Anisotropic In-plane Strain Research Articles

In-plane anisotropic effect of magnetoelectric coupled PMN-PT/FePt multiferroic heterostructure: Static and microwave properties

The effects of the electric and magnetic field variation on multiferroic heterostructure were studied in this work. Thin films of polycrystalline Fe50Pt50 (FePt) were grown by dc-sputtering on top of the commercial slabs of lead magnesium niobate-lead titanate (PMN-PT). The sample was a (011)-cut single crystal and had one side polished. In this condition, the PMN-PT/FePt operates in the L-T (longitudinal magnetized-transverse polarized) mode. A FePt thin film of 20 nm was used in this study to avoid the characteristic broad microwave absorption line associated with these films above thicknesses of 40 nm. For the in-plane easy magnetization axis (01-1), a microwave magnetoelectric (ME) coupling of 28 Oe cm kV −1 was estimated, whereas a value of 42 Oe cm kV −1 was obtained through the hard magnetization axis (100). Insight into the effects of the in-plane strain anisotropy on the ME coupling is obtained from the dc-magnetization loops. It was observed that the trend was opposite along the easy and hard magnetic directions. In particular, along the easy-magnetic axis (01-1), a square and narrow loop with a factor of Mr/MS of 0.96 was measured at 10 kV/cm. Along the hard-magnetic axis, a factor of 0.16 at 10 kV/cm was obtained. Using electric tuning via microwave absorption at X-band (9.78 GHz), we observe completely different trends along the easy and hard magnetic directions; Multiple absorption lines along the latter axis compared to a single and narrower absorption line along the former. In spite of its intrinsic complexity, we propose a model which gives good agreement both for static and microwave properties. These observations are of fundamental interest for future ME microwave components, such as filters, phase-shifters, and resonators.

Voltage control of metal-insulator transition and non-volatile ferroelastic switching of resistance in VOx/PMN-PT heterostructures.

The central challenge in realizing electronics based on strongly correlated electronic states, or ‘Mottronics', lies in finding an energy efficient way to switch between the distinct collective phases with a control voltage in a reversible and reproducible manner. In this work, we demonstrate that a voltage-impulse-induced ferroelastic domain switching in the (011)-oriented 0.71Pb(Mg1/3Nb2/3)O3-0.29PbTiO3 (PMN-PT) substrates allows a robust non-volatile tuning of the metal-insulator transition in the VOx films deposited onto them. In such a VOx/PMN-PT heterostructure, the unique two-step electric polarization switching covers up to 90% of the entire poled area and contributes to a homogeneous in-plane anisotropic biaxial strain, which, in turn, enables the lattice changes and results in the suppression of metal-insulator transition in the mechanically coupled VOx films by 6 K with a resistance change up to 40% over a broad range of temperature. These findings provide a framework for realizing in situ and non-volatile tuning of strain-sensitive order parameters in strongly correlated materials, and demonstrate great potentials in delivering reconfigurable, compactable, and energy-efficient electronic devices.

Optical characteristics of nonpolar a-plane ZnO thin film on (010) LiGaO2 substrate

We investigate the optical properties of non-polar a-plane ZnO film grown on (010) LiGaO2 substrate by chemical vapor deposition. X-ray diffraction indicates the a-plane orientation and Raman spectroscopy reveals the phase purity. Four distinct features in the near band-edge range are identified as neutral-donor-bound-exciton (D°X), free-exciton (FX), free-to-bound, and donor-acceptor pair transitions. The thermal activation energy of D°X is 12 meV while an acceptor energy level of 124 meV is estimated. Temperature evolution of photoluminescence (PL) shows that the room-temperature luminescence is a mixture of free-to-bound and FX transitions. The polarization dependence of the near-band-edge transitions were investigated. The largest degree of polarization of 95.5% occurs in FX transition. It can be attributed to anisotropic in-plane strain and the nonpolar a-plane orientation.

Strain-induced anisotropic transport properties of LaBaCo₂O₅.₅+δ thin films on NdGaO₃ substrates.

Thin films of double-perovskite structural LaBaCo2O5.5+δ were epitaxially grown on (110) NdGaO3 substrates by pulsed laser deposition. Microstructural studies by high-resolution X-ray diffraction and transmission electron microscopy revealed that the films have an excellent quality epitaxial structure. In addition, strong in-plane anisotropic strains were measured. Electrical transport properties of the films were characterized by an ultra-high-vacuum four-probe scanning tunneling microscopy system at different temperatures. It was found that the anisotropic in-plane strain can significantly tune the values of film resistance up to 590%.

Effect of “symmetry mismatch” on the domain structure of rhombohedral BiFeO3 thin films

Considerable work has focused on the use of epitaxial strain to engineer domain structures in ferroic materials. Here, we revisit the observed reduction of domain variants in rhombohedral BiFeO3 films on rare-earth scandate substrates. Prior work has attributed the reduction of domain variants to anisotropic in-plane strain, but our findings suggest that the monoclinic distortion of the substrate, resulting from oxygen octahedral rotation, is the driving force for variant selection. We study epitaxial BiFeO3/DyScO3 (110)O heterostructures with and without ultrathin, cubic SrTiO3 buffer layers as a means to isolate the effect of “symmetry mismatch” on the domain formation. Two-variant stripe domains are observed in films grown directly on DyScO3, while four-variant domains are observed in films grown on SrTiO3-buffered DyScO3 when the buffer layer is >2 nm thick. This work provides insights into the role of the substrate—beyond just lattice mismatch—in manipulating and controlling domain structure evolution in materials.

In-plane uniaxial magnetic anisotropy induced by anisotropic strain relaxation in high lattice-mismatched Dy/Sc superlattices

We report on the magnetic and structural characterization of high lattice-mismatched [Dy${}_{2nm}$/Sc${}_{{t}_{\text{Sc}}}$] superlattices, with variable Sc thickness ${t}_{\text{Sc}}$= 2--6 nm. We find that the characteristic in-plane effective hexagonal magnetic anisotropy K${}_{6}^{6,ef}$ reverses sign and undergoes a dramatic reduction, attaining values of $\ensuremath{\approx}$13--24 kJm${}^{\ensuremath{-}3}$, when compared to K${}_{6}^{6}=\ensuremath{-}0.76$ MJm${}^{\ensuremath{-}3}$ in bulk Dy. As a result, the basal plane magnetic anisotropy is dominated by a uniaxial magnetic anisotropy (UMA) unfound in bulk Dy, which amounts to $\ensuremath{\approx}$175--142 kJm${}^{\ensuremath{-}3}$. We attribute the large downsizing in K${}_{6}^{6,ef}$ to the compression epitaxial strain, which generates a competing sixfold magnetoelastic (MEL) contribution to the magnetocrystalline (strain-free) magnetic anisotropy. Our study proves that the in-plane UMA is caused by the coupling between a giant symmetry-breaking MEL constant M${}_{\ensuremath{\gamma},2}^{2}\ensuremath{\approx}1$ GPa and a morphic orthorhombiclike strain ${\ensuremath{\varepsilon}}_{\ensuremath{\gamma},1}\ensuremath{\approx}{10}^{\ensuremath{-}4}$, whose origin resides on the arising of an in-plane anisotropic strain relaxation process of the pseudoepitaxial registry between the nonmagnetic bottom layers in the superstructure. This investigation shows a broader perspective on the crucial role played by epitaxial strains at engineering the magnetic anisotropy in multilayers.

Optical in-plane anisotropy of ZnO/(Zn,Mg)O quantum wells grown ona-plane sapphire: Implications for optical spin control

Utilizing the anisotropic thermal expansion of $a$-plane sapphire, we study the role of anisotropic in-plane strain on ZnO quantum well structures. X-ray data reveal a deformation of the hexagonal unit cell. The symmetry reduction shows up in the optical spectra by distinct linear-polarization features, both for the neutral and charged exciton transition. We elaborate the excitonic fine structure behind this observation, accounting for spin-orbit coupling and electron-hole exchange in the presence of anisotropic strain. The consequences for optical spin control are discussed.

Polarization of emission from non-polar III-nitride quantum wells: the influence of confinement

Taking M-plane oriented GaN quantum wells (QW) as an example, it is shown that the finite out-of-plane crystal momentum arising from quantum confinement modifies valence band mixing in a way that can significantly alter the emission polarization properties of strained non-polar oriented wurtzite group III-nitride QWs. For certain values of strain, the emission polarization direction can rotate by 90° either within the QW plane, or from being out-of-plane to being in-plane which is desirable for light emission applications. The study based on a k · p type perturbation theory simultaneously accounts for the influence of anisotropic in-plane strain which arises in such QWs and also affects the optical polarization properties. An important practical implication of these results is that M-plane oriented AlxGa1−xN QWs under anisotropic in-plane tensile strain can work as efficient ultra-violet light emitters, unlike bulk AlxGa1−xN films with identical composition and strain. After including the influence of the out-of-plane crystal momentum, the emission polarization criterion allows for larger concentration of Al in such QW active layers if the well width is kept sufficiently small. These results are also applicable to A-plane oriented QWs.

Anisotropic strain relaxation and the resulting degree of polarization by one- and two-step growth in nonpolar a-plane GaN grown on r-sapphire substrate

Anisotropic strain relaxation and the resulting degree of polarization of the electronic transition in nonpolar a-plane GaN using one- and two-step growth are studied. By using two-step growth, a slower coalescence and a longer roughening-recovery process lead to larger anisotropic strain relaxation, a less striated surface, and lower densities of basal stacking fault (BSF) and prismatic stacking fault (PSF). It is suggested that anisotropic in-plane strains, surface striation, and BSF and PSF densities in nonpolar a-GaN are consequences of the rate of coalescence, the period of roughening-recovery process, and the degree of anisotropic strain relaxation. In addition, the two-step growth mode can enhance the degree of polarization of the electronic transition. The simulation results of the k⋅p perturbation approach show that the oscillator strength and degree of polarization of the electronic transition strongly depend on the in-plane strains upon anisotropic in-plane strain relaxation. The research results provide important information for optimized growth of nonpolar III-nitrides. By using two-step growth and by fabricating the devices on the high-quality nonpolar free-standing GaN substrates, high-efficiency nonpolar a-plane InGaN LEDs can be realized. Nonpolar a-plane InGaN/GaN LEDs can exhibit a strongly polarized light to improve the contrast, glare, eye discomfort and eye strain, and efficiency in display application.

Giant electrocaloric effect of PbTiO3 thin film tuned in a wide temperature range by the anisotropic misfit strain

The influence of anisotropic in-plane strains on the electrocaloric effect (ECE) in PbTiO3 (PT) epitaxial ferroelectric thin films is investigated by using a Landau-Devonshire thermodynamic theory. The calculation results show that the anisotropic strain can tune the ECE of PT ferroelectric thin films to obtain a large adiabatic temperature change in a wide temperature range which is attributed to the shift of c-phase boundary of PT thin films under the anisotropic strains with an external electric field. These results indicate that the anisotropic strain can provide an efficient way to adjust the ECE of ferroelectric thin films to refrigerate in a wide temperature range.

Engineering domain structures in nanoscale magnetic thin films via strain

We study the strain effects on magnetic domain stability and dynamics in nanoscale magnetic thin films using phase-field simulations. Numerous strain-stabilized single-/multi-domain states are discovered, including various magnetic vortices with circular in-plane domains. Furthermore, a strain-domain stability map was constructed, displaying the stable magnetic domain and domain wall structures as a function of biaxial isotropic and anisotropic in-plane strains at room temperature. The present work provides useful guidelines for a precise engineering and experimental observation of domain structures in nanoscale magnetic thin films and a promising scheme towards a low-power and local control over magnetic domain structures.

Strain Determination in Quasi-Lattice-Matched LWIR HgCdTe/CdZnTe Layers

We take advantage of the zinc distribution of (211)B CdZnTe substrates to probe the lattice-mismatch-induced stress in long-wave infrared HgCdTe layers grown by molecular beam epitaxy. High-resolution x-ray diffraction is used to accurately determine the strain-free lattice parameters of both CdZnTe and HgCdTe, together with the in-plane components of the stress tensor. By using several wafers, the stress evolution is derived over a broad range of lattice mismatch. In particular, stress relaxation is evidenced for mismatch greater than 0.02% and 0.04% for tensile and compressively strained HgCdTe, respectively. In-plane strain anisotropy, expected for the (211) orientation, is only evidenced for the compressive configuration. Strain relaxation is correlated with substrate curvature and rocking-curve peak broadening, providing indirect evidence for plastic relaxation.

Large exciton g-factors in anisotropically strained A-plane GaN film measured using magneto-optical Kerr effect spectroscopy

Exciton Landé g-factors in wurtzite GaN epitaxial films with (0001) C-plane and (112¯0) A-plane orientations have been measured in magnetic fields B up to 1.8 T, using polar magneto-optical Kerr effect (MOKE) spectroscopy. A procedure is developed for extracting the Zeeman splitting and thereby the g-factor, from Kerr ellipticity and rotation spectra of A-plane films, which have in-plane polarization anisotropy. In the C-plane film the measured g-factors for the A, B, and C exciton transitions were gA=0.09±0.02, gB=0.74±0.05, and gC=3.9±0.2, respectively, with B∥c-axis and comparable to earlier reports. The MOKE spectra of the A-plane film have one dominant exciton feature each for analyzer axis ⊥ and ∥ to the c-axis of GaN, and they arise at different energies. The measured g-factors for these were much larger, with values g⊥c=4.7±1 and g||c=7.1±1.2 with B⊥c-axis. Comparison with a k·p perturbation theory based calculation, which included the influence of strain, indicates that the features in the A-plane film are associated with exciton transitions involving bands that are strongly mixed by the anisotropic in-plane strain.

Electric field driven dynamic percolation in electronically phase separated (La0.4Pr0.6)0.67Ca0.33MnO3thin films

Competing ferromagnetic metallic (FMM) and insulating phases in the manganite (La${}_{1\ensuremath{-}y}$Pr${}_{y}$)${}_{1\ensuremath{-}x}$Ca${}_{x}$MnO${}_{3}$ lead to a phase separated state in which micrometer scale FMM regions behave in a fluidlike manner over a narrow temperature range. Here we show that an electric field can realign the fluidlike FMM phases embedded in an insulating matrix, resulting in anisotropic in-plane resistance in microstructures of (La${}_{0.4}$Pr${}_{0.6}$)${}_{0.67}$Ca${}_{0.33}$MnO${}_{3}$ thin films. Time and voltage dependent resistance and magnetization measurements show that the dynamic percolation of the FMM regions leads to an insulator to metal transition due to electric-field induced realignment of the FMM regions, which is analogous to the dielectrophoresis of metallic particles suspended in fluid media. In-plane strain anisotropy plays an important role in determining the speed of dynamic percolation of the FMM regions by modifying the local electric fields in the phase separated state.

Microarea Strain Analysis in InGaN/GaN Multiple Quantum Wells on m-Plane Using High-Resolution Microbeam X-ray Diffraction

Using high-resolution microbeam X-ray diffraction, we investigated in-plane anisotropic strain distributions within InGaN/GaN multiple quantum well structures on an m-plane GaN substrate. With this strain analysis, the micro-reciprocal space map (micro-RSM) and transmission electron microscopy measurements supported a structure without any dislocations and strain relaxations. From examining the microarea two-dimensional intensity profile map of the InGaN reflection peaks in the micro-RSMs, we consider that the in-plane anisotropic strain in the m-plane inclines toward the [1120] direction rather than the [0001] direction. On the basis of the slip system in the m-plane via the {1010} prism plane with <1120>-type slip directions that we have advocated, this anisotropy is in agreement with the causes of the strain relaxation in the prismatic plane with slip systems in the m-plane.

Effects of thickness on the polarization states in epitaxial ferroelectric thin films

The effect of thickness and depolarization field on the polarization states of epitaxial ferroelectric single-domain thin films with anisotropic in-plane misfit strains is studied by using a nonlinear thermodynamic theory with the thickness-dependent Curie temperature. The “thickness-misfit strain” phase diagrams for single-domain PbTiO3 thin films grown on tetragonal substrates are developed with and without depolarization field through minimization of the Helmholtz free energy. The results show that the depolarization field always reduces the region of the c-phase (P3 ≠ 0, P1 = P2 = 0) in the phase diagrams. The critical thickness, at which the ferroelectricity disappears, decreases when the tensile or compressive misfit strain increases.

In-Plane Optical Anisotropy of a-Plane GaN Film on r-Plane Sapphire Grown by Metal Organic Chemical vapour Deposition

The in-plane optical anisotropic properties of the non-polar a-plane GaN films grown by metal organic chemical vapour deposition are investigated by using polarised photoluminescence (PL), optical transmission and Raman scattering measurements. Through polarised PL and transmission spectra, the in-plane optical anisotropic properties of a-plane GaN film are found, which are attributed to the topmost valance band (at Γ point) split into three sub-bands under anisotropic strain. The PL spectra also exhibit that the light hole band moves up more rapidly than the spin-orbit crystal-field spilt-off hole band with the increasing in-plane anisotropic compressive strain. Raman scattering spectra under different configurations further indicate the in-plane anisotropy and the hexagonal crystalline structure of these a-plane GaN films.

The influence of in-plane ferroelectric crystal orientation on electrical modulation of magnetic properties in Co60Fe20B20/SiO2/(011) xPb(Mg1/3Nb2/3)O3-(1 − x)PbTiO3 heterostructures

(011) cut xPb(Mg1/3Nb2/3)O3-(1 − x)PbTiO3 (PMN-PT) ferroelectric crystal is usually used in ferroelectric/ferromagnetic (FE/FM) heterostructures due to its strong voltage-induced anisotropic in-plane strain. (011) PMN-PT crystal includes two in-plane crystal orientations, 〈100〉 and 〈01 − 1〉, with different piezoelectric strength resulting in anisotropic in-plane strain. Few systematic studies have been conducted to determine the influence of in-plane orientation on magnetoelectric (ME) coupling characteristic in ferroelectric/ferromagnetic composites. In this paper, we report our work to distinguish the contributions of in-plane orientations on electric modulation of magnetic properties. Magneto-optical Kerr effect and propagating spin wave spectroscopy are measured to define the influence of in-plane orientations on electric control of magnetic parameters and spin wave propagation. Magnetoelectric coupling coefficients and frequency modulation coefficients are also calculated.

Strain-driven transition fromE-type toA-type magnetic order in YMnO3epitaxial films

Single-crystal (100)-oriented YMnO${}_{3}$ thin films grown on (110)-oriented SrTiO${}_{3}$ substrates have been studied by polarized extended x-ray absorption fine structure to determine the Mn-O bond lengths. Using a simple geometrical model and previously reported x-ray diffraction data on the same samples, the Mn-O-Mn bonding angles are calculated. We show that the epitaxy-induced in-plane anisotropic strain has a dramatic impact on the bonding angles, allowing the rationalization of the reported existence of cycloidal magnetic order and concomitant ferroelectricity in moderately strained films and the gradual suppression by larger strains. We shall argue that epitaxial strain allows shifting YMnO${}_{3}$ from an $E$-type to $A$-type antiferromagnetic ground state, crossing a cycloidal magnetic region.

Large optical polarization anisotropy due to anisotropic in-plane strain in m-plane GaInN quantum well structures grown on m-plane 6H-SiC

We investigated the optical polarization anisotropy of m-plane GaInN/GaN quantum well structures on m-plane SiC and bulk GaN substrates. On bulk GaN, the degree of polarization increases with increasing indium content according to the larger strain-induced separation of the topmost valence bands. On m-plane SiC, however, we observe constantly large polarization ratios of around 90% and more. From an x-ray strain state analysis and calculations of the valence band energies, we find that an anisotropic strain of the GaN buffer layer leads to a very strong separation of the topmost valence bands resulting in a large degree of polarization.