among the most fascinating phenomena to be manipulated using interface effects. Hence, in this review we compare and contrast the ferroic properties that arise in these two different film forms, highlighting exemplary materials combinations which demonstrate novel, enhanced and/or emergent ferroic functionalities. We discuss the origins of the observed functionalities and propose where knowledge can be translated from one materials form to another, to potentially produce new functionalities. Finally, for the two different film forms we present a perspective on underexplored/emerging research directions.

Heteroepitaxial Control of Fermi Liquid, Hund Metal, and Mott Insulator Phases in Single-Atomic-Layer Ruthenates

Interfaces between dissimilar correlated oxides can offer devices with versatile functionalities, and great efforts have been made to manipulate interfacial electronic phases. However, realizing such phases is often hampered by the inability to directly access the electronic structure information; most correlated interfacial phenomena appear within a few atomic layers from the interface. Here, atomic-scale epitaxy and photoemission spectroscopy are utilized to realize the interface control of correlated electronic phases in atomic-scale ruthenate-titanate heterostructures. While bulk SrRuO₃ is a ferromagnetic metal, the heterointerfaces exclusively generate three distinct correlated phases in the single-atomic-layer limit. The theoretical analysis reveals that atomic-scale structural proximity effects yield Fermi liquid, Hund metal, and Mott insulator phases in the quantum-confined SrRuO₃ . These results highlight the extensive interfacial tunability of electronic phases, hitherto hidden in the atomically thin correlated heterostructure. Moreover, this experimental platform suggests a way to control interfacial electronic phases of various correlated materials.

Room-temperature valence transition in a strain-tuned perovskite oxide

Cobalt oxides have long been understood to display intriguing phenomena known as spin-state crossovers, where the cobalt ion spin changes vs. temperature, pressure, etc. A very different situation was recently uncovered in praseodymium-containing cobalt oxides, where a first-order coupled spin-state/structural/metal-insulator transition occurs, driven by a remarkable praseodymium valence transition. Such valence transitions, particularly when triggering spin-state and metal-insulator transitions, offer highly appealing functionality, but have thus far been confined to cryogenic temperatures in bulk materials (e.g., 90 K in Pr_1-xCa_xCoO₃). Here, we show that in thin films of the complex perovskite (Pr_1-yY_y)_1-xCa_xCoO_3-δ, heteroepitaxial strain tuning enables stabilization of valence-driven spin-state/structural/metal-insulator transitions to at least 291 K, i.e., around room temperature. The technological implications of this result are accompanied by fundamental prospects, as complete strain control of the electronic ground state is demonstrated, from ferromagnetic metal under tension to nonmagnetic insulator under compression, thereby exposing a potential novel quantum critical point.

Atomically engineered cobaltite layers for robust ferromagnetism

Emergent phenomena at heterointerfaces are directly associated with the bonding geometry of adjacent layers. Effective control of accessible parameters, such as the bond length and bonding angles, offers an elegant method to tailor competing energies of the electronic and magnetic ground states. In this study, we construct unit-thick syntactic layers of cobaltites within a strongly tilted octahedral matrix via atomically precise synthesis. The octahedral tilt patterns of adjacent layers propagate into cobaltites, leading to a continuation of octahedral tilting while maintaining substantial misfit tensile strain. These effects induce severe rumpling within an atomic plane of neighboring layers, further triggering the electronic reconstruction between the splitting orbitals. First-principles calculations reveal that the cobalt ions transit to a higher spin state level upon octahedral tilting, resulting in robust ferromagnetism in ultrathin cobaltites. This work demonstrates a design methodology for fine-tuning the lattice and spin degrees of freedom in correlated quantum heterostructures by exploiting epitaxial geometric engineering.

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.

Direct Observation of Strong Anomalous Hall Effect and Proximity-Induced Ferromagnetic State in SrIrO3

The 5d iridium-based transition metal oxides have gained broad interest because of their strong spin-orbit coupling, which favors new or exotic quantum electronic states. On the other hand, they rarely exhibit more mainstream orders like ferromagnetism due to generally weak electron-electron correlation strength. Here, a proximity-induced ferromagnetic (FM) state with T_C  ≈ 100 K and strong magnetocrystalline anisotropy is shown in a SrIrO₃ (SIO) heterostructure via interfacial charge transfer by using a ferromagnetic insulator in contact with SIO. Electrical transport allows to selectively probe the FM state of the SIO layer and the direct observation of a strong, intrinsic, and positive anomalous Hall effect (AHE). For T ≤ 20 K, the AHE displays unusually large coercive and saturation field, a fingerprint of a strong pseudospin-lattice coupling. A Hall angle, σ_xy ^AHE /σ_{xx ,} larger by an order of magnitude than in typical 3d metals and an FM net moment of about 0.1 μ_B /Ir, is reported. This emphasizes how efficiently the nontrivial topological band properties of SIO can be manipulated by structural modifications and the exchange interaction with 3d TMOs.

Strain-Induced Atomic-Scale Building Blocks for Ferromagnetism in Epitaxial LaCoO3

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.

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.

Strong Ferromagnetism Achieved via Breathing Lattices in Atomically Thin Cobaltites

Low-dimensional quantum materials that remain strongly ferromagnetic down to monolayer thickness are highly desired for spintronic applications. Although oxide materials are important candidates for the next generation of spintronics, ferromagnetism decays severely when the thickness is scaled to the nanometer regime, leading to deterioration of device performance. Here, a methodology is reported for maintaining strong ferromagnetism in insulating LaCoO₃ (LCO) layers down to the thickness of a single unit cell. It is found that the magnetic and electronic states of LCO are linked intimately to the structural parameters of adjacent "breathing lattice" SrCuO₂ (SCO). As the dimensionality of SCO is reduced, the lattice constant elongates over 10% along the growth direction, leading to a significant distortion of the CoO₆ octahedra, and promoting a higher spin state and long-range spin ordering. For atomically thin LCO layers, surprisingly large magnetic moment (0.5 μ_B /Co) and Curie temperature (75 K), values larger than previously reported for any monolayer oxides are observed. The results demonstrate a strategy for creating ultrathin ferromagnetic oxides by exploiting atomic heterointerface engineering, confinement-driven structural transformation, and spin-lattice entanglement in strongly correlated materials.

High-Performance Photovoltaic Materials Based on the Superlattice Structures of Organic-Inorganic Halide Perovskite and Superhalogen Hybrid Perovskite

The use of superlattice structures is an attractive strategy for expanding the family of perovskites and obtaining excellent optoelectronic materials. Mixing of cations and partial replacement of halogens by superhalogens are advantageous for improving the stability and optoelectronic properties of hybrid perovskites. Herein, the superlattice structures of the (CsPbI₃)_n/MAPbI₂BF₄, (FAPbI₃)_n/MAPbI₂BF₄, (MAPbI₃)_n/CsPbI₂BF₄, and (FAPbI₃)_n/CsPbI₂BF₄ hybrid perovskites were investigated using first-principles density functional theory calculations. The results show that these superlattice structures have tunable direct band gaps and small effective electron and hole masses. Additionally, the charge densities for the valence band maximum and conduction band minimum states are located in different regions of the superlattices. Suggesting that these structures are type-II superlattices that show greatly reduced electron-hole recombination rates. Excellent optical absorption properties for all of perovskite superlattices and the calculated power conversion efficiency of 22.77% for the single-junction solar cells based on the (FAPbI₃)₃/MAPbI₂BF₄ and (FAPbI₃)₃/CsPbI₂BF₄ perovskites were obtained.

Ferromagnetism and Charge Order from a Frozen Electron Configuration in Strained Epitaxial LaCoO_{3}

We report ordering of the cobalt electron configuration in ferromagnetic strained epitaxial LaCoO_{3}. Specifically, the presence of charge order is demonstrated from distinct features of the resonant cobalt contribution to superstructure reflections. Density functional theory calculations show that the observed order is consistent with the spin-state periodicity predicted to give rise to ferromagnetism in LaCoO_{3}. Through the modification of symmetry by strain, concurrent frozen charge and spin-state order are stabilized, giving rise to long-range magnetic order.

Co2+ substituted La2CuO4/LaCoO3 perovskite nanocomposites: synthesis, properties and heterogeneous catalytic performance

Cobalt substituted La₂CuO₄/LaCoO₃ perovskite nanocomposites were prepared using a microwave combustion method.

Engineering the strongly correlated properties of bulk Ruddlesden-Popper transition metal oxides via self-doping

We demonstrate via first-principles calculations a novel method of tuning the electron-electron interactions in bulk oxide materials via controlling the cationic layer arrangement. Using the Ruddlesden-Popper oxides LaSrMnO₄ and LaSrTiO₄ as examples, our study demonstrates that a self-doping effect can be induced by changing the stacking of the neutral and charged cationic layers. It is believed that such a phenomenon is associated with different movements of apical oxygen atoms, resulting in diverse bandgaps, magnetism and orbital degrees of freedom in the same stoichiometric strongly-correlated material. This finding may open up a new direction to engineer the transition metal oxides for practical applications requiring tunable electronic properties without external doping.

Interfacial Coupling-Induced Ferromagnetic Insulator Phase in Manganite Film

Interfaces with subtle differences in atomic and electronic structures in perovskite ABO3 heterostructures often yield intriguingly different properties, yet their exact roles remain elusive. Here, we report an integrated study of unusual transport, magnetic, and structural properties of Pr0.67Sr0.33MnO3 film on SrTiO3 substrate. The variations in the out-of-plane lattice constant and BO6 octahedral rotation across the Pr0.67Sr0.33MnO3/SrTiO3 interface strongly depend on the thickness of the Pr0.67Sr0.33MnO3 film. In the 12 nm film, a new interface-sensitive ferromagnetic polaronic insulator (FI') phase is formed during the cubic-to-tetragonal phase transition of SrTiO3, apparently due to the enhanced electron-phonon interaction and atomic disorder in the film. The transport properties of the FI' phase in the 30 nm film are masked because of the reduced interfacial coupling and smaller interface-to-volume ratio. This work demonstrates how thickness-dependent interfacial coupling leads to the formation of a theoretically predicted ferromagnetic-polaronic insulator, as illustrated in a new phase diagram, that is otherwise ferromagnetic metal (FM) in bulk form.

Independent Control of the Magnetization in Ferromagnetic La2/3Sr1/3MnO3/SrTiO3/LaCoO3 Heterostructures Achieved by Epitaxial Lattice Mismatch

We report the effect of interface symmetry-mismatch on the magnetic properties of LaCoO3 (LCO) thin films. Growing epitaxial LCO under tensile strain on top of cubic SrTiO3 (STO) produces a contraction along the c axis and a characteristic ferromagnetic response. However, we report here that ferromagnetism in LCO is completely suppressed when grown on top of a buffer layer of rhombohedral La2/3Sr1/3MnO3 (LSMO), in spite of identical in-plane and out-of-plane lattice deformation. This confirms that it is the lattice symmetry mismatch and not just the total strain, which determines the magnetism of LCO. On the basis of this control over the magnetic properties of LCO, we designed a multilayered structure to achieve independent rotation of the magnetization in ferromagnetic insulating LCO and half-metallic ferromagnet LSMO. This is an important step forward for the design of spin-filtering tunnel barriers based on LCO.