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The concept of multiphysics, where materials respond to diverse external stimuli, such as magnetic fields, electric fields, light irradiation, stress, heat, and chemical reactions, plays a fundamental role in the development of innovative devices. Nanomanufacturing, especially in low-dimensional systems, enhances the synergistic interactions taking place on the nanoscale. Light-matter interaction, rather than electric fields, holds great promise for achieving low-power, wireless control over magnetism, solving two major technological problems: the feasibility of electrical contacts at smaller scales and the undesired heating of the devices. Here, we shed light on the remarkable reversible modulation of magnetism using visible light in epitaxial Fe 3 O 4 /BaTiO 3 heterostructure. This achievement is underpinned by the convergence of two distinct mechanisms. First, the magnetoelastic effect, triggered by ferroelectric domain switching, induces a proportional change in coercivity and remanence upon laser illumination. Second, light-matter interaction induces charged ferroelectric domain walls' electrostatic decompensations, acting intimately on the magnetization of the epitaxial Fe 3 O 4 film by magnetoelectric coupling. Crucially, our experimental results vividly illustrate the capability to manipulate magnetic properties using visible light. This concomitant mechanism provides a promising avenue for low-intensity visible-light manipulation of magnetism, offering potential applications in multiferroic devices.

Ba-rich compositions in the Ba x Sr 1- x MnO 3 (BSMO) cubic perovskite (3C) system are magnetic ferroelectrics and are of interest for their strong magnetoelectric coupling. Beyond x = 0.5, they only form in hexagonal polymorphs. Here, the 3C phase boundary is pushed to Ba 0.6 Sr 0.4 MnO 3 for the thin films. Using regular pulsed laser deposition (rPLD), 3C Ba 0.6 Sr 0.4 MnO 3 could be epitaxially stabilized on DyScO 3 (101) o substrates by using a 0.1% O 2 /99.9% N 2 gas mixture. However, the 3C phase was mixed with the 4H polymorph for films 24 nm thick and above, and the films were relatively rough. To improve flatness and phase purity, changes in growth kinetics were investigated and interval PLD (iPLD) was especially effective. In iPLD, deposition is interrupted after completion of approximately one monolayer, and the deposit is annealed for a specific period of time before repeating. Both film flatness and, more importantly, the volume of the 3C polymorph improved with iPLD, resulting in 40 nm single-phase films. The results imply that iPLD improves the persistent nucleation of highly metastable phases and offers a new approach to epitaxial stabilization of novel materials, including more Ba-rich BSMO compositions in the 3C structure.

The discovery of superconductivity in the infinite-layer nickelate Nd 0.8 Sr 0.2 NiO 2 has motivated tremendous efforts for its significance toward the understanding of high-temperature superconductivity. However, the synthesis of infinite-layer nickelates is instable and has become a hindrance to experimental progress. Optimizing the growth of precursor Nd 0.8 Sr 0.2 NiO 3 by pulsed laser deposition is crucial for obtaining infinite-layer nickelates. By systematically investigating the growth of Nd 0.8 Sr 0.2 NiO 3 with wide range of conditions, we found that the laser fluence plays a critical role in determining the stoichiometry, lattice structure, and electronic properties. A higher Ni deficiency and larger c -axis lattice constant appeared with the lower laser fluence. At 0.6 J/cm 2 , the Ni deficiency is as large as 25%. According to X-ray absorption spectra and X-ray linear dichroism, we further find that (i) there are no obvious changes of the Ni valence and (ii) the energy level of the d x 2 - y 2 orbital gradually increases relative to the d 3 z 2 - r 2 orbital with increasing Ni deficiency. What is more, the onset temperature and magnitude of the resistivity change at the metal-to-insulator transitions (MITs) also are found to decrease with increasing laser fluence during the growth.

Antiferroelectric PbZrO 3 has attracted renewed interest in recent years because of its unique properties and wide range of potential applications. However, the nature of antiferroelectricity and its evolution with the electric field and temperature remain controversial, mostly due to the difficulty of obtaining high-quality single-crystal samples. The lack of consensus regarding the phase transition in PbZrO 3 is not only important on a fundamental side but also greatly hinders further applications. Herein, high-quality PbZrO 3 epitaxial thin films are successfully fabricated by pulsed laser deposition. The structural and physical properties of the films are systematically studied via a combination of electric property measurements, X-ray diffraction, scanning transmission electron microscopy imaging, and second-harmonic generation studies. Our studies unveil the noncentrosymmetric nature of PbZrO 3 films at room temperature. Moreover, the Curie temperature increased to 270°, ∼40° higher than that in the bulk, and no intermediate ferroelectric phase was observed. Besides, an incipient ferroelectric with relaxor-like behavior above the Curie temperature due to the existence of a local polar cluster in the high-temperature paraelectric phase is experimentally observed for the first time. Our studies provide a better understanding of PbZrO 3 thin films and pave the way for practical applications of antiferroelectric material in modern electronic devices.

Atomic layer deposition (ALD) was used to control the stoichiometry of thin lithium aluminosilicate films, thereby enabling crystallization into the ion-conducting β-eucryptite LiAlSiO 4 phase. The rapid thermal annealed ALD film developed a well-defined epitaxial relationship to the silicon substrate: β-LiAlSiO 4 (12̅10)||Si (100) and β-LiAlSiO 4 (101̅0)||Si (001). The extrapolated room temperature ionic conductivity was found to be 1.2 × 10 -7 S/cm in the [12̅10] direction. Because of the unique 1-D channel along the c axis of β-LiAlSiO 4 , the epitaxial thin film has the potential to facilitate ionic transport if oriented with the c axis normal to the electrode surface, making it a promising electrolyte material for three-dimensional lithium-ion microbatteries.

The study of ionic materials on nanometer scale is of great relevance for efficient miniaturized devices for energy applications. The epitaxial growth of thin films can be a valid route to tune the properties of the materials and thus obtain new degrees of freedom in materials design. High crystal quality Sm x Ce 1- x O 2-δ films are here reported at a high doping level up to x = 0.4, thanks to the good lattice matching with the (110) oriented NdGaO 3 substrate. X-ray diffraction and transmission electron microscopy demonstrate the ordered structural quality and absence of Sm segregation at the macroscopic and atomic level, respectively. Therefore, in epitaxial thin films, the homogeneous doping can be obtained even with the high dopant content not always approachable in bulk form, getting even an improvement of the structural properties. In situ spectroscopic measurements by X-ray photoemission and X-ray absorption show the O 2p band shift toward the Fermi level, which can favor the oxygen exchange and vacancy formation on the surface when the Sm doping is increased to x = 0.4. X-ray absorption spectroscopy also confirms the absence of ordered oxygen vacancy clusters and further reveals that the 5d e g and t 2g states are well separated by the crystal field in the undistorted local structure even in the case of a high doping level up to x = 0.4.

With the advances in artificial intelligence and communication technologies, flexible microwave magnetic materials have become essential for flexible microwave detectors, wearable microwave sensors, and flexible spintronics. Here, a highly stable in-plane (IP) ferromagnetic resonance (FMR) and a tunable out-of-plane (OOP) FMR character are demonstrated in the flexible microwave magnetic Li 0.35 Zn 0.3 Fe 2.35 O 4 (LZFO) (111) epitaxial thin films under external mechanical bending. Both the IP FMR line width and resonance field ( H r ) are basically unchanged when the sample was bent under an external tensile or compressive bending strain and deformation. However, the OOP FMR spectra (including H r and absorption peak) could be tuned by mechanical bending, i.e., the LZFO sample possesses two OOP FMR absorption peaks at an external bending curvature. Meanwhile, excellent mechanical antifatigue and mechanical retention characteristics have also been obtained in the LZFO sample. The highly stable IP and the tunable OOP FMR spectra in the same LZFO sample with excellent mechanical antifatigue character have a promising prospect in microwave magnetic devices and flexible spintronics.

Perovskite multiferroic oxides are promising materials for the realization of sensitive and switchable photodiodes because of their favorable band gap (<3.0 eV), high absorption coefficient, and tunable internal ferroelectric (FE) polarization. A high-speed switchable photodiode based on multiferroic Bi 2 FeCrO 6 (BFCO)/SrRuO 3 (SRO)-layered heterojunction was fabricated by pulsed laser deposition. The heterojunction photodiode exhibits a large ideality factor ( n = ∼5.0) and a response time as fast as 68 ms, thanks to the effective charge carrier transport and collection at the BFCO/SRO interface. The diode can switch direction when the electric polarization is reversed by an external voltage pulse. The time-resolved photoluminescence decay of the device measured at ∼500 nm demonstrates an ultrafast charge transfer (lifetime = ∼6.4 ns) in BFCO/SRO heteroepitaxial structures. The estimated responsivity value at 500 nm and zero bias is 0.38 mA W -1 , which is so far the highest reported for any FE thin film photodiode. Our work highlights the huge potential for using multiferroic oxides to fabricate highly sensitive and switchable photodiodes.

Oxygen vacancy dynamic behavior at room temperature in complex oxides was carefully explored by using a combined approach of ion liquid gating technique and resistance measurements. Heterostructures of PrBaCo 2 O 5+δ /Gd 2 O 3 -doped CeO 2 epitaxial thin films were fabricated on (001) Y 2 O 3 -stabilized ZrO 2 single crystal substrates for systematically investigating the oxygen redox dynamics. The oxygen dynamic changes as response to the gating voltage and duration were precisely detected by in situ resistance measurements. A reversible and nonvolatile resistive switching dynamics was detected at room temperature under the gating voltage >13.5 V with pulse duration >1 s.

Metastable polymorphs, many of which have never been fabricated, have been predicted to exhibit interesting and technologically relevant properties. Epitaxial synthesis is a powerful structure-directing method that can produce metastable polymorphs but is typically done in a trial and error fashion. Unfortunately, the relevant thermodynamic terms governing epitaxial synthesis of new materials are unknown. Accurate calculation of the relevant thermodynamic terms and their incorporation into predictive models would accelerate the synthesis of metastable polymorphs by identifying thermodynamically favorable paths. Using density functional theory with three different functionals, we computed several relevant terms for TiO 2 anatase (A) and rutile (R) film growth on low-index surfaces of SrTiO 3 (STO) and BaTiO 3 (BTO) cubic perovskites. After identifying potential coherent epitaxial interfaces based on experimental observations, the volumetric formation, volumetric strain, and areal substrate-film interface energies were calculated for (001) A ∥(001) (S/B)TO , (102) A ∥(011) (S/B)TO , (100) R ∥(111) (S/B)TO , and (112) A ∥(111) (S/B)TO coherent interfaces. These terms were integrated into a standard model of epitaxial nucleation, and the results yielded reasonable agreement between experimental observations and DFT predictions of the preferred epitaxial polymorph. Predicted trends in epitaxial stability were essentially independent of the three functionals used in the calculations. These results are discussed in light of their promise that DFT-informed epitaxial film growth can accelerate fabrication of new polymorphs. These results also validate the recently proposed 20 kJ/mol stability window for predicting which polymorphs could be epitaxially stabilized.

Samaria-doped ceria (SDC) thin films are particularly important for energy and electronic applications such as microsolid oxide fuel cells, electrolyzers, sensors, and memristors. In this paper, we report a comparative study investigating ionic conductivity and surface reactions for well-grown epitaxial SDC films varying the samaria doping concentration. With increasing doping above 20 mol % of samaria, an enhancement in the defect association is observed by Raman spectroscopy. The role of such associated defects on the films̀ oxygen ion transport and exchange is investigated by electrochemical impedance spectroscopy and electrochemical strain microscopy (ESM). The measurements reveal that the ionic transport has a sharp maximum in ionic conductivity and drops in its activation energy down to 0.6 eV for 20 mol % doping. Increasing the doping concentration further up to 40 mol %, it raises the activation energy substantially by a factor of 2. We ascribe the sluggish transport kinetics to the "bulk" ionic-near ordering in case of the heavily doped epitaxial films. Analysis of the ESM first-order reversal curve measurements indicates that these associated defects may have a beneficial role by lowering the activation of the oxygen exchange "surface" reaction for heavily doped 40 mol % of samaria. In a model experiment, through a solid solution series of samaria doped ceria epitaxial films, we reveal that the occurrence of associated defects in the bulk affects the surface charging state of the SDC films to increase the exchange rates. The implication of these findings is the design of coatings with tuned oxygen surface exchange by controlling the bulk associated clusters for future electrocatalytic applications.

Employing atomic layer deposition, we have grown p-type epitaxial undoped and N-doped anatase TiO2(001) thin films on c-axis Al2O3 substrate. From X-ray diffraction and transmission electron microscopy studies, crystallographic relationships between the film and the substrate are found to be (001)TiO2//(0001)Al2O3 and [1̅10]TiO2//[011̅0]Al2O3. N-doping in TiO2 thin films enhances the hole concentration and mobility. The optical band gap of anatase TiO2 (3.23 eV) decreases to 3.07 eV upon N-doping. The epitaxial films exhibit room-temperature ferromagnetism and photoresponse. A TiO2-based homojunction diode was fabricated with rectification from the p-n junction formed between N-doped p-TiO2 and n-TiO2.

The evolution of interface between the epitaxial thin film LiCoO2 (LCO) electrode and liquid electrolyte and inside the LCO film during electrochemical cycling has been analyzed by high resolution scanning transmission electron microscopy. Relaxation of sharp translational domain boundaries with mismatched layers of CoO2 octahedra occurs during cycling and results in formation of continuous CoO2 layers across the boundaries. The original trigonal layered structure of LiCoO2 tends to change into a spinel structure at the electrode/electrolyte interface after significant extraction of Li from LCO. This change is more pronounced at 4.2 V peak of CV, indicating lower stability of the layered LCO structure near its surface after Li is extracted above 60%. The transformed structure is identified to be close to Co3O4, with Co both on tetrahedral and octahedral sites, rather than to LiCo2O4 as it was suggested in earlier publications. Electron energy-loss spectroscopy measurements also show that Co ions oxidation state is reduced to mixed valence state Co(2+)/Co(3+) during the structure changes to spinel rather than oxidized.

Epitaxial LiCoO2 (LCO) thin films of different orientations were fabricated by pulsed laser deposition (PLD) in order to model single-crystal behavior during electrochemical reaction. This paper demonstrates that deposition of conductive SrRuO3 between a SrTiO3 (STO) substrate and an LCO film allows (1) epitaxial growth of LCO with orientation determined by STO and (2) electrochemical measurements, such as cyclic voltammetry and impedance spectroscopy. Scanning transmission electron microscopy (S/TEM and SEM) has demonstrated an orientation relationship between LCO and STO of three orientations, (111), (110) and (100), and identified a LCO/electrolyte surface as consisting of two crystallographic facets of LCO, (001) and {104}. The difference in the orientation of LCO accounts for the difference in the exposed area of {104} planes to the electrolyte, where lithium ions have easy access to fast diffusion planes. The resistance for lithium ion transfer measured by electrochemical impedance spectroscopy had inverse correlation with exposed area of {104} plane measured by TEM. Chemical diffusivity of lithium ions in LCO was measured by fitting electrochemical impedance spectroscopy data to a modified Randles equivalent circuit and allowed us to determine its dependence on film orientation.

Next-generation devices will rely on exotic functional properties not found in traditional systems. One class of materials of particular interest for applications are those possessing metal-to-insulator transitions (MITs). In this work, we probe the relationship between variations in the growth process, subsequent variations in cation stoichiometry, and the MIT in NdNiO3 thin films. Slight variations in the growth conditions, in particular the laser fluence, during pulsed-laser deposition growth of NdNiO3 produces films that are both single-phase and coherently strained to a range of substrates despite possessing as much as 15% Nd-excess. Subsequent study of the temperature-dependence of the electronic transport reveals dramatic changes in both the onset and magnitude of the resistivity change at the MIT with increasing cation nonstoichiometry giving rise to a decrease (and ultimately a suppression) of the transition and the magnitude of the resistivity change. From there, the electronic transport of nearly ideal NdNiO3 thin films are studied as a function of epitaxial strain, thickness, and orientation. Overall, transitioning from tensile to compressive strain results in a systematic reduction of the onset and magnitude of the resistivity change across the MIT, thinner films are found to possess sharper MITs with larger changes in the resistivity at the transition, and (001)-oriented films exhibit sharper and larger MITs as compared to (110)- and (111)-oriented films as a result of highly anisotropic in-plane transport in the latter.