Complex Oxide Heterostructures Research Articles

Engineering the Coherent Phonon Transport in Polar Ferromagnetic Oxide Superlattices.

Artificial superlattices composed of perovskite oxides serves as an essential platform for engineering coherent phonon transport by redefining the lattice periodicity, which strongly influences the lattice-coupled phase transitions in charge and spin degrees of freedom. However, previous methods of manipulating phonons have been limited to controlling the periodicity of superlattice, rather than utilizing complex mutual interactions that are prominent in transition metal oxides. In this study onoxide superlattices composed of ferromagnetic metallic SrRuO3 and quantum paraelectric SrTiO3, phonon modulation by controlling the geometry of superlattice in atomic-scale precision is realized, demonstrating the coherent phonon engineering using structural and magnetic phase transitions. By modulating the interface density, coherent-incoherent crossover of the phonon transport at room temperature is observed, which is coupled with a change in interfacial structural continuity. Upon cooling, the close relation between phonon transport and multiple phase transitions is identified. In particular, the enhancement of the polar state in SrTiO3 layer at ≈200K leads to the weakening of phonon coherence and a further reduction of thermal conductivity in superlattices compared to the bulk limit. These findings provide a guide to developing future thermoelectric nanodevices by engineering the coherence of phonons via the design of complex oxide heterostructures.

Strain Programming of Oxygen Octahedral Symmetry in Perovskite Oxide Thin Films

AbstractThe collective rotations of oxygen octahedra play an important role in determining the physical properties of functional perovskite oxides. The epitaxial strain can serve as an effective means to modify the oxygen octahedral symmetry (OOS), i.e., oxygen octahedral rotation or tilt (OOR/OOT). However, the strain‐OOS coupling that may alter the details of the OOS, thereby the physical properties, has not been fully understood. In this work, it is demonstrated that epitaxial strain can not only induce a structural phase transition but also precisely tune the degree of OOR. The correlated metal CaNbO3, which is orthorhombic, is studied by growing as epitaxial thin films. By imposing epitaxial strain, it is found that the film undergoes a structural phase transition from orthorhombic to tetragonal upon fully straining (i.e., from a+b−b− to a0a0c−). In unstrained films, the octahedral rotation along the c‐axis is as large as 15.7° that can be tuned to 6.6° by strain. This finding offers a general approach to manipulating OOR/OOT via strain engineering in complex oxide heterostructures.

High electroresistance in all-oxide ferroelectric tunnel junctions enabled by a narrow bandgap Mott insulator electrode

Ferroelectric tunnel junctions (FTJs) based on epitaxial complex oxide heterostructures are promising building blocks for developing low power nanoelectronics and neuromorphic computing. FTJs consisting of correlated oxide electrodes have distinct advantages in size scaling but only yield moderate electroresistance (ER) at room temperature due to the challenge in imposing asymmetric interfacial screening and large modulation of the tunneling potential profile. Here, we achieve large ER in all-oxide FTJs by paring a correlated metal with a narrow bandgap Mott insulator as electrodes. We fabricate epitaxial FTJs composed of 2.8 and 4 nm PbZr0.2Ti0.8O3 tunnel barriers sandwiched between correlated oxides LaNiO3 and Sr3Ir2O7 electrodes. An ER of 6500% has been observed at room temperature, which increases to over 105% at 100 K. The high ER can be attributed to ferroelectric polarization induced metal–insulator transition in interfacial Sr3Ir2O7, which enhances the potential asymmetry for the tunnel barrier. The temperature dependence of tunneling current shows that direct tunneling dominates in the on state, while the off-state conduction transitions from thermally activated behavior at high temperatures to Glazman–Matveev defect-mediated inelastic tunneling at low temperatures. Our study provides a viable material strategy for designing all-oxide FTJs with high ER, facilitating their implementation in nonvolatile memories and energy-efficient computing devices.

Demonstration of β-(Al x Ga1−x )2O3/β-Ga2O3 superlattice growth by mist chemical vapor deposition

This study demonstrates the successful growth of a β-(Al x Ga1−x )2O3/β-Ga2O3 superlattice structure with six periods using mist CVD. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) analysis revealed that the superlattice consisted of six periods of β-(Al x Ga1−x )2O3/β-Ga2O3 with an individual layer thickness of 12.9 nm and 9.1 nm, respectively. XRD analysis further confirmed the periodicity of the structure, yielding a period of 22.7 nm, which is in good agreement with the STEM result. Additionally, the Al composition was determined to be x = 0.085 based on XRD peak positions. Both atomic force microscopy and HAADF-STEM observations revealed atomically flat surfaces and sharp interfaces. This achievement highlights the potential of mist CVD for fabricating complex oxide heterostructures, offering a cost-effective and scalable alternative to conventional methods. The findings open new avenues for developing advanced electronic and optoelectronic devices based on wide-bandgap oxides.

Oxygen Ion Pumping across Atomically Designed Oxide Heterointerfaces

AbstractComplex oxide heterointerfaces and heterostructures have demonstrated enormous emergent phenomena over the last decades, attributed to the reconstructions of mis‐matched crystalline structure, polarity, and spin ordering across the heterointerfaces. This work employs the heterostructures of La0.7Sr0.3MnO3 and CaFeO2.5 as model system to demonstrate an interface‐specific oxygen migration/reconstruction across the interfaces due to the mismatched chemical potential, which dramatically influences the ferromagnetic and electronic states of La0.7Sr0.3MnO3 layer. Specifically, the alternative stacking of octahedral (Oh) and tetrahedral (Td) layers in CaFeO2.5 are used to form two distinct heterointerfaces, namely the Oh‐Td and the Oh‐Oh interfaces with the adjacent La0.7Sr0.3MnO3 layer. Interestingly, the oxygen ion migrates toward opposite directions across the interface for these two cases, in which the CaFeO2.5 layer acts as an “oxygen pump” and manipulates the oxygen contents of its adjacent La0.7Sr0.3MnO3 layers. Such manipulation leads to a dramatically changed ferromagnetic transition temperature for the heterostructure with the Oh‐Td and Oh‐Oh interface. This work establishes a feasible and efficient strategy to control the oxygen ionic distribution through atomic‐scale interface design and opens up new opportunities to exploit emergent states at the complex oxide heterostructures through selective oxygen ion evolution.

The Role of Interfacial Interactions and Oxygen Vacancies in Tuning Magnetic Anisotropy in LaCrO3/LaMnO3 Heterostructures

AbstractThe interplay of lattice, electronic, and spin degrees of freedom at epitaxial complex oxide interfaces provides a route to tune their magnetic ground states. Unraveling the competing contributions is critical for tuning their functional properties. The relationship between magnetic ordering and magnetic anisotropy and the lattice symmetry, oxygen content, and film thickness in compressively strained LaMnO3 (LMO)/LaCrO3 (LCO) superlattices is investigated. Mn–O–Cr antiferromagnetic superexchange interactions across the heterointerface result in a net ferrimagnetic magnetic structure. Bulk magnetometry measurements reveal isotropic in‐plane magnetism for as‐grown oxygen‐deficient thin samples due to equal fractions of orthorhombic a+a‐c‐, and a‐a+c‐ twin domains. As the superlattice thickness is increased, in‐plane magnetic anisotropy emerges as the fraction of the a+a‐c‐ domain increases. On annealing in oxygen, the suppression of oxygen vacancies results in a contraction of the lattice volume, and an orthorhombic to rhombohedral transition leads to isotropic magnetism independent of the film thickness. The complex interactions are investigated using high‐resolution synchrotron diffraction and X‐ray absorption spectroscopy. These results highlight the role of the evolution of structural domains with film thickness, interfacial spin interactions, and oxygen‐vacancy‐induced structural phase transitions in tuning the magnetic properties of complex oxide heterostructures.

Strain-insensitive ferromagnetic SrRuO3 thin films with ferrimagnetic CoFe2O4 buffer layer

Flexible electronics, such as wearable devices and biosensors, require materials that maintain their properties under mechanical stress. A recent study addresses this by focusing on SrRuO3 (SRO) thin films, which typically suffer reduced coercivity under strain. Herein, we introduce a novel approach by using CoFe2O4 (CFO) as a buffer layer in SRO/CFO/F-mica heterostructures to address this issue. When subjected to a strain of up to ±0.553 %, these heterostructures displayed a mere 11 % variation in saturation magnetic moment and coercive field, significantly outperforming SRO/BaTiO3 configurations, which showed a 95 % reduction in coercivity at only −0.3 % strain. This result demonstrates the effectiveness of the CFO layer in stabilizing the magnetic properties of SRO films against external mechanical deformations. These findings mark a significant advancement in the development of mechanically robust thin films for complex oxide heterostructures in flexible device applications.

Epitaxial superlattices as a framework for stabilizing different LaNiO3 structures

Targeted material design is at the center of research efforts on epitaxial, complex oxide heterostructures. Due to the strong electron-lattice coupling, small structural modifications are often decisive for the resulting macroscopic, physical properties of artificially layered materials. Here we report on a detailed structural analysis of a set of differently stacked LaNiO3−LaGaO3 superlattices by transmission electron microscopy. We find that the relative thickness ratio of the two superlattice sublayers affects the structural network, resulting in different Ni–O bond lengths and Ni–O–Ni angles under the same epitaxial strain conditions. Whereas the bond length values depend mainly on the LaNiO3 layer thickness, bond angles are mainly influenced by the LaGaO3 layer thickness. The orbital polarization determined from x-ray absorption spectroscopy measurements shows that the superlattice with the smallest deviation of both values compared to bulk shows the highest orbital polarization. Therefore, the thickness ratio of the two superlattice components can be regarded as an additional effective tool to tune the functional properties of nickelates. Published by the American Physical Society 2024

Novel 2DEG System at the HfO2/STO Interface.

Multifunctional integration in a single device has always been a hot research topic, especially for contradictory phenomena, one of which is the coexistence of ferroelectricity and metallicity. The complex oxide heterostructures, as symmetric breaking systems, provide a great possibility to incorporate different properties. Moreover, finding a series of oxide heterostructures to achieve this goal remains as a challenge. Here, taking the advantage of different physical phenomena, we use H2 plasma to pretreat the SrTiO3 (STO) substrate and then fabricate HfO2/STO heterostructures with it. The novel, well-repeatable metallic two-dimensional electron gas (2DEG) is directly obtained at the heterointerfaces without any further complex procedures, while the obvious ferroelectric-like behavior and Rashba spin-orbit coupling are also observed. The understanding of the mechanism, as well as the modified facile preparation procedure, would be meaningful for further development of ferroelectric metal in complex oxide heterostructures.

High-mobility spin-polarized quasi-two-dimensional electron gas and large low-field magnetoresistance at the interface of EuTiO3/SrTiO3 (110) heterostructures

High-mobility spin-polarized two-dimensional electron gas (2DEG) at the interfaces of complex oxide heterostructures provide great potential for spintronic device applications. Unfortunately, the interfacial ferromagnetism and its associated spin polarization of mobile electrons and negative magnetoresistance (MR) are too weak. As of now, obtaining enhanced interfacial ferromagnetism and MR and strong spin-polarized 2DEG is still a great challenge. In this paper, we report on the realization of strong spin-polarized 2DEG at the interface of EuTiO3/SrTiO3 (110) heterostructures, which were prepared by directly depositing 39-nm EuTiO3 films onto as-received SrTiO3 (110) substrates. Hall and Kondo effects, low-field MR, Shubnikov–de Haas (SdH) oscillation, and magnetic hysteresis loop measurements demonstrate that high mobility electrons (1.4 × 104 cm2 V−1 s−1) accumulate at the interface of the heterostructures, which are not only highly conducting and show SdH oscillations with a non-zero Berry phase but also show a large out-of-plane and in-plane butterfly-like negative low-field MR whose magnitude is unprecedentedly large (46%–59% at 500 Oe and 1.8 K), approximately one to two orders higher than those of previously reported spin-polarized 2DEG systems. The strong spin polarization of the interfacial 2DEG is attributed to the presence of interfacial Eu2+ 4f (3.6–4 μB/f.u.) and Ti3+ 3d moments. Our results may provide guidance for exploring strong spin-polarized 2DEG at the interface of rare-earth titanate–strontium titanate heterostructures.

Low-frequency noise behaviors of quasi-two-dimensional electron systems based on complex oxide heterostructures

We report the low-frequency noise behaviors in quasi-two-dimensional (quasi-2D) electron systems based on complex oxide heterostructures. First, the surface 2D electron gas (2DEG) on SrTiO3 (STO) exhibits the 1/fα-type current power spectral density (PSD) with α∼1.39. The non-unity exponent α indicates the discrepancy between the depth distributions of electrons and oxygen vacancies in the STO substrate. Second, the amorphous LaAlO3/KTaO3 (LAO/KTO) interface, another quasi-2D electron system, shows Lorentzian components of PSD at a high-frequency region around 1 kHz, implying that the amorphous overlayer can provide additional shallow charge-trapping sites to the quasi-2D electrons in the crystalline KTO substrate. Lastly, ultrathin SrRuO3 (SRO) film grown on STO substrate exhibits the Lorentzian components of PSD at a low-frequency region around 200 Hz. The slight suppression of the fast charge trapping is attributed to the intrinsic band bending at the interface between film and substrate. These results will provide a guideline for understanding the defect-induced charge trapping and the relevant electron dynamics in the quasi-2D electron systems as well as the oxide-based electronic materials in general.

Influence of misfit dislocations on ionic conductivity at oxide interfaces

Mismatched complex oxide thin films and heterostructures have gained significant traction for use as electrolytes in intermediate temperature solid oxide fuel cells, wherein interfaces exhibit variation in ionic conductivity as...

Epitaxial-orientation-controlled magnetic anisotropy in the La0.67Sr0.33MnO3/SrTiO3 heterostructures

Magnetic anisotropy has significant importance in both designing high-efficiency magnetic devices and understanding noncollinear spin textures. Here, La0.67Sr0.33MnO3 layers are epitaxially fabricated on the (001)-, (110)-, and (111)-orientated SrTiO3 substrates to manipulate magnetic anisotropy. While the [11¯0] axis is the in-plane magnetic easy axis for the (001) La0.67Sr0.33MnO3 layer, it becomes the hard axis for the (110) and (111) samples. Interestingly, the (110) samples exhibit a great enhancement in magnetic anisotropy energy, reaching at 6.1 × 105 erg/cm3 that is 1–2 orders of magnitude higher than ones in (001) and (111) films. It is also noted that such improved magnetic anisotropy of (110) samples is maintained up to 30 nm. Those observations can be interpreted in terms of the orientation-modified orbital symmetry. While the energetically favored 3dx2−y2 orbital and degenerated eg orbitals drive the two-dimensional and bulk-like ferromagnetism in (001) and (111) La0.67Sr0.33MnO3 layers, respectively, the 3d3z2−r2 orbital provides the lowest orbital symmetry and results in the highest magnetic anisotropy energy in (110) samples. Meanwhile, this model can also explain the thickness-dependent Curie temperatures with different epitaxial orientations, deepening the understanding of the complex oxide heterostructures for magnetic modulations.

Intermittent Defect Fluctuations in Oxide Heterostructures.

The heterogeneous nature, local presence, and dynamic evolution of defects typically govern the ionic and electronic properties of a wide variety of functional materials. While the last 50 years have seen considerable efforts into development of new methods to identify the nature of defects in complex materials, such as the perovskite oxides, very little is known about defect dynamics and their influence on the functionality of a material. Here, the discovery of the intermittent behavior of point defects (oxygen vacancies) in oxide heterostructures employing X-ray photon correlation spectroscopy is reported. Local fluctuations between two ordered phases in strained SrCoOx with different degrees of stability of the oxygen vacancies are observed. Ab-initio-informed phase-field modeling reveals that fluctuations between the competing ordered phases are modulated by the oxygen ion/vacancy interaction energy and epitaxial strain. The results demonstrate how defect dynamics, evidenced by measurement and modeling of their temporal fluctuations, give rise to stochastic properties that now can be fully characterized using coherent X-rays, coupled for the first time to multiscale modeling in functional complex oxide heterostructures. The study and its findings open new avenues for engineering the dynamical response of functional materials used in neuromorphic and electrochemicalapplications.

Hybrid organic/inorganic interface: in situ and ex situ characterization in terms of structure, morphology, electronic and magnetic properties

Organic conjugated molecules are promising media to host spin-polarized carriers, due to their weak spin relaxation mechanisms. Here, we fabricated a metal–hybrid organic–inorganic complex oxide heterostructure made of Fe–Phthalocyanine (FePc): Co(0.5 nm)/FePc(0; 0.5; 1 nm)/La0.67Sr0.33MnO3/SrTiO3 and investigated the interface structure and chemical bonding, and overall magnetic properties. Details about the Co/FePc and FePc/LSMO interfaces were obtained through X-ray photoelectron and absorption characterizations. The combined in-situ and ex-situ magnetic measurements of the heterostructure indicates the contribution of FePc and Co moments to the overall magnetic response. Moreover, these measurements indicate that the LSMO bulk magnetic properties were not canceled out by the surface treatment, but favored the formation of a magnetic dead layer. The hybrid heterostructure presents a modification of the saturation magnetization attributed to the contribution of the molecular layer. The present results are a step forward in the engineering of hybrid organic/inorganic interfaces.

Controlled Electronic and Magnetic Landscape in Self-Assembled Complex Oxide Heterostructures.

Complex oxide heterointerfaces contain a rich playground of novel physical properties and functionalities, which give rise to emerging technologies. Among designing and controlling the functional properties of complex oxide film heterostructures, vertically aligned nanostructure (VAN) films using a self-assembling bottom-up deposition method presents great promise in terms of structural flexibility and property tunability. Here, the bottom-up self-assembly is extended to a new approach using a mixture containing a 2Dlayer-by-layer film growth, followed by a 3D VAN film growth. In this work, the two-phase nanocomposite thin films are based on LaAlO3 :LaBO3 , grown on a lattice-mismatched SrTiO3001 (001) single crystal.The 2D-to-3D transient structural assembly is primarily controlled by the composition ratio, leading to the coexistence of multiple interfacial properties, 2D electron gas, and magnetic anisotropy. This approach provides multidimensional film heterostructures which enrich the emergent phenomena for multifunctional applications.

Coexistence of surface oxygen vacancy and interface conducting states in LaAlO3/SrTiO3 revealed by grazing-angle resonant soft x-ray scattering

Oxide heterostructures have shown rich physics phenomena, particularly in the conjunction of exotic insulator–metal transition (IMT) at the interface between polar insulator LaAlO3 and non-polar insulator SrTiO3 (LaAlO3/SrTiO3). The polarization catastrophe model has suggested an electronic reconstruction, yielding to metallicity at both the interface and surface. Another scenario is the occurrence of surface oxygen vacancy at LaAlO3 (surface-Ov), which has predicted surface-to-interface charge transfer, yielding metallic interface but insulating surface. To clarify the origin of IMT, one should probe surface-Ov and the associated electronic structures at both the surface and the buried interface simultaneously. Here, using grazing-angle resonant soft x-ray scattering (GA-RSXS) supported with first-principles calculations, we reveal the co-existence of the surface-Ov state and the interface conducting state only in conducting LaAlO3/SrTiO3 (001) films. Interestingly, both the surface-Ov state and the interface conducting state are absent for the insulating film. As a function of Ov density, while the surface-Ov state is responsible for the IMT, the spatial charge distribution is found responsible for a transition from two-dimensional-like to three-dimensional-like conductivity accompanied by spectral weight transfer, revealing the importance of electronic correlation. Our results show the importance of surface-Ov in determining interface properties and provide a new strategy in utilizing GA-RSXS to directly probe the surface and buried interface electronic properties in complex oxide heterostructures.

Polar Perturbations in Functional Oxide Heterostructures

AbstractGrowth and characterization of metal‐oxide thin films foster successful development of oxide‐material‐integrated thin‐film devices represented by metal‐oxide‐semiconductor field‐effect transistors (MOSFET), drawing enormous technological and scientific interest for several decades. In recent years, functional oxide heterostructures have demonstrated remarkable achievements in modern technologies and provided deeper insights into condensed‐matter physics and materials science owing to their versatile tunability and selective amplification of the functionalities. One of the most critical aspects of their physical properties is the polar perturbation stemming from the ionic framework of an oxide. By engineering and exploiting the structural, electrical, magnetic, and optical characteristics through various routes, numerous perceptive studies have clearly shown how polar perturbations advance functionalities or drive exotic physical phenomena in complex oxide heterostructures. In this review, both intrinsic (engraved by thin‐film heteroepitaxy) and extrinsic (reversibly controllable defect‐mediated disorder and polar adsorbates) elements of polar perturbations, highlighting their abilities for the development of highly tunable functional properties are summarized. Scientifically, the recent approaches of polar perturbations render one to consolidate a prospect of atomic‐level manipulation of polar order in epitaxial oxide thin films. Technologically, this review also offers useful guidelines for rational design to heterogeneously integrated oxide‐based multi‐functional devices with high performances.

Dimensionality-driven in-plane antiferromagnetism in two-dimensional SrRuO3

Control of dimensionality is a powerful tool to unlock hidden electronic and magnetic phases. In particular, reduced dimensionality in strongly correlated oxides leads to an intriguing ``dead-layer'' behavior that a transition from (ferromagnetic) metal to (antiferromagnetic) insulator occurs as the thickness approaches the two-dimensional limit. However, the origin of such transition has been a subject of debate that both the intrinsic dimensionality effect and the extrinsic disorder effect are proposed to be the driving force. Here, we reveal a transition from ferromagnetic metal with perpendicular magnetic anisotropy to antiferromagnetic insulator with in-plane magnetic anisotropy in ${\mathrm{SrRuO}}_{3}$ epitaxial films down to the monolayer limit. Experimentally, we demonstrate that ${\mathrm{SrRuO}}_{3}$ films below 3 unit cells become magnetic insulators and the spin easy axis changes to the in-plane ⟨110⟩ directions. First-principles calculations reveal that the interplay of the orbital-selective quantum confinement on Ru $4d$ orbitals and the oxygen octahedral rotation drives the ultrathin films from ferromagnetic metal to antiferromagnetic insulator, reorienting Ru spins from the perpendicular to the ⟨110⟩ directions. Our findings demonstrate how reduced dimensionality can tailor the magnetic state and provide significant advances in one of the debated topics in complex oxide heterostructures that dimensionality effect alone can be the driving force of dead-layer phenomenon in two-dimensional systems.

Origin of the giant persistent photoconductivity in LaAlO3/SrTiO3 heterostructures probed by noise spectroscopy

• The origin of the giant persistent photoconductivity in LaAlO 3 /SrTiO 3 heterostructures is explained by the photo-induced valence change in oxygen vacancies. • The change in the ionization state distribution is successfully probed by the noise spectroscopy technique. • The noise spectra correlate the ionization state distribution and the charge trapping behavior of oxygen vacancies. • The neutralization of the ionized oxygen vacancies near the LaAlO 3 /SrTiO 3 interface hinders the fast and complete charge relaxation, resulting in the strong persistent photoconductivity. LaAlO 3 /SrTiO 3 (LAO/STO) heterostructures have shown a strong persistent photoconductivity (PPC) at room temperature. The abnormally strong PPC has attracted immense research interest due to its possible applications in optically-tunable electronic devices. Despite its promise, the fundamental understanding of the PPC in the LAO/STO heterostructures is still elusive. Herein, we report that the giant PPC originates from the photo-induced valence change in oxygen vacancies near the LAO/STO interface. Our spectral analysis of the photocurrent and the model-fitting study consistently show that the ionized oxygen vacancies near the interface are neutralized during the electron relaxation process. They hinder the complete relaxation of the photoexcited electrons by the deeply-located oxygen vacancies and result in the strong PPC. The change in the ionization state distribution of the oxygen vacancies is probed by the persistent noise behavior at the frequency between 1 kHz and 20 kHz regime. These results provide insight into the role of oxygen vacancies in influencing the internal charge distribution and triggering the PPC phenomena in complex oxide heterostructures.