Artificial Oxide Heterostructures Research Articles

The different types of magnetic coupling achieved via dimensionality control of interfacial charge transfer in cobaltite-cuprate superlattices

The interface effect in artificial oxide heterostructures provides opportunities to engineer and explore desired functionalities associated with spintronic devices. Understanding the basic mechanism behind the interface effect is crucial for possible applications. Herein, charge-transfer-mediated magnetic coupling in LaCoO3/SrCuO2 (LCO/SCO) superlattices is investigated by changing the thickness of the SCO layer, which exhibits dimensional control of the structure transformation from planar-type to chain-type configuration. We found that the LCO/SCO superlattices display obvious soft-hard magnetic coupling when the SCO layer is thin, but the exchange bias effect gradually becomes dominant as thickness increases. As suggested by the x-ray absorption spectrum, electronic charge transfer dependent on SCO thickness is present at the interface of LCO/SCO superlattices. As the SCO is thin, the interaction of interfacial Co4+ and Co3+ results in the soft magnetic layer strongly coupling with the intrinsic hard magnetic layer of the LCO layer. But as the SCO layer thickens, charge transfer reduces, and the soft magnetic properties of the interface are suppressed. Instead, the long-range antiferromagnetic order of the bulk-like planar-type SCO layer couples with the adjacent ferromagnetic layer, leading to the exchange bias effect. Our results reveal an extraordinary spectrum of possibilities to design and engineer multifunctionalities of advanced oxide electronics.

Enhancing Interfacial Ferromagnetism and Magnetic Anisotropy of CaRuO3/SrTiO3 Superlattices via Substrate Orientation.

Artificial oxide heterostructures have provided promising platforms for the exploration of emergent quantum phases with extraordinary properties. One of the most interesting phenomena is the interfacial magnetism formed between two non-magnetic compounds. Here, a robust ferromagnetic phase emerged at the (111)-oriented heterointerface between paramagnetic CaRuO3 and diamagnetic SrTiO3 is reported. The Curie temperature is as high as ≈155K and the saturation magnetization is as large as ≈1.3µB per formula unit for the (111)-CaRuO3/SrTiO3 superlattices, which are obviously superior to those of the (001)-oriented counterparts and are comparable to the typical itinerant ferromagnet SrRuO3. A strong in-plane magnetic anisotropy with six-fold symmetry is further revealed by the anisotropic magnetoresistance measurements, presenting a large in-plane anisotropic field of 3.0-3.6T. More importantly, the magnetic easy axis of the (111)-oriented superlattices can be effectively tuned from 〈 1〉 to 〈 〉 directions by increasing the layer thickness of SrTiO3. The findings demonstrate a feasible approach to enhance the interface coupling effect by varying the stacking orientation of oxide heterostructures. The tunable magnetic anisotropy also shows potential applications in low-power-consumption or exchange spring devices.

Observation of Moiré Patterns in Twisted Stacks of Bilayer Perovskite Oxide Nanomembranes with Various Lattice Symmetries.

The design and fabrication of novel quantum devices in which exotic phenomena arise from moiré physics have sparked a new race of conceptualization and creation of artificial lattice structures. This interest is further extended to the research on thin-film transition metal oxides, with the goal of synthesizing twisted layers of perovskite oxides concurrently revealing moiré landscapes. By utilizing a sacrificial-layer-based approach, we show that such high-quality twisted bilayer oxide nanomembrane structures can be achieved. We observe atomic-scale distinct moiré patterns directly formed with different twist angles, and the symmetry-inequivalent nanomembranes can be stacked together to constitute new complex moiré configurations. This study paves the way to the construction of higher-order artificial oxide heterostructures based on different materials/symmetries and provides the materials foundation for investigating moiré-related electronic effects in an expanded selection of twisted oxide thin films.

Artificial oxide heterostructures with non-trivial topology

In the quest for topological insulators with large band gaps, heterostructures with Rashba spin–orbit interactions come into play. Transition metal oxides with heavy ions are especially interesting in this respect. We discuss the design principles for stacking oxide Rashba layers. Assuming a single layer with a two-dimensional electron gas (2DEG) on both interfaces as a building block, a two-dimensional topological insulating phase is present when negative coupling between the 2DEGs exists. When stacking multiple building blocks, a two-dimensional or three-dimensional topological insulator is artificially created, depending on the intra- and interlayer coupling strengths and the number of building blocks. We show that the three-dimensional topological insulator is protected by reflection symmetry, and can therefore be classified as a topological crystalline insulator. In order to isolate the topological states from bulk states, the intralayer coupling term needs to be quadratic in momentum. It is described how such a quadratic coupling could potentially be realized by taking buckling within the layers into account. The buckling, thereby, brings the idea of stacked Rashba system very close to the alternative approach of realizing the buckled honeycomb lattice in [111]-oriented perovskite oxides.

Large Polarization and Susceptibilities in Artificial Morphotropic Phase Boundary PbZr1−xTixO3 Superlattices

AbstractThe ability to produce atomically precise, artificial oxide heterostructures allows for the possibility of producing exotic phases and enhanced susceptibilities not found in parent materials. Typical ferroelectric materials either exhibit large saturation polarization away from a phase boundary or large dielectric susceptibility near a phase boundary. Both large ferroelectric polarization and dielectric permittivity are attained wherein fully epitaxial (PbZr0.8Ti0.2O3)n/(PbZr0.4Ti0.6O3)2n (n = 2, 4, 6, 8, 16 unit cells) superlattices are produced such that the overall film chemistry is at the morphotropic phase boundary, but constitutive layers are not. Long‐ (n ≥ 6) and short‐period (n = 2) superlattices reveal large ferroelectric saturation polarization (Ps = 64 µC cm−2) and small dielectric permittivity (εr ≈ 400 at 10 kHz). Intermediate‐period (n = 4) superlattices, however, exhibit both large ferroelectric saturation polarization (Ps = 64 µC cm−2) and dielectric permittivity (εr = 776 at 10 kHz). First‐order reversal curve analysis reveals the presence of switching distributions for each parent layer and a third, interfacial layer wherein superlattice periodicity modulates the volume fraction of each switching distribution and thus the overall material response. This reveals that deterministic creation of artificial superlattices is an effective pathway for designing materials with enhanced responses to applied bias.

Influence of chemical composition and crystallographic orientation on the interfacial magnetism in BiFeO3 / La1−xSrxMnO3 superlattices

The emergence of magnetism unique to the interface between the multiferroic $\mathrm{BiFe}{\mathrm{O}}_{3}$ (BFO) and ferromagnetic $\mathrm{L}{\mathrm{a}}_{1\ensuremath{-}\mathrm{x}}\mathrm{S}{\mathrm{r}}_{\mathrm{x}}\mathrm{Mn}{\mathrm{O}}_{3}$ (LSMO) offers an opportunity to control magnetism in nanoscale heterostructures with electric fields. In this paper, we investigate the influence of chemical composition and crystallographic orientation on the interfacial magnetism of BFO/LSMO superlattices. Our results reveal that the induced net magnetic moment in the BFO layers increases monotonically with increasing saturation magnetization of the LSMO layers. For the (100)-BFO/LSMO ($x=0.2$) superlattice, the induced moment reaches a record high value of $\ensuremath{\sim}2.8\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}/\mathrm{Fe}$. No interfacial magnetization is observed at the (100)-BFO/LSMO interface when LSMO is an antiferromagnet. In contrast to (100)-oriented superlattices, no induced moment is observed in (111)-BFO layers. Our results suggest the interfacial structural reconstruction may not be a sufficient condition for the enhanced net moment in BFO layer. Instead, spin canting induced by interfacial exchange coupling is proposed in the (100)- but not in the (111)-BFO, leading to the large net magnetization at the (100)-oriented interface. This work further demonstrates the importance of exchange coupling across heterointerfaces for spin canting in nominally antiferromagnets, providing a pathway to control the magnetic properties of artificial oxide heterostructures.

Atomic Level Engineering of Structural and Functional Properties of Transition Metal Oxides

Metal-oxygen bonds are responsible for a broad spectrum of functional properties of transition-metal oxides, and engineering the bonds is a key for exploring their properties. Artificial oxide heterostructures with chemically abrupt interfaces have provided a platform for engineering bonding geometries that could lead to emergent phenomena not seen in bulk oxides. In this paper, we demonstrate that interfacially engineering oxygen displacement is a good route for exploring structural and functional properties of oxides. Our high-resolution annular bright-field scanning transmission electron microscopy observations revealed that interfacial oxygen displacement determines metal-oxygen bond angles in entire heterostructures and the magnitude of the displacements can be adjusted by controlling propagations of the oxygen octahedral rotations across the heterointerface. We also show that by heterostructuring an itinerant ferromagnet SrRuO3 with Ca0.5Sr0.5TiO3 (0–4 monolayers thick) grown on a GdScO3 substrate, structural phase and magnetic anisotropy of SrRuO3 can be controlled through the interfacial engineering of its oxygen displacement.

Tuning the interfacial charge, orbital, and spin polarization properties in La0.67Sr0.33MnO3/La1−xSrxMnO3 bilayers

The possibility of controlling the interfacial properties of artificial oxide heterostructures is still attracting researchers in the field of materials engineering. Here, we used surface sensitive techniques and high-resolution transmission electron microscopy to investigate the evolution of the surface spin-polarization and lattice strains across the interfaces between La0.66Sr0.33MnO3 thin films and low-doped manganites as capping layers. We have been able to fine tune the interfacial spin-polarization by changing the capping layer thickness and composition. The spin-polarization was found to be the highest at a critical capping thickness that depends on the Sr doping. We explain the non-trivial magnetic profile by the combined effect of two mechanisms: On the one hand, the extra carriers supplied by the low-doped manganites that tend to compensate the overdoped interface, favouring locally a ferromagnetic double-exchange coupling. On the other hand, the evolution from a tensile-strained structure of the inner layers to a compressed structure at the surface that changes gradually the orbital occupation and hybridization of the 3d-Mn orbitals, being detrimental for the spin polarization. The finding of an intrinsic spin-polarization at the A-site cation observed in x-ray magnetic circular dichroism (XMCD) measurements also reveals the existence of a complex magnetic configuration at the interface, different from the magnetic phases observed at the inner layers.

Quantum anomalous Hall phase and half-metallic phase in ferromagnetic (111) bilayers of 4d and 5d transition metal perovskites

Extraordinary electronic phases can form in artificial oxide heterostructures, which will provide a fertile ground for new physics and also give rise to novel device functions. Based on a systematic first-principles density functional theory study of the magnetic and electronic properties of the (111) superlattices ${({AB\text{O}}_{3})}_{2}/{({AB}^{\ensuremath{'}}{\mathrm{O}}_{3})}_{10}$ of $4d$ and $5d$ transition metal perovskite ($B$ = Ru, Rh, Ag, Re, Os, Ir, Au; ${AB}^{\ensuremath{'}}{\mathrm{O}}_{3}={\mathrm{LaAlO}}_{3}, {\mathrm{SrTiO}}_{3})$, we demonstrate that due to quantum confinement, bilayers ${({\text{La}B\text{O}}_{3})}_{2}$ ($B$ = Ru, Re, Os) and ${({\text{Sr}B\text{O}}_{3})}_{2}$ ($B$ = Rh, Os, Ir) are ferromagnetic with ordering temperatures up to room temperature. In particular, bilayer ${({\mathrm{LaOsO}}_{3})}_{2}$ is an exotic spin-polarized quantum anomalous Hall insulator, while the other ferromagnetic bilayers are metallic with large Hall conductances comparable to the conductance quantum. Furthermore, bilayers ${({\mathrm{LaRuO}}_{3})}_{2}$ and ${({\mathrm{SrRhO}}_{3})}_{2}$ are half metallic, while the bilayer ${({\mathrm{SrIrO}}_{3})}_{2}$ exhibits a peculiar colossal magnetic anisotropy. Our findings thus show that $4d$ and $5d$ metal perovskite (111) bilayers are a class of quasi-two-dimensional materials for exploring exotic quantum phases and also for advanced applications such as low-power nanoelectronics and oxide spintronics.

Interface Control of Ferroelectricity in an SrRuO3 /BaTiO3 /SrRuO3 Capacitor and its Critical Thickness.

The atomic-scale synthesis of artificial oxide heterostructures offers new opportunities to create novel states that do not occur in nature. The main challenge related to synthesizing these structures is obtaining atomically sharp interfaces with designed termination sequences. In this study, it is demonstrated that the oxygen pressure (PO2) during growth plays an important role in controlling the interfacial terminations of SrRuO3 /BaTiO3 /SrRuO3 (SRO/BTO/SRO) ferroelectric (FE) capacitors. The SRO/BTO/SRO heterostructures are grown by a pulsed laser deposition method. The top SRO/BTO interface, grown at high PO2 (around 150 mTorr), usually exhibits a mixture of RuO2 -BaO and SrO-TiO2 terminations. By reducing PO2, the authors obtain atomically sharp SRO/BTO top interfaces with uniform SrO-TiO2 termination. Using capacitor devices with symmetric and uniform interfacial termination, it is demonstrated for the first time that the FE critical thickness can reach the theoretical limit of 3.5 unit cells.

Field-effect control of superconductivity and Rashba spin-orbit coupling in top-gated LaAlO3/SrTiO3 devices.

The recent development in the fabrication of artificial oxide heterostructures opens new avenues in the field of quantum materials by enabling the manipulation of the charge, spin and orbital degrees of freedom. In this context, the discovery of two-dimensional electron gases (2-DEGs) at LaAlO3/SrTiO3 interfaces, which exhibit both superconductivity and strong Rashba spin-orbit coupling (SOC), represents a major breakthrough. Here, we report on the realisation of a field-effect LaAlO3/SrTiO3 device, whose physical properties, including superconductivity and SOC, can be tuned over a wide range by a top-gate voltage. We derive a phase diagram, which emphasises a field-effect-induced superconductor-to-insulator quantum phase transition. Magneto-transport measurements show that the Rashba coupling constant increases linearly with the interfacial electric field. Our results pave the way for the realisation of mesoscopic devices, where these two properties can be manipulated on a local scale by means of top-gates.