Real-space observation of ferroelectrically induced magnetic spin crystal in SrRuO3

Orientation-modulated oxygen evolution reaction in epitaxial SrRuO3 films

The SrRuO3 films were grown on SrTiO3 using a lattice matching strategy. Scanning electrochemical microscopy imaged local oxygen evolution reaction (OER) performance, exploring the relationship between micro-area activity and the termination layer. Combined with traditional electrochemical methods and DFT calculations, the OER activity was correlated with the electronic structure.

Light-Induced Mott-Insulator-to-Metal Phase Transition in Ultrathin Intermediate-Spin Ferromagnetic Perovskite Ruthenates

Light control of emergent quantum phenomena is a widely used external stimulus for quantum materials. Generally, perovskite strontium ruthenate SrRuO₃ has an itinerant ferromagnetism with a low-spin state. However, the phase of intermediate-spin (IS) ferromagnetic metallic state has never been seen. Here, by means of UV-light irradiation, a photocarrier-doping-induced Mott-insulator-to-metal phase transition is shown in a few atomic layers of perovskite IS ferromagnetic SrRuO_{3- δ} . This new metastable IS metallic phase can be reversibly regulated due to the convenient photocharge transfer from SrTiO₃ substrates to SrRuO_{3- δ} ultrathin films. These dynamical mean-field theory calculations further verify such photoinduced electronic phase transformation, owing to oxygen vacancies and orbital reconstruction. The optical manipulation of charge-transfer finesse is an alternative pathway toward discovering novel metastable phases in strongly correlated systems and facilitates potential light-controlled device applications in optoelectronics and spintronics.

High-Mobility Magnetic Two-Dimensional Electron Gas in Engineered Oxide Interfaces

The engineered interfaces of complex oxides have abundant physical properties and provide a powerful platform for the exploration of fundamental physics and emergent phenomena. In particular, research on the two-dimensional magnetic systems with high mobility remains a long-standing challenge for the discovery of quantum phase and spintronic applications. Here, we introduce a few atomic layers of the delta doping layer at LaAlO₃/SrTiO₃ interfaces through elaborately controllable epitaxial growth of SrRuO₃. After inserting a SrRuO₃ buffer layer, the interfaces exhibit a well-defined anomalous Hall effect up to 100 K and their mobility is enhanced by 3 orders of magnitude at low temperatures. More intriguingly, a large unsaturated positive magnetoresistance is created at interfaces. Combining with the density functional theory calculation, we attribute our findings to the electron transfer at interfaces and the magnetic moment of Ru⁴⁺ 4d bands. The results pave a way for further research of two-dimensional ferromagnetism and quantum transport in all-oxide systems.

Strong Interfacial Charge Trapping in Ultrathin SrRuO3 on SrTiO3 Probed by Noise Spectroscopy

SrRuO₃ (SRO) has emerged as a promising quantum material due to its exotic electron correlations and topological properties. In epitaxial SRO films, electron scattering against lattice phonons or defects has been considered as only a predominant mechanism accounting for electronic properties. Although the charge trapping by polar defects can also strongly influence the electronic behavior, it has often been neglected. Herein, we report strong interfacial charge trapping in ultrathin SRO films on SrTiO₃ (STO) substrates probed by noise spectroscopy. We find that oxygen vacancies in the STO cause stochastic interfacial charge trapping, resulting in high electrical noise. Spectral analyses of the photoinduced noise prove that the oxygen vacancies buried deep in the STO can effectively contribute to the charge trapping process. These results unambiguously reveal that electron transport in ultrathin SRO films is dominated by the carrier number fluctuation that correlates with interfacial charge trapping.

First-principles study of the crystal and magnetic structures of multiferroic Cu2OCl2

Recently, the discovery of multiferroicity in pyrochlore-like compound Cu₂OCl₂has generated significant interest, and several studies have been performed in this area. This transition metal oxychloride is unique because the divalent copper atoms create anS=1/2correlated insulator and the pyrochlore lattice tends to frustrate spins. From neutron powder diffraction measurements, an incommensurate magnetic order of the ordering vectorq=(0.827,0,0)emerges below the Néel temperature of 70 K. At this temperature or slightly above, ferroelectricity (FE) or antiferroelectricity, accompanying a lattice distortion, has been observed. Experimentally, some discrepancies remain. In this paper, we report our first-principles simulation results by evaluating the possible lattice and spin spiral states. We found that theFdddstructure is not more stable thanFdd2(a), which is supported by our reexamination of the x-ray diffraction data. In addition, we find that after we include magnetism in the calculation, it predicts that theFdd2(a)lattice with a helical (proper screw) spin structure is energetically more stable than other spin configurations. Our results indicate charge-order-driven FE that subsequently induces magnetism.

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.

Ferroelectric incommensurate spin crystals

Ferroics, especially ferromagnets, can form complex topological spin structures such as vortices¹ and skyrmions^2,3 when subjected to particular electrical and mechanical boundary conditions. Simple vortex-like, electric-dipole-based topological structures have been observed in dedicated ferroelectric systems, especially ferroelectric-insulator superlattices such as PbTiO₃/SrTiO₃, which was later shown to be a model system owing to its high depolarizing field^4-8. To date, the electric dipole equivalent of ordered magnetic spin lattices driven by the Dzyaloshinskii-Moriya interaction (DMi)^9,10 has not been experimentally observed. Here we examine a domain structure in a single PbTiO₃ epitaxial layer sandwiched between SrRuO₃ electrodes. We observe periodic clockwise and anticlockwise ferroelectric vortices that are modulated by a second ordering along their toroidal core. The resulting topology, supported by calculations, is a labyrinth-like pattern with two orthogonal periodic modulations that form an incommensurate polar crystal that provides a ferroelectric analogue to the recently discovered incommensurate spin crystals in ferromagnetic materials^11-13. These findings further blur the border between emergent ferromagnetic and ferroelectric topologies, clearing the way for experimental realization of further electric counterparts of magnetic DMi-driven phases.

Ferroelectric Proximity Effect and Topological Hall Effect in SrRuO3/BiFeO3 Multilayers

Interfaces between complex oxides provide a unique opportunity to discover novel interfacial physics and functionalities. Here, we fabricate the multilayers of itinerant ferromagnet SrRuO₃ (SRO) and multiferroic BiFeO₃ (BFO) with atomically sharp interfaces. Atomically resolved transmission electron microscopy reveals that a large ionic displacement in BFO can penetrate into SRO layers near the BFO/SRO interfaces to a depth of 2-3 unit cells, indicating the ferroelectric proximity effect. A topological Hall effect is indicated by hump-like anomalies in the Hall measurements of the multilayer with a moderate thickness of the SRO layer. With magnetic measurements, it can be further confirmed that each SRO layer in the multilayers can be divided into interfacial and middle regions, which possess different magnetic ground states. Our work highlights the key role of functional heterointerfaces in exotic properties and provides an important guideline to design spintronic devices based on magnetic skyrmions.