Substrate Clamping Effect Research Articles

A Flexible Magnetic Field Sensor Based on PZT/CFO Bilayer via van der Waals Oxide Heteroepitaxy.

Magnetoelectric (ME) magnetic field sensors utilize ME effects in ferroelectric ferromagnetic layered heterostructures to convert magnetic signals into electrical signals. However, the substrate clamping effect greatly limits the design and fabrication of ME composites with high ME coefficients. To reduce the clamping effect and improve the ME response, a flexible ME sensor based on PbZr0.2Ti0.8O3 (PZT)/CoFe2O4 (CFO) ME bilayered heterostructure was deposited on mica substrates via van der Waals oxide heteroepitaxy. A saturated magnetization of 114.5 emu/cm3 was observed in the bilayers. The flexible sensor exhibited a strong ME coefficient of 6.12 V/cm·Oe. The local ME coupling has been confirmed by the evolution of the ferroelectric domain under applied magnetic fields. The flexible ME sensor possessed a stable response with high sensitivity to both AC and DC weak magnetic fields. A high linearity of 0.9988 and sensitivity of 72.65 mV/Oe of the ME sensor were obtained under flat states. The ME output and limit-of-detection under different bending states showed an inferior trend as the bending radius increased. A flexible proximity sensor has been demonstrated, indicating a promising avenue for wearable device applications and significantly broadening the potential application of the flexible ME magnetic field sensors.

Thickness-dependent microstructure, resistive switching, ferroelectric, and energy storage properties of pulsed laser deposited 0.85[0.6Ba(Zr0.2Ti0.8)O3-0.4(Ba0.7Ca0.3)TiO3]-0.15SrTiO3 thin films

In this work, we have studied the effect of thickness on structural, morphological, resistive switching (RS), ferroelectric, and energy storage properties of 0.85[0.6Ba (Zr0.2Ti0.8)O3-0.4(Ba0.7Ca0.3)TiO3]-0.15SrTiO3 (BZCT-STO) thin films deposited by the pulsed laser deposition technique. X-ray diffraction (XRD) analysis suggests that BZCT-STO thin films exhibit a polycrystalline tetragonal structure. The lattice parameters and tetragonality ratio approaching to bulk value with an increase of thickness, confirm strain relaxation in thicker films. SEM analysis reveals a dense columnar structure with a different grain size that varies from 3 nm to 60 nm as thickness increased from 160 to 360 nm. The P-E loops suggest that the relaxor behaviour increases with a decrease of thickness due to interface low dielectric layer, substrate clamping effect, small grain size, and high back switching ratio. The RS effect observed in Pt/BZCT-STO/Au capacitors is attributed to polarization modulation of the Schottky barrier and is found to be significant at low thickness. The effect of electric field and frequency on energy storage properties of BZCT-STO films is also investigated. The Pt/BZCT-STO/Au capacitor with a thickness of 160 nm exhibited 6.0 J/cm3 with an efficiency of 72% at a field of 800 kV/cm. It also exhibited a robust energy storage performance in the frequency range of 50–2000 Hz and also up to 108 cycles passing through the capacitor.

In Situ Synchrotron XRD Characterization of Piezoelectric Al1−xScxN Thin Films for MEMS Applications

Aluminum scandium nitride (Al1-xScxN) film has drawn considerable attention owing to its enhanced piezoelectric response for micro-electromechanical system (MEMS) applications. Understanding the fundamentals of piezoelectricity would require a precise characterization of the piezoelectric coefficient, which is also crucial for MEMS device design. In this study, we proposed an in situ method based on a synchrotron X-ray diffraction (XRD) system to characterize the longitudinal piezoelectric constant d33 of Al1-xScxN film. The measurement results quantitatively demonstrated the piezoelectric effect of Al1-xScxN films by lattice spacing variation upon applied external voltage. The as-extracted d33 had a reasonable accuracy compared with the conventional high over-tone bulk acoustic resonators (HBAR) devices and Berlincourt methods. It was also found that the substrate clamping effect, leading to underestimation of d33 from in situ synchrotron XRD measurement while overestimation using Berlincourt method, should be thoroughly corrected in the data extraction process. The d33 of AlN and Al0.9Sc0.1N obtained by synchronous XRD method were 4.76 pC/N and 7.79 pC/N, respectively, matching well with traditional HBAR and Berlincourt methods. Our findings prove the in situ synchrotron XRD measurement as an effective method for precise piezoelectric coefficient d33 characterization.

Flexible all-inorganic Sm-doped PMN-PT film with ultrahigh piezoelectric coefficient for mechanical energy harvesting, motion sensing, and human-machine interaction

In the era of artificial intelligence and the internet of things, the emergence of flexible MICAtronics, heterostructures made by van der Waals epitaxy on layered mica, has attracted great research interest due to applications in the fields of flexible ferroelectric memory, dielectric capacitors, and others, but less effort has been dedicated to the realization of flexible piezoelectric inorganic films for energy harvesters and sensors. In this work, an all-inorganic transparent Sm-doped Pb(Mg1/3Nb2/3)O3-PbTiO3 (Sm:PMN-PT) film has been successfully grown on Mica substrate via a simple one-step sol-gel process. The flexible film shows an ultrahigh piezoelectric coefficient d33 of 380 pm V−1 owing to the enhanced local structural heterogeneity and the weak substrate clamping effect. Simultaneously, superior flexibility is realized without sacrificing its electrical performance, as reflected by withstanding a small radius of curvature of 2 mm and even 103 mechanical bending cycles. Utilizing this Sm:PMN-PT film, the fabricated device exhibits an excellent energy harvesting performance with output voltage of 6 V, current density of 150 μA cm−2, andultrahigh force sensitivity of 5.86 V N−1. Besides, it also possesses a certain temperature resistance and favorable hydrophobicity. In practical applications, it can not only be employed to monitor body motions, including finger pressing, hand tapping, foot stepping, finger/wrist bending, and hand grasping, but also serves as a tactile sensor to record the touch of a finger to unlock a smartphone. This work demonstrates that the flexible PMN-PT based film on Mica substrate has full application potential in mechanical energy harvesting, motion sensing, and human-machine interaction.

Effect of substrate clamping on evolution of properties in homovalent and heterovalent relaxor thin films

Relaxor ferroelectrics, well known for their large dielectric and piezoelectric response, are attracting growing interest in thin-film form driven by a need for miniaturized devices. Fundamental understanding and control of the performance of relaxors as thin films, however, is underdeveloped relative to studies of bulk versions. One obvious challenge is substrate clamping, which reduces the dielectric and piezoelectric response, yet systematic comparisons of the effect of substrate clamping on different relaxor materials is missing. Here, the impact of epitaxial constraint on the relaxor behavior and properties of both homovalent ${\mathrm{BaZr}}_{0.5}{\mathrm{Ti}}_{0.5}{\mathrm{O}}_{3}$ and heterovalent $\mathrm{Pb}{\mathrm{Mg}}_{1/3}{\mathrm{Nb}}_{2/3}{\mathrm{O}}_{3}$ relaxors is studied in (001)-, (011)-, and (111)-oriented thin films. While the different orientations of $\mathrm{Pb}{\mathrm{Mg}}_{1/3}{\mathrm{Nb}}_{2/3}{\mathrm{O}}_{3}$ heterostructures show little variation in dielectric permittivity, a strong orientational effect is observed in ${\mathrm{BaZr}}_{0.5}{\mathrm{Ti}}_{0.5}{\mathrm{O}}_{3}$ heterostructures, with (111)-oriented films exhibiting a 67% higher dielectric permittivity than (001)-oriented films, a difference that is attributed to how substrate clamping along different crystallographic axes interacts with the polar-structure nucleation and growth. Measurements of dielectric anisotropy in (001)-oriented films confirm the trend---with $\mathrm{Pb}{\mathrm{Mg}}_{1/3}{\mathrm{Nb}}_{2/3}{\mathrm{O}}_{3}$ heterostructures exhibiting just half as much anisotropy in dielectric permittivity than ${\mathrm{BaZr}}_{0.5}{\mathrm{Ti}}_{0.5}{\mathrm{O}}_{3}$ heterostructures. These results indicate a higher susceptibility to substrate clamping in homovalent relaxors as compared to heterovalent relaxors, implying that the random fields in the heterovalent relaxors could be advantageous for materials in such geometries and applications.

Strain-Induced Magnetoelectric Coupling in Fe3O4/BaTiO3 Nanopillar Composites.

Magnetoelectric coupling properties are limited to the substrate clamping effect in traditional ferroelectric/ferromagnetic heterostructures. Here, Fe3O4/BaTiO3 nanopillar composites are successfully constructed. The well-ordered BaTiO3 nanopillar arrays are prepared through template-assisted pulsed laser deposition. The Fe3O4 layer is coated on BaTiO3 nanopillar arrays by atomic layer deposition. The nanopillar arrays and heterostructure are confirmed by scanning electron microscopy and transmission electron microscopy. A large thermally driven magnetoelectric coupling coefficient of 395 Oe °C-1 near the phase transition of BaTiO3 (orthorhombic to rhombohedral) is obtained, indicating a strong strain-induced magnetoelectric coupling effect. The enhanced magnetoelectric coupling effect originated from the reduced substrate clamping effect and increased the interface area in nanopillar structures. This work opens a door toward cutting-edge potential applications in spintronic devices.

Electrostatically Driven Polarization Flop and Strain-Induced Curvature in Free-Standing Ferroelectric Superlattices.

The combination of strain and electrostatic engineering in epitaxial heterostructures of ferroelectric oxides offers many possibilities for inducing new phases, complex polar topologies, and enhanced electrical properties. However, the dominant effect of substrate clamping can also limit the electromechanical response and often leaves electrostatics to play a secondary role. Releasing the mechanical constraint imposed by the substrate can not only dramatically alter the balance between elastic and electrostatic forces, enabling them to compete on par with each other, but also activates new mechanical degrees of freedom, such as the macroscopic curvature of the heterostructure. In this work, an electrostatically driven transition from a predominantly out-of-plane polarized to an in-plane polarized state is observed when a PbTiO3 /SrTiO3 superlattice with a SrRuO3 bottom electrode is released from its substrate. In turn, this polarization rotation modifies the lattice parameter mismatch between the superlattice and the thin SrRuO3 layer, causing the heterostructure to curl up into microtubes. Through a combination of synchrotron-based scanning X-ray diffraction imaging, Raman scattering, piezoresponse force microscopy, and scanning transmission electron microscopy, the crystalline structure and domain patterns of the curved superlattices are investigated, revealing a strong anisotropy in the domain structure and a complex mechanism for strain accommodation.

Magnetoelectric coupling in micropatterned BaTiO3/CoFe2O4 epitaxial thin film structures: Augmentation and site-dependency

The prominent magnetoelectric (ME) effect in ME composites usually originates from an interfacial strain-mediated coupling between ferroelectric and ferromagnetic components. While ME composite bilayer thin films have been proposed for microdevice applications, the ME coupling is usually diminished because of the substrate clamping effect. Here, we apply simple CMOS/MEMS compatible top-down fabrication techniques to ferroelectric BaTiO3 (BTO) and ferromagnetic CoFe2O4 (CFO) bilayer epitaxial thin films to control the substrate clamping. We found augmented ME coupling in micro-patterned bilayer thin films compared to the as-deposited films due to the reduced substrate clamping. In addition, a site-dependent ME coupling within the microstructure was observed. Larger ME coupling was obtained near the edge of the microstructure, and site-dependent ferroelectric imprints were observed on the micropattern. This can be attributed to the non-uniform substrate clamping across the film, and the strain gradient developed in the BTO layer due to the magnetostriction of CFO. Our findings provide additional insights to the design of micro- and nanoscale devices based on 2–2 ME composite thin films.

Advances in strain engineering on oxide heteroepitaxy

Advances in strain engineering on oxide heteroepitaxy

Wide-Range Epitaxial Strain Control of Electrical and Magnetic Properties in High-Quality SrRuO3 Films

Epitaxial strain in 4d ferromagnet SrRuO3 films is directly linked to the physical properties through the strong coupling between lattices, electrons, and spins. It provides an excellent opportunity to tune the functionalities of SrRuO3 in electronic and spintronic devices. However, a thorough understanding of the epitaxial strain effect in SrRuO3 has remained elusive due to the lack of systematic studies. This study demonstrates wide-range epitaxial strain control of electrical and magnetic properties in high-quality SrRuO3 films. The epitaxial strain was imposed by cubic or pseudocubic perovskite substrates having a lattice mismatch of -1.6 to 2.3% with reference to bulk SrRuO3. The Poisson ratio, which describes the two orthogonal distortions due to the substrate clamping effect, is estimated to be 0.33. The Curie temperature (TC) and residual resistivity ratios of the series of films are higher than or comparable to the highest reported values for SrRuO3 on each substrate, confirming the high crystalline quality of the films. A TC of 169 K is achieved in a tensile-strained SrRuO3 film on the DyScO3 (110) substrate, which is the highest value ever reported for SrRuO3. The TC (146-169 K), magnetic anisotropy (perpendicular or in-plane magnetic easy axis), and metallic conduction (residual resistivity at 2 K of 2.10 - 373 {\mu}{\Omega}cm) of SrRuO3 are widely controlled by epitaxial strain. These results provide guidelines to design SrRuO3-based heterostructures for device applications.

Multiferroic Heterostructures: Ultraflexible and Malleable Fe/BaTiO3 Multiferroic Heterostructures for Functional Devices (Adv. Funct. Mater. 16/2021)

In article number 2009376, Zhenlin Luo, Ming Liu, and co-workers present ultra-flexible and malleable Fe/BaTiO3 heterostructures with a good multiferroic pro perty, which show a large thermally driven magnetoelectric coupling effect by avoiding the substrate clamping effect, as well as other fascinating flexible-tuning interfacial phenomena. The research provides broader opportunities for low-power-driven flexible applications, and contributes to the community of flexible oxide materials application in a significant manner.

Flexible Epsilon Iron Oxide Thin Films

Metastable ε-Fe2O3 is a unique phase of iron oxide, which exhibits a giant coercivity field. In this work, we grew epitaxial ε-Fe2O3 films on flexible two-dimensional muscovite substrates via quasi van der Waals epitaxy. It turns out that twinning and interface energies have been playing essential roles in stabilizing metastable ε-Fe2O3 on mica substrates. Moreover, the weak interfacial bonding between ε-Fe2O3 and mica is expected to relieve the substrate clamping effect ubiquitously encountered in films epitaxially grown on rigid substrates, such as SrTiO3. It is anticipated that these flexible ε-Fe2O3 thin films can serve as a platform for exploring possible interesting emergent physical properties and eventually be integrated as flexible functional devices.

Voltage tunable low damping YIG/PMN-PT multiferroic heterostructure for low-power RF/microwave devices

Voltage-tunable RF/microwave devices attract great attention due to their low power consumption; however, they still suffer from the fundamental dilemma between large tunability and low insertion loss. In this paper, yttrium iron garnet (YIG) polycrystalline thin films with low microwave loss (<3 × 10−4) were directly deposited on mica substrates via a van der Waals force and then transferred onto PMN-PT substrates. The in-plane magnetic anisotropy of YIG was tuned by the in-plane biaxial stress of PMN-PT, and an improved voltage tunability (175 MHz) of in-plane FMR fr was obtained due to the removal of the substrate clamping effect. Moreover, our samples still represent good frequency stability (4–12 GHz) during the tuning process. It is of significance for exploiting novel, portable, low-loss RF/microwave devices in the future.

The microstructure and ferroelectric properties of PbZr0.52Ti0.48O3 films on mica substrates

PbZr0.52Ti0.48O3 (PZT) tetragonal ferroelectric films have been fabricated on flexible mica and rigid silicon substrates respectively with a bottom electrode of LaNiO3 (LNO) films via a low-cost sol-gel process. The microstructure and ferroelectric properties of both films were systematically investigated. The quantitative analysis results of XRD and TEM indicate that the PZT film on Si substrate suffer from residual in-plane tensile stress, while a virtually strain-free film can be obtained on mica substrate. The PZT film on mica substrate has a lower local coercive voltage studied by a piezoelectric response force microscope. It is the weak van der Waals interaction between the film and the two-dimensional mica substrate that reduce the substrate clamping effect and contribute to a more free response to the electric field for domain walls. A large measured residual polarization of ~52 μC/cm2 was obtained for the film on mica substrate, and the P−E loops can remain stable after bending cycles of 1000 times. The flexible PZT thin film is expected to have a strong potential application in the next generation of wearable devices.

Heterogeneous integration of single-crystalline complex-oxide membranes.

Complex-oxide materials exhibit a vast range of functional properties desirable for next-generation electronic, spintronic, magnetoelectric, neuromorphic, and energy conversion storage devices1-4. Their physical functionalities can be coupled by stacking layers of such materials to create heterostructures and can be further boosted by applying strain5-7. The predominant method for heterogeneous integration and application of strain has been through heteroepitaxy, which drastically limits the possible material combinations and the ability to integrate complex oxides with mature semiconductor technologies. Moreover, key physical properties of complex-oxide thin films, such as piezoelectricity and magnetostriction, are severely reduced by the substrate clamping effect. Here we demonstrate a universal mechanical exfoliation method of producing freestanding single-crystalline membranes made from a wide range of complex-oxide materials including perovskite, spinel and garnet crystal structures with varying crystallographic orientations. In addition, we create artificial heterostructures and hybridize their physical properties by directly stacking such freestanding membranes with different crystal structures and orientations, which is not possible using conventional methods. Our results establish a platform for stacking and coupling three-dimensional structures, akin to two-dimensional material-based heterostructures, for enhancing device functionalities8,9.

The preparation of flexible Ni-Mn-In thin films on mica substrates by pulsed laser deposition

Abstract The large volume variation of ferromagnetic shape memory alloy is strongly restricted in the film format on rigid substrates. Due to the high growth temperature (>673 K), it is difficult to grow Heusler alloys with high crystalline quality on flexible polymers. We successfully synthesize high-quality 100 nm thick Ni51.6Mn33.5In14.9 films on transparent flexible mica substrates by pulsed laser deposition, and compare the crystalline structure, morphology and magnetic properties with the films grown on Si (1 0 0) substrates. Due to the weak Van der Waals inter-layer interaction in mica substrates, the clamping effect of substrate can be significantly weakened by thinning the thickness of substrates using the simple mechanical exfoliation. This makes the final sample with Ni-Mn-In film on ultrathin mica substrate highly flexible with smallest curvature radius down to 0.745 cm. In addition, the mechanical properties of flexible Ni-Mn-In films were tested by bending in various curvature radiuses repeatedly. The resistance only changes slightly after more than 10,000 cycles, indicating the highly stable flexibility. These results demonstrate that flexible ferromagnetic shape memory alloy film can be obtained by using mica substrates and has great potential applications in flexible electronics.

Origin of giant negative piezoelectricity in a layered van der Waals ferroelectric.

Recent research on piezoelectric materials is predominantly devoted to enhancing the piezoelectric coefficient, but overlooks its sign, largely because almost all of them exhibit positive longitudinal piezoelectricity. The only experimentally known exception is ferroelectric polymer poly(vinylidene fluoride) and its copolymers, which condense via weak van der Waals (vdW) interaction and show negative piezoelectricity. Here we report quantitative determination of giant intrinsic negative longitudinal piezoelectricity and electrostriction in another class of vdW solids-two-dimensional (2D) layered ferroelectric CuInP2S6. With the help of single crystal x-ray crystallography and density-functional theory calculations, we unravel the atomistic origin of negative piezoelectricity in this system, which arises from the large displacive instability of Cu ions coupled with its reduced lattice dimensionality. Furthermore, the sizable piezoelectric response and negligible substrate clamping effect of the 2D vdW piezoelectric materials warrant their great potential in nanoscale, flexible electromechanical devices.

Magnetoelectric properties of NiFe2O4/0.65BaTiO3–0.35Na0.5Bi0.5TiO3 bilayer thin films deposited via pulsed laser deposition

Lead-free magnetoelectric (ME) NiFe2O4/0.65BaTiO3–0.35Na0.5Bi0.5TiO3 (NFO/BT–NBT) bilayer thin films grown on (001)-SrRuO3/SrTiO3 (SRO/STO) substrates were prepared via pulsed laser deposition. Different deposition sequences of the BT–NBT and NFO layers on the substrates yielded the following layered structures: NFO/BT–NBT/SRO/STO and BT–NBT/NFO/SRO/STO. Structural characterization via X-ray diffraction indicated that all of the samples are good crystallization and show c-axis orientation. Physical properties testing indicated that such heteroepitaxial composite thin films simultaneously exhibit excellent ferroelectric, dielectric, ferromagnetic, and ME coupling properties. Meanwhile, the ME coupling behavior of the bilayer thin films is influenced by growth sequences of the composite films. The ME coupling coefficient of the BT–NBT/NFO/SRO/STO (αE ∼ 120 mV cm─1 Oe─1) thin film is smaller than that of the NFO/BT–NBT/SRO/STO (αE ∼ 140 mV cm─1 Oe─1) thin film, which may be due to the different substrate clamping effect.

Voltage control of magnetism in NiZn ferrite/mica/PMNPT heterostructure with giant tunability and narrow linewidth

Recently, large magnetoelectric coupling of a spinel/piezoelectric heterostructure has been reported. However, the linewidth of the spinel is very large due to lattice mismatch when ferrite is directly deposited on piezoelectric substrates. This indicates a large magnetic loss, which impedes the spinel/piezoelectric heterostructure from useful device applications. Mica is a well-known 2D material, which can be split manually layer by layer without the substrate clamping effect. In this report, NiZn ferrite was deposited on a mica substrate by a spin-spray deposition technique. Spin-spray deposition is a wet chemical synthesis technique involving several chemical reactions for generating high-quality crystalline spinel ferrite films with various compositions directly from an aqueous solution. The thickness of ferrite is 2 μm, and the linewidth of the ferromagnetic resonance (FMR) is 115 Oe which is suitable for RF/microwave devices. The large FMR field tuning of 605 Oe was observed in NiZn ferrite/mica/PMN-PT heterostructures with minimal substrate clamping effect by reducing the thickness of the mica substrate. These multiferroic heterostructures exhibiting combined giant magnetoelectric coupling and narrow ferromagnetic resonance linewidth offer great opportunities for flexible RF magnetic devices.

Thermal Driven Giant Spin Dynamics at Three-Dimensional Heteroepitaxial Interface in Ni0.5Zn0.5Fe2O4/BaTiO3-Pillar Nanocomposites

Traditional magnetostrictive/piezoelectric laminated composites rely on the two-dimensional interface that transfers stress/strain to achieve the large magnetoelectric (ME) coupling, nevertheless, they suffer from the theoretical limitation of the strain effect and of the substrate clamping effect in real ME applications. In this work, 3D NZFO/BTO-pillar nanocomposite films were grown on SrTiO3 by template-assisted pulsed laser deposition, where BaTiO3 (BTO) nanopillars appeared in an array with distinct phase transitions as the cores were covered by NiZn ferrite (NZFO) layer. The perfect 3D heteroepitaxial interface between BTO and NZFO phases can be identified without any edge dislocations, which allows effective strain transfer at the 3D interface. The 3D structure nanocomposites enable the strong two magnon scattering (TMS) effect that enhances ME coupling at the interface and reduces the clamping effect by strain relaxation. Thereby, a large FMR field shift of 1866 Oe in NZFO/BTO-pillar nanocomposite was obtained at the TMS critical angle near the BTO nanopillars phase transition of 255 K.