Ru Vacancies Research Articles

Unusual anomalous Hall effect in SrRuO3 films with linear out-of-plane Ru vacancies gradient

The manipulation of magnetic transport properties has been one of the central problems in spintronics. However, the modulation of Hall signals in thin films has stringent requirements on their thickness and means of growth. Here, a series of inhomogeneous SrRuO3 thin films with different paths of linear out-of-plane Ru vacancies gradient was designed to generate an unusual anomalous Hall effect (UAHE) under broad growth conditions. Combining x-ray diffraction and magnetic data, it was concluded that the appearance of UAHE was not a simple superposition of AHE caused by multiple magnetic phases. The interaction between these magnetic phases in the linear-vacancies-gradient SrRuO3 films was analyzed by the first-order reversal curve (FORC) method, and it was found that the change trend of FORC was the same as that of UAHE. Such out-of-plane linear-vacancies-gradient thin film provides a way to regulate the different phases by introducing the cation vacancies distribution in an orderly way to control their magnetic and transport properties in oxide films. Furthermore, a distinctive perspective on the origin of UAHE was obtained by combining FORC with UAHE.

Point defects in two-dimensional RuCl3

Defects are crucial in determining a variety of material properties especially in low dimensions. In this work, we study point defects in monolayer \ensuremath{\alpha}-phase Ruthenium (III) chloride $(\ensuremath{\alpha}-\mathrm{Ru}{\mathrm{Cl}}_{3})$, a promising candidate to realize quantum spin liquid with nearly degenerate magnetic states. Our first-principles simulations reveal that Cl vacancies, Ru vacancies, and oxygen substitutional defects are the most energetically stable point defects. Besides, these point defects break the magnetic degeneracy: Cl vacancies and oxygen substitutional defects energetically favor the zigzag-antiferromagnetic configuration while Ru vacancies favor the ferromagnetic configuration, shedding light on understanding the observed magnetic structures and further defect engineering of magnetism in monolayer $\ensuremath{\alpha}-\mathrm{Ru}{\mathrm{Cl}}_{3}$. We further calculated their electronic structures and optical absorption spectra. The polarization symmetry of optical responses provides a convenient signature to identify the point defect types and long-range magnetic orders.

Tuning magnetic anisotropy in SrRuO3 thin film by Ru vacancies induced phase transition

Effective control of magnetic anisotropy, including perpendicular magnetic anisotropy (PMA) and lateral magnetic anisotropy, is important for the design of low-power and high-density spintronic devices. However, the rarity of oxide materials with PMA and stringent conditions required to control magnetic anisotropy have prevented its large-scale application. Here, we demonstrate that the magnetic anisotropy of SrRuO3 films can be specified on-demand by adjusting the content of Ru vacancies to control the structure of the films. With the increase in Ru vacancies, the structure of SrRuO3 changes from orthorhombic to tetragonal. The field angle dependence of the Hall resistance confirmed that the uniaxial magnetic easy axis of SrRuO3 thin films continuously rotates in the (100)pc crystallographic plane, which is identical to the continuous phase transition. Our results not only provide a way to continuously tune the physical properties of epitaxial oxide films by continuously changing the composition but also help to provide guidance for the on-demand design of spintronic devices.

A well-fabricated Ru@C material derived from Ru/Zn-MOF with high activity and stability in the hydrogenation of 4-chloronitrobenzene.

4-Chloroaniline (4-CAN) plays an important role in chemical and industrial production. However, it remains a challenge to avoid the hydrogenation of the C-Cl bond in the synthesis process to improve selectivity under high activity conditions. In this study, we in situ fabricated ruthenium nanoparticles (Ru NPs) containing vacancies inserted into porous carbon (Ru@C-2) as a highly efficient catalyst for the catalytic hydrogenation of 4-chloronitrobenzene (4-CNB) with remarkable conversion (99.9%), selectivity (99.9%), and stability. Experiments and theoretical calculations indicate that the appropriate Ru vacancies affect the charge distribution of the Ru@C-2 catalyst, promote the electron transfer between the Ru metal and support, and increase the active sites of the Ru metal, thus facilitating the adsorption of 4-CNB and the desorption of 4-CAN to improve the activity and stability of the catalyst. This study can provide some enlightenment for the development of new 4-CNB hydrogenation catalysts.

Photocatalytic reduction of CO2 into CH4 over Ru-doped TiO2: Synergy of Ru and oxygen vacancies

Photocatalytic conversion of CO2 and H2O into CH4 is an intriguing approach to achieve solar energy utilization and CO2 conversion, yet remains challenging in conversion efficiency. In this study, we present a synthesis of defected TiO2 nanocrystal with oxygen vacancies (Vo) by a facile Ru doping-induced strategy under hydrothermal condition. The synergistic effect of Ru and oxygen vacancies contributed to the enhanced photocatalytic reduction of CO2 toward CH4. Oxygen vacancies and doped Ru not only can synergistically promote the separation of photogenerated carriers, but also promote the CO2 adsorption, thus enhancing the photocatalytic activities. The optimal Ru-doped TiO2 (denoted as 1% Ru-TiO2-x) exhibited a remarkable enhanced photocatalytic performance with a CH4 yield of 31.63 μmol·g−1·h−1, which is significantly higher than Ru-TiO2 and TiO2-x counterparts. This study systematically investigates the multiple roles of Ru in CO2 reduction and provides new insights for the construction of metal oxide photocatalysts with oxygen vacancies by simple doping of metal ions.

Ru nanoparticles supported on partially reduced TiO2 as highly efficient catalyst for hydrogen evolution

The development of low-cost yet highly efficient catalysts for hydrogen evolution reaction (HER) is crucial for large-scale clean and sustainable hydrogen production from water splitting. Tuning the interfacial structure of catalyst has emerged as an effective strategy to optimize the intrinsic catalytic activity. In this study, we demonstrated the deposition of Ru nanoparticles by freshly prepared strong reductive Ti(III) oxide, resulting in Ru/reduced TiO 2 interface with oxygen vacancies. The as-prepared Ru/r-TiO 2 exhibited a superior HER performance over commercial Pt/C in alkaline media, with only a small overpotential of 15 mV required to deliver the benchmark current density of 10 mA cm −2 and a high turnover frequency of 8.74 s −1 achieved at an overpotential of 100 mV. Density functional theory calculation indicates that high electrocatalytic activity of Ru/r-TiO 2 is originated from the promotion of water dissociation and weakening OH adsorption by reduced TiO 2 , which facilitate the conversion of water to H 2 . This work provides an efficient strategy for the design of high-performance HER catalysts. • Reduced TiO 2 -supported Ru nanocatalyst with negatively charged Ru and oxygen vacancies was prepared. • Ru/r-TiO 2 exhibited a superior hydrogen evolution activity in alkaline media. • Only an overpotential of 15 mV is need to deliver a benchmark current density of 10 mA cm −2 . • High HER activity was attributed to the synergy of promoting water dissociation and weakening OH* adsorption.

Structural and transport properties of highly Ru-deficient SrRu0.7O3 thin films prepared by molecular beam epitaxy: Comparison with stoichiometric SrRuO3

We investigate structural and transport properties of highly Ru-deficient SrRu0.7O3 thin films prepared by molecular beam epitaxy on (001) SrTiO3 substrates. To distinguish the influence of the two types of disorders in the films—Ru vacancies within lattices and disorders near the interface—SrRu0.7O3 thin films with various thicknesses (t = 1–60 nm) were prepared. It was found that the influence of the former dominates the electrical and magnetic properties when t ≥ 5–10 nm while that of the latter does when t ≤ 5–10 nm. Structural characterizations revealed that the crystallinity, in terms of the Sr and O sublattices, of SrRu0.7O3 thin films is as high as that of the ultrahigh-quality SrRuO3 ones. The Curie temperature (TC) analysis elucidated that SrRu0.7O3 (TC ≈ 140 K) is a material distinct from SrRuO3 (TC ≈ 150 K). Despite the large Ru deficiency (∼30%), the SrRu0.7O3 films showed metallic conduction when t ≥ 5 nm. In high-field magnetoresistance measurements, the fascinating phenomenon of Weyl fermion transport was not observed for the SrRu0.7O3 thin films irrespective of thickness, which is in contrast to the stoichiometric SrRuO3 films. The (magneto)transport properties suggest that a picture of carrier scattering due to the Ru vacancies is appropriate for SrRu0.7O3 and also that proper stoichiometry control is a prerequisite to utilizing the full potential of SrRuO3 as a magnetic Weyl semimetal and two-dimensional spin-polarized system. Nevertheless, the large tolerance in Ru composition (∼30%) to metallic conduction is advantageous for some practical applications where SrRu1−xO3 is used as an epitaxial conducting layer.

Inhomogeneous ferromagnetism mimics signatures of the topological Hall effect in SrRuO3 films

Topological transport phenomena in magnetic materials are a major topic of current condensed matter research. One of the most widely studied phenomena is the ``topological Hall effect'' (THE), which is generated via spin-orbit interactions between conduction electrons and topological spin textures such as skyrmions. We report a comprehensive set of Hall effect and magnetization measurements on epitaxial films of the prototypical ferromagnetic metal SrRuO$_3$ the magnetic and transport properties of which were systematically modulated by varying the concentration of Ru vacancies. We observe Hall effect anomalies that closely resemble signatures of the THE, but a quantitative analysis demonstrates that they result from inhomogeneities in the ferromagnetic magnetization caused by a non-random distribution of Ru vacancies. As such inhomogeneities are difficult to avoid and are rarely characterized independently, our results call into question the identification of topological spin textures in numerous prior transport studies of quantum materials, heterostructures, and devices. Firm conclusions regarding the presence of such textures must meet stringent conditions such as probes that couple directly to the non-collinear magnetization on the atomic scale.

Oxygen Vacancy Engineering for Highly Tunable Ferromagnetic Properties: A Case of SrRuO3 Ultrathin Film with a SrTiO3 Capping Layer

AbstractOxide heterostructures have great potential for spintronics applications due to their well‐defined heterointerfaces and vast functionalities. To integrate such compelling features into practical spintronics devices, effective control of the magnetic switching behavior is key. Here, continuous control of the magnetic coercive field in SrTiO3/SrRuO3 ultrathin heterostructures is achieved by oxygen vacancy (VO) engineering. Pulsed laser deposition of an oxygen‐deficient SrTiO3 capping layer can trigger VO migration into the SrRuO3 layer while avoiding the formation of Ru vacancies. Moreover, by varying the thickness and growth conditions of the SrTiO3 capping layer, the value of the coercive field (HC) in the ferromagnetic SrRuO3 layer can be continuously tuned. The maximum enhancement of HC at 5 K is 3.2 T. Such a wide‐range tunability of HC may originate from a VO‐induced enhancement of perpendicular magnetic anisotropy and domain wall pinning. This study offers effective approaches for controlling physical properties of oxide heterostructures via VO engineering, which may facilitate the development of oxide‐based functional devices.

Electronic and vibrational signatures of ruthenium vacancies in Sr2RuO4 thin films

The synthesis of stoichiometric ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$ thin films has been a challenge because of the high volatility of ruthenium oxide precursors, which gives rise to ruthenium vacancies in the films. Ru vacancies greatly affect the transport properties and electronic phase behavior of ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$, but their direct detection is difficult due to their atomic dimensions and low concentration. We applied polarized x-ray absorption spectroscopy at the oxygen $K$ edge and confocal Raman spectroscopy to ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$ thin films synthesized under different conditions. The results show that these methods can serve as sensitive probes of the electronic and vibrational properties of Ru vacancies, respectively. The intensities of the vacancy-related spectroscopic features extracted from these measurements are well correlated with the transport properties of the films. The methodology introduced here can thus help one to understand and control the stoichiometry and transport properties in films of ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$ and other ruthenates.

The effect of pressure and disorder on half-metallicity of CoRuFeSi Quaternary Heusler alloy

Heusler alloys possess some exceptional properties, such as high spin polarization and large Curie temperatures. These properties are quite sensitive to disorders. Therefore, it is quite important to investigate the effect of different disorders on electronic and magnetic properties of Heusler alloys. Keeping in view sensitivity of properties of Heusler alloys to disorder, effect of hydrostatic pressure, vacancies, swap and antisite disorder on electronic and magnetic properties of quaternary Heusler alloy CoRuFeSi is presented in this paper. Two concentrations of vacancy disorders, six type of swap disorders and twelve type of antisite disorders at four different concentrations have been studied. Under the effect of pressure, half-metallicity of CoRuFeSi is retained up to 30 GPa. A transition from perfect half-metallic to nearly half-metallic compound is observed above this pressure. In case of vacancy disorder, Co, Ru and Si vacancies are more probable to occur as compared to Fe vacancies. Among various swap disorders, Co-Ru, Co-Fe and Fe-Si swaps are most favourable due to low value of formation energies. Co-Ru and Fe-Si disordered structures shows 100% spin polarization at all concentrations which shows robustness of half-metallicity against these swap disorders. Among various antisite disorders, Co(Ru), Co(Si), Ru(Co), Ru(Si), Si(Co) and Si(Ru) have low values of formation energies and are most likely to occur. The value of energy gap decreased on introduction of various disorders which is due to emergence of new minority-spin states. These results would help in incorporating CoRuFeSi Heusler alloy in spintronic devices.

Role of electron-magnon interaction in non-Fermi liquid behavior of SrRuO3

SrRuO3 is a popular material extensively used as a bottom electrode in various applications, however, a few problems which will certainly change the interface band structure and greatly alter the device’s property are still not fully understood, such as the change of carrier types at a certain temperature and the quasiparticle scattering for non-Fermi liquid behavior below ferromagnetic transition temperature. In this study, magnetic, transport (electrical and thermal) properties and x-ray photoemission spectra have been used to understand the role of quasiparticle interactions in the SrRuO3 bulk system. At the Fermi level, the hybridization of Ru4dt2g ↓ and O2p bands form a typical two band system. In order to explain the problems as mentioned, our present work reveals that there must be an impurity band that couples with the bands around Fermi level and serves as a charge reservoir. In the present case, the impurity is attributed to the Ru vacancies. As a result, the conduction electrons scatter strongly with the Ru vacancies and couple with the Ru magnons to give rise to a dominant electron-magnon coupling that overwhelms the electron-phonon coupling in the temperature range of 90–150 K.

Vacancy-coordinated hydrogen evolution reaction on MoO3−x anchored atomically dispersed MoRu pairs

Enhanced electrocatalytic HER is demonstrated by constructing atomically dispersed MoRu pairs with abundant dangling unsaturated O and Ru vacancies on rGO-MoO3−x using an in situ versatile electrostatic assembly strategy.

Defect‐Induced Anomalous Transverse Resistivity in an Itinerant Ferromagnetic Oxide

Phenomena giving rise to voltages transverse to electric currents, such as anomalous and topological Hall effects, have been at the heart of modern condensed matter physics. Here, we show that epitaxial thin films of the itinerant ferromagnetic oxide SrRuO3 exhibit anomalous transverse resistivity associated with magnetization reversal, which is reminiscent of the emergence of the topological Hall effect. The behavior of the anomalous transverse resistivity of SrRuO3 films is found to depend on Ru vacancies. The anomalous peaking behavior in the transverse resistivity is enhanced when the film is thin enough that Ru vacancies significantly influence the magneto‐transport properties. Furthermore, films with fewer Ru vacancies exhibit no anomaly in transverse resistivity. Our observations imply that the Ru vacancies significantly influence the transverse resistivity in SrRuO3 and that defect‐induced changes in electronic structure are a plausible origin for the emergence of the anomalous peaking behavior in the transverse resistivity.

Tuning electromagnetic properties of SrRuO3 epitaxial thin films via atomic control of cation vacancies

Elemental defect in transition metal oxides is an important and intriguing subject that result in modifications in variety of physical properties including atomic and electronic structure, optical and magnetic properties. Understanding the formation of elemental vacancies and their influence on different physical properties is essential in studying the complex oxide thin films. In this study, we investigated the physical properties of epitaxial SrRuO3 thin films by systematically manipulating cation and/or oxygen vacancies, via changing the oxygen partial pressure (P(O2)) during the pulsed laser epitaxy (PLE) growth. Ru vacancies in the low-P(O2)-grown SrRuO3 thin films induce lattice expansion with the suppression of the ferromagnetic TC down to ~120 K. Sr vacancies also disturb the ferromagnetic ordering, even though Sr is not a magnetic element. Our results indicate that both A and B cation vacancies in an ABO3 perovskite can be systematically engineered via PLE, and the structural, electrical, and magnetic properties can be tailored accordingly.

Origin of abnormal structural transformation in a (BiPb)FeO3/SrRuO3/SrTiO3 hetero-structure probed by Rutherford backscattering

Scientific efforts are growing to understand artificial BiFeO3/SrRuO3/SrTiO3-heterostructures, wherein an altered environment at each interface, caused by epitaxial strains, broken symmetry, off-stoichiometry and charge transfer, can generate a rich spectrum of exotic properties. Herein, (BiPb)FeO3/SrRuO3/SrTiO3-heterostructures were sputtered with various top (BiPb)FeO3-layers at different growth temperatures (Tg). Strain relaxation at each interface changes with Tg and generates an additional peak alongside with (BiPb)FeO3 at a high Tg of 700 °C. Rutherford backscattering (RBS) was employed to understand this unusual behavior as to whether it is a mixture of two phases, layer splitting or inter-diffusion of elements. Surprisingly, complete overlapping of random and aligned RBS spectra from the sample with Tg = 700 °C indicates the presence of a large amount of defects/distortions at the interfaces. The RBS compositional analysis gives clear evidence of Fe and Ru vacancies to an extent that the structural integrity may not be maintained. This abnormal condition can be explained by the inter-diffusion of Pb and Bi elements into whole films and even into the top layer of the SrTiO3 substrate, which compensates for these vacancies by substitutional replacement and is responsible for the generation of the additional SrTi(BiPb)O3—peak. Below TcSrRuO3, the magnetic properties change significantly with Tg.

Hydrogen production by autothermal reforming of methane over lanthanum chromites modified with Ru and Sr

The partial substitution of La by Sr in lanthanum chromites (La1−ySryCr0.85Ru0.15O3, y = 0.01, 0.02, 0.05) produced changes in the size, insertion degree of Ru and oxygen vacancies of the perovskite structure. The surface and structural modifications of the La1−ySryCr0.85Ru0.15O3 perovskites brought about by substitution of La by Sr directly affected the catalytic behaviour of the perovskites in the autothermal reforming of methane (ATR). It was found that all La1−ySryCr0.85Ru0.15O3 perovskites showed excellent ATR activity that it increased with the Sr substitution degree in the perovskite. The most active Sr-substituted chromite also showed high stability and thiotolerance in the long ATR durability test in the presence of 50 ppm of sulphur. Characterization of used samples revealed not only good structural stability of the Sr-substituted perovskites under the ATR conditions but also absence of carbon deposition on the surface of used catalyst in the ATR reaction.

Hydrogen diffusion behavior and vacancy interaction behavior in OsO2 and RuO2 by ab initio calculations

The behavior of atomic hydrogen in osmium dioxide and ruthenium dioxide is investigated through ab initio calculations using density functional theory. Interstitial hydrogen behaves as a donor in both metal oxides and only has one stable interstitial position, bonding with an O atom to form an OHO defect. The rates of diffusion parallel to the c-axis are estimated to be 3.95 × 10−7exp(−0.793/kBT) m2/s and 3.64 × 10−7exp(−0.659/kBT) m2/s in OsO2 and RuO2, respectively. The rates of diffusion perpendicular to the c-axis are estimated to be 1.80 × 10−6exp(−0.941/kBT) m2/s and 1.10 × 10−6exp(−1.21/kBT) m2/s in OsO2 and RuO2, respectively. Additionally, the binding energy of multiple H atoms in (O, Os, Ru) vacancies have been calculated. High binding energies of hydrogen in metal vacancies result in multiple H atoms associating with a single metal vacancy. In RuO2, the binding energy of multiple H atoms outweighs the formation energy of a single Ru vacancy as well as that of a Ru–O divacancy.

Ruthenia-Based Electrochemical Supercapacitors: Insights from First-Principles Calculations

Electrochemical supercapacitors (ECs) have important applications in areas wherethe need for fast charging rates and high energy density intersect, including in hybrid and electric vehicles, consumer electronics, solar cell based devices, and other technologies. In contrast to carbon-based supercapacitors, where energy is stored in the electrochemical double-layer at the electrode/electrolyte interface, ECs involve reversible faradaic ion intercalation into the electrode material. However, this intercalation does not lead to phase change. As a result, ECs can be charged and discharged for thousands of cycles without loss of capacity. ECs based on hydrous ruthenia, RuO2·xH2O, exhibit some of the highest specific capacitances attained in real devices. Although RuO2 is too expensive for widespread practical use, chemists have long used it as a model material for investigating the fundamental mechanisms of electrochemical supercapacitance and heterogeneous catalysis. In this Account, we discuss progress in first-principles density-functional theory (DFT) based studies of the electronic structure, thermodynamics, and kinetics of hydrous and anhydrous RuO2. We find that DFT correctly reproduces the metallic character of the RuO2 band structure. In addition, electron-proton double-insertion into bulk RuO2 leads to the formation of a polar covalent O-H bond with a fractional increase of the Ru charge in delocalized d-band states by only 0.3 electrons. This is in slight conflict with the common assumption of a Ru valence change from Ru(4+) to Ru(3+). Using the prototype electrostatic ground state (PEGS) search method, we predict a crystalline RuOOH compound with a formation energy of only 0.15 eV per proton. The calculated voltage for the onset of bulk proton insertion in the dilute limit is only 0.1 V with respect to the reversible hydrogen electrode (RHE), in reasonable agreement with the 0.4 V threshold for a large diffusion-limited contribution measured experimentally. DFT calculations also predict that proton diffusion in RuO2 is hindered by a migration barrier of 0.8 eV, qualitatively explaining the observed strong charging rate-dependence of the diffusion-limited contribution. We found that reversible adsorption of up to 1.5 protons per Ru on the (110) surface contributes to the measured capacitive current at higher voltages. PEGS-derived models of the crystal structure of hydrated ruthenia show that incorporation of water in Ru vacancies or in bulk crystals is energetically much more costly than segregation of water molecules between slabs of crystalline RuO2. These results lend support to the so-called "water at grain boundaries" model for the structure of hydrous RuO2·xH2O. This occurs where metallic nanocrystals of RuO2 are separated by grain boundary regions filled with water molecules. Chemists have attributed the superior charge storage properties of hydrous ruthenia to the resulting composite structure. This facilitates fast electronic transport through the metallic RuO2 nanocrystals and fast protonic transport through the regions of structural water at grain boundaries.

Strain Relaxation in Atomic Flat SrRu$_{1-{\rm x}}$O$_{3}$/SrTiO$_{3}$ Layers Grown by Off-Axis RF-Sputtering

Strain-relaxed atomically flat SrRu <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$_{1-{\rm x}}$</tex></formula> O <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$_{3}$</tex> </formula> layers can be grown on SrTiO <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$_{3}$</tex></formula> substrate by a standard off-axis RF sputtering technique without using buffer layers. High RF power generated a high concentration of Ru vacancies proportional to the enlarged unit cell volume. The strong interface strain is relaxed by nano-crystal-defects induced by the large Ru vacancies and thin amorphous layer sitting in between film-substrate interface. Because of the structural defects and large unit cell expansion, the <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$T_{C}$</tex></formula> is lowered to 110–137 K as compared to bulk value of 161 K. Strong uniaxial anisotropy along [001] direction indicates that the intrinsic bulk SrRuO <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$_{3}$</tex> </formula> anisotropy dominating over the lattice expansion induced cubic anisotropy.