SrRuO3 Films Research Articles

Anomalous Hall effect superimposed in polycrystalline SrRuO3 thick film

The electric, magnetic, and thermal properties of transition metal oxide films can be modulated by introducing polycrystalline at the macroscopic grain boundaries. Based on these points, in this work, we studied the two-channel anomalous Hall effect (AHE) in polycrystalline ferromagnetic SrRuO3 (SRO) films. The magnetic regions with different crystal directions have different coercivities, resulting in two opposite AHE channels in the polycrystalline SRO layer. However, single-crystal SRO films prepared under the same conditions are found to exhibit only one AHE. The superposition of the two AHE leads to the hump-like behavior of the Hall resistance loop, which is caused by the change of crystalline. This observation provides a new way to explain the hump-like feature of SRO.

Disentangling types of lattice disorder impacting superconductivity in Sr2RuO4 by quantitative local probes

The unconventional superconductivity in Sr2RuO4 is infamously susceptible to suppression by small levels of disorder such that it has been most commonly studied in extremely high-purity bulk crystals. Here, we harness local structural and spectroscopic scanning transmission electron microscopy measurements in epitaxial thin films of Sr2RuO4 to disentangle the impact of different types of crystalline disorder on superconductivity. We find that cation off-stoichiometry during growth gives rise to two distinct types of disorder: mixed-phase structural inclusions that accommodate excess ruthenium and ruthenium vacancies when the growth is ruthenium-deficient. Several superconducting films host mixed-phase intergrowths, suggesting this microstructural disorder has relatively little impact on superconductivity. In a non-superconducting film, on the other hand, we measure a high density of ruthenium-vacancies (∼14%) with no significant reduction in the crystallinity of the film. The results suggest that ruthenium vacancy disorder, which is hidden to many structural probes, plays an important role in suppressing superconductivity. We discuss the broader implications of our findings to guide the future synthesis of this and other layered systems.

Imaging of strain driven magnetic domains and strong spin-phonon coupling in epitaxial thin films of SrRuO3

SrRuO3 epitaxial thin films with large strain disorder have been grown using pulsed laser deposition on (LaAlO3)0.3(Sr2TaAlO6)0.7 (LSAT) and SrTiO3 (STO) substrates. The films demonstrate two distinct transition temperatures in magnetic measurements. We present a visual evolution of magnetic domains across the two transitions using magnetic force microscopy in these films. The study clearly shows that the magnetic anisotropy corresponding to the two transitions is different. The perpendicular magnetic anisotropy is found dominating in films which results in domain spin orientation preferably in out-of-plane direction. The Raman study shows that the magnetic order strongly influences the lattice. The analysis of the phonon spectra around the magnetic transitions reveals the existence of strong spin-phonon coupling and the calculations resulted in the spin-phonon coupling constant λ ∼ 0.65 cm−1 and 1.96 cm−1, for SrRuO3 films grown on LSAT and SrTiO3 single crystal substrates, respectively.

Magnetoresistance of epitaxial SrRuO3 thin films on a flexible CoFe2O4-buffered mica substrate

We have investigated the magnetoresistance of epitaxial SrRuO3 (SRO) thin films on a flexible CoFe2O4 (CFO)-buffered mica substrate. High-resolution X-ray diffraction and transmission electron microscopy revealed that the SRO film could be epitaxially grown on a mica substrate with a 22-nm-thick CFO buffer layer. The epitaxial relationships were SRO [1–10] || CFO [1–10] || mica [010] and SRO [111] || CFO [111] || mica [001]. Epitaxial SRO thin films exhibited two magnetoresistance (MR) peaks; one peak occurred at a Curie temperature of 160 K (HT-MR) and the other at a low temperature of 40 K (LT-MR). The LT-MR increased more rapidly with an increase of the buffer layer thickness than the HT-MR. The LT-MR was similar for the two orthogonal current directions with respect to the magnetic field. We explained the HT-MR and LT-MR in terms of the suppression of spin fluctuations and the magnetic rotation of crystallographic domains, respectively.

Anomalous Hall effect in spatially inhomogeneous SrRuO3 films

The Topological Hall Effect (THE) has been considered a hallmark of magnetic skyrmions in SrRuO3 single films and heterostructures. Alternative interpretations suggest that the THE-like anomalies could also originate from multi-channel anomalous Hall effects (AHEs) induced by structural inhomogeneity. Here, we reveal another type of THE-like anomaly in SrRuO3 films that was rarely reported previously. By scanning minor loops of the Hall resistivity of ultrathin SrRuO3 films, we demonstrate that the increased saturation resistivity and reversed polarity with decreasing a scanning field cannot be explained by THE, while it shows consistence with spatially inhomogeneous AHE. Our research provides a simple but effective way to quantitatively distinguish between THE and inhomogeneous AHE in those magnetic systems even beyond SrRuO3.

Tunable Two‐Channel Magnetotransport in SrRuO3 Ultrathin Films Achieved by Controlling the Kinetics of Heterostructure Deposition

AbstractIn the field of oxide heterostructure engineering, there are extensive efforts to couple the various functionalities of each material. The Berry curvature‐driven magnetotransport of SrRuO3 ultrathin films is currently receiving a great deal of attention because it is extremely sensitive to the electronic structures near the Fermi surface driven by extensive physical parameters such as spin–orbit coupling and inversion symmetry breaking. Although this is beneficial in terms of heterostructure engineering, it renders transport behavior vulnerable to nanoscale inhomogeneity, resulting in artifacts called “hump anomalies.” Here, a method to tune the magnetotransport properties of SrRuO3 ultrathin films capped by LaAlO3 layers is developed. The kinetic process of pulsed laser deposition by varying the growth pressure during LaAlO3 layer deposition is systematically controlled. Furthermore, the effects of nanoscale inhomogeneity on the Berry curvature near the Fermi surface in SrRuO3 films are investigated. It is found that the high kinetic energy of the capping layer adatoms induces stoichiometric modification and nanoscale lattice deformation of the underlying SrRuO3 layer. The control of kinetics provides a way to modulate magnetization and the associated magnetotransport of the SrRuO3 layer.

Improved dielectric constant and leakage current characteristics of BaTiO3 thin film on SrRuO3 seed layer

The fabrication of high-k BaTiO3 (BTO) films for applications such as multilayer ceramic capacitors or dynamic random access memory has been achieved via the ex-situ crystallization of amorphous BTO into perovskite structure through post-deposition annealing (PDA). However, high-temperature PDA at temperatures exceeding 700 °C results in the severe degradation of BTO properties. In other words, ex-situ crystallization via PDA results in an increased dielectric constant but is accompanied by inferior leakage current characteristics caused by rough surface morphologies and defects such as voids and cracks. In this study, we demonstrate the in-situ crystallization of BTO films by introducing a high-quality SrRuO3 (SRO) film as a seed layer. The SRO film is deposited using rf magnetron sputtering at 270–350 °C, and stoichiometric SRO films are obtained on SiO2, Al2O3, and TiN substrates by controlling the growth temperature; subsequently, rapid thermal annealing at 700 °C is conducted to crystallize the SRO film. The polycrystalline SRO film on the TiN substrate exhibits excellent morphology with uniform grains as well as a continuous and smooth surface, whereas the SRO films on the SiO2 and Al2O3 substrates exhibit rough and discontinuous surfaces. The SRO seed layer on TiN permits the in-situ growth of crystalline BTO films at a relatively low temperature of 500 °C featuring a high dielectric constant of 160 and a low leakage current density of 9.5 × 10−7 A/cm2 at 0.8 V, which are 2.2 and 3.4 × 107 times improved compared with those of the ex-situ crystallized BTO film on bare TiN, respectively.

Observation of metallic electronic structure in a single-atomic-layer oxide

Correlated electrons in transition metal oxides exhibit a variety of emergent phases. When transition metal oxides are confined to a single-atomic-layer thickness, experiments so far have shown that they usually lose diverse properties and become insulators. In an attempt to extend the range of electronic phases of the single-atomic-layer oxide, we search for a metallic phase in a monolayer-thick epitaxial SrRuO3 film. Combining atomic-scale epitaxy and angle-resolved photoemission measurements, we show that the monolayer SrRuO3 is a strongly correlated metal. Systematic investigation reveals that the interplay between dimensionality and electronic correlation makes the monolayer SrRuO3 an incoherent metal with orbital-selective correlation. Furthermore, the unique electronic phase of the monolayer SrRuO3 is found to be highly tunable, as charge modulation demonstrates an incoherent-to-coherent crossover of the two-dimensional metal. Our work emphasizes the potentially rich phases of single-atomic-layer oxides and provides a guide to the manipulation of their two-dimensional correlated electron systems.

Dielectric function and band gap modulation in perovskite SrRuO3 thin film

An effective method for studying the relationship of dielectric functions and thickness in perovskite SrRuO3 (SRO) thin film was proposed by spectroscopic ellipsometry measurement. A wedge-shaped SRO film with various thickness was prepared on SrTiO3 substrate by pulse laser deposition, whose dielectric functions and thickness were obtained by using the Drude–Lorentz dispersive model. The acquired band gap is slightly enhanced with film thickness reducing due to the one-dimensional quantum confinement effect and the disorder effect. The proposed method is expected to be applied for dielectric modulation in perovskite photonic device.

High-sensitivity of initial SrO growth on the residual resistivity in epitaxial thin films of SrRuO_3 on SrTiO_3 (001)

The growth of SrRuO_3 (SRO) thin film with high-crystallinity and low residual resistivity (RR) is essential to explore its intrinsic properties. Here, utilizing the adsorption-controlled growth technique, the growth condition of initial SrO layer on TiO_2-terminated SrTiO_3 (STO) (001) substrate was found to be crucial for achieving a low RR in the resulting SRO film grown afterward. The optimized initial SrO layer shows a c(2 times 2) superstructure that was characterized by electron diffraction, and a series of SRO films with different thicknesses (ts) were then grown. The resulting SRO films exhibit excellent crystallinity with orthorhombic-phase down to t approx 4.3 nm, which was confirmed by high resolution X-ray measurements. From X-ray azimuthal scan across SRO orthorhombic (02 ± 1) reflections, we uncover four structural domains with a dominant domain of orthorhombic SRO [001] along cubic STO [010] direction. The dominant domain population depends on t, STO miscut angle (alpha ), and miscut direction (beta ), giving a volume fraction of about 92 % for t approx 26.6 nm and (alpha , beta ) approx (0.14^{mathrm{o}}, 5^{mathrm{o}}). On the other hand, metallic and ferromagnetic properties were well preserved down to tapprox 1.2 nm. Residual resistivity ratio (RRR = rho ({mathrm{300 K}})/rho ({mathrm{5K}})) reduces from 77.1 for tapprox 28.5 nm to 2.5 for tapprox 1.2 nm, while rho ({mathrm{5K}}) increases from 2.5 upmu Omega cm for tapprox 28.5 nm to 131.0 upmu Omega cm for tapprox 1.2 nm. The ferromagnetic onset temperature (T'_{mathrm{c}}) of around 151 K remains nearly unchanged down to tapprox 9.0 nm and decreases to 90 K for tapprox 1.2 nm. Our finding thus provides a practical guideline to achieve high crystallinity and low RR in ultra-thin SRO films by simply adjusting the growth of initial SrO layer.

Selective Growth and Micropatterning Technique for Oxide Thin Films by Sacrificial a-CaO Layer

Selective growth and micropatterning through a water lift-off (WLO) process utilizing a sacrificial amorphous CaO (a-CaO) layer were demonstrated to fabricate hetero-epitaxial all-oxides film capacitors. Patterned Pb(Zr,Ti)O3 (PZT) and SrRuO3 (SRO) films were grown on (100) SrTiO3 (STO) substrate by pulsed laser deposition, and the subsequent lift-off process for target oxides was carried out using pure water. The results showed that selective growth and micropatterning of an all-oxides capacitor structure could be realized through WLO process using a-CaO as the sacrificial layer. Electron backscattered diffraction results revealed that SRO/PZT/SRO capacitor structures were grown hetero-epitaxially on the STO substrate. Furthermore, the electrical properties of patterned SRO/PZT/SRO film capacitors could be improved by suppressing the height of overgrown parts of the SRO bottom electrodes with a thick a-CaO sacrificial layer.

Electric Field Control of the Magnetic Weyl Fermion in an Epitaxial SrRuO3 (111) Thin Film.

The magnetic Weyl fermion originates from the time reversal symmetry (TRS)-breaking in magnetic crystalline structures, where the topology and magnetism entangle with each other. Therefore, the magnetic Weyl fermion is expected to be effectively tuned by the magnetic field and electrical field, which holds promise for future topologically protected electronics. However, the electrical field control of the magnetic Weyl fermion has rarely been reported, which is prevented by the limited number of identified magnetic Weyl solids. Here, the electric field control of the magnetic Weyl fermion is demonstrated in an epitaxial SrRuO3 (111) thin film. The magnetic Weyl fermion in the SrRuO3 films is indicated by the chiral anomaly induced magnetotransport, and is verified by the observed Weyl nodes in the electronic structures characterized by the angle-resolved photoemission spectroscopy (ARPES) and first-principles calculations. Through the ionic-liquid gating experiment, the effective manipulation of the Weyl fermion by electric field is demonstrated, in terms of the sign-change of the ordinary Hall effect, the nonmonotonic tuning of the anomalous Hall effect, and the observation of the linear magnetoresistance under proper gating voltages. The work may stimulate the searching and tuning of Weyl fermions in other magnetic materials, which are promising in energy-efficient electronics.

Epitaxial lift-off of freestanding (011) and (111) SrRuO3 thin films using a water sacrificial layer

Two-dimensional freestanding thin films of single crystalline oxide perovskites are expected to have great potential in integration of new features to the current Si-based technology. Here, we showed the ability to create freestanding single crystalline (011)- and (111)-oriented SrRuO3 thin films using Sr3Al2O6 water-sacrificial layer. The epitaxial Sr3Al2O6(011) and Sr3Al2O6(111) layers were realized on SrTiO3(011) and SrTiO3(111), respectively. Subsequently, SrRuO3 films were epitaxially grown on these sacrificial layers. The freestanding single crystalline SrRuO3(011)pc and SrRuO3(111)pc films were successfully transferred on Si substrates, demonstrating possibilities to transfer desirable oriented oxide perovskite films on Si and arbitrary substrates.

Wide-Range Epitaxial Strain Control of Electrical and Magnetic Properties in High-Quality SrRuO3 Films

Epitaxial strain in 4d ferromagnet SrRuO3 films is directly linked to the physical properties through the strong coupling between lattices, electrons, and spins. It provides an excellent opportunity to tune the functionalities of SrRuO3 in electronic and spintronic devices. However, a thorough understanding of the epitaxial strain effect in SrRuO3 has remained elusive due to the lack of systematic studies. This study demonstrates wide-range epitaxial strain control of electrical and magnetic properties in high-quality SrRuO3 films. The epitaxial strain was imposed by cubic or pseudocubic perovskite substrates having a lattice mismatch of -1.6 to 2.3% with reference to bulk SrRuO3. The Poisson ratio, which describes the two orthogonal distortions due to the substrate clamping effect, is estimated to be 0.33. The Curie temperature (TC) and residual resistivity ratios of the series of films are higher than or comparable to the highest reported values for SrRuO3 on each substrate, confirming the high crystalline quality of the films. A TC of 169 K is achieved in a tensile-strained SrRuO3 film on the DyScO3 (110) substrate, which is the highest value ever reported for SrRuO3. The TC (146-169 K), magnetic anisotropy (perpendicular or in-plane magnetic easy axis), and metallic conduction (residual resistivity at 2 K of 2.10 - 373 {\mu}{\Omega}cm) of SrRuO3 are widely controlled by epitaxial strain. These results provide guidelines to design SrRuO3-based heterostructures for device applications.

First-Principles Calculations Predict Tunable Large Magnetic Anisotropy Due to Spin-Polarized Quantum-Well Resonances in Nanometer-Thick SrRuO3 Films: Implications for Spintronic Devices

We demonstrate that the magnetic anisotropy in SrRuO3 (SRO) ultrathin films can be well controlled by thickness or electrical modulations: The easy axis (preferred magnetization direction) of a SRO ultrathin film can be switched between out-of-plane and in-plane magnetization either by altering the film thickness or by tuning the doping level through applying a gate voltage or chemical doping. Such a capability to manipulate the magnetocrystalline anisotropy energy (MCAE) is given by the spin–orbit coupling (SOC)-induced energy splitting in the spin-polarized quantum well states (QWSs) near the Fermi level. These QWSs are susceptible to the intrinsic spin–orbit interaction and thus drive a large energy discrepancy when an SRO ultrathin film rotates its magnetization direction. As a result of the MCAE nature, a SRO film has a magnetic anisotropy approximately an order of magnitude larger than those in 3d-ferromagnet ultrathin films and its easy-axis direction depends on the film thickness, gate voltage, and substrate. This tunable large magnetic anisotropy in SRO ultrathin films provides an excellent route toward advanced spintronic devices such as storage and memory devices.

Superconducting Sr2RuO4 Thin Films without Out-of-Phase Boundaries by Higher-Order Ruddlesden-Popper Intergrowth.

Ruddlesden-Popper (RP) phases (An+1BnO3n+1, n = 1, 2,···) have attracted intensive research with diverse functionalities for device applications. However, the realization of a high-quality RP-phase film is hindered by the formation of out-of-phase boundaries (OPBs) that occur at terrace edges, originating from lattice mismatch in the c-axis direction with the A'B'O3 (n = ∞) substrate. Here, using strontium ruthenate RP-phase Sr2RuO4 (n = 1) as a model system, an experimental approach for suppressing OPBs was developed. By tuning the growth parameters, the Sr3Ru2O7 (n = 2) phase was formed in a controlled manner near the film-substrate interface. This higher-order RP-phase then blocked the subsequent formation of OPBs, resulting in nearly defect-free Sr2RuO4 layer at the upper region of the film. Consequently, the Sr2RuO4 thin films exhibited superconductivity up to 1.15 K, which is the highest among Sr2RuO4 films grown by pulsed laser deposition. This work paves the way for synthesizing pristine RP-phase heterostructures and exploring their unique physical properties.

Capping and gate control of anomalous Hall effect and hump structure in ultra-thin SrRuO3 films

Ferromagnetism and exotic topological structures in SrRuO3 (SRO) induce sign-changing anomalous Hall effect (AHE). Recently, hump structures have been reported in the Hall resistivity of SRO thin films, especially in the ultra-thin regime. We investigate the AHE and hump structure in the Hall resistivity of SRO ultra-thin films with an SrTiO3 (STO) capping layer and ionic liquid gating. STO capping results in sign changes in the AHE and modulation of the hump structure. In particular, the hump structure in the Hall resistivity is strongly modulated and even vanishes in STO-capped 4 unit cell films. In addition, the conductivity of STO-capped SRO ultra-thin films is greatly enhanced with restored ferromagnetism. We also performed ionic liquid gating to modulate the electric field at SRO/STO interface. Drastic changes in the AHE and hump structure are observed with different gate voltages. Our study shows that the hump structure as well as the AHE can be controlled by tuning inversion symmetry and the electric field at the interface.

Tailoring the anomalous Hall effect of SrRuO3 thin films by strain: A first principles study

Motivated by the recently observed unconventional Hall effect in ultrathin films of ferromagnetic SrRuO3 (SRO), we investigate the effect of strain-induced oxygen octahedral distortion in the electronic structure and anomalous Hall response of the SRO ultrathin films by virtue of density functional theory calculations. Our findings reveal that the ferromagnetic SRO films grown on SrTiO3 (in-plane strain of −0.47%) have an orthorhombic (both tilting and rotation) distorted structure, and with an increasing amount of substrate induced compressive strain the octahedral tilting angle is found to be suppressed gradually, with SRO films grown on NdGaO3 (in-plane strain of −1.7%) stabilized in the tetragonal distorted structure (with zero tilting). Our Berry curvature calculations predict a positive value of the anomalous Hall conductivity of +76 S/cm at −1.7% strain, whereas it is found to be negative (−156 S/cm) at −0.47% strain. We attribute the found behavior of the anomalous Hall effect to the nodal point dynamics in the electronic structure arising in response to tailoring the oxygen octahedral distortion driven by the substrate induced strain. We also calculate strain-mediated anomalous Hall conductivity as a function of reduced magnetization obtained by scaling down the magnitude of the exchange field inside Ru atoms finding good qualitative agreement with experimental observations, which indicates a strong impact of longitudinal thermal fluctuations of Ru spin moments on the anomalous Hall effect in this system.

Thickness-dependent quantum transport of Weyl fermions in ultra-high-quality SrRuO3 films

The recent observation of Weyl fermions in the itinerant 4d ferromagnetic perovskite SrRuO3 points to this material being a good platform for exploring novel physics related to a pair of Weyl nodes in epitaxial heterostructures. In this Letter, we report the thickness-dependent magnetotransport properties of ultra-high-quality epitaxial SrRuO3 films grown under optimized conditions on SrTiO3 substrates. Signatures of Weyl fermion transport, i.e., unsaturated linear positive magnetoresistance accompanied by a quantum oscillation having a high quantum mobility of 10 000 cm2/V s and a π Berry phase, were observed in films with thicknesses as small as 10 nm. Residual resistivity increased with the decreasing film thickness indicating disorder near the interface between SrRuO3 and the SrTiO3 substrate. Since this disorder affects the magnetic and electrical properties of the films, the Curie temperature decreases and the coercive field increases with the decreasing thickness. Thickness-dependent magnetotransport measurements revealed that the threshold residual resistivity ratio to observe Weyl fermion transport is 21. These results provide guidelines for realizing quantum transport of Weyl fermions in SrRuO3 near heterointerfaces.

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.