Film-substrate Lattice Mismatch Research Articles

Strain Control of Magnetic Anisotropy in Yttrium Iron Garnet Films in a Composite Structure with Yttrium Aluminum Garnet Substrate

This report is on the nature of strain in thin films of yttrium iron garnet (YIG) on yttrium aluminum garnet (YAG) substrates due to film-substrate lattice mismatch and the resulting induced magnetic anisotropy. Films with thickness 55 nm to 380 nm were deposited on (100), (110), and (111) YAG substrates using pulsed laser deposition (PLD) techniques and characterized by structural and magnetic characterization techniques. The in-plane strain determined to be compressive using X-ray diffraction (XRD). It varied from −0.12% to −0.98% and increased in magnitude with increasing film thickness and was relatively large in films on (100) YAG. The out-of-plane strain was tensile and also increased with increasing film thickness. The estimated strain-induced magnetic anisotropy field, found from XRD data, was out of plane; its value increased with film thickness and ranged from 0.47 kOe to 3.96 kOe. Ferromagnetic resonance (FMR) measurements at 5 to 21 GHz also revealed the presence of a perpendicular magnetic anisotropy that decreased with increasing film thickness and its values were smaller than values obtained from XRD data. The PLD YIG films on YAG substrates exhibiting a perpendicular anisotropy field have the potential for use in self-biased sensors and high-frequency devices.

Effect of substrate-induced lattice strain on the electrochemical properties of pulsed laser deposited nickel oxide thin film

The storage of renewable energy is an important step toward the global effort to combat air contamination and climate change. In this work, the influence of substrate-induced strain on the electrocatalytic properties of nickel oxide (NiO) films toward the hydrogen evolution reaction (HER) is studied. Using pulsed laser deposition, NiO thin films were deposited on strontium titanate, lanthanum aluminate, and sapphire substrates to examine how the substrate–film lattice mismatch influences the electrochemical properties. It was observed that the electrocatalytic activities of the NiO thin films exhibited strong sensitivity to strain; the NiO film with the smallest strain recorded the lowest overpotential for the HER. The NiO films were further explored to estimate the charge storage capacity and surface roughness. This work shows the use of simple thin-film synthesis as a way to evaluate the strain effect in electrocatalysis.

High room-temperature TCR and MR of La1-xSrxMnO3 thin films for advanced uncooled infrared bolometers and magnetic sensors

Perovskite manganese films had promising applications in the field of photoelectronic materials and devices. However, the utilization of manganese films remained severely challenging due to high fabrication cost and inferior electrical and magnetic properties. Herein, La1-xSrxMnO3 (LSMO) thin films were fabricated on La0.3Sr0.7Al0.65Ta0.35O3 (LSATO) substrates using a facile sol–gel spin-coating technique. All the LSMO films displayed (001)-orientation with orthorhombic structure. The LSMO films exhibited high temperature coefficient of resistivity (TCR) and magnetoresistance (MR) values at room-temperature. The maximum TCR value was 10.4 % K−1 (301.2 K, for x = 0.175). This result can be attributed to high carrier concentration and mobility of optimal × content LSMO films with low film-substrate lattice mismatch (-0.4%), leading to low resistivity, and thus, increased TCR value. The maximum MR value reached 42.85% (290.95 K, for x = 0.150). The LSMO films with high room-temperature TCR and MR values are potential materials for advanced uncooled infrared bolometers and magnetic sensors.

Domain boundaries in incommensurate epitaxial layers on weakly interacting substrates

There has been increasing interest in the fabrication of thin film materials with mixed dimensions, in particular, 2D to 3D and 3D to 2D heterostructures. Often, if the interface interaction is weak, the lattice matching criterion between the substrate and overlayer can be lifted. If the overlayer lattice is completely relaxed, it can form an incommensurate film on the mismatched substrate. In this work, we show that domain boundaries are inherent in the incommensurate epitaxial films due to random nucleation sites of domains in an overlayer. The nature and origin of the incommensurate domain boundaries are different from the conventional dislocation boundaries that come from the relaxation of strain due to film–substrate lattice mismatch. We propose that the formation of such domain boundaries can be studied through Voronoi tessellation. Using a case study of monolayer WS2 on sapphire (2D on 3D), we show the formation of domain boundaries that compared well with a recent experiment reported in the literature. In the Voronoi tessellation, we also show quantitatively that the average domain size depends on the density of nucleation sites. The conclusion of this case study may be generalized to any incommensurate epitaxial films when the interface interaction is weak.

Interface-driven magnetic anisotropy in relaxed La0.7Sr0.3CrO3/La0.7Sr0.3MnO3 heterostructures on MgO

We investigate the structural and magnetic properties of La0.7Sr0.3CrO3 (LSCO)/La0.7Sr0.3MnO3 (LSMO) heterostructures grown on (001)-oriented MgO by molecular beam epitaxy. Due to the large film-substrate lattice mismatch, strain relaxation is found to occur within the first 2–3 unit cells (uc) of the film as evidenced by reflection high energy electron diffraction and high-resolution synchrotron x-ray reciprocal space mapping. We find that the presence of the LSCO spacer and capping layers leads to ferromagnetism in ultra-thin LSMO layers with thicknesses of the order of 2 uc with the magnetic easy axis oriented in the film plane. Net magnetic moments of 1.4 and 2.4 μB/Mn are measured for (2 uc LSCO/2 uc LSMO) and (2 uc LSCO/4 uc LSMO) superlattices, respectively, by superconducting quantum interference device magnetometry. The effective magnetic anisotropy of the relaxed (2 uc LSCO/4 uc LSMO) heterostructure is found to be an order of magnitude higher than bulk LSMO highlighting the critical role of interfacial interactions in tuning magnetic anisotropy at complex oxide interfaces.

The role of iron in magnetic damping of Mg(Al,Fe)2O4 spinel ferrite thin films

We have investigated magnesium aluminum ferrite thin films with a range of iron concentrations and identified the optimal iron content to obtain high crystalline quality thin films with the low magnetic damping required for spin current-based applications. Epitaxial MgAl2−x FexO4 films with 0.8 < x < 2.0 were grown by pulsed laser deposition on single crystal MgAl2O4 substrates and were characterized structurally and magnetically. We find that the x = 1.5 composition minimizes the room-temperature magnetic damping with a typical Gilbert damping parameter of αeff=1.8×10−3. This minimized damping is governed by a competition between the more robust magnetic ordering with increased iron content, x, and the more defective structure due to larger film-substrate lattice mismatch with increased iron content. The temperature-dependent magnetization curves indicate that Tc is suppressed below room temperature for iron content x≤1.2 and eventually suppressed entirely for x = 0.8. X-ray magnetic circular dichroism results indicate that for all x the magnetic moment is dominated by Fe3+ cations distributed in a 60:40 ratio on the octahedral and tetrahedral sites, with minimal contribution from Fe2+ cations. Films with x=1.4−1.6 exhibit very strong ferromagnetic resonance and low Gilbert damping with αeff=(1.8−6)×10−3, making them ideal candidates for microwave and spintronic applications.

Nontrivial magnetic field related phenomena in the singlelayer graphene on ferroelectric substrate (Review Article)

The review is focused on our predictions of nontrivial physical phenomena taking place in the nanostructure single-layer graphene on ferroelectric substrate, which are related with magnetic field. In particular we predicted that 180-degree domain walls in a strained ferroelectric film can induce p-n junctions in a graphene channel and lead to the unusual temperature and gate voltage dependences of the perpendicular modes v┴ of the integer quantum Hall effect. The non-integer numbers and their irregular sequence principally differ from the conventional sequence v┴ = 3/2, 5/3, … The unusual v┴-numbers originate from significantly different numbers of the edge modes, v1 and v2, corresponding to different concentration of carriers in the left (n1) and right (n2) ferroelectric domains of p-n junction boundary. The difference between n1 and n2 disappears with the vanishing of the film spontaneous polarization in a paraelectric phase, which can be varied in a wide temperature range by an appropriate choice of misfit strain originated from the film-substrate lattice mismatch. Next we studied the electric conductivity of the system ferromagnetic dielectric-graphene channel-ferroelectric substrate. The magnetic dielectric locally transforms the band spectrum of graphene by inducing an energy gap in it and making it spin-asymmetric with respect to the free electrons. It was demonstrated, that if the Fermi level in the graphene channel belongs to energy intervals, where the graphene band spectrum, modified by EuO, becomes sharply spin-asymmetric, such a device can be an ideal non-volatile spin filter. The practical application of the system under consideration would be restricted by a Curie temperature of a ferromagnet. Controlling of the Fermi level (e.g., by temperature that changes ferroelectric polarization) can convert a spin filter to a spin valve.

Enhanced Ferroelectric Functionality in Flexible Lead Zirconate Titanate Films with In Situ Substrate‐Clamping Compensation

AbstractMuch attention has recently been given to flexible and wearable integrated electronic devices, with a strong emphasis on real‐time sensing, computing, and communication technologies. Thin ferroelectric films exhibit switchable polarization and strong electromechanical coupling, and hence are in widespread use in such technologies, albeit not when flexed. Effects of extrinsic strain on thin ferroelectric films are still unclear, mainly due to the lack of suitable experimental systems that allow cross structural–functional characterization with in situ straining. Moreover, although the effects of intrinsic strain on ferroelectric films, e.g., due to film–substrate lattice mismatch, have been extensively investigated, it is unclear how these effects are influenced by external strain. A method to strain thin films homogenously in situ is developed, allowing structural characterization as well as functional switching and piezorsponse measurements at the nanoscale, while retaining the sample under constant straining conditions. Using this method, thin films of PbZr0.2Ti0.8O3, which were grown on a flexible mica substrate, are strained to reduce substrate clamping effects and increase the tetragonality. Consequently, the domain stability is increased, the coercive field value is decreased, and imprint effects are reduced. This method also allows direct characterization of the relationship between the lattice parameters and nanoscale properties of other flexible materials.

Surface acoustic waves in strain-engineered K0.7Na0.3NbO3 thin films

Epitaxial K0.7Na0.3NbO3 thin films are grown via metal-organic chemical vapor deposition on (110)-oriented TbScO3. The films are strained due to the substrate–film lattice mismatch and therefore exhibit a strong and anisotropic modification of all its ferroelectric properties. The compressive in-plane strain leads to a reduction of the ferroelectric transition temperature from approximately 700 K for unstrained K0.7Na0.3NbO3 to 324 K and 330 K with maximum permittivities of 10 270 and 13 695 for the main crystallographic directions [001]TSO and [11¯0]TSO, respectively. Moreover, the quite thin films (approx. 30 nm thick) exhibit very large piezoelectric properties. For instance, surface acoustic waves with intensities of up to 4.7 dB are recorded for wave propagation along the [11¯0]TSO direction. The signal is smaller (up to 1.3 dB) along [001]TSO, whilst for the intermediate direction [11¯2]TSO, the signal seems to vanish (<0.1 dB). The results indicate that the choice of material, (K,Na)NbO3, in combination with strain-engineering via epitaxial growth onto lattice-mismatched substrates represents a promising way to optimize ferroelectric materials for piezoelectric thin-film applications.

Amorphousness induced significant room temperature ferromagnetism of TiO2 thin films

TiO2 films have been grown on Si(100) substrates via pulsed laser deposition. Amorphousness has been achieved for films <30 nm due to the large film-substrate lattice mismatch. With the increased film thickness, crystallization occurs and the amorphous TiO2 evolves into the anatase and rutile phases. Compared with the crystallized phases, the amorphous structure contains more oxygen vacancies (Vo) for large room temperature ferromagnetism. Significant magnetization (up to 180.4 emu/cc) has been achieved for the film in the complete amorphous state. Theoretical calculations indicate that the magnetic moments distribute around the Ti3+ ions induced by Vo in the amorphous structure.

Structural and electrical properties of large area epitaxial VO2 films grown by electron beam evaporation

Large area (up to 4 squared inches) epitaxial VO2 films, with a uniform thickness and exhibiting an abrupt metal-insulator transition with a resistivity ratio as high as 2.85×104, have been grown on (001)-oriented sapphire substrates by electron beam evaporation. The lattice distortions (mosaicity) and the level of strain in the films have been assessed by X-ray diffraction. It is demonstrated that the films grow in a domain-matching mode where the distortions are confined close to the interface which allows growth of high-quality materials despite the high film-substrate lattice mismatch. It is further shown that a post-deposition high-temperature oxygen annealing step is crucial to ensure the correct film stoichiometry and provide the best structural and electrical properties. Alternatively, it is possible to obtain high quality films with a RF discharge during deposition, which hence do not require the additional annealing step. Such films exhibit similar electrical properties and only slightly degraded structural properties.

Magnetic anisotropy of polycrystalline high-temperature ferromagnetic MnxSi1-x ([formula omitted]) alloy films

A set of thin film MnxSi1-x alloy samples with different manganese concentration x≈0.44−0.63 grown by the pulsed laser deposition (PLD) method onto the Al2O3 (0001) substrate was investigated in the temperature range 4–300K using ferromagnetic resonance (FMR) measurements in the wide range of frequencies (f=7−60GHz) and magnetic fields (H=0−30kOe). For samples with x≈0.52−0.55, FMR data show clear evidence of ferromagnetism with high Curie temperatures TC∼300K. These samples demonstrate a complex magnetic anisotropy described phenomenologically as a combination of the essential second order easy plane anisotropy contribution and the additional fourth order uniaxial anisotropy contribution with easy direction normal to the film plane. The observed anisotropy is attributed to a polycrystalline (mosaic) structure of the films caused by the film-substrate lattice mismatch. The existence of extra strains at the crystallite boundaries initiates a random distribution of local in-plane anisotropy axes in the samples. As a result, the symmetry of the net magnetic anisotropy is axial with the symmetry axis normal to the film plane. The principal features of the observed anisotropy are explained qualitatively within the proposed microscopic model.

Magnetic anisotropy peculiarities of high-temperature ferromagnetic MnxSi1−x (x ≈ 0.5) alloy films

Thin films of MnxSi1−x alloys with different Mn concentration grown by the pulsed-laser deposition (PLD) method onto the Al2O3 (0001) substrate were investigated in the temperature range 4–300 K using ferromagnetic resonance (FMR) measurements in the wide range of frequencies and magnetic fields . For samples with , FMR data show clear evidence of ferromagnetism (FM) with high Curie temperatures . These samples demonstrate the complex and unusual character of magnetic anisotropy described in the frame of phenomenological model as a combination of the essential second-order easy-plane anisotropy contribution and the additional fourth-order anisotropy contribution with the easy direction normal to the film plane. We explain the obtained results by a polycrystalline (mosaic) structure of the films caused by the film-substrate lattice mismatch.

Growth and morphological evolution of Co3-x Mn x O4 (x = 1, 2) thin films

We grew epitaxial CoMn2O4 and Co2MnO4 thin films on Nbdoped (0.1 wt.%) SrTiO3(100) singlecrystal substrates by using pulsed laser deposition and studied how the growth temperature affected their crystal structures and surface morphologies. For Co2MnO4, increasing the growth temperature induced no significant changes except for an enlarged grain size. In contrast for CoMn2O4, increasing the growth temperature caused its surface morphology to evolve from spherical grains to rectangular nanorods; additionally, X-ray diffraction showed that these rectangular nanorod grains scattered to domains with different crystal orientations. We attribute this rectangular nanorod pattern to self-assembled domains originating from Jahn-Teller distortions and film-substrate lattice mismatch

Pulsed laser deposition of La1−xSrxMnO3: thin-film properties and spintronic applications

Materials engineering on the nanoscale by precise control of growth parameters can lead to many unusual and fascinating physical properties. The development of pulsed laser deposition (PLD) 25 years ago has enabled atomistic control of thin films and interfaces and as such it has contributed significantly to advances in fundamental material science. One application area is the research field of spintronics, which requires optimized nanomaterials for the generation and transport of spin-polarized carriers. The mixed-valence manganite La1−xSrxMnO3 (LSMO) is an interesting material for spintronics due to its intrinsic magnetoresistance properties, electric-field tunable metal–insulator transitions, and half-metallic band structure. Studies on LSMO thin-film growth by PLD show that the deposition temperature, oxygen pressure, laser fluence, strain due to substrate–film lattice mismatch and post-deposition annealing conditions significantly influence the magnetic and electrical transport properties of LSMO. For spintronic structures, robust ferromagnetic exchange interactions and metallic conductivity are desirable properties. In this paper, we review the physics of LSMO thin films and the important role that PLD played in advancing the field of LSMO-based spintronics. Some specific application areas including magnetic tunnel junctions, multiferroic tunnel junctions and organic spintronic devices are highlighted, and the advantages, drawbacks and opportunities of PLD-grown LSMO for next-generation spintronic devices are discussed.

Ferroelectric nanodomains in epitaxial PbTiO3 films grown on SmScO3 and TbScO3 substrates

Domain structures of 320 nm thin epitaxial films of ferroelectric PbTiO3 grown by metal-organic chemical vapor deposition technique in identical conditions on SmScO3 and TbScO3 perovskite substrates have been investigated by Raman spectroscopy and piezoresponse force microscopy techniques. Phonon frequency shifts and typical domain structure motifs are discussed. The results reveal strikingly different domain structure architecture: domain structures of the PbTiO3 film grown on SmScO3 have dominantly a-domain orientation while strongly preferential c-domain orientation was found in the PbTiO3 film grown on the TbScO3 substrate. Differences between the two cases are traced back to the film-substrate lattice mismatch at the deposition temperature.

Self-Assembled Nanostructures as Templates for the Integration of Nanoparticles in Oxide Surfaces

Under properly selected growth conditions, complex oxide thin films might exhibit a tendency towards selforganization allowing obtaining regular arrays of three-dimensional nanostructures. This behavior, together with their rich physics, offers enormous potential for the implementation of new nanodevices. Among complex oxides, manganese perovskites exhibiting colossal magnetoresistance and half-metallic character have emerged as promising candidates for the implementation of new spintronic devices. Manganite thin films are often elastically strained, due to film-substrate lattice mismatch, and this lattice strain can, in some cases, select preferential growth modes leading to the appearance of different self-organized nanostructured morphologies. It is shown that under properly chosen growth conditions long range ordered arrays of nanoobjects, running along the steps direction defined by the miscut angle of the underlying substrate, can be obtained in highly epitaxial La2/3Sr 1/3MnO3 (LSMO) thin films. These results suggest that self-organization process is directly guided by the topological features of the underlying substrate and highlight the relevance of growth kinetic effects. The use of those nanoobjects arrays as nanostencils for fabricating arrays of nanoparticles is also explored.

Growth Aspects of Thin-Film Composite Heterostructures of Oxide Multicomponent Perovskites for Electronics

We review, based on our results, the problems and solutions for the growth of thin films and composite heterostructures emphasizing the general growth aspects and principles vs specifics for each material or heterostructure. The materials used in our examples are Bi2Sr2Ca2Cu3O10, Bi2Sr2CaCu2O8, YBa2Cu3O7, (Sr, Ca)CuO2, (Ba, Ca)CuO2, and Bi4Ti3O12. The growth method was metal organic chemical vapor deposition (MOCVD). The presented thin films or heterostructures have c- and non-c-axis orientations. We discuss the implications of the film–substrate lattice relationships, paying attention to film–substrate lattice mismatch anisotropy and to film–film lattice mismatch, which has a significant influence on the quality of the non-c-axis heterostructures. We also present growth control through the use of vicinal substrates and two-temperature (template) and interrupted growth routes allowing significant quality improvements or optimization. Other key aspects of the growth mechanism, that is, roughness, morphology, and interdiffusion, are addressed. It is concluded that the requirements for the growth of non-c-axis heterostructures are more severe than those for the c-axis ones.

Lattice distortion and strain relaxation in epitaxial thin films of multiferroic TbMnO3 probed by X-ray diffractometry and micro-Raman spectroscopy

A detailed structural XRD analysis of (110)-oriented TbMnO3 thin films grown on (110)-YAlO3 substrates shows the co-existence of a strained and relaxed “sublayer” within the films due to strain relaxation during epitaxial growth by pulsed laser deposition. The substrate-film lattice mismatch yields a compressive strain anisotropy along the two in-plane directions, i.e. [1−10] and [001] and a monoclinic distortion. A further manifestation of the growth-induced strain is the hardening of Raman active modes as a result of changed atomic motions along the [1−10] and [001] directions.

Effects of film thickness and composition on the structure and martensitic transition of epitaxial off-stoichiometric Ni–Mn–Ga magnetic shape memory films

Epitaxial Ni–Mn–Ga magnetic shape memory films with varied thickness and variable stoichiometry were prepared by magnetron sputtering on MgO(001) substrates and investigated by using x-ray diffraction (XRD), scanning tunneling microscopy (STM) and scanning electron microscopy (SEM). The results demonstrate that the structure of the films is either a non-modulated martensite or a seven-layer-modulated martensite. At small film thicknesses, we observe a preferential alignment of the {110} twin planes titled 45° from the substrate surface, which gives rise to the surface corrugation, whereas at large thicknesses, twin planes align additionally along the perpendicular planes. The biaxial tensile stress due to the film–substrate lattice mismatch is shown to have an important role in the selection of the possible twinning planes. An exponential relation between the stress and the film thickness is identified. In contrast, the magnetic properties of the films are found to be independent of the stress. A resistance maximum is measured close to the Curie temperature, which can be attributed to the relaxation of the lattice distortion induced by magnetoelastic coupling.