Epitaxial Strain Research Articles

Highly Energy-Efficient Spin Current Generation in SrIrO3 by Manipulating the Octahedral Rotation.

Materials with strong spin-orbit coupling (SOC) have been continuously attracting intensive attention due to their promising application in energy-efficient, high-density, and nonvolatile spintronic devices. Particularly, transition-metal perovskite oxides with strong SOC have been demonstrated to exhibit efficient charge-spin interconversion. In this study, we systematically investigated the impact of epitaxial strain on the spin-orbit torque (SOT) efficiency in the SrIrO3(SIO)/Ni81Fe19(Py) bilayer. The results reveal that the SOT efficiency is strongly related to the octahedral rotation around the in-plane axes of the single-crystal SIO. By modulating the epitaxial strain using different substrates, the SOT efficiency can be remarkably improved from 0.15 to 1.45. This 10-fold enhancement of SOT efficiency suggests that modulating the epitaxial strain is an efficient approach to control the SOT efficiency in complex oxide-based heterostructures. Our work may have the potential to advance the application of complex oxides in energy-efficient spintronic devices.

Thin Films of α-Quartz GeO2 on TiO2-Buffered Quartz Substrates.

α-Quartz (SiO2) is one of the most widely used piezoelectric materials. However, the challenges associated with the control of the crystallization and the growth process limit its production to the hydrothermal growth of bulk crystals. GeO2 can also crystallize into the α-quartz phase, with a higher piezoelectric response and better thermal stability than SiO2. In a previous study, we have found that GeO2 crystallization on nonquartz substrates shows a tendency to form spherulites with a randomized orientation; while epitaxial growth of crystalline GeO2 thin films can take place on quartz (SiO2) substrates. However, in the latter case, the α-β phase transition that takes place in both substrates and thin films during heating deteriorates the long-range order and, thus, the piezoelectric properties. Here, we report the ousting of spherulitic growth by using a buffer layer. Using TiO2 as a buffer layer, the epitaxial strain of the substrates can be transferred to the growing films, leading to the oriented crystallization of GeO2 in the α-quartz phase. Moreover, since the TiO2 separates the substrates and the thin films, the thermal stability of the GeO2 is kept across the substrate's phase transitions. Our findings reveal the complexity of the crystallization process of quartz thin films and present a way to eliminate the tendency for spherulitic growth of quartz thin films by epitaxial strain.

Significant effects of epitaxial strain on the nonlinear transport properties in Ca2RuO4 thin films with the current-driven transition

Control of the nonlinear transport properties in strongly correlated materials with metal–insulator transitions has become an important task in the development of next-generation computing elements. Here, we demonstrate that the nonlinear transport properties in the Mott insulator Ca2RuO4, which are caused by the current-driven transition with a nonthermal-type mechanism, can be significantly affected by epitaxial strain in thin films. Ca2RuO4 epitaxial thin films were grown on various single-crystal substrates by the solid-phase epitaxy method under nonvacuum conditions, and the effects of epitaxial strain on the transport properties were investigated. While nonlinear transport properties with negative differential resistance behavior were observed in [001]-oriented Ca2RuO4/LaAlO3 (001) thin films, the current transport nonlinearity was critically diminished in the current–voltage characteristics of Ca2RuO4/NdCaAlO4 (100) thin films with the [110] orientation. Through structural characterization of the thin films, a possible correlation between the strain states and nonlinear transport properties was discussed.

Impact of Structural Strain in Perovskite Epitaxial Thin Films on Their Functional Properties

The strain engineering effects induced by different means, e.g., the substrate lattice mismatch and/or chemical doping, on the functional properties of perovskite thin films have triggered interest in the use of these materials in different applications such as energy storage/generation or photonics. The effects of the film’s thickness and strain state of the structure for the lead-free perovskite ferrite-based materials (BiFeO3-BFO; Y-doped BiFeO3-BYFO; LaFeO3-LFO) on their functional properties are highlighted here. As was previously demonstrated, the dielectric properties of BFO epitaxial thin films are strongly affected by the film thickness and by the epitaxial strain induced by the lattice mismatch between substrate and film. Doping the BiFeO3 ferroelectric perovskite with rare-earth elements or inducing a high level of structural deformation into the crystalline structure of LaFeO3 thin films have allowed the tuning of functional properties of these materials, such as dielectric, optical or photocatalytic ones. These changes are presented in relation to the appearance of complex ensembles of nanoscale phase/nanodomains within the epitaxial films due to strain engineering. However, it is a challenge to maintain the same level of epitaxial strain present in ultrathin films (<10 nm) and to preserve or tune the positive effects in films of thicknesses usually higher than 30 nm.

Stabilizing Crystal Framework of an Overlithiated Li1+xMn2O4 Cathode by Heterointerfacial Epitaxial Strain for High-Performance Microbatteries.

To meet the increasing demands of high-energy and high-power-density lithium-ion microbatteries, overlithiated Li1+xMn2O4 (0 ≤ x ≤ 1) is an attractive cathode candidate due to the high theoretical capacity of 296 mAh g-1 and the interconnected lithium-ion diffusion pathways. However, overlithiation triggers the irreversible cubic-tetragonal phase transition due to Jahn-Teller distortion, causing rapid capacity degradation. In contrast to conventional lithium-ion batteries, microbatteries offer the opportunity to develop specific thin-film-based modification strategies. Here, heterointerfacial lattice strain is proposed to stabilize the spinel crystal framework of an overlithiated Li1+xMn2O4 (LMO) cathode by epitaxial thin film growth on an underlying SrRuO3 (SRO) electronic conductor layer. It is demonstrated that the lattice misfit at the LMO/SRO heterointerface results in an in-plane epitaxial constraint in the full LMO film. This suppresses the lattice expansion during overlithiation that typically occurs in the in-plane direction. It is proposed by density functional theory modeling that the epitaxial constraint can accommodate the internal lattice stress originating from the cubic-tetragonal transition during overlithiation. As a result, a doubling of the capacity is achieved by reversibly intercalating a second lithium ion in a LiMn2O4 epitaxial cathode with a complete reversible phase transition. An impressive cycling stability can be obtained with reversible capacity retentions of above 90.3 and 77.4% for the 4 and 3 V range, respectively. This provides an effective strategy toward a stable overlithiated Li1+xMn2O4 epitaxial cathode for high-performance microbatteries.

Nucleation of threading dislocations in atomistic simulations of strained layer epitaxy of III-nitrides

The early stage of heteroepitaxial growth of wurtzite III-nitrides on lattice-mismatched substrates produces epitaxial strain that is relieved by nucleating a network of misfit dislocations. However, these epilayers contain also a large density of threading dislocations that are nearly perpendicular to the interface, despite the fact that they do not relieve the epitaxial strain. To shed light onto the mechanism of nucleation of threading dislocations, we employ atomistic simulations to explore the heteroepitaxy between a GaN nanoisland and a rigid substrate whose lattice parameter is adjusted to produce the desired misfit. We observe that the ends of misfit dislocations experience large image forces that pull them toward the pyramidal facets of the nanoisland. This results in bending of the terminating ends of misfit dislocations which constitute segments of threading dislocations. The spacing of these threading dislocations would increase (and thus their density decrease) if the epitaxial growth could be manipulated such that the film was deposited in a layer-by-layer fashion.

The dynamics of glassy behavior in SrRu1-xFexO3 heterostructure

We report the magnetic interactions in SrRu1-xFexO3 epitaxial thin films. All the films showed a single-phase and good step-terrace structure with minimal surface roughness. The X-ray photoelectron spectra of Fe and Ru revealed that both transition metal elements exist in the films as Fe3+ and Ru4+. With Fe doping, the itinerant ferromagnetic behavior and ferromagnetic transition temperature gradually decreased, but the coercivity field increased. The magnetic interplay between Fe3+-O-Fe3+ is coined as antiferromagnetic. There was another interaction between Ru4+-O-Fe3+ which is best described as ferromagnetic. Furthermore, the magnetic frustration in our films caused by the doped Fe resulted in dynamic glassy behavior. On the other hand, the epitaxial strain from the imminent SrTiO3 substrate along with change in oxygen vacancy might have strong evidence for the spin-glass nature in our thin films.

Tuning the band topology of GdSb by epitaxial strain

Rare-earth monopnictide (RE-V) semimetal crystals subjected to hydrostatic pressure have shown interesting trends in magnetoresistance, magnetic ordering, and superconductivity, with theory predicting pressure-induced band inversion. Yet, thus far, there have been no direct experimental reports of interchanged band order in RE-Vs due to strain. This work studies the evolution of band topology in biaxially strained GdSb(001) epitaxial films using angle-resolved photoemission spectroscopy (ARPES) and density functional theory (DFT). As biaxial strain is tuned from tensile to compressive strain, the gap between the hole and the electron bands dispersed along [001] decreases. The conduction and valence band shifts seen in DFT and ARPES measurements are explained by a tight-binding model that accounts for the orbital symmetry of each band. Finally, we discuss the effect of biaxial strain on carrier compensation and magnetic ordering temperature.

Aluminum substitution in low damping epitaxial lithium ferrite films

Ferromagnetic insulators with ultra-low damping and tunable magnetic properties are of great interest for the study of spin wave-based phenomena as well as for applications in spin wave spintronics. We have developed epitaxial spinel structure ferrite thin films of Li0.5(AlxFe2.5−x)O4 (LAFO) with ultra-low magnetic damping. The degree of Al substitution tunes the saturation magnetization and in-plane magnetic anisotropy while maintaining excellent epitaxy and crystalline quality as well as low magnetic damping. All LAFO films, deposited on (001)-oriented MgAl2O4 (MAO), are under epitaxial compressive strain, giving rise to an easy magnetic axis in-plane and a hard out-of-plane axis. Element-specific x-ray magnetic circular dichroism measurements confirm that the magnetism originates from the majority of Fe3+ cations occupying the octahedral sites, which are displaced by increased Al3+ substitution. Broadband ferromagnetic resonance reveals the lowest Gilbert damping parameters of 5×10−4 for films 10–15 nm thick. Our films demonstrate the robustness of the low loss dynamic properties of Li0.5(AlxFe2.5−x)O4 that are promising for spin wave applications.

Impact of epitaxial strain relaxation on ferromagnetism in a freestanding La2 / 3Sr1 / 3MnO3 membrane

Manganite perovskites host emerging physical properties of strongly-correlated electrons with charge, spin, and lattice degrees of freedom. Using the epitaxial lift-off technique, we report enhancement of saturation magnetization and the ferromagnetic transition temperature of freestanding La2/3Sr1/3MnO3 membrane compared with as-grown film on SrTiO3 substrate involving lateral tensile strain. Structural analysis reveals shrinkage of unit-cell volume by tensile strain relaxation in the freestanding membrane, which causes enhancement of the ferromagnetic interaction. The impact of the microscopic lattice deformation on the ferromagnetism of La2/3Sr1/3MnO3 indicates a high potential of this material for flexible electronics applications with intriguing functionalities in strongly-correlated electron systems.

Dislocation interactions during plastic relaxation of epitaxial colloidal crystals

The severe difficulty to resolve simultaneously both the macroscopic deformation process and the dislocation dynamics on the atomic scale limits our understanding of crystal plasticity. Here we use colloidal crystals, imaged on the single particle level by high-speed three-dimensional (3D) confocal microscopy, and resolve in real-time both the relaxation of the epitaxial misfit strain and the accompanying evolution of dislocations. We show how dislocation interactions give rise to the formation of complex dislocation networks in 3D and to unexpectedly sharp plastic relaxation. The sharp relaxation is facilitated by attractive interactions that promote the formation of new dislocations that are more efficient in mediating strain. Dislocation networks form fragmented structures, as dislocation growth is blocked by either attractive interactions, which result in the formation of sessile dislocation junctions, or by repulsion from perpendicular segments. The strength of these blocking mechanisms decreases with the thickness of the crystal film. These results reveal the critical role of dislocation interactions in plastic deformation of thin films and can be readily generalized from the colloidal to the atomic scale.

Intermittent Defect Fluctuations in Oxide Heterostructures.

The heterogeneous nature, local presence, and dynamic evolution of defects typically govern the ionic and electronic properties of a wide variety of functional materials. While the last 50 years have seen considerable efforts into development of new methods to identify the nature of defects in complex materials, such as the perovskite oxides, very little is known about defect dynamics and their influence on the functionality of a material. Here, the discovery of the intermittent behavior of point defects (oxygen vacancies) in oxide heterostructures employing X-ray photon correlation spectroscopy is reported. Local fluctuations between two ordered phases in strained SrCoOx with different degrees of stability of the oxygen vacancies are observed. Ab-initio-informed phase-field modeling reveals that fluctuations between the competing ordered phases are modulated by the oxygen ion/vacancy interaction energy and epitaxial strain. The results demonstrate how defect dynamics, evidenced by measurement and modeling of their temporal fluctuations, give rise to stochastic properties that now can be fully characterized using coherent X-rays, coupled for the first time to multiscale modeling in functional complex oxide heterostructures. The study and its findings open new avenues for engineering the dynamical response of functional materials used in neuromorphic and electrochemicalapplications.

Ferromagnetism and ferroelectricity in a superlattice of antiferromagnetic perovskite oxides without ferroelectric polarization

We study the structural, electronic, and magnetic properties of the SrCrO3/YCrO3 superlattice and their dependence on epitaxial strain. We discover that the superlattice adopts A-type antiferromagnetic (A-AFM) ordering in contrast to its constituents (SrCrO3: C-AFM; YCrO3: G-AFM) and retains it under compressive strain while becoming ferromagnetic (5 μB per formula unit) at +1% strain. The obtained ferroelectric polarization is significantly higher than that of the R2NiMnO6/La2NiMnO6 (R = Ce to Er) series of superlattices [Nat. Commun. 5, 4021 (2014)] due to a large difference between the antipolar displacements of the Sr and Y cations. The superlattice is a hybrid-improper multiferroic material with a spontaneous ferroelectric polarization (13.5 μC/cm2) approaching that of bulk BaTiO3 (19 μC/cm2). The combination of ferromagnetism with ferroelectricity enables multistate memory applications. In addition, the charge-order-driven p-type semiconducting state of the ferromagnetic phase (despite the metallic nature of SrCrO3) is a rare property and interesting for spintronics. Monte Carlo simulations demonstrate a magnetic critical temperature of 90 K for the A-AFM phase without strain and of 115 K for the ferromagnetic phase at +5% strain, for example.

The significant magnetic attenuation with submicrometer scale magnetic phase separation in tensile-strained LaCoO3 films

It is well known that the epitaxial strain plays a vital role in tuning the magnetic states in transition metal oxide LaCoO3 films. Here, we reported a robust long-range ferromagnetic (FM) ground state in a tensile-strained perovskite LaCoO3 film on a SrTiO3 (STO) substrate, which has a very significant attenuation when the thickness ranges from 10 to 50 nm. It is speculated that such attenuation may be caused by the appearance of the cross-hatched grain boundary, which relaxes the tensile strain around the crosshatch, resulting in the local non-FM phases. Magnetic force microscope observation reveals non-FM patterns correlated with the structural crosshatches in the strain-relaxed film even down to a temperature of 2 K and up to a magnetic field of 7 T, suggesting the phase separation origin of magnetization attenuation. Furthermore, the investigations of the temperature-dependent inverse magnetic susceptibility show a deviation from the Curie–Weiss law above the transition temperature in a 50-nm-thick LaCoO3/STO film but not in the LaCoO3/LaAlO3 film, which is ascribed to the Griffiths phase due to the crosshatch-line grain boundaries. These results demonstrated that the local strain effect due to structural defects is important to affect the ferromagnetism in strain-engineered LaCoO3 films, which may have potential implications for future oxide-based spintronics.

Anisotropic strain-modulated monoclinic phases with giant piezoelectricity in high-index-oriented PbTiO3 films

By combining phase-field simulations and first-principles calculations, we proposed to modulate the monoclinic phases of high-index-oriented PbTiO3 films through epitaxial anisotropic strain to obtain superior piezoelectric performance. It was found that the piezoelectric coefficients e22 were remarkably enhanced (up to 2400%) in both (111)- and (110)-oriented PbTiO3 films. The excellent piezoelectric property is mainly attributed to the polarization rotation due to the flatter energy landscapes in the monoclinic phases, while the variation of polarization magnitude contributes little to piezoelectricity. Our research suggests modulating material properties through anisotropic strain, which might bring some insight to other functional materials.

Thermodynamics of the solid-liquid phase equilibrium of a binary system: Effect of a chemical reaction in the liquid and epitaxial strain in the solid

We introduce two thermodynamics models of solid-liquid phase equilibrium of a binary system in the presence of (a) a chemical reaction in the liquid involving only one of the binary components and, (b) strain-inducing epitaxial conditions on the solid in addition to the chemical reaction in the liquid. These models show the existence of the counter intuitive behavior that epitaxy may promote recrystallization of the solid in spite of the reaction driving dissolution. The conditions for this recrystallization behavior are determined by the value of the epitaxial lattice matching composition, the temperature and the overall composition of the binary solid-liquid system.

Strain-Induced Structural Phase Transitions in Epitaxial (001) BiCoO3 Films: A First-Principles Study.

We have simulated BiCoO3 films epitaxially grown along (001) direction with density functional theory computations. Leading candidates for the lowest-energy phases have been identified. The tensile strains induce magnetic phase transition in the ground state (P4mm symmetry) from a C-type antiferromagnetic order to a G-type order for the in-plane lattice parameter above 3.922 Å. The G-type antiferromagnetic order will be maintained with larger tensile strains; however, a continuous structural phase transition will occur, combining the ferroelectric and antiferrodistortive modes. In particular, the larger tensile strain allows an isostructural transition, the so-called Cowley's ''Type Zero'' phase transitions, from Cc-(I) to Cc-(II), with a slight volume collapse. The orientation of ferroelectric polarization changes from the out-of-plane direction in the P4mm to the in-plane direction in the Pmc21 state under epitaxial tensile strain; meanwhile, the magnetic ordering temperature TN can be strikingly affected by the variation of misfit strain.

Engineering the electro-optic effect in HfO2 and ZrO2 through strain and polarization control

The ability to control the optical properties of a material with an electric field has led to optical memory devices, communication systems, optical signal processing, or quantum cryptography. Understanding electro-optic effects, especially in thin films, would improve the efficiency of these applications. In particular, the influence of epitaxial strains is of prime importance. In addition, the active control of these effects would be of great interest to tailor the material to the desired performance. Here, we demonstrate through first-principle calculations that the linear electro-optic response (Pockels effect) of two silicon-compatible ferroelectrics is stable with respect to bi-axial strain and that the electro-optic response can be strongly enhanced through the electrical control of the polarization. We attribute the former to the lack of optical phonon softening and a weak elasto-optic response and the latter to the externally induced softening of a phonon of symmetry A1. Our results are readily applicable to other polar materials and show that the electro-optic effect can be efficiently engineered to meet the performance criteria of future technologies.

Pressure modulated spin-gapless semiconductivity in FeCrTiAl

Spin-gapless semiconductors (SGS) represent a new type of compounds with potential applications in novel spintronic devices. Here, we performed a comprehensive computational and theoretical study of FeCrTiAl, a quaternary Heusler compound that was recently predicted to exhibit nearly SGS properties. Our calculations indicate that this material undergoes several band structure transitions from essentially semimetallic phase at smaller lattice constants to nearly type-II SGS at the ground state, then to nearly type-III SGS and further to nearly type-I SGS, as the lattice parameter is increased. Another interesting feature of FeCrTiAl is that its spin polarization changes sign from negative to positive as the volume of the cell increases. At the largest considered lattice parameters, this compound exhibits nearly 100% spin polarization. The mechanical expansion discussed in this text may be achieved, in principle, either by applying an epitaxial strain in thin-film geometry, or by chemical substitution, for example with non-magnetic element of larger atomic radius. We hope that the presented results may provide guidance for further research on mechanical strain induced manipulation of electronic and magnetic properties of spin-gapless semiconductors.

The anomalous Hall effect controlled by residual epitaxial strain in antiferromagnetic Weyl semimetal Mn3Sn thin films grown by molecular beam epitaxy

The large anomalous Hall effect (AHE) in antiferromagnetic(AFM) Weyl semimetal Mn3Sn attracts intensive attentions in spintronics. Here, we report the structural property of high quality Mn3Sn thin film on insulator substrate MgO(110) by molecular beam epitaxy (MBE), and AHE in control of residual mismatch strain between Mn3Sn film and substrate. We are able to grow strain-free Mn3Sn(10 1¯ 0) films or alternatively strained Mn3Sn(11 2¯ 0) films via a three-step process. The strain-free Mn3Sn film has large anomalous Hall conductivity up to 30 Ω-1cm−1 at room temperature, which is comparable to bulk Mn3Sn. In contrast, AHE is switched off in strained Mn3Sn film due to piezomagnetic effect under a uniaxial compress strain of ∼2.0%. These findings provide a deeper understanding on AFM spintronic applications.