Interfacial charge-transfer Mott state in iridate-nickelate superlattices

Charge-Transfer-Induced Interfacial Ferromagnetism in Ferromagnet-Free Oxide Heterostructures

Due to the strong interlayer coupling between multiple degrees of freedom, oxide heterostructures have demonstrated exotic properties that are not shown by their bulk counterparts. One of the most interesting properties is ferromagnetism at the interface formed between "nonferromagnetic" compounds. Here we report on the interfacial ferromagnetic phase induced in the superlattices consisting of the two paramagnetic oxides CaRuO3 (CRO) and LaNiO3 (LNO). By varying the sublayer thickness in the superlattice period, we demonstrate that the ferromagnetic order has been established in both CaRuO3 and LaNiO3 sublayers, exhibiting an identical Curie temperature of ∼75 K. The X-ray absorption spectra suggest a strong charge transfer from Ru to Ni at the interface, triggering superexchange interactions between Ru/Ni ions and giving rise to the emergent ferromagnetic phase. Moreover, the X-ray linear dichroism spectra reveal the preferential occupancy of the d3z2-r2 orbital for the Ru ions and the dx2-y2 orbital for the Ni ions in the heterostructure. This leads to different magnetic anisotropy of the superlattices when they are dominated by CRO or LNO sublayers. This work clearly demonstrates a charge-transfer-induced interfacial ferromagnetic phase in the whole ferromagnet-free oxide heterostructures, offering a feasible way to tailor oxide materials for desired functionalities.

Hundred-Fold Enhancement in the Anomalous Hall Effect Induced by Hydrogenation

The anomalous Hall effect (AHE) is one of the most fascinating transport properties in condensed matter physics. However, the AHE magnitude, which mainly depends on net spin polarization and band topology, is generally small in oxides and thus limits potential applications. Here, we demonstrate a giant enhancement of AHE in a LaCoO3-induced 5d itinerant ferromagnet SrIrO3 by hydrogenation. The anomalous Hall resistivity and anomalous Hall angle, which are two of the most critical parameters in AHE-based devices, are found to increase to 62.2 μΩ·cm and 3%, respectively, showing an unprecedentedly large enhancement ratio of ∼10000%. Theoretical analysis suggests the key roles of Berry curvature in enhancing AHE. Furthermore, the hydrogenation concomitantly induces the significant elevation of Curie temperature from 75 to 160 K and 40-fold reinforcement of coercivity. Such giant regulation and very large AHE magnitude observed in SrIrO3 could pave the path for 5d oxide devices.

Emergent quantum phenomena in atomically engineered iridate heterostructures

Over the last few years, researches in iridates have developed into an exciting field with the discovery of numerous emergent phenomena, interesting physics, and intriguing functionalities. Among the studies, iridate-based artificial structures play a crucial role owing to their extreme flexibility and tunability in lattice symmetry, chemical composition, and crystal dimensionality. In this article, we present an overview of the recent progress regarding iridate-based artificial structures. We first explicitly introduce several essential concepts in iridates. Then, we illustrate important findings on representative SrIrO3/SrTiO3 superlattices, heterostructures comprised of SrIrO3 and magnetic oxides, and their response to external electric-field stimuli. Finally, we comment on existing problems and promising future directions in this exciting field.

Nd:YAG infrared laser as a viable alternative to excimer laser: YBCO case study

We report on the growth and characterization of epitaxial YBa[Formula: see text]Cu[Formula: see text]O[Formula: see text] (YBCO) complex oxide thin films and related heterostructures exclusively by Pulsed Laser Deposition (PLD) and using first harmonic Nd:Y[Formula: see text]Al[Formula: see text]O[Formula: see text] (Nd:YAG) pulsed laser source ([Formula: see text] = 1064  nm). High-quality epitaxial YBCO thin film heterostructures display superconducting properties with transition temperature [Formula: see text] 80 K. Compared with the excimer lasers, when using Nd:YAG lasers, the optimal growth conditions are achieved at a large target-to-substrate distance d. These results clearly demonstrate the potential use of the first harmonic Nd:YAG laser source as an alternative to the excimer lasers for the PLD thin film community. Its compactness as well as the absence of any safety issues related to poisonous gas represent a major breakthrough in the deposition of complex multi-element compounds in form of thin films.

Controllable Itinerant Ferromagnetism in Weakly Correlated 5d SrIrO3

The weakly correlated nature of 5d oxide SrIrO₃ determines its rare ferromagnetism, and the control of its magnetic order is even less studied. Tailoring structure distortion is currently a main route to tune the magnetic order of 5d iridates, but only for the spatially confined insulating counterparts. Here, we have realized ferromagnetic order in metallic SrIrO₃ by construction of SrIrO₃/ferromagnetic-insulator (LaCoO₃) superlattices, which reveal a giant coercivity of ∼10 T and saturation field of ∼25 T with strong perpendicular magnetic anisotropy. The Curie temperature of SrIrO₃ can be controlled by engineering interface charge transfer, which is confirmed by Hall effect measurements collaborating with EELS and XAS. Besides, the noncoplanar spin texture is captured, which is caused by interfacial Dzyaloshinskii-Moriya interactions as well. These results indicate controllable itinerant ferromagnetism and an emergent topological magnetic state in strong spin-orbit coupled semimetal SrIrO₃, showing great potential to develop efficient spintronic devices.

Oxide Spintronics as a Knot of Physics and Chemistry: Recent Progress and Opportunities

Transition-metal oxides (TMOs) constitute a key material family in spintronics because of mutually coupled degrees of freedom and tunable magneto-ionic properties. In this Perspective, we consider oxide spintronics as a knot of physics and chemistry and mainly discuss two current hot topics: spin-charge interconversion and magneto-ionics. First, spin-charge interconversion is focused on oxide films and heterostructures including 4d/5d heavy metal oxides (e.g., SrIrO₃) and two-dimensional electron gases. Based on spin-charge interconversion, charge currents can be transformed to spin currents and generate spin-orbit torque in oxide/metal and all-oxide heterostructures. Additionally, the voltage control of magnetism in TMOs by the magneto-ionic pathway has rapidly accelerated during the past few years due to the versatile advantages of effective control, nonvolatile nature, low power cost, etc. Typical magneto-ionic oxide systems and corresponding physicochemical mechanisms will be discussed. Finally, further developments of oxide spintronics are envisioned, including material discovery, physics exploration, device design, and manipulation methods.

Dimensionality-Controlled Evolution of Charge-Transfer Energy in Digital Nickelates Superlattices

Fundamental understanding and control of the electronic structure evolution in rare-earth nickelates is a fascinating and meaningful issue, as well as being helpful to understand the mechanism of recently discovered superconductivity. Here the dimensionality effect on the ground electronic state in high-quality (NdNiO₃ ) _m /(SrTiO₃ )₁ superlattices is systematically studied through transport and soft X-ray absorption spectroscopy. The metal-to-insulator transition temperature decreases with the thickness of the NdNiO₃ slab decreasing from bulk to 7 unit cells, then increases gradually as m further reduces to 1 unit cell. Spectral evidence demonstrates that the stabilization of insulating phase can be attributed to the increase of the charge-transfer energy between O 2p and Ni 3d bands. The prominent multiplet feature on the Ni L₃ edge develops with the decrease of NdNiO₃ slab thickness, suggesting the strengthening of the charge disproportionate state under the dimensional confinement. This work provides convincing evidence that dimensionality is an effective knob to modulate the charge-transfer energy and thus the collective ground state in nickelates.

Visualizing Spatial Evolution of Electron-Correlated Interface in Two-Dimensional Heterostructures

Microscopically visualizing the evolution of electronic structures at the interface between two electron-correlated domains shows fundamental importance in both material science and physics. Here, we report scanning tunneling microscopy and spectroscopy studies of the interfacial electronic structures evolution in a phase-engineered monolayer NbSe₂ heterostructure. The H-NbSe₂ metallic state penetrates the Mott insulating T-NbSe₂ at the H/T phase interface, with a prominent 2D charge density wave (CDW) proximity effect. Moreover, an insulating Mott gap collapse with the disappearance of the upper Hubbard band is detected at the electronic phase transition region. Theoretical calculations reveal that such insulating Mott gap collapse can be attributed to the electron doping effect induced by the interface. Our findings promote a microscopical understanding of the interactions between different electron-correlated systems and provide an effective method for controlling the Mott insulating states with phase engineering.

Novel Spin-Orbit Torque Generation at Room Temperature in an All-Oxide Epitaxial La0.7 Sr0.3 MnO3 /SrIrO3 System

Spin-orbit torques (SOTs) that arise from materials with large spin-orbit coupling offer a new pathway for energy-efficient and fast magnetic information storage. SOTs in conventional heavy metals and topological insulators are explored extensively, while 5d transition metal oxides, which also host ions with strong spin-orbit coupling, are a relatively new territory in the field of spintronics. An all-oxide, SrTiO₃ (STO)//La_0.7 Sr_0.3 MnO₃ (LSMO)/SrIrO₃ (SIO) heterostructure with lattice-matched crystal structure is synthesized, exhibiting an epitaxial and atomically sharp interface between the ferromagnetic LSMO and the high spin-orbit-coupled metal SIO. Spin-torque ferromagnetic resonance (ST-FMR) is used to probe the effective magnetization and the SOT efficiency in LSMO/SIO heterostructures grown on STO substrates. Remarkably, epitaxial LSMO/SIO exhibits a large SOT efficiency, ξ_||  = 1, while retaining a reasonably low shunting factor and increasing the effective magnetization of LSMO by ≈50%. The findings highlight the significance of epitaxy as a powerful tool to achieve a high SOT efficiency, explore the rich physics at the epitaxial interface, and open up a new pathway for designing next-generation energy-efficient spintronic devices.

Hole-Trapping-Induced Stabilization of Ni4 + in SrNiO3 /LaFeO3 Superlattices

Creating new functionality in materials containing transition metals is predicated on the ability to control the associated charge states. For a given transition metal, there is an upper limit on valence that is not exceeded under normal conditions. Here, it is demonstrated that this limit of 3+ for Ni and Fe can be exceeded via synthesis of (SrNiO₃ )_m /(LaFeO₃ )_n superlattices by tuning n and m. The Goldschmidt tolerance constraints are lifted, and SrNi⁴⁺ O₃ with holes on adjacent O anions is stabilized as a perovskite at the single-unit-cell level (m = 1). Holding m = 1, spectroscopy reveals that the n = 1 superlattice contains Ni³⁺ and Fe⁴⁺ , whereas Ni⁴⁺ and Fe³⁺ are observed in the n = 5 superlattice. It is revealed that the B-site cation valences can be tuned by controlling the magnitude of the FeO₆ octahedral rotations, which, in turn, determine the energy balance between Ni³⁺ /Fe⁴⁺ and Ni⁴⁺ /Fe³⁺ , thus controlling emergent electrical properties such as the band alignment and resulting hole confinement. This approach can be extended to other systems for synthesizing novel, metastable layered structures with new functionalities.

Emergent electric field control of phase transformation in oxide superlattices

Electric fields can transform materials with respect to their structure and properties, enabling various applications ranging from batteries to spintronics. Recently electrolytic gating, which can generate large electric fields and voltage-driven ion transfer, has been identified as a powerful means to achieve electric-field-controlled phase transformations. The class of transition metal oxides provide many potential candidates that present a strong response under electrolytic gating. However, very few show a reversible structural transformation at room-temperature. Here, we report the realization of a digitally synthesized transition metal oxide that shows a reversible, electric-field-controlled transformation between distinct crystalline phases at room-temperature. In superlattices comprised of alternating one-unit-cell of SrIrO3 and La0.2Sr0.8MnO3, we find a reversible phase transformation with a 7% lattice change and dramatic modulation in chemical, electronic, magnetic and optical properties, mediated by the reversible transfer of oxygen and hydrogen ions. Strikingly, this phase transformation is absent in the constituent oxides, solid solutions and larger period superlattices. Our findings open up this class of materials for voltage-controlled functionality.

Carrier localization in perovskite nickelates from oxygen vacancies

Point defects, such as oxygen vacancies, control the physical properties of complex oxides, relevant in active areas of research from superconductivity to resistive memory to catalysis. In most oxide semiconductors, electrons that are associated with oxygen vacancies occupy the conduction band, leading to an increase in the electrical conductivity. Here we demonstrate, in contrast, that in the correlated-electron perovskite rare-earth nickelates, RNiO₃ (R is a rare-earth element such as Sm or Nd), electrons associated with oxygen vacancies strongly localize, leading to a dramatic decrease in the electrical conductivity by several orders of magnitude. This unusual behavior is found to stem from the combination of crystal field splitting and filling-controlled Mott-Hubbard electron-electron correlations in the Ni 3d orbitals. Furthermore, we show the distribution of oxygen vacancies in NdNiO₃ can be controlled via an electric field, leading to analog resistance switching behavior. This study demonstrates the potential of nickelates as testbeds to better understand emergent physics in oxide heterostructures as well as candidate systems in the emerging fields of artificial intelligence.