SrRuO3 Films Research Articles

Strain-controlled electrical and magnetic properties of SrRuO3 thin films with Sr3Al2O6 buffer layers

SrRuO3 (SRO) thin films have been grown on a (001)-oriented SrTiO3 substrate with Sr3Al2O6 (SAO) buffer layers using pulsed laser deposition. X-ray diffraction results reveal that the epitaxial strain of SRO changes from compressive to relaxed or tensile ones by tuning the thickness of the SAO buffer layer (tSAO). We have demonstrated that the variation of strain has a strong influence on the microstructure and electrical and magnetic properties of SRO. When tSAO < 10 nm, the epitaxial strain is relaxed and the SRO film possesses higher Curie temperature resembling that of SRO bulk. Upon further increasing tSAO ≥ 10 nm, the SRO films are subjected to tensile strain, showing a typical step-and-terrace surface and coherent epitaxy characteristic on the SAO buffer layer. The electrical and magnetic properties of SRO are very sensitive to buffer layer-controlled epitaxial strain. The tensile strained SRO films show quite different electrical transport properties at low temperature, i.e., appearance of metal-insulator transition and positive magnetoresistances and the absence of non-Fermi-liquid behavior. Additionally, magnetic anisotropy is found in both the tensile and the compressively strained SRO, while the strain-relaxed film shows isotropic magnetization. Based on the electrical and magnetic properties, a phase diagram of SRO on the SAO buffer layer has been constructed.

Local Kondo scattering in 4d-electron RuOx nanoclusters on atomically-resolved ultrathin SrRuO3 films.

Perovskite SrRuO3 is a unique 4d transition metal oxide with coexisting spin-orbit coupling (SOC) and electron-electron correlation. However, the intrinsic, non-reconstructed surface structure of SrRuO3 has not been reported so far. Here we report an atomic imaging of the non-reconstructed, SrO-terminated SrRuO3 surface by scanning tunneling microscopy/spectroscopy. Moreover, a Kondo resonant behavior is revealed in RuOx clusters located on top of the nonmagnetic SrO surface layer. The density functional theory calculations confirm that RuOx clusters possess localized 4d-electron-involved spin moments and hybridize with the conduction electrons in the metal host, resulting in the appearance of the Kondo resonance features around the Fermi level. Our work demonstrates that artificially-engineered transition metal oxides provide new opportunities to explore the Kondo physics in 4d multi-orbital systems.

Low-resistivity SrRuO3 thin films formed on SiO2 substrates without buffer layer by RF magnetron sputtering

Thin films of SrRuO3, which is a conductive oxide, have the potential for use as electrode layers in versatile electronic applications, particularly with perovskite structured dielectric and ferroelectric oxide thin films. Moreover, for practical device applications, it is important to develop the deposition process of low-resistivity SrRuO3 thin films on SiO2 substrates. Therefore, in this study, the growth and characteristics of SrRuO3 thin films deposited by radiofrequency magnetron sputtering on SiO2 substrates without a buffer layer were investigated. It was found that the oxygen flow ratio in the growth ambient, post-deposition annealing temperature, and post-deposition annealing time are key parameters in realizing high-quality SrRuO3 thin films. When SrRuO3 thin films were deposited with a gas flow ratio of Ar:O2 = 1:1.5 and subjected to post-deposition annealing in an oxygen ambient at 750 °C for 3 min, they exhibited an excellent resistivity of 148 μΩ-cm. This value is comparable to that of SrRuO3 thin films epitaxially grown on single-crystal SrTiO3 or LaAlO3 substrates.

Magnetic domain engineering in SrRuO3 thin films

Magnetic domain engineering in ferromagnetic thin films is a very important route toward the rational design of spintronics and memory devices. Although the magnetic domain formation has been extensively studied, artificial control of magnetic domain remains challenging. Here, we present the control of magnetic domain formation in paradigmatic SrRuO3/SrTiO3 heterostructures via structural domain engineering. The formation of structural twin domains in SrRuO3 films can be well controlled by breaking the SrTiO3 substrate symmetry through engineering miscut direction. The combination of x-ray diffraction analysis of structural twin domains and magnetic imaging of reversal process demonstrates a one-to-one correspondence between structural domains and magnetic domains, which results in multi-step magnetization switching and anomalous Hall effect in films with twin domains. Our work sheds light on the control of the magnetic domain formation via structural domain engineering, which will pave a path toward desired properties and devices applications.

Quantum transport evidence of Weyl fermions in an epitaxial ferromagnetic oxide

Magnetic Weyl semimetals have novel transport phenomena related to pairs of Weyl nodes in the band structure. Although the existence of Weyl fermions is expected in various oxides, the evidence of Weyl fermions in oxide materials remains elusive. Here we show direct quantum transport evidence of Weyl fermions in an epitaxial 4d ferromagnetic oxide SrRuO3. We employ machine-learning-assisted molecular beam epitaxy to synthesize SrRuO3 films whose quality is sufficiently high to probe their intrinsic transport properties. Experimental observation of the five transport signatures of Weyl fermions—the linear positive magnetoresistance, chiral-anomaly-induced negative magnetoresistance, π phase shift in a quantum oscillation, light cyclotron mass, and high quantum mobility of about 10,000 cm2V−1s−1—combined with first-principles electronic structure calculations establishes SrRuO3 as a magnetic Weyl semimetal. We also clarify the disorder dependence of the transport of the Weyl fermions, which gives a clear guideline for accessing the topologically nontrivial transport phenomena.

Polar Metal Phase Induced by Oxygen Octahedral Network Relaxation in Oxide Thin Films

ABO3 perovskite materials and their derivatives have inherent structural flexibility due to the corner sharing network of the BO6 octahedron, and the large variety of possible structural distortions and strong coupling between lattice and charge/spin degrees of freedom have led to the emergence of intriguing properties, such as high-temperature superconductivity, colossal magnetoresistance, and improper ferroelectricity. Here, an unprecedented polar ferromagnetic metal phase in SrRuO3 (SRO) thin films is presented, arising from the strain-controlled oxygen octahedral rotation (OOR) pattern. For compressively strained SRO films grown on SrTiO3 substrate, oxygen octahedral network relaxation is accompanied by structural phase separation into strained tetragonal and bulk-like orthorhombic phases, and the asymmetric OOR evolution across the phase boundary allows formation of the polar phase, while bulk metallic and ferromagnetic properties are maintained. From the results, it is expected that other oxide perovskite thin films will also yield similar structural environments with variation of OOR patterns, and thereby provide promising opportunities for atomic scale control of material properties through strain engineering.

Detection of the Chiral Spin Structure in Ferromagnetic SrRuO3 Thin Film.

SrRuO3 (SRO) thin films and their heterostructure have attracted much attention because of the recently demonstrated fascinating properties, such as topological Hall effect and skyrmions. Critical to the understanding of those SRO properties is the study of the spin configuration. Here, we conduct resonant soft X-ray scattering (RSXS) at the oxygen K edge to investigate the spin configuration of a four-unit-cell SRO film that was grown epitaxially on a single-crystal SrTiO3. The RSXS signal under a magnetic field (∼0.4 tesla) clearly shows a magnetic dichroism pattern around the specular reflection. Model calculations on the RSXS signal demonstrate that the magnetic dichroism pattern originates from a Néel-type chiral spin structure in this SRO thin film. We believe that the observed spin structure of the SRO system is a critical piece of information for understanding its intriguing magnetic and transport properties.

Structural transformations and desirable electronic properties of low-temperature grown ruthenium oxide films under nitrogen atmosphere

Sr2RuO4 has recently attracted intense attentions for its intriguing physical properties. However, the ultra-high growth temperature (≥1000 °C) poses a major challenge for fabricating high-quality Sr2RuO4 films. In sharp contrast to effort previously attempted in traditional oxygen atmosphere, herein, a nitrogen gas is employed to grow ruthenium oxide films during pulsed laser depositions (PLD). It is shown that the present method facilitates a successive SrRuO3–Sr2RuO4 structural transformation via increasing growth temperatures from 700 °C to 900 °C. The high-quality Sr2RuO4 film grown at a relative low deposition temperature exhibits a comparable electrical resistivity, in comparison with counterparts deposited at oxygen atmosphere. Combined X-ray photoelectron and X-ray absorption measurements suggest that the nonreactive nitrogen atmosphere involved during the deposition yields a relative lower kinetic energy of laser ablated plume reaching to substrates, and effectively regulates structural and electronic properties of ruthenium oxide films. These findings may offer a new route for synthesizing other oxide functional films with high deposition-temperature.

Negligible Substrate‐Induced Strain Effects on Magnetic Properties of SrRuO3 Thin Films

SrRuO3 is known to exhibit strong magnetocrystalline effect, which enables us to control its magnetic properties using lattice engineering. The magnetocrystalline effect of SrRuO3 is systematically investigated under both compressive and tensile strain by depositing SrRuO3 thin films on various oxide substrates with different in‐plane lattice mismatch. All SrRuO3 films have high crystallinity with flat interface and surface. It is found that out‐of‐plane lattice constant is decreased with increasing strain. Moreover, unit cell volume decreases with strain due to structural distortion attributable to RuO6 octahedral distortion as theoretically expected. However, magnetization does not match to any of the theoretical models only considering structural changes; at current stage, internal interactions rather than structural changes seem critical in determining the magnetization of the SrRuO3 films. This study can play a crucial role in understanding internal correlations of magnetic properties in ruthenate films and can trigger research on magnetic functionality.

Electronic nematicity in Sr2RuO4

We have measured the angle-resolved transverse resistivity (ARTR), a sensitive indicator of electronic anisotropy, in high-quality thin films of the unconventional superconductor Sr2RuO4 grown on various substrates. The ARTR signal, heralding the electronic nematicity or a large nematic susceptibility, is present and substantial already at room temperature and grows by an order of magnitude upon cooling down to 4 K. In Sr2RuO4 films deposited on tetragonal substrates the highest-conductivity direction does not coincide with any crystallographic axis. In films deposited on orthorhombic substrates it tends to align with the shorter axis; however, the magnitude of the anisotropy stays the same despite the large lattice distortion. These are strong indications of actual or incipient electronic nematicity in Sr2RuO4.

Topological Hall Effect in Single Thick SrRuO3 Layers Induced by Defect Engineering

AbstractThe topological Hall effect (THE) has been discovered in ultrathin SrRuO3 (SRO) films, where the interface between the SRO layer and another oxide layer breaks the inversion symmetry resulting in the appearance of THE. Thus, THE only occurs in ultrathin SRO films of several unit cells. In addition to employing a heterostructure, the inversion symmetry can be broken intrinsically in bulk SRO by introducing defects. In this study, THE is observed in 60‐nm‐thick SRO films, in which defects and lattice distortions are introduced by helium ion irradiation. The irradiated SRO films exhibit a pronounced THE in a wide temperature range from 5 to 80 K. These observations can be attributed to the emergence of Dzyaloshinskii–Moriya interaction as a result of artificial inversion symmetry breaking associated with the lattice defect engineering. The creation and control of the THE in oxide single layers can be realized by ex situ film processing. Therefore, this work provides new insights into the THE and illustrates a promising strategy to design novel spintronic devices.

Controllable Thickness Inhomogeneity and Berry Curvature Engineering of Anomalous Hall Effect in SrRuO3 Ultrathin Films.

In quantum matters hosting electron-electron correlation and spin-orbit coupling, spatial inhomogeneities, arising from competing ground states, can be essential for understanding exotic topological properties. A prominent example is Hall anomalies observed in SrRuO3 films, which were interpreted in terms of either magnetic skyrmion-induced topological Hall effect or inhomogeneous anomalous Hall effect (AHE). To clarify this ambiguity, we systematically investigated the evolution of AHE with controllable inhomogeneities in SrRuO3 film thickness (tSRO). By exploiting the step-flow growth of SrRuO3 films, we induced a microscopically ordered stripe pattern with one-unit-cell differences in tSRO. The associated spatial distribution of momentum-space Berry curvatures enables a two-channel AHE with hump-like Hall anomalies, which can be continuously engineered according to non-integer tSRO. We further microscopically characterized the stripe-like ferromagnetic domains and two-step magnetic switching behavior in the inhomogeneous SrRuO3 film. These unique features can be utilized to identify the two-channel AHE model and understand its microscopic origin.

Magnon profile on SrRuO3 films studied by inelastic neutron scattering.

In this study, the inelastic neutron scattering probe of SIKA in ANSTO is employed to investigate the magnon dispersion curve in ferromagnetic SrRuO3 single crystal epitaxial films and to better understand the underlying mechanisms. This report presents the successful measurement of a magnon peak from the SrRuO3 films which contained an amount of material of only 0.9 mg. We reveal one significant magnon dispersion curve along [002] following the quadratic relation, which shows a magnon gap of 0.32 meV. We have discussed several possible mechanisms, such as the higher symmetry structure and the impurity levels, which may contribute to this smaller gap.

Interfacial Control of Ferromagnetism in Ultrathin SrRuO3 Films Sandwiched between Ferroelectric BaTiO3 Layers.

Interfaces between materials provide an intellectually rich arena for fundamental scientific discovery and device design. However, the frustration of magnetization and conductivity of perovskite oxide films under reduced dimensionality is detrimental to their device performance, preventing their active low-dimensional application. Herein, by inserting the ultrathin 4d ferromagnetic SrRuO3 layer between ferroelectric BaTiO3 layers to form a sandwich heterostructure, we observe enhanced physical properties in ultrathin SrRuO3 films, including longitudinal conductivity, Curie temperature, and saturated magnetic moment. Especially, the saturated magnetization can be enhanced to ∼3.12 μB/Ru in ultrathin BaTiO3/SrRuO3/BaTiO3 trilayers, which is beyond the theoretical limit of bulk value (2 μB/Ru). This observation is attributed to the synergistic ferroelectric proximity effect (SFPE) at upper and lower BaTiO3/SrRuO3 heterointerfaces, as revealed by the high-resolution lattice structure analysis. This SFPE in dual-ferroelectric interface cooperatively induces ferroelectric-like lattice distortions in RuO6 oxygen octahedra and subsequent spin-state crossover in SrRuO3, which in turn accounts for the observed enhanced magnetization. Besides the fundamental significance of interface-induced spin-lattice coupling, our findings also provide a viable route to the electrical control of magnetic ordering, taking a step toward low-power applications in all-oxide spintronics.

Correlating the Nanoscale Structural, Magnetic, and Magneto-Transport Properties in SrRuO3-Based Perovskite Thin Films: Implications for Oxide Skyrmion Devices

We investigated the structural and magnetic properties of bare SrRuO3 (SRO) ultrathin films and SrRuO3/SrIrO3/SrZrO3 (SRO/SIO/SZO: RIZ) trilayer heterostructures between 10 and 80 K, by comparing macroscopic data using the magneto-optical Kerr effect (MOKE) and magneto-transport (anomalous Hall effect: AHE), with nanoscale fingerprints when applying noncontact scanning force microscopy (nc-SFM) and magnetic force microscopy (MFM). SRO and RIZ ultrathin films were epitaxially grown at 650 °C onto vicinal SrTiO3 (100) single-crystalline substrates to a nominal thickness of 4 and 4/2/2 unit cells (uc), respectively. Our correlated analysis allows associating topographic sample features of overgrown individual layers to their residual magnetization, as is shown here to be relevant for interpreting the macroscopic AHE data. Although the hump-like features in the AHE suggest a magnetically textured skyrmion phase to exist around 55 K associated with the topological Hall effect (THE), both our MOKE and MFM data ...

Characterization of Sr2RuO4 Josephson junctions made of epitaxial films

We have studied fundamental properties of weak-link Sr2RuO4/Sr2RuO4 Josephson junctions fabricated by making a narrow constriction on superconducting Sr2RuO4 films through laser micro-patterning. The junctions show a typical overdamped behavior with much higher critical current density, compared with those previously reported for bulk Sr2RuO4/s-wave superconductor junctions. Observed magnetic field and temperature dependences of the Josephson critical current suggest that the chiral p-wave is unlikely for the superconducting symmetry, encouraging further theoretical calculations of the Sr2RuO4/Sr2RuO4 type junctions.

Topological Signatures in the Hall Effect of SrRuO3/La0.7Sr0.3MnO3 SLs

Noncollinear spin textures such as skyrmions lead to a novel Hall effect contribution called the topological Hall effect. In recent years, research has focused on the Weyl metal SrRuO3 with topological signatures reported for SrRuO3 layers adjacent to layers with strong spin–orbit coupling. However, SrRuO3 films are known to be prone to structural modifications when interfaced with different materials. In this work the Hall effect of SrRuO3/La0.7Sr0.3MnO3 superlattices (SLs) is correlated with structural and magnetic information to study whether a topological Hall effect is induced by interfacing to ferromagnetic layers. High‐quality SLs with ultrathin layers are fabricated by pulsed laser deposition on SrTiO3 substrates. The symmetry of the SrRuO3 layers is studied by angular magnetoresistance measurements and the magnetocrystalline anisotropy by magnetization measurements. Two SLs are compared in detail, both with 8 unit cell thick SrRuO3 layers, but with La0.7Sr0.3MnO3 layers 2 and 4 unit cell thick. Only the sample with 2 unit cell thick manganite layers shows features resembling a topological Hall effect. Depending on the La0.7Sr0.3MnO3 layer thickness the SrRuO3 crystalline symmetry is orthorhombic or tetragonal. The results indicate that the topological Hall effect features arise from an intricate interplay between magnetocrystalline anisotropy and antiferromagnetic interlayer coupling.

Interface-induced magnetic polar metal phase in complex oxides

Polar metals are commonly defined as metals with polar structural distortions. Strict symmetry restrictions make them an extremely rare breed as the structural constraints favor insulating over metallic phase. Moreover, no polar metals are known to be magnetic. Here we report on the realization of a magnetic polar metal phase in a BaTiO3/SrRuO3/BaTiO3 heterostructure. Electron microscopy reveals polar lattice distortions in three-unit-cells thick SrRuO3 between BaTiO3 layers. Electrical transport and magnetization measurements reveal that this heterostructure possesses a metallic phase with high conductivity and ferromagnetic ordering with high saturation moment. The high conductivity in the SrRuO3 layer can be attributed to the effect of electrostatic carrier accumulation induced by the BaTiO3 layers. Density-functional-theory calculations provide insights into the origin of the observed properties of the thin SrRuO3 film. The present results pave a way to design materials with desired functionalities at oxide interfaces.

Hump-like structure in Hall signal from ultra-thin SrRuO3 films without inhomogeneous anomalous Hall effect

A controversy arose over the interpretation of the recently observed hump features in Hall resistivity ρxy from ultra-thin SrRuO3 (SRO) film; it was initially interpreted to be due to topological Hall effect but was later proposed to be from existence of regions with different anomalous Hall effect (AHE). In order to settle down the issue, we performed Hall effect as well as magneto-optic Kerr-effect measurements on 4 unit cell SRO films. Clear hump features are observed in ρxy, whereas neither hump feature nor double hysteresis loop is seen in the Kerr rotation which should be proportional to the magnetization. In addition, magnetization measurement by superconducting quantum interference device shows no sign of multiple coercive fields. These results show that inhomogeneous AHE alone cannot explain the observed hump behavior in ρxy data. We suggest that emergence of the hump structure in ρxy is closely related to the growth condition.

Machine-learning-assisted thin-film growth: Bayesian optimization in molecular beam epitaxy of SrRuO3 thin films

Materials informatics exploiting machine learning techniques, e.g., Bayesian optimization (BO), have the potential to reduce the number of thin-film growth runs for optimization of thin-film growth conditions through incremental updates of machine learning models in accordance with newly measured data. Here, we demonstrated BO-based molecular beam epitaxy (MBE) of SrRuO3, one of the most intensively studied materials in the research field of oxide electronics, mainly owing to its unique nature as a ferromagnetic metal. To simplify the intricate search space of entangled growth conditions, we ran the BO for a single condition while keeping the other conditions fixed. As a result, high-crystalline-quality SrRuO3 film exhibiting a high residual resistivity ratio of over 50 as well as strong perpendicular magnetic anisotropy was developed in only 24 MBE growth runs in which the Ru flux rate, growth temperature, and O3-nozzle-to-substrate distance were optimized. Our BO-based search method provides an efficient experimental design that is not as dependent on the experience and skills of individual researchers, and it reduces experimental time and cost, which will accelerate materials research.