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This study offers a comprehensive investigation into the fabrication of all-oxide-based magnetic tunnel junctions (MTJs) utilizing the focused ion beam (FIB) nanomachining method. The oxide thin films of SrTiO3 (STO), La 1 − x CaxMnO3 (LCMO) and La 1 − x SrxMnO3 (LSMO) were grown using pulsed laser deposition (PLD). Magnetization measurements at 5K revealed a notable difference in coercivity between LCMO and LSMO films, highlighting their suitability as electrodes in MTJ device fabrication. The deposited trilayer structure of LCMO/STO/LSMO exhibited heteroepitaxial growth, enabling the independent magnetic switching of electrode layers in the MTJ with the STO barrier layer. Conventional photolithography and ion milling were used to create a series of 4 μ m Hall bar tracks in the deposited films. Nanopillar devices, sized 250 nm × 250 nm, were successfully fabricated on these 4 μ m tracks by optimizing the FIB nanomachining technique. The fabricated MTJs demonstrated a high tunnel magnetoresistance (TMR) response, leading to the conclusion that the FIB nanomachining technique holds significant promise for the development of high-performance oxide-based spintronic devices. [ABSTRACT FROM AUTHOR]

Multiferroic BiFeO3 exhibits a morphotropic phase boundary at large compressive strain that merges metastable phases of tetragonal (T) and rhombohedral (R) character resulting in giant ferroelectric and electromechanical responses. To utilize this functionality in devices, it is essential to understand the response of these ferroelectric phases to the environment of a nanoscale heterostructure. Here, the emergence of ferroelectricity near the morphotropic phase boundary in BiFeO3 is explored directly during thin‐film growth, using optical second harmonic generation. It is found that the epitaxial films form at the growth temperature purely in the T phase with zero critical thickness for the spontaneous polarization. Signatures of monoclinic T‐like and R‐like phases only appear upon sample cooling. The robustness of a single‐domain configuration in the high‐temperature T phase is furthermore demonstrated during growth of capacitor‐like metal | ferroelectric | metal heterostructures. Here, a reduction in tetragonality, rather than multidomain formation, lowers the electrostatic energy in the few‐unit‐cell thickness regime. For this lower tetragonality, density‐functional calculations and scanning transmission electron microscopy point to the stabilization of a novel metastable monoclinic structure upon cooling toward room temperature. The synergistic combination of strain and electrostatic phase stabilization in BiFeO3 heterostructures hence provides a basis for designing new ferroelectric phases and ultrathin ferroelectric devices. [ABSTRACT FROM AUTHOR]

Piezoelectric and ferroelectric wurtzite are promising to reshape modern microelectronics because they can be easily integrated with mainstream semiconductor technology. Sc doped AlN (Al1‐xScxN) has attracted much attention for its enhanced piezoelectric and emerging ferroelectric properties, yet the commonly used sputtering results in polycrystalline Al1‐xScxN films with high leakage current. Here, the pulsed laser deposition of single crystalline epitaxial Al1‐xScxN thin films on sapphire and 4H‐SiC substrates is reported. Pure wurtzite phase is maintained up to x = 0.3 with ≤0.1 at% oxygen contamination. Polarization is estimated to be 140 µC cm−2 via atomic scale microscopy imaging and found to be switchable via a scanning probe. The piezoelectric coefficient is found to be five times of the undoped one when x = 0.3, making it desirable for high‐frequency radiofrequency (RF) filters and 3D nonvolatile memories. [ABSTRACT FROM AUTHOR]

Establishing a reliable control of perpendicular magnetic anisotropy (PMA) is challenging but essential for the full utilization of rare‐earth iron garnets in spintronic devices. In this study, a feasible approach to enhance the PMA of ferrimagnetic thulium iron garnet (TmIG) films is presented. This approach involves precise adjustments in cation stoichiometry and epitaxial strain state. By fine‐tuning the pre‐ablation process and oxygen partial pressure during pulsed laser deposition, a series of high‐quality TmIG films can grow with variable cation stoichiometry, i.e., the Tm/Fe molar ratio. The finding reveals that cation stoichiometry plays a crucial role in determining the magnetic properties of the TmIG films. Particularly, the stoichiometric TmIG film has the strongest PMA due to the maximized magnetostriction coefficient. Combining this stoichiometry optimization and strain engineering, an unprecedented PMA strength of ≈30 kJ m−3 for TmIG is achieved. This achievement demonstrates a simple and effective method for harnessing the magnetic properties of rare‐earth iron garnet films, paving the way for their advanced applications in next‐generation spintronic devices. [ABSTRACT FROM AUTHOR]

The cubic perovskite SrMoO3 with a paramagnetic ground state and remarkably low room-temperature resistivity has been considered as a suitable candidate for the new-era oxide-based technology. However, the difficulty of preparing single-phase SrMoO3 thin films by hydrogen-free sputtering has hindered their practical use, especially due to the formation of thermodynamically favorable SrMoO4 impurity. In this work, we developed a radio frequency sputtering technology enabling the reduction reaction and achieved conductive epitaxial SrMoO3 films with pure phase from a SrMoO4 target in a hydrogen-free, pure argon environment. We demonstrated the significance of controlling the target-to-substrate distance (TSD) on the synthesis of SrMoO3; the film resistivity drastically changes from 1.46 × 105 μΩ·cm to 250 μΩ·cm by adjusting the TSD. Cross-sectional microstructural analyses demonstrated that films with the lowest resistivity, deposited for TSD = 2.5 cm, possess a single-phase SrMoO3 with an epitaxial perovskite structure. The formation mechanism of the conductive single-phase SrMoO3 films can be attributed to the plasma-assisted growth process by tuning the TSD. Temperature-dependent resistivity and Hall effect studies revealed metal-like conducting properties for low-resistive SrMoO3 films, while the high-resistive ones displayed semiconductor-like behavior. Our approach makes hydrogen-free, reliable and cost-efficient scalable deposition of SrMoO3 films possible, which may open up promising prospects for a wide range of future applications of oxide materials.

Abstract Multiferroic BiFeO3 exhibits a morphotropic phase boundary at large compressive strain that merges metastable phases of tetragonal (T) and rhombohedral (R) character resulting in giant ferroelectric and electromechanical responses. To utilize this functionality in devices, it is essential to understand the response of these ferroelectric phases to the environment of a nanoscale heterostructure. Here, the emergence of ferroelectricity near the morphotropic phase boundary in BiFeO3 is explored directly during thin‐film growth, using optical second harmonic generation. It is found that the epitaxial films form at the growth temperature purely in the T phase with zero critical thickness for the spontaneous polarization. Signatures of monoclinic T‐like and R‐like phases only appear upon sample cooling. The robustness of a single‐domain configuration in the high‐temperature T phase is furthermore demonstrated during growth of capacitor‐like metal | ferroelectric | metal heterostructures. Here, a reduction in tetragonality, rather than multidomain formation, lowers the electrostatic energy in the few‐unit‐cell thickness regime. For this lower tetragonality, density‐functional calculations and scanning transmission electron microscopy point to the stabilization of a novel metastable monoclinic structure upon cooling toward room temperature. The synergistic combination of strain and electrostatic phase stabilization in BiFeO3 heterostructures hence provides a basis for designing new ferroelectric phases and ultrathin ferroelectric devices.

Abstract Hafnium‐oxide‐based ferroelectrics have garnered considerable research interest, primarily for their robust ferroelectricity at the nanoscale and their high compatibility with complementary metal‐oxide‐semiconductors processes. However, the impact of electrodes on the ferroelectric properties of hafnium‐oxide layer, particularly that of top electrodes, is not yet fully understood even in the simplest capacitor geometry. In this study, the La0.67Sr0.33MnO3/Hf0.5Zr0.5O2 (LSMO/HZO) epitaxial heterostructure is utilized as a model system to conduct a systematic comparative study on ferroelectricity between the LSMO/HZO (H‐LS) bilayer and LSMO/HZO/LSMO (LS‐H‐LS) trilayer samples. In comparison to the H‐LS sample, the LS‐H‐LS sample exhibits a more uniform polar domain configuration and larger ferroelectric polarization. Moreover, the LS‐H‐LS sample exhibits significant improvements in leakage, endurance, and retention. These substantial enhancements in ferroelectricity are likely due to interfacial stress imposed by the LSMO capping layer and its capacity to accommodate extra oxygen vacancies. These results underscore the pivotal role of oxide‐based top electrodes in determining the ferroelectricity of hafnium‐oxide‐based heterostructures, providing crucial insights for optimizing the performance of innovative ferroelectric devices.

Within the huge perovskite materials family, thin films of highly conducting materials such as SrMoO3, SrNbO3, and SrVO3 are candidates for low‐loss bottom electrodes in epitaxial all‐oxide devices, in particular for high‐frequency applications. Recently, the fully coherent growth of more than 5 µm thick SrMoO3 electrodes in a varactor device prototype epitaxial heterostructure has been reported. This result raises the question of the strain mechanism in such anomalously thick coherent epitaxial layers. Here, this question is addressed by comparing the lattice constants of coherently strained layers and their free‐standing membranes. SrMoO3 is mainly elastically strained within the heterostructure and fully relaxed after removal of the substrate. These results indicate a giant critical thickness, making highly conducting perovskites even more outstanding materials for high‐frequency applications that require electrode thicknesses beyond the skin depth. The described technology of lifting off thick SrMoO3 membranes joins the emerging field of freestanding oxide layer technology, opening unexplored avenues for single crystal investigations, novel perovskite nanostructures, and wafer transfer of functional oxides, walking in the footsteps of recent developments in van der Waals epitaxial heterostructures. [ABSTRACT FROM AUTHOR]

The attractive physical properties of perovskite manganese make them fundamental building blocks for oxide spintronic devices. However, defects and lattice distortions can lead to extrinsic properties, especially cation vacancies, severely compromising device performance. In particularly, many intriguing properties, such as the emergence of magnetism and two-dimensional high-mobility electron gases, are closely related to cation vacancies. In this study, we deposited a series of La x MnO 3 epitaxial thin films with varying contents of La3+ vacancies on SrTiO 3 (100) substrates, with the presence of La3+ vacancies verified by using X-ray diffraction, X-ray photoemission spectroscopy, X-ray absorption spectroscopy, and energy dispersive spectroscopy spectrum. The reduction in La3+ vacancies led to stronger ferromagnetism with an easy axis oriented along the (110) direction in LaMnO 3 thin films, along with the appearance of a monoclinic phase, indicating that the ferromagnetism possibly originated from the three-dimensional d 3 z 2 − r 2 / d x 2 − y 2 -alternated orbital order rather than the Mn3+/Mn4+ double exchange effect. This result implies that the consistent regulation of cation vacancies by pulsed laser deposition can modulate both the structure and physical properties of manganate films, thereby enabling control over device properties. [ABSTRACT FROM AUTHOR]

Understanding the origin and mechanisms of magnetic damping in complex oxide materials is crucial for optimizing spin dynamics and tailoring their properties for specific spintronic applications. Ferromagnetic resonance spectroscopy (FMR) technique has been used to investigate the magnetic damping of multiple La2/3Sr1/3MnO3 (LSMO) epitaxial thin films with similar thickness and identical DC magnetic properties. However, the dynamic magnetic properties exhibit noticeable variations among samples. Microstructural analyses using X-ray diffraction (XRD) and atomic force microscopy (AFM), confirm that the samples are structurally identical, except for minute differences in the miscut angles of the substrates. Nevertheless, when examining the samples using backscattered electron (BSE) images in scanning electron microscopy (SEM), significant disparities in the twin distribution are observed. These variations in the twin distribution directly correlate with the observed differences in the damping values. A careful image analysis of BSE images allows to demonstrate that the increase of damping is due to the pinning of the magnetization in the twin boundaries. [ABSTRACT FROM AUTHOR]

The growth of epitaxial thin films from the Ruddlesden–Popper series of strontium iridates by magnetron sputtering is analyzed. It was found that, even using a non-stoichiometric target, the films formed under various conditions were consistently of the perovskite-like n = ∞ SrIrO3 phase, with no evidence of other RP series phases. A detailed inspection of the temperature–oxygen phase diagram underscored that kinetics mechanisms prevail over thermodynamics considerations. The analysis of the angular distribution of sputtered iridium and strontium species indicated clearly different spatial distribution patterns. Additionally, significant backsputtering was detected at elevated temperatures. Thus, it is assumed that the interplay between these two kinetic phenomena is at the origin of the preferential nucleation of the SrIrO3 phase. In addition, strategies for controlling cation stoichiometry off-axis have also been explored. Finally, the long-term stability of the films has been demonstrated. [ABSTRACT FROM AUTHOR]

Auger electron spectroscopy was used to determine the phase composition of Cr2GeC MAX phase thin films. A distinctive feature of the formation of carbon-containing MAX phases is the shape of carbon Auger peaks, which is characteristic of metal carbides spectra. Features of the Auger spectra in the presence of secondary phases of chromium germanides are found. Their presence can manifest itself in an increase in the energy of the germanium peaks, which is caused by a chemical shift during the formation of the Cr–Ge bond. Moreover, we have detected the accumulation of electronic charge, which can be explained by the features of the surface morphology. [ABSTRACT FROM AUTHOR]

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

A novel class of transition metal oxides, capable of reversible topotactic phase transition between the oxygen‐deficient brownmillerite and oxygen‐rich perovskite, has emerged as a promising material for memristive and magnetoelectric devices. However, the absence of a local oxygen source in the device structure necessitates an oxygen exchange process between the surrounding atmosphere and the switching layer during operation, which can lead to unreliable device performance. In this study, graphene quantum dots (GQDs) are introduced into a SrFe0.5Co0.5Ox memristive device as an oxygen reservoir for the nanoscale topotactic redox process. The SrFe0.5Co0.5Ox memristive devices with GQDs exhibit reliable resistive switching performance compared to SrFe0.5Co0.5Ox devices without GQDs. To understand the effect of GQDs on the device structure, a pulse endurance test is carried out in a high vacuum. The devices with GQDs show rather good endurance behavior, while devices without GQDs exhibit endurance failure. These results provide a deeper understanding of the potential use of GQDs in enhancing the performance of SrFe0.5Co0.5Ox memristive devices, with implications for tuning nanoscale topotactic phase transition for multi‐functional properties. [ABSTRACT FROM AUTHOR]

High energy and high power density rechargeable micro‐batteries are a necessity for powering the next generation of flexible electronics, internet of things, and medical technology devices. In theory, significant improvements in the capacity, current and power densities of micro‐batteries would result if 3D architectures with enhanced interdigitated component interface areas and shortened ion diffusion path‐lengths were used. Further gains are achievable if the materials utilized have high crystalline quality and are preferentially oriented for fast lithium intercalation. In this work, this is achieved by creating epitaxial thin film cathodes comprised of nanopillars of LiMn2O4 (LMO) embedded in a supporting matrix of electronically conducting SrRuO3 (SRO). The first electrochemical study of such a 3D vertically aligned nanocomposite (VAN) that displays clear cathode redox signatures is provided, and demonstrates remarkable capacity retention under high‐rate regimes. The electrochemical performance is shown to be dependent on the nanopillar topography, namely the crystallographic orientation, nanopillar dimensions, and electrode/electrolyte interfacial surface area. This work offers a pathway to realizing 3D architectured micro‐batteries with high‐capacity retention under high‐rate conditions enabling fast charge capabilities. [ABSTRACT FROM AUTHOR]

Solid-gas interactions at electrode surfaces determine the efficiency of solid-oxide fuel cells and electrolyzers. Here, the correlation between surface-gas kinetics and the crystal orientation of perovskite electrodes is studied in the model system La0.8 Sr0.2 Co0.2 Fe0.8 O3 . The gas-exchange kinetics are characterized by synthesizing epitaxial half-cell geometries where three single-variant surfaces are produced [i.e., La0.8 Sr0.2 Co0.2 Fe0.8 O3 /La0.9 Sr0.1 Ga0.95 Mg0.05 O3-δ /SrRuO3 /SrTiO3 (001), (110), and (111)]. Electrochemical impedance spectroscopy and electrical conductivity relaxation measurements reveal a strong surface-orientation dependency of the gas-exchange kinetics, wherein (111)-oriented surfaces exhibit an activity >3-times higher as compared to (001)-oriented surfaces. Oxygen partial pressure ( pO2 )-dependent electrochemical impedance spectroscopy studies reveal that while the three surfaces have different gas-exchange kinetics, the reaction mechanisms and rate-limiting steps are the same (i.e., charge-transfer to the diatomic oxygen species). First-principles calculations suggest that the formation energy of vacancies and adsorption at the various surfaces is different and influenced by the surface polarity. Finally, synchrotron-based, ambient-pressure X-ray spectroscopies reveal distinct electronic changes and surface chemistry among the different surface orientations. Taken together, thin-film epitaxy provides an efficient approach to control and understand the electrode reactivity ultimately demonstrating that the (111)-surface exhibits a high density of active surface sites which leads to higher activity.

Abstract A novel class of transition metal oxides, capable of reversible topotactic phase transition between the oxygen‐deficient brownmillerite and oxygen‐rich perovskite, has emerged as a promising material for memristive and magnetoelectric devices. However, the absence of a local oxygen source in the device structure necessitates an oxygen exchange process between the surrounding atmosphere and the switching layer during operation, which can lead to unreliable device performance. In this study, graphene quantum dots (GQDs) are introduced into a SrFe0.5Co0.5Ox memristive device as an oxygen reservoir for the nanoscale topotactic redox process. The SrFe0.5Co0.5Ox memristive devices with GQDs exhibit reliable resistive switching performance compared to SrFe0.5Co0.5Ox devices without GQDs. To understand the effect of GQDs on the device structure, a pulse endurance test is carried out in a high vacuum. The devices with GQDs show rather good endurance behavior, while devices without GQDs exhibit endurance failure. These results provide a deeper understanding of the potential use of GQDs in enhancing the performance of SrFe0.5Co0.5Ox memristive devices, with implications for tuning nanoscale topotactic phase transition for multi‐functional properties.

Electrocaloric effect is the adiabatic temperature change in a dielectric material when an electric field is applied or removed, and it can be considered as an alternative refrigeration method. Materials with ferroelectric order exhibit large temperature variations in the vicinity of a phase transition, while antiferroelectrics and relaxors may exhibit a negative electrocaloric effect. In this study, the temperature variation in polarization was investigated for epitaxial ferroelectric thin film structures based on PbZrTiO3 materials in simple or complex multilayered structures. We propose the intriguing possibility of a giant negative electrocaloric effect (ΔT = −3.7 K at room temperature and ΔT = −5.5 K at 370 K) in a simple epitaxial Pb(ZrTi)O3 capacitor. Furthermore, it was shown that abnormal temperature variation in polarization is dependent on the non-FE component introduced in a multilayered structure. No significant variation in polarization with temperature was obtained for PZT/STON multilayered structures around room temperature. However, for PZT/BST or PZT/Nb2O5 multilayers, an abnormal temperature variation in polarization was revealed, which was similar to a simple PZT layer. The giant and negative ∆T values were attributed to internal fields and defects formed due to the large depolarization fields when the high polarization of the FE component was not fully compensated either by the electrodes or by the interface with an insulator layer. The presented results make Pb(ZrTi)O3-based structures promising for cooling applications operating near room temperature.

The luminescent properties of epitaxial Cu2O thin films were studied in 10–300 K temperature range and compared with the luminescent properties of Cu2O single crystals. Cu2O thin films were deposited epitaxially via the electrodeposition method on either Cu or Ag substrates at different processing parameters, which determined the epitaxial orientation relationships. Cu2O (100) and (111) single crystal samples were cut from a crystal rod grown using the floating zone method. Luminescence spectra of thin films contain the same emission bands as single crystals around 720, 810 and 910 nm, characterizing V O 2 + , V O + and V Cu defects, correspondingly. Additional emission bands, whose origin is under discussion, are observed around 650–680 nm, while the exciton features are negligibly small. The relative mutual contribution of the emission bands varies depending on the thin film sample. The existence of the domains of crystallites with different orientations determines the polarization of luminescence. The PL of both Cu2O thin films and single crystals is characterized by negative thermal quenching in the low-temperature region; the reason of this phenomenon is discussed. [ABSTRACT FROM AUTHOR]

Valence change memories are novel data storage devices in which the resistance is determined by a reversible redox reaction triggered by voltage. The oxygen content and mobility within the active materials of these devices play a crucial role in their performance. Therefore, materials which present fast oxygen migration properties and can accommodate variable oxygen stoichiometry are promising candidates. In this work, the perovskite La0.5Sr0.5MnO3‐δ (LSM50) as memristive material is studied, which presents a more facile oxygen vacancy formation and faster oxygen migration compared to other strontium‐substituted manganites. For the first time reproducible resistive switching is reported in epitaxial LSM50‐based devices with active Ti electrodes, which show large operating window and stable multilevel states. Based on the structural, chemical, and electrical results, a simple phenomenological description of the resistive switching phenomena taking place in these novel LSM50‐based memristive devices is proposed. [ABSTRACT FROM AUTHOR]

Abstract Thin ferroelectric layers are capable of generating large dielectric and electromechanical responses at relatively low voltages, thus can be utilized in various applications including energy storage, energy harvesting, sensors, and actuators. Among ferroelectrics, relaxor ferroelectrics (1 − x)Pb(Mg1/3Nb2/3)O3 − xPbTiO3 are particularly interesting due to their superior dielectric and piezoelectric properties. However, dielectric and piezoelectric responses of the ferroelectric thin films are significantly suppressed relative to their bulk counterparts. The physical mechanism of the properties degradation still remains elusive, which greatly hinders their technological applications. Herein, this work systematically investigates the dielectric nonlinearity and the relationship between composition and intrinsic/extrinsic contributions of high‐quality (1 − x)Pb(Mg1/3Nb2/3)O3 − xPbTiO3 epitaxial thin films (x = 0, 0.1, 0.32) fabricated by pulsed laser deposition. Through the Rayleigh analysis, it is found that the suppression of domain wall motion by substrate clamping is the main cause of thin film performance degradation and is further confirmed by continuous application of background dc bias to extract the dielectric response's intrinsic and extrinsic contributions. This work will pave the way for practical applications of (1 − x)Pb(Mg1/3Nb2/3)O3 − xPbTiO3 thin films in energy storage and electromechanical devices.

The growth of epitaxial thin films from the Ruddlesden–Popper series of strontium iridates by magnetron sputtering is analyzed. It was found that, even using a non-stoichiometric target, the films formed under various conditions were consistently of the perovskite-like n = ∞ SrIrO3 phase, with no evidence of other RP series phases. A detailed inspection of the temperature–oxygen phase diagram underscored that kinetics mechanisms prevail over thermodynamics considerations. The analysis of the angular distribution of sputtered iridium and strontium species indicated clearly different spatial distribution patterns. Additionally, significant backsputtering was detected at elevated temperatures. Thus, it is assumed that the interplay between these two kinetic phenomena is at the origin of the preferential nucleation of the SrIrO3 phase. In addition, strategies for controlling cation stoichiometry off-axis have also been explored. Finally, the long-term stability of the films has been demonstrated.

Abstract Valence change memories are novel data storage devices in which the resistance is determined by a reversible redox reaction triggered by voltage. The oxygen content and mobility within the active materials of these devices play a crucial role in their performance. Therefore, materials which present fast oxygen migration properties and can accommodate variable oxygen stoichiometry are promising candidates. In this work, the perovskite La0.5Sr0.5MnO3‐δ (LSM50) as memristive material is studied, which presents a more facile oxygen vacancy formation and faster oxygen migration compared to other strontium‐substituted manganites. For the first time reproducible resistive switching is reported in epitaxial LSM50‐based devices with active Ti electrodes, which show large operating window and stable multilevel states. Based on the structural, chemical, and electrical results, a simple phenomenological description of the resistive switching phenomena taking place in these novel LSM50‐based memristive devices is proposed.

Multiferroic thin films are a promising class of multifunctional materials, since they allow the integration of multiple functionalities within a single device. In order to overcome the scarcity of single phase multiferroics, it is crucial to develop novel multiferroic heterostructures, combining good ferroelectric and ferromagnetic properties as well as a strong coupling between them. For this purpose, Ba2EuFeNb4O15/BaFe12O19 multiferroic magnetoelectric bilayers have been epitaxially grown on niobium doped SrTiO3 (100) single crystal substrates by pulsed laser deposition. The simultaneous presence of both ferroelectric and magnetic properties—due, respectively, to the Ba2EuFeNb4O15 and BaFe12O19 components—was demonstrated at room temperature, attesting the multiferroic nature of the heterostructure. More interestingly, a strong magnetoelectric coupling was demonstrated (i) by manipulating the ferroelectric properties via an external magnetic field, and conversely, (ii) by tuning the magnetic properties via an external electric field. This strong magnetoelectric coupling shows the high interdependence of both ferroic orders in the Ba2EuFeNb4O15/BaFe12O19 heterostructure, mediated by elastic (epitaxial) strain at the interfaces. [ABSTRACT FROM AUTHOR]

Designing oxide materials to achieve the high oxygen reduction reaction (ORR) activity is a key requirement to facilitate the development of energy and environmental applications. Manipulating the crystallographic planes of layered oxides has an impact on determining the electrocatalytic activity. However, the correlation between the ORR kinetics and the crystallographic orientations is not fully understood in a single material system. Here, a superconducting oxide, La 1.85 Sr 0.15 CuO 4-δ (LSC 214) is used to demonstrate that the crystallographic orientation plays a crucial role in controlling the ORR activity. The (114)-oriented epitaxial LSC 214 films show dramatically enhanced ORR activity up to two orders of magnitude compared to the (001)- and (103)-oriented LSC 214 films. We attribute the enhanced ORR activity of the LSC 214 films to both the exposed oxygen migration channels and the increased oxygen vacancies. Our study provides a new design strategy to enhance the ORR activity for high-performance energy applications and illustrates that the control of orientation is a simple means to tune the electrocatalytic activity. [Display omitted] • The orientation of epitaxial Ruddlesden-Popper (RP) oxide thin films was controlled. • The k q values of RP oxides were enhanced (~2 orders) by controlling the orientation. • Controlling the crystalline anisotropy modulated the electrocatalytic activity. • The relationship of orientation-electrochemical property in RP oxides was revealed. [ABSTRACT FROM AUTHOR]

Understanding how defect chemistry of oxide material influences the thermal stability of noble metal dopant ions plays an important role in designing high-performance heterogeneous catalytic systems. Here we use in-situ ambient-pressure X-ray photoemission spectroscopy (APXPS) to experimentally determine the role of grain boundary in the thermal stability of platinum doped cerium oxide (Pt/CeO2). The grain boundaries were introduced in Pt/CeO2 thin films by pulsed laser deposition without significantly change of the surface microstructure. The defect level was tuned by the strain field obtained using a highly/low mismatched substrate. The Pt/CeO2 thin film models having well defined crystallographic properties but different grain boundary structural defect levels provide an ideal platform for exploring the evolution of Pt—O—Ce bond with changing the temperature in reducing conditions. We have direct demonstration and explanation of the role of Ce3+ induced by grain boundaries in enhancing Pt2+ stability. We observe that the Pt2+—O—Ce3+ bond provides an ideal coordinated site for anchoring of Pt2+ ions and limits the further formation of oxygen vacancies during the reduction with H2. Our findings demonstrate the importance of grain boundary in the atomic-scale design of thermally stable catalytic active sites. [ABSTRACT FROM AUTHOR]

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

In condensed matter physics, oxide materials show various intriguing physical properties. Therefore, many efforts are made in this field to develop functional oxides. Due to the excellent potential for tin‐based perovskite oxides, an expansion of new related functional compounds is crucial. This work uses a heteroepitaxial approach supported by theoretical calculation to stabilize PbSnO3 thin films with different orientations. The analyses of X‐ray diffraction and transmission electron microscopy unveil the structural information. A typical antiferroelectric feature with double hysteresis and butterfly loops is observed through electrical characterizations consistent with the theoretical prediction. The phase transition is monitored, and the transition temperatures are determined based on temperature‐dependent structural and electrical characterizations. Furthermore, the microscopic antiferroelectric order is noticed under atomic resolution images via scanning transmission electron microscopy. This work offers a breakthrough in synthesizing epitaxial PbSnO3 thin films and comprehensively understanding its anisotropic antiferroelectric behavior. [ABSTRACT FROM AUTHOR]

Electrocaloric effect is the adiabatic temperature change in a dielectric material when an electric field is applied or removed, and it can be considered as an alternative refrigeration method. Materials with ferroelectric order exhibit large temperature variations in the vicinity of a phase transition, while antiferroelectrics and relaxors may exhibit a negative electrocaloric effect. In this study, the temperature variation in polarization was investigated for epitaxial ferroelectric thin film structures based on PbZrTiO3 materials in simple or complex multilayered structures. We propose the intriguing possibility of a giant negative electrocaloric effect (ΔT = −3.7 K at room temperature and ΔT = −5.5 K at 370 K) in a simple epitaxial Pb(ZrTi)O3 capacitor. Furthermore, it was shown that abnormal temperature variation in polarization is dependent on the non-FE component introduced in a multilayered structure. No significant variation in polarization with temperature was obtained for PZT/STON multilayered structures around room temperature. However, for PZT/BST or PZT/Nb2O5 multilayers, an abnormal temperature variation in polarization was revealed, which was similar to a simple PZT layer. The giant and negative ∆T values were attributed to internal fields and defects formed due to the large depolarization fields when the high polarization of the FE component was not fully compensated either by the electrodes or by the interface with an insulator layer. The presented results make Pb(ZrTi)O3-based structures promising for cooling applications operating near room temperature. [ABSTRACT FROM AUTHOR]

Magnetic perovskite films have promising properties for use in energy-efficient spintronic devices and magnetic refrigeration. Here, an epitaxial ferromagnetic La0.67Ba0.33Mn0.95Ti0.05O3 (LBMTO-5) thin film was grown on SrTiO3(001) single crystal substrate by pulsed laser deposition. High-resolution X-ray diffraction proved the high crystallinity of the film with tetragonal symmetry. The magnetic, magnetocaloric and magnetoresistance properties at different directions of the applied magnetic field with respect to the ab plane of the film were investigated. An in-plane uni-axial magnetic anisotropy was evidenced. The LBMTO-5 epilayer exhibits a second-order ferromagnetic-paramagnetic phase transition around 234 K together with a metal–semiconductor transition close to this Curie temperature (TC). The magnetic entropy variation under 5 T induction of a magnetic field applied parallel to the film surface reaches a maximum of 17.27 mJ/cm3 K. The relative cooling power is 1400 mJ/cm3 K (53% of the reference value reported for bulk Gd) for the same applied magnetic field. Giant magnetoresistance of about 82% under 5 T is obtained at a temperature close to TC. Defined as the difference between specific resistivity obtained under 5 T with the current flowing along the magnetic easy axis and the magnetic field oriented transversally to the current, parallel and perpendicular to the sample plane, respectively, the in-plane magneto-resistance anisotropy in 5 T is about 9% near the TC. [ABSTRACT FROM AUTHOR]

Spin Hall magnetoresistance (SMR) is used to study interfacial magnetic features in La2CoMnO6 (LCMO)/Pt bilayers. LCMO is a ferromagnetic insulator with a Curie temperature Tc = 250 K and a strong magnetic anisotropy that can be tuned by microstructural strain. However, SMR measurements do not show any dependence on the strained state of the LCMO films. In addition, contrary to what may be expected, SMR signal persists above TC exhibiting a strong magnetic field dependence well above the saturation of the magnetization while the typical hysteresis on crossing H = 0 is absent. All these results suggest that the uppermost LCMO interfacial layers are magnetically uncoupled from the rest of the LCMO film and they do not show long range magnetic ordering. This fact is confirmed by a complete scanning transmission electron microscopy study of the interface that makes evident that the last atomic layers of the LCMO film do not contain Co and have a LaMnO3+δ simple perovskite‐like structure with the last atomic layer being lanthanum oxide. The analysis of the experimental results according to recent SMR theories indicates, in fact, that the LCMO interfacial layer behaves as a 2D Heisenberg ferromagnet. [ABSTRACT FROM AUTHOR]

Epitaxial strain modifies the physical properties of thin films deposited on single-crystal substrates. In a previous work, we demonstrated that in the case of La2/3Ca1/3MnO3 thin films the strain induced by the substrate can produce the segregation of a non-ferromagnetic layer (NFL) at the top surface of ferromagnetic epitaxial La2/3Ca1/3MnO3 for a critical value of the tetragonality τ, defined as τ = |c − a|a, of τC ≈ 0.024. Although preliminary analysis suggested its antiferromagnetic nature, to date a complete characterization of the magnetic state of such an NFL has not been performed. Here, we present a comprehensive magnetic characterization of the strain-induced segregated NFL. The field-cooled magnetic hysteresis loops exhibit an exchange bias mechanism below T ≈ 80 K, which is well below the Curie temperature of the ferromagnetic La2/3Ca1/3MnO3 layer. The exchange bias and coercive fields decay exponentially with temperature, which is commonly accepted to describe spin-glass (SG) behavior. The signatures of slow dynamics were confirmed by slow spin relaxation over a wide temperature regime. Low-energy muon spectroscopy experiments directly evidence the slowing down of the magnetic moments below ~100 K in the NFL. The experimental results indicate the SG nature of the NFL. This SG state can be understood within the context of the competing ferromagnetic and antiferromagnetic interactions of similar energies. [ABSTRACT FROM AUTHOR]

Multi-functional magnetic sensors with tunable measure range can help cover different requirements from detecting weak field to sensing strong field. However, additional control system is often required to realize the tunability, which increases the complexity of device. In this work, we found the current controlled sensitivity and linear range in the NiCo2O4 thin film based anomalous hall sensor. The device owns not only simple structure but also interesting anomalous hall signal whose linearity range and sensitivity can be controlled by current. Ultrahigh sensitivity of 4900 V/(AT) in the range of ±0.2 mT and competitive sensitivity of 20 V/(AT) in the range of ±23 mT are simultaneously realized by tunning the work current. In addition, a memory switching characteristic controlled by magnetic field is further found in the device. These results show that the anomalous hall device has the potential for multifunctional compact device. [ABSTRACT FROM AUTHOR]

Understanding how doping influences physicochemical properties of ABO 3 perovskite oxides is critical for tailoring their functionalities. In this study, SrFe 0.67 Cr 0.33 O 3-δ epitaxial thin films were used to examine the effects of Fe and Cr competition on structure and B-site cation oxidation states. The films exhibit a perovskite-like structure near the film/substrate interface, while a brownmillerite-like structure with horizontal oxygen vacancy channels predominates near the surface. Electron energy loss spectroscopy shows Fe remains Fe 3+ , while Cr varies from ∼Cr 3+ (tetrahedral layers) to ∼Cr 4+ (octahedral layers) within brownmillerite phases and becomes ∼Cr 4.5+ in perovskite-like phases. Theoretical simulations indicate that Cr-O bond arrangements and the way oxygen vacancies interact with Cr and Fe drive Cr charge disproportionation. High-valent Cr cations introduce additional densities of states near the Fermi level, reducing the optical bandgap from ∼2.0 eV (SrFeO 2.5 ) to ∼1.7 eV (SrFe 0.67 Cr 0.33 O 3-δ ). These findings offer insights into B-site cation doping in the perovskite oxide framework.

Abstract In condensed matter physics, oxide materials show various intriguing physical properties. Therefore, many efforts are made in this field to develop functional oxides. Due to the excellent potential for tin‐based perovskite oxides, an expansion of new related functional compounds is crucial. This work uses a heteroepitaxial approach supported by theoretical calculation to stabilize PbSnO3 thin films with different orientations. The analyses of X‐ray diffraction and transmission electron microscopy unveil the structural information. A typical antiferroelectric feature with double hysteresis and butterfly loops is observed through electrical characterizations consistent with the theoretical prediction. The phase transition is monitored, and the transition temperatures are determined based on temperature‐dependent structural and electrical characterizations. Furthermore, the microscopic antiferroelectric order is noticed under atomic resolution images via scanning transmission electron microscopy. This work offers a breakthrough in synthesizing epitaxial PbSnO3 thin films and comprehensively understanding its anisotropic antiferroelectric behavior.

In this work, we experimentally studied the structure and magnetic properties of epitaxially grown ultrathin Pd/Co/Ta films. We have studied the effect of Ta on the structural and magnetic properties of Pd/Co epitaxial films. The deposition of a Ta layer on an epitaxial Co layer leads to a decrease in the saturation magnetization of the film due to strong magnetic disordering and the formation of a dead magnetic layer with a thickness of about 0.5 nm. Perpendicular magnetic anisotropy is observed for Co layer thicknesses less than 1.3 nm. The volume and surface components of magnetic anisotropy are determined. Additionally, the Dzyaloshinskii–Moriya interaction (DMI) is observed in epitaxial films with perpendicular magnetic anisotropy, with a DMI field value of about 30 mT.

Interface‐type valence change memories (VCMs) are exciting candidates for multilevel storage in resistive random access memories (RRAM) and as artificial synapses for neuromorphic computing. Several materials have been proposed as VCM candidates and, depending on the materials and electrodes of choice, different switching mechanisms take place leading to the change in resistance. Here, the focus is on La0.8Sr0.2MnO3–δ (LSM) perovskite and, particularly, the role of its nanostructure on the memristive device performance. The nanostructural details of the layers are modified by growing LSM epitaxial thin films on different substrates, i.e., SrTiO3 (STO) and LaAlO3 (LAO), by metal‐organic chemical vapor deposition (MOCVD). An interface‐type memristive response is observed using Ti as active electrode and Pt as inert electrode. The modifications in the nanostructure of LSM (strain and dislocations) determine the memristive performance, leading to differences in cycle to cycle reproducibility and multilevel capabilities by the modification of the LSM's oxygen migration properties. The results show that nanostructure engineering is a promising approach for optimizing the performance of memristive devices, an approach which can also be extended and applied to other nanoionic electrochemical devices. [ABSTRACT FROM AUTHOR]

Epitaxial Cr2AlC MAX phase thin films were grown on MgO(111) and Al2O3(0001) by pulsed laser deposition (PLD) at 600°C. X-ray diffraction and morphology studies of Cr2AlC thin films on MgO (111) reveal phase purity, columnar growth, the epitaxial relation Cr2AlC(0001) || MgO(111) and Cr2AlC [11-20] || MgO[10-1] and similar growth behaviour on Al2O3(0001). Resistivity measurements show semiconductor-like behaviour for 10 and 20 nm thick films, and metallic-like behaviour for thicker films, suggesting a percolation thickness slightly above 20 nm. Our results demonstrate the potential of PLD as a novel method for the growth of epitaxial MAX phase thin films.

Metal--organic frameworks (MOFs) have gained a large amount of interest because of their periodic and modular structure. These features allow easy prediction of the physical and chemical properties of an organic chromophore, acting as a linker in the MOF. In the present work, a bianthryl (BA) chromophore, equipped with metal-coordinating carboxylate groups, is studied to construct a photoluminescent Zn-BA surface-anchored MOF (SURMOF) thin film. The Zn-BA SURMOF, in response to prolonged UV light irradiation under ambient conditions, exhibits prominent changes in the ground and excited state optical properties, without losing its crystalline structure. A detailed spectroscopic study using UV--vis, infra-red, Raman, and electron paramagnetic resonance (EPR) reveals that in the presence of O2 a photoinduced topotactic transformation is initiated by the formation of singlet oxygen, which then reacts with the BA linkers to form endoperoxide. [ABSTRACT FROM AUTHOR]

Self-assembly via nanoscale phase separation offers an elegant route to fabricate nanocomposites with physical properties unattainable in single-component systems. One important class of nanocomposites are optical metamaterials which exhibit exotic properties and lead to opportunities for agile control of light propagation. Such metamaterials are typically fabricated via expensive and hard-to-scale top-down processes requiring precise integration of dissimilar materials. In turn, there is a need for alternative, more efficient routes to fabricate large-scale metamaterials for practical applications with deep-subwavelength resolution. Here, we demonstrate a bottom-up approach to fabricate scalable nanostructured metamaterials via spinodal decomposition. To demonstrate the potential of such an approach, we leverage the innate spinodal decomposition of the VO2-TiO2 system, the metal-to-insulator transition in VO2, and thin-film epitaxy, to produce self-organized nanostructures with coherent interfaces and a structural unit cell down to 15 nm (tunable between horizontally and vertically aligned lamellae) wherein the iso-frequency surface is temperature-tunable from elliptic to hyperbolic dispersion producing metamaterial behavior. These results provide an efficient route for the fabrication of nanostructured metamaterials and other nanocomposites for desired functionalities.

Over the past four years, the magnetism of 2-D magnets has been extensively studied by the full arsenal of probing techniques. Two-dimensional magnets can be incorporated to form heterostructures with clean and sharp interfaces, which gives rise to exotic phenomena as a result of the interfacial proximity effect. Here, we report a detailed study of the spin ($m_{s}$) and orbital ($m_{l}$) moments of an epitaxial (CrSb/Fe3GeTe2)6 superlattice. The synchrotron-radiation-based X-ray magnetic circular dichroism (XMCD) technique was performed to probe the microscopic magnetic properties of the superlattices in an elemental resolved manner. We unambiguously obtained a bulk-like moment of Fe3GeTe2, i.e., $m_{s} = 1.58~\mu _{\text {B}}/\text {Fe} \pm 0.2~\mu _{\text {B}}$ /Fe and $m_{l} = 0.22~\mu _{\text {B}}/\text {Fe} \pm 0.02~\mu _{\text {B}}$ /Fe. Future works to explore the tuning of the spin-polarized band structure of 2-D ferromagnetic superlattices will be of great interest and can have strong implications for both fundamental physics and the emerging spintronics technology. [ABSTRACT FROM AUTHOR]

In the field of oxide heterostructure engineering, there are extensive efforts to couple the various functionalities of each material. The Berry curvature‐driven magnetotransport of SrRuO3 ultrathin films is currently receiving a great deal of attention because it is extremely sensitive to the electronic structures near the Fermi surface driven by extensive physical parameters such as spin–orbit coupling and inversion symmetry breaking. Although this is beneficial in terms of heterostructure engineering, it renders transport behavior vulnerable to nanoscale inhomogeneity, resulting in artifacts called "hump anomalies." Here, a method to tune the magnetotransport properties of SrRuO3 ultrathin films capped by LaAlO3 layers is developed. The kinetic process of pulsed laser deposition by varying the growth pressure during LaAlO3 layer deposition is systematically controlled. Furthermore, the effects of nanoscale inhomogeneity on the Berry curvature near the Fermi surface in SrRuO3 films are investigated. It is found that the high kinetic energy of the capping layer adatoms induces stoichiometric modification and nanoscale lattice deformation of the underlying SrRuO3 layer. The control of kinetics provides a way to modulate magnetization and the associated magnetotransport of the SrRuO3 layer. [ABSTRACT FROM AUTHOR]