SrRuO3 Thin Films Research Articles

Fabrication of (110)-one-axis-oriented perovskite-type oxide thin films and their application to buffer layer

BaCe0.9Y0.1O3−δ (BCYO) and SrZr0.8Y0.2O3−δ (SZYO) thin films of perovskite-type oxides were deposited on (111)Pt/TiOx/SiO2/(100)Si substrates. X-ray diffraction patterns showed that the (110)-oriented BCYO and SZYO thin films were grown on (111)Pt/Si substrates directly without using any buffer layers. Thin films of SrRuO3 (SRO), a conductive perovskite-type oxide, were also deposited on those films and highly (110)-oriented SRO thin films were obtained. We believe that this (110)-oriented SRO works as a buffer layer to deposit (110)-oriented perovskite-type ferroelectric oxide thin films as well as a bottom electrode and can modify the ferroelectric properties of the oxide thin films by controlling their crystallographic orientations.

Formation of V-grooves in SrRuO3 epitaxial film

SrRuO3 thin films were epitaxially grown on a (001) SrTiO3 substrate using pulsed laser deposition technique. Various defects such as V-grooves, threading dislocations and dislocation dipoles are observed in the SrRuO3 epitaxial film. It is found that the sidewalls of most V-grooves are {101} facets, and the dominant angle between the sidewalls is 90°. Some threading dislocations end at the apexes of the V-grooves while the others penetrate the entire film. The threading dislocations and V-grooves can partially relieve the strain in the epitaxial SrRuO3 film. During the relaxation process, a two-dimensional growth mode transforms into a three-dimensional one, along with the formation of mesa-like islands separated by V-grooves. The dimensions and distributions of V-grooves are associated with the growth conditions. The control of growth mechanism and surface morphology are very important for the fabrication of novel perovskite oxide devices.

Antiferromagnetic and xy ferro-orbital order in insulating SrRuO3 thin films with SrO termination

By means of first-principles calculations we study the structural, magnetic and electronic properties of SrRuO3 surface for the SrO termination. We find that the RuO6 octahedra and the structure of the SrO layers at the surface are strongly modified as well as the Ru–O–Ru bond angles. We find in the thin films a dxy ferro-orbital order. The dxy orbital becomes the lowest in energy as in other quasitwodimensional ruthenates. Such structural rearrangement, together with a band reduction, leads to a modification of the magnetic properties. We compare the Jahn–Teller effect between the ferromagnetic and antiferromagnetic phases. We show that an insulating G-type antiferromagnetic phase takes place in SrRuO3 thin films, substituting the metallic phase experimentally found in every bulk Sr-ruthenates. The single layer SrRuO3 presents many similarities with the Ca2RuO4 low temperature phase, these similarities disappear with a larger number of layers. A study of the ground state of the as function of the number of layers is presented, the competition between bandwidth and Coulomb repulsion determines the ground state. We propose the disorder as responsible for the exchange bias effect observed.

Orientation control of SrRuO3 thin film on a Si substrate by chemical solution deposition for an electrode of lead zirconate titanate thin films

A strontium ruthenium oxide (SRO) electrode was deposited on a Si substrate using chemical solution deposition (CSD). For this study, the crystal orientation of the SRO electrode was controlled using a CSD-derived seeding layer deposited on a Si substrate. Thereby, SRO thin film with a-axis preferred orientation was obtained on the Si substrate. Furthermore, lead zirconate titanate (PZT) thin film with Zr/Ti=50/50 composition (PZT50) was deposited onto the a-axis oriented SRO layer. The deposited PZT thin film also exhibited preferred orientation in the direction of the a-axis and c-axis. The estimated remanent polarization and the effective piezoelectric constant (d33eff) were, respectively, 20μC/cm2 and 298pm/V. These values were nearly equal to those of the reported bulk PZT of the same composition.

Growth control of RF magnetron sputtered SrRuO3 thin films through the thickness of LaNiO3 seed layers

SrRuO3 (SRO) thin films were grown on SiO2/Si substrates with different thickness of LaNiO3 (LNO) seed layers by RF magnetron sputtering. Effects of LNO thickness on the grain orientation, surface morphology, magnetic behavior and electrical transport properties of SRO films were investigated. The orientation of SRO films transformed from (110)pc to (001)pc and the residual stress was released gradually with increasing the thickness (pc refers to the pseudo-cubic unit cell of SrRuO3). SRO films with higher orientation grown on LNO exhibited more flat surface, higher saturation magnetization, and lower coercive field. The magnetic anisotropy was enhanced on thicker LNO due to the different states of residual stress. In addition, the temperature dependence of resistivity was promoted by the microstructural disorder. (110)pc-oriented SRO monolayer electrode and (001)pc-oriented SRO/LNO300 bilayer electrode own low room temperature sheet resistance of 0.38Ω/□ and 0.26Ω/□, respectively. The results indicate that the controllable SRO films can be used as not only good bottom electrodes but also promising templates to control the crystallographic orientations of various other perovskite-based functional materials.

Tunable Ferromagnetic Transition Temperature and Vertical Hysteretic Shift in SrRuO3 Films Integrated on Si(001).

SrRuO3 thin films have been epitaxially integrated on complementary metal oxide semiconductor (CMOS) compatible Si(001) substrates via pulsed laser deposition using a unique buffer layer (SrTiO3/TiN) approach. When the oxygen pressure during deposition was controlled, a dramatic suppression in the ferromagnetic transition temperature (TC) of up to 53 K was observed, caused by the growth-induced ruthenium vacancies rather than the oxygen vacancies. The ruthenium vacancies can also effectively tune the vertical magnetization shift (Mshift) in hysteresis loops, and thus we achieved a giant Mshift of 240%. Transport and magnetic measurements reveal that these appreciable physical phenomena are closely related to the ruthenium defect-induced local disorder and complex effects due to the strongly hybridized p-d orbitals as well as the induced lattice distortion. These observations indicate the importance of ruthenium defects in controlling the vertical magnetization shift and ferromagnetic transition temperature in this transitional metal oxide.

Epitaxial growth of SrRuO3 thin films with different orientation by chemical solution deposition

Epitaxial SrRuO3 (SRO) thin films have been prepared on (100), (110), and (111) LaAlO3 substrates by chemical solution deposition method, and the microstructures as well as the properties are investigated. The SRO (110) thin films display much more denser, homogeneous and well-connected grains. All the derived SRO thin films show a metallic behavior with low room-temperature resistivity (mΩ·cm). All derived SRO films are ferromagnetic at low temperatures showing different Curie temperature due to the different residual stain, and the SRO (110) film has the largest magnetization and coercivity due to the enhanced morphology. These results will provide a low-cost route to fabricate epitaxial SRO thin films with different orientation.

In situ X-ray diffraction and the evolution of polarization during the growth of ferroelectric superlattices.

In epitaxially strained ferroelectric thin films and superlattices, the ferroelectric transition temperature can lie above the growth temperature. Ferroelectric polarization and domains should then evolve during the growth of a sample, and electrostatic boundary conditions may play an important role. In this work, ferroelectric domains, surface termination, average lattice parameter and bilayer thickness are simultaneously monitored using in situ synchrotron X-ray diffraction during the growth of BaTiO3/SrTiO3 superlattices on SrTiO3 substrates by off-axis radio frequency magnetron sputtering. The technique used allows for scan times substantially faster than the growth of a single layer of material. Effects of electric boundary conditions are investigated by growing the same superlattice alternatively on SrTiO3 substrates and 20 nm SrRuO3 thin films on SrTiO3 substrates. These experiments provide important insights into the formation and evolution of ferroelectric domains when the sample is ferroelectric during the growth process.

Thickness-dependent magnetic properties and strain-induced orbital magnetic moment inSrRuO3thin films

Thin films of the ferromagnetic metal SrRuO3 (SRO) show a varying easy magnetization axis depending on the epitaxial strain and undergo a metal-to-insulator transition with decreasing film thickness. We have investigated the magnetic properties of SRO thin films with varying thicknesses fabricated on SrTiO3(001) substrates by soft x-ray magnetic circular dichroism (XMCD) at the Ru M2,3 edge. Results have shown that, with decreasing film thickness, the film changes from ferromagnetic to non-magnetic around 3monolayer thickness, consistent with previous magnetization and magneto-optical Kerr effect measurements. The orbital magnetic moment perpendicular to the film was found to be ~ 0.1{\mu}B/Ru atom, and remained nearly unchanged with decreasing film thickness while the spin magnetic moment decreases. Mechanism for the formation of the orbital magnetic moment is discussed based on the electronic structure of the compressively strained SRO film.

Epitaxy on Demand

Perovskite oxide heteroepitaxy is realized on the top of inorganic nanosheets that are covering the amorphous oxide surfaces of Si substrates. Utilizing pulsed laser deposition, thin films of SrRuO3 in a (001)pc and (110)pc orientation on nanosheets of Ca2Nb3O10 and Ti0.87O2 are grown, respectively. The two types of nanosheets are patterned to locally tailor the crystallographic orientation and properties of SrRuO3. The success of our approach is demonstrated by electron backscatter diffraction and spatial magnetization maps. An unprecedented control of perovskite film growth on arbitrary substrates is illustrated in this work, and the methods that are developed to deposit SrRuO3 thin films are a viable starting point for growth of artificial heteroepitaxial thin films that require a bottom electrode. Control is not just reached in the direction of film growth, as the crystal orientation and film properties are regulated laterally on the surface of micropatterned nanosheets. Local control of magnetic properties is illustrated, which holds out prospects for the fabrication of next‐generation devices like noncollinear magnetic random access memories.

Investigation of the growth mechanism of SrRuO3 thin films fabricated by pulsed laser deposition

SrRuO3 thin films were prepared on (100) SrTiO3 substrates by pulsed laser deposition (PLD), and the dependences of film morphology and growth mode on the defocusing distance, repetition, oxygen pressure and substrate temperature were systematically investigated. Our results suggest that the growth mode transforms from three-dimensional (3D) island formation to two-dimensional (2D) layered growth as the defocusing distance increases. Under high laser repetition, adatoms do not have enough time to diffuse to step edges to promote step advancement; consequently, adatoms aggregate to form islands. The effective diffusivity was estimated as D* = 5 × 104 nm2/s, which represents the average diffusion including adatoms and small clusters. Decreasing oxygen pressure leads to a change in growth mode from 2D layered to 3D island formation. The X-ray diffraction (XRD) patterns show that under low oxygen pressure, the (00l) peaks disappear when l is odd, indicating that oxygen vacancies are preferentially produced in the SrO plane. With decreasing substrate temperature, the growth mode changes from step flow to multilayered growth. The relationship between step width and substrate temperature was investigated, and an activation energy of Ea = 0.8 eV was obtained by fitting.

Optimizing strontium ruthenate thin films for near-infrared plasmonic applications.

Several new plasmonic materials have recently been introduced in order to achieve better temperature stability than conventional plasmonic metals and control field localization with a choice of plasma frequencies in a wide spectral range. Here, epitaxial SrRuO3 thin films with low surface roughness fabricated by pulsed laser deposition are studied. The influence of the oxygen deposition pressure (20–300 mTorr) on the charge carrier dynamics and optical constants of the thin films in the near-infrared spectral range is elucidated. It is demonstrated that SrRuO3 thin films exhibit plasmonic behavior of the thin films in the near-infrared spectral range with the plasma frequency in 3.16–3.86 eV range and epsilon-near-zero wavelength in 1.11–1.47 μm range that could be controlled by the deposition conditions. The possible applications of these films range from the heat-generating nanostructures in the near-infrared spectral range, to metamaterial-based ideal absorbers and epsilon-near-zero components, where the interplay between real and imaginary parts of the permittivity in a given spectral range is needed for optimizing the spectral performance.

Anisotropy of electrical and magnetic transport properties of epitaxial SrRuO3 thin films

SrRuO3 (SRO) thin films with different thickness (2–70 nm) have been grown on (001), (110), and (111) SrTiO3 substrates. The (001)-SRO films (2–8 nm) exhibited smooth flat surfaces whereas the (110)- and (111)-SRO films featured a faceted island structure. Room temperature resistivity and residual resistivity are the lowest for the (111)-SRO films (30–70 nm). Over all thicknesses, we observed enhanced magnetization in the (111)-SRO films (∼4 μB/Ru) compared with that for the (001)- and (110)-SRO films (∼2 μB/Ru and ∼3 μB/Ru, respectively), suggesting a low-spin state t2g(3↑,1↓), high-spin state t2g(3↑)eg(1↑), and mixed low- and high-spin states for the (001)-, (111)-, and (110)-SRO films, respectively. The dependence of resistivity on temperatures near TC follows a power law with exponent β = 0.312 and β = 0.363 for the (110)- and (111)-SRO films, respectively. These critical exponents are consistent with magnetic data with scaling law M = C (TC − T)β. At low temperatures, dM/dT and dρ/dT show a linear relationship in the temperature range for the Fermi liquid. These results suggest that the intrinsic electrical and magnetic transport properties are coupled.

Ferromagnetic SrRuO3 thin-film deposition on a spin-triplet superconductor Sr2RuO4 with a highly conducting interface

Ferromagnetic SrRuO3 thin films are deposited on the ab surface of single crystals of the spin-triplet superconductor (TSC) Sr2RuO4 as substrates using pulsed laser deposition. The films are under a severe in-plane compressive strain. Nevertheless, the films exhibit ferromagnetic order with the easy axis along the c-direction below the Curie temperature of 158 K. The electrical transport reveals that the SrRuO3/Sr2RuO4 interface is highly conducting, in contrast with the interface between other normal metals and the ab surface of Sr2RuO4. Our results stimulate investigations on proximity effects between a ferromagnet and a TSC.

Topological electromagnetic response in the chiral superconductor Sr2RuO4

We report the observation of a fractional topological magneto-electric effect in Sr2RuO4 single crystal thin films by measuring the electric transport properties. In the absence of an external magnetic field, the surface transport in Sr2RuO4 thin films exhibited a fractional quantum Hall conductance in the superconducting state. The fractional magnetic-field-induced electric polarization was observed under zero bias current. We can understand the fractional quantum Hall conductance as a consequence of the observation of the (2+1)-dimensional topological surface state in the (3+1)-dimensional fractional topological magneto-electric effect in Sr2RuO4 thin films.

Exchange bias in strained SrRuO3 thin films

Recently, it was found that the ferromagnetic SrRuO3 when combined with another ferromagnet in thin film form gives rise to exchange bias (EB) effect. However, we observed EB in single, strained, SrRuO3 thin films grown on diamagnetic LaAlO3 (100) substrates. It displays the training effect, which essentially confirms EB. The temperature dependence of the EB reveals the blocking temperature to be around ∼75 K. The strength of the exchange bias decreases with the increase in thickness of the film. We observe tensile strain in the out of plane direction. Further, the presence of in-plane compressive strain is observed through asymmetric reciprocal space mapping. Finally, we find a direct link between strain and EB. The evolution of strain with thickness matches well with the nature of scaled EB. It has been shown earlier by first principle calculations that this strain can induce EB in thin films.

Strong Dependence of Oxygen Octahedral Distortions in SrRuO3 Films on Types of Substrate-Induced Epitaxial Strain

This work investigates the effects of types of epitaxial strain on the structural and magneto-transport properties of SrRuO3 (SRO) thin films grown on (110)ortho NdGaO3 (NGO) and (110)ortho SmScO3 (SSO) substrates that result in a −1.66% compressive strain and a +1.63% tensile strain, respectively. Although the epitaxial strains induced by the NGO and SSO substrates are almost equal in magnitude, the film properties were found to be strongly dependent on the type of strain. High-resolution scanning transmission electron microscopy revealed that the compressively strained SRO films possess a tetragonal structure with no octahedral tilts, while the tensilely strained SRO films undergo a thickness-dependent transition from a monoclinic structure with octahedral tilts to a tetragonal structure with small tilts. These findings indicate that octahedral tilt propagation from the substrate into the film is preferentially induced under tensile rather than compressive strain. We further found that the magneto-transport properties of SRO films exhibit a significant dependence on the type of the epitaxial strain, demonstrating the close correlation between strain-induced octahedral distortions and magnetic anisotropy. These results highlight the important role of the type of the epitaxial strain on the structural and physical properties of epitaxial thin films.

Gate tuning of anomalous Hall effect in ferromagnetic metal SrRuO3

The electric field effect on ferromagnetism offers a new dimension in the recent advancement of spintronics. We report on the gate control of transport properties in thin films of oxide-based ferromagnetic metal, SrRuO3. An electric double layer transistor configuration was utilized with an ionic liquid dielectric to apply a strong electric field on a SrRuO3 thin film of 5 monolayers in thickness. The application of gate voltage induced a clear electroresistance effect, despite a considerably-large initial carrier density of the order of 1022 cm−3. Furthermore, we found that the gate modulation of the anomalous Hall conductivity σxy, which was as large as ∼±40% at low temperatures, was about three times larger than that of the longitudinal conductivity σxx. The variation of σxy is characterized by the power-law scaling relation with σxx, which is widely observed in a bad metal regime of the charge transport.

Augmentation of coercivity in nanocrystalline SrRuO3 thin film and its spin glass behavior

SrRuO3 films were prepared using pulsed laser deposition on different substrates (z-cut Quartz, (001) SrTiO3 and (001) LaAlO3) in this work. We have observed greatly enhanced perpendicular coercivity over 2T in nanocrystalline SrRuO3 thin films with a thickness of 60nm deposited at 600°C on quartz substrates compared to epitaxial thin films (60nm) on (001) SrTiO3 and (001) LaAlO3 substrates. Our experiments have also shown that a lattice strain may result in reduced Curie temperature and enhanced saturation magnetization. The spin glass like behavior is observed in the nanocrystalline thin films. The augmentation of the coercivity is possibly associated with nanocrystalline nature and the spin glass like phenomena in the films.

Nanoscale monoclinic domains in epitaxial SrRuO3 thin films deposited by pulsed laser deposition

In this paper, we analyze the structural distortions observed by transmission electron microscopy in thin epitaxial SrRuO3 layers used as bottom electrodes in multiferroic coatings onto SrTiO3 substrates for future multiferroic devices. Regardless of the nature and architecture of the multilayer oxides deposited on the top of the SrRuO3 thin films, selected area electron diffraction patterns systematically revealed the presence of faint diffraction spots appearing in forbidden positions for the SrRuO3 orthorhombic structure. High-resolution transmission electron microscopy (HRTEM) combined with Geometric Phase Analysis (GPA) evidenced the origin of these forbidden diffraction spots in the presence of structurally disordered nanometric domains in the SrRuO3 bottom layers, resulting from a strain-driven phase transformation. The local high compressive strain (−4% ÷ −5%) measured by GPA in the HRTEM images induces a local orthorhombic to monoclinic phase transition by a cooperative rotation of the RuO6 octahedra. A further confirmation of the origin of the forbidden diffraction spots comes from the simulated diffraction patterns obtained from a monoclinic disordered SrRuO3 structure.