Influence Of Epitaxial Strain Research Articles

Large Spin Coherence Length and High Photovoltaic Efficiency of the Room Temperature Ferrimagnet Ca2FeOsO6 by Strain Engineering

The influence of epitaxial strain on the electronic, magnetic, and optical properties of the distorted double perovskite Ca2FeOsO6 is studied. These calculations show that the compound realizes a monoclinic structure with P21/n space group from −6% to +6% strain. While it retains ferrimagnetic ordering with a net magnetic moment of 2 μB per formula unit at low strain, it undergoes transitions into E‐antiferromagnetic and C‐antiferromagnetic phases at −5% and +5% strain, respectively. It is shown that spin frustration reduces the critical temperature of the ferrimagnetic ordering from the mean field value of 600–350 K, in excellent agreement with the experimental value of 320 K. It is also shown that the critical temperature can be tuned efficiently through strain and that the spin coherence length surpasses that of Sr2FeMoO6 under tensile strain. An indirect‐to‐direct bandgap transition is observed at +5% strain. Localization of the valence and conduction states on different transition metal sublattices enables efficient electron–hole separation upon photoexcitation. The calculated spectroscopic limited maximum efficiency of up to 33% points to excellent potential of Ca2FeOsO6 in solar cell applications.

Strain Adjustment Realizes the Photocatalytic Overall Water Splitting on Tetragonal Zircon BiVO4.

Overall water splitting to generate H2 and O2 is vital in solving energy problem. It is still a great challenge to seek efficient visible light photocatalyst to realize overall water splitting. In this work, the tetragonal zircon BiVO4 is prepared by epitaxial growth on FTO substrate and its overall water splitting reaction is studied. Under the influence of epitaxial strain, the conduction band position shifts negatively and beyond H+/H2 reduction potential (0 V vs NHE), which enables it to possess the photocatalytic hydrogen evolution activity. After loading cocatalysts, the overall water splitting (λ > 400 nm) is realized (H2: ≈65.7 µmol g−1 h−1, O2: ≈32.6 µmol g−1 h−1), and the value of solar hydrogen conversion efficiency is 0.012%. The single‐particle photoluminescence (PL) spectra and PL decay kinetics tests demonstrate the cocatalysts are beneficial to the separation and transfer of carriers. The new strategy of adjusting the band structure by strain is provided.

Impact of compressive and tensile epitaxial strain on transport and nonlinear optical properties of magnetoelectric BaTiO3-(LaCa)MnO3 tunnel junction

This work is devoted to disclosing the effect of epitaxial strain on crystallographic, transport, and nonlinear optical properties of a bilayer structure consisting of one ferroelectric (FE) (BaTiO3 with a thickness of 4 nm) and one ferromagnetic (La0.7Ca0.3MnO3 with the thickness of 12 nm) layers. Both tensile (of about ∼6% caused by the MgO substrate) and compressive (of about ∼1% caused by the SrTiO3 substrate) strains reduce the bulk metal-to-insulator phase transition temperature of the La0.7Ca0.3MnO3 layer from 240 K towards 195 K and 160 K, respectively. The appearance of the effect of colossal electroresistance, linear and nonlinear current–voltage behaviours and contribution of FE polarization switching in the BaTiO3 layer to the conductivity are associated with the influence of epitaxial strain. The predominance of the compressive over the tensile strain leads to the hysteresis behaviour of electroresistance, maximum conductivity caused by the resonant-tunnelling mechanism, and strong nonlinear optical dependence on the applied electric field.

Electronic structure of La2/3Sr1/3MnO3 : Interplay of oxygen octahedra rotations and epitaxial strain

Influence of epitaxial strain and oxygen octahedra rotations on electronic structure of ${\mathrm{La}}_{2/3}{\mathrm{Sr}}_{1/3}{\mathrm{MnO}}_{3}$ ultrathin films was systematically studied. The investigated films were grown by pulsed laser deposition on four different substrates: cubic (001)-oriented ${\mathrm{LaAlO}}_{3}$, (001) ${({\mathrm{LaAlO}}_{3})}_{1/3}{({\mathrm{Sr}}_{2}{\mathrm{AlTaO}}_{6})}_{2/3}$, (001) ${\mathrm{SrTiO}}_{3}$, and orthorhombic (110) ${\mathrm{DyScO}}_{3}$, providing a broad range of induced epitaxial strains. Magnetic properties were found to deteriorate with increasing value of the epitaxial strain, as expected due to unit cell distortion increasingly deviating from the bulk and effect of the magnetically inert layer. A combination of spectroscopic ellipsometry and magneto-optical Kerr effect spectroscopy was used to determine spectra of the diagonal and off-diagonal elements of permittivity tensor. The off-diagonal elements at room temperature confirmed the presence of two previously reported electronic transitions in the spectra of all films. Moreover they revealed another electronic transition around 4.3 eV only in the spectra of films grown under compressive strain. We proposed classification of this transition as a crystal field paramagnetic Mn ${t}_{2g}\ensuremath{\rightarrow}{e}_{g}$ transition. Ab initio calculations were employed to distinguish between the potential influence of oxygen octahedra rotations and distortions. The ab initio calculations indicated a negligible influence of oxygen octahedra rotations on magneto-optical properties of ${\mathrm{La}}_{2/3}{\mathrm{Sr}}_{1/3}{\mathrm{MnO}}_{3}$. They further supported the proposed classification of the additional electronic transition, showing a key role of strain in controlling the electronic structure of ultrathin perovskite films.

Epitaxial strain dependence of band gaps in perovskite oxynitrides compared to perovskite oxides

Perovskite oxynitrides are a promising class of materials for photocatalytic water splitting due to their small band gaps and suitably aligned band edges. Recently, epitaxially strained perovskite oxynitrides started to attract interest due to the possibility to engineer the anion order and ferroelectric distortions. However, contrary to perovskite oxides there have been no studies on the band-gap evolution in perovskite oxynitrides with epitaxial strain. Here, we investigate, using density functional theory calculations, the influence of epitaxial strain on the band gaps of two different perovskite oxynitrides and compare our results with perovskite oxides. As opposed to cubic oxides where both compressive and tensile strains narrow the band gap, we find that the anion order leads to nondegenerate N $2p$ bands already at zero strain, which leads to opposite and anion order-dependent evolutions of the band gap under compressive and tensile strain. Furthermore, we find that in perovskite oxynitrides polar distortions increase the band gap by up to 1 eV for 4% strain whereas for perovskite oxides the increase is only about 0.1 eV for the same amount of strain. The reason for this difference is the larger ionic radius and smaller electronegativity of nitrogen leading to larger ferroelectric distortions and a stronger dependence of the band gap on the distortion amplitude. These results imply that ferroelectric distortions are strongly detrimental to light absorption and that their suppression, for example, above a critical temperature, should result in a marked decrease in the band gap and enhanced absorption of visible light.

Influence of epitaxial strain on multiple-mode compounds: The case of SrBi2Nb2O9

We study the effect of a tetragonal, symmetry-conserving epitaxial strain on the phase transition sequence and the onset of three symmetry-breaking modes in ${\mathrm{SrBi}}_{2}{\mathrm{Nb}}_{2}{\mathrm{O}}_{9}$ by first-principles calculations. The interplay of the strain variation of energy expansion coefficients results in an interesting phase diagram which includes proper, hybrid improper, and a different kind of improper ferroelectric phase transitions. The symmetries of the epitaxially stabilized phases and the possible phase-transition sequences are compared with the unstrained case.

Sub-band level-assisted photoconduction in epitaxial BiFeO3 films

Sub-band level assisted conduction mechanisms are well known in the field of semiconducting materials. In this work, we explicitly show the validity of such a mechanism in the multiferrroic material BiFeO3 (BFO). Our study is based on two different systems of epitaxial thin films of BFO, relaxed and strained. By analyzing the spectral distribution of the photoresponse from both the systems, the role of the sub-band levels in the photoconductive phenomena becomes evident. Additionally, the influence of epitaxial strain on the trapping activity of these levels is also observed. A model is proposed by taking into account the reversal of the role of a sub-band gap level, i.e., from a trapping to a ground state.

Magnetoelastic coupling induced magnetic anisotropy in Co2(Fe/Mn)Si thin films

The influence of epitaxial strain on uniaxial magnetic anisotropy of Co$_{2}$FeSi (CFS) and Co$_{2}$MnSi (CMS) Heusler alloy thin films grown on (001) SrTiO$_3$ (STO) and MgO is reported. The in-plane biaxial strain is susceptible to tune by varying the thickness of the films on STO, while on MgO the films show in-plane easy axis for magnetization (\overrightarrow{M}) irrespective of their thickness. A variational analysis of magnetic free energy functional within the Stoner-Wohlfarth coherent rotation model with out-of-plane uniaxial anisotropy for the films on STO showed the presence of magnetoelastic anisotropy with magnetostriction constant $\approx$ (12.22$\pm$0.07)$\times 10^{-6}$ and (2.02$\pm$0.06)$\times 10^{-6}$, in addition to intrinsic magnetocrystalline anisotropy $\approx$ -1.72$\times 10^{6}$ erg/cm$^{3}$ and -3.94$\times 10^{6}$ erg/cm$^{3}$ for CFS and CMS, respectively. The single-domain phase diagram reveals a gradual transition from in-plane to out-of-plane orientation of magnetization with the decreasing film thickness. A maximum canting angle of 41.5$^{\circ}$ with respect to film plane is predicted for the magnetization of the thinnest (12 nm) CFS film on STO. The distinct behaviour of \overrightarrow{M} in the films with lower thickness on STO is attributed to strain-induced tetragonal distortion.

Epitaxial strain induced magnetic transitions and phonon instabilities in tetragonal SrRuO3

Using density-functional theory calculations, we investigate the magnetic as well as the dynamical properties of tetragonal SrRuO3 (SRO) under the influence of epitaxial strain. It is found that both tensile and compressive strain in the xy-plane can induce an abrupt change in the magnetic moment of the Ru atom. In particular, under an in-plane compressive strain of ∼4%, a ferromagnetic to nonmagnetic transition is induced, whereas for a tensile strain larger than 3%, the magnetic moment of Ru drops gradually with increase of the strain, exhibiting a weak ferromagnetic state. We find that these magnetic transitions can be qualitatively explained by the Stoner model. In addition, frozen-phonon calculations at the Γ point and phonon dispersion calculations reveal that structural instabilities can occur under both compressive and tensile strain. These instabilities are very similar to those of the ferroelectric perovskite oxides, even though SRO remains metallic in the range we studied. This might have an influence on the physical properties of oxide supercells having SRO as a constituent.

Control of magnetic and transport properties in Nd0.45Sr0.55MnO3 films through epitaxial strain

The influence of epitaxial strain, tuned with tetragonal distortion c/a from 0.965 to 1.034 by choosing different substrates and varying film thickness, on the magnetic and transport properties of Nd0.45Sr0.55MnO3 thin films has been investigated. It is found that epitaxial strain, depending on the value of c/a, can enhance or decrease the stability of A-type AF to induce ferromagnetic fluctuation and C-type AF instead of A-type AF.

The influence of epitaxial strain on magnetic and electrical properties of BiFeO 3 thin films

Strained epitaxial BiFeO 3 films deposited on (001) SrTiO 3 substrates by metal organic chemical vapor deposition were studied by optical second harmonic generation (SHG) and SQUID magnetometry. The observed SHG intensity vs temperature dependencies indicate that for less strained films ( σ < 0.6 GPa) a strong interplay between the ferroelectric and magnetic subsystems exists, while for the films with larger σ-values strain-induced destruction of the magnetic cycloidal ordering takes place.

Epitaxial Heterostructures of KNbO3/LaNiO3/SrTiO3 and KNbO3/La1-xKxMnO3/SrTiO3 : MOCVD preparation and properties

Epitaxial conducting electrodes (60-300 nm thick) of LaNiO3, La1-xKxMnO3 and ferroelectric KNbO3 thin films were deposited by MOCVD using thd-volatile precursors on SrTiO3 and MgO single crystalline substrates. Influence of epitaxial strain on properties is discussed. Ferroelectric properties of the title heterostructures were characterized by hysteresis measurements, piezo-response force microscopy and second harmonic generation.

Magnetic anisotropy in (Ga,Mn)As: Influence of epitaxial strain and hole concentration

We present a systematic study on the influence of epitaxial strain and hole concentration on the magnetic anisotropy in (Ga,Mn)As at 4.2 K. The strain was gradually varied over a wide range from tensile to compressive by growing a series of (Ga,Mn)As layers with 5% Mn on relaxed graded (In,Ga)As/GaAs templates with different In concentration. The hole density, the Curie temperature, and the relaxed lattice constant of the as-grown and annealed (Ga,Mn)As layers turned out to be essentially unaffected by the strain. Angle-dependent magnetotransport measurements performed at different magnetic-field strengths were used to probe the magnetic anisotropy. The measurements reveal a pronounced linear dependence of the uniaxial out-of-plane anisotropy on both strain and hole density. Whereas the uniaxial and cubic in-plane anisotropies are nearly constant, the cubic out-of-plane anisotropy changes sign when the magnetic easy axis flips from in-plane to out-of-plane. The experimental results for the magnetic anisotropy are quantitatively compared with calculations of the free energy based on a mean-field Zener model. Almost perfect agreement between experiment and theory is found for the uniaxial out-of-plane and cubic in-plane anisotropy parameters of the as-grown samples. In addition, magnetostriction constants are derived from the anisotropy data.

Influence of epitaxial strain on the magnetic properties of (110) SmFe2 thin film

A novel nonlinear influence of the magnetoelastic energy because of the epitaxial strain allows us to explain the spontaneous magnetization and the cubic magnetostriction of a molecular-beam-epitaxy-grown SmFe2 thin film. Under this scope, new crystal-field parameters A4 and A6 and exchange coupling parameter λex have been found. These new parameters could account for the change of the energy balance in the spin reorientation transition and explain the softening it has with respect to the bulk case.

Phase transition and ferroelectric properties of epitaxially strained KNbO3/NaNbO3 superlattice

The influence of epitaxial strain on the crystal structure and polarization of KNbO3/NaNbO3 (KNO/NNO) superlattice has been quantified using density functional theory based on pseudopotential and plane-wave basis. A sequence of structural phase transitions with changing the in-plane misfit strain has been identified. If the compressive strain is more than −0.8%, tetragonal phase with polarization along [001] is stable. For the misfit strain between −0.8% and 0.36%, the stable phase is monoclinic while the stable phase becomes orthorhombic with polarization along [110] when the tensile strain is more than 0.36%. The spontaneous polarization in the orthorhombic phase reaches more than 70 μC/cm2.

Quantum-size effects in ultrathin Mg films

Recent experiments on the oxidation of ultrathin Mg films have revealed the existence of a correlation between surface reactivity and quantum-size effects. Using ab initio density-functional calculations, we have investigated the electronic properties of epitaxial Mg(0001) films, 5--17 atomic-layer thick, on a W(110) substrate to clarify the origin of this correlation. We find that the decay length in vacuum of the thin-film local density of states at the Fermi energy exhibits a pronounced oscillatory behavior as a function of film thickness. This is expected to have a major impact on the electron transfer rate by resonant tunneling, which is believed to control the initial sticking of ${\text{O}}_{2}$ molecules in the oxidation process. We have also examined the atomic-scale properties of the surface, interface, and quantum-well states of the Mg(0001) films on tungsten and the influence of epitaxial strain on the electronic states, in connection with the interpretation of recent photoemission spectra on these systems. In particular, we find strongly coupled surface-interface resonant states that originate from the Shockley surface states of the films. Comparison with photoemission measurements allows an unambiguous identification of the corresponding surface-interface-state splitting.

Dynamic investigations on the influence of epitaxial strain on the superconducting transition inYBa2Cu3O7−x

YBa2Cu3O7−x (YBCO) films have been prepared on piezoelectric (001)Pb(Mg1/3Nb2/3)0.72Ti0.28O3 (PMN-PT) substrates for dynamic investigations on the influence of strain on thesuperconducting transition temperature. The YBCO films deposited by off-axis pulsed laserdeposition showed a perfect epitaxial growth on the piezocrystals but also significantsurface roughness due to the miscut of the substrate. The in-plane lattice constant of thePMN-PT substrate was varied dynamically by the application of an electric field of≤10 kV cm−1. As a result, a reversible shift of the superconducting transition by about 0.1 K was foundon an optimally doped YBCO film for an applied strain of 0.05%. The results show for thefirst time that this approach is suitable for studying the strain dependence ofsuperconducting properties in detail.

Control of the magnetic and magnetotransport properties of La0.67Sr0.33MnO3 thin films through epitaxial strain

The influence of epitaxial strain, in the form of tetragonal distortions, on the magnetic and magnetotransport properties of La0.67Sr0.33MnO3 thin films was studied. The tetragonal distortion (c∕a ratio) was modulated through the choice of the substrate, ranging from c∕a=1.007 on (001)-oriented (LaAlO3)0.3(Sr2AlTaO6)0.7 substrates to 0.952 on (110)-oriented GdScO3 substrates. In agreement with previous theoretical predictions, these large values of tensile strain cause the Curie temperature and the saturation magnetization to decrease, alter the temperature dependence of the resistivity and magnetoresistance, and increase the resistivity by several orders of magnitude.

Magnetization of free standing GaMnAs

We compared the magnetization curves between as-grown and free standing GaMnAs samples, which were obtained from the same epitaxially grown wafer, to study the influence of epitaxial strain on their ferromagnetic properties. We found that the magnetization and coercive field for the free standing sample have significantly increased about twice as large as those of the as-grown GaMnAs. In addition, the uniaxial ferromagnetic anisotropies, which would not be expected on the basis of crystal symmetry, are remarkably different between the two samples. These results undoubtedly demonstrate that the magnetic properties are strongly influenced by the epitaxially induced strain.

Influence of epitaxial strain on the terrace and inter-layer diffusions in metal epitaxy

The influence of epitaxial strain on the surface and inter-layer diffusions are investigated using molecular statics and transition state theory with several types of embedded atom method potentials for Ag and Ni. Quantitatively to analyze the competition of the surface and inter-layer diffusions in the instability to the multi-layer growth, Ehrlich–Schwoebel barrier and the attempt frequencies of the surface and inter-layer diffusions by both hopping and exchange mechanisms are considered simultaneously. The attempt frequencies of exchange mechanism are larger by order one than those of hopping mechanism and especially, the difference becomes more severe for the inter-layer diffusion. Considering both the attempt frequency and activation energy barrier shows that the layer-by-layer growth is enhanced by compressive strain for Ag(0 0 1), which is confirmed by the existing linear stability theory and kinetic Monte Carlo simulations.