Abstract This study investigates the control of the domain configuration in tetragonal Pb(Zr 0.3 Ti 0.7 )O 3 (PZT) thin films based on strain using Sr(Zr y Ti 1− y )O 3 (SZTO) buffer layers grown on (100) [(LaAlO 3 ) 0.3 –(SrAl 0.5 Ta 0.5 O 3 ) 0.7 ] substrates. The high-temperature heat treatment of SZTO layers was used to improve crystallinity and increase thermal stability during the subsequent high-temperature deposition of the bottom electrode layer and PZT films. Conductive La-SrSnO 3 and (Ba 0.4 Sr 0.6 )RuO 3 bottom electrodes with lattice parameters compatible with SZTO were introduced between the PZT film and SZTO layer for electrical characterization and strain control. Reciprocal space mapping revealed that predominantly a -domain [(100) orientation] and c -domain [(001) orientation] structures were achieved through strain control during film growth using these buffer layers. The present approach can be applied to a wide range of ferroelectric thin films by tuning the lattice parameters of the strain-control layers.

Rare-earth-ferrimagnetic oxides are emerging as attractive platforms for investigating ultrafast spin dynamics. Here, we study the photoinduced magnetization dynamics of epitaxial NiCo2O4 (NCO) thin films by time-resolved magneto-optical Faraday effect using two independent pump-probe configurations: 1030/515 nm and 800/400 nm. In both measurements, photoexcitation induces an immediate reduction of the magneto-optical signal within the experimental time resolution, followed by a reproducible slower demagnetization component with a characteristic timescale of approximately 5-6 ps and a subsequent recovery on the 100 ps timescale. Importantly, this picosecond demagnetization component is observed consistently across the two experimental configurations and excitation wavelengths, demonstrating that it is an intrinsic feature of the ultrafast magnetic response of NCO thin films. Because the earliest-time dip may contain a transient optical contribution, we describe the overall response as type-II-like, rather than assigning a definitive textbook type-II classification solely on the basis of the sub-resolution signal. These results establish a robust two-step ultrafast demagnetization behavior in NCO and highlight rare-earth-free oxide ferrimagnets as promising systems for exploring Mult sublattice spin dynamics on ultrafast timescales.

Fluorite ferroelectrics are exciting candidates for next-generation non-volatile memory devices because their unique ferroelectric mechanism, which arises from unconventional oxygen displacements, permits ferroelectricity with minimal thickness constraints. However, the polarisation switching mechanism remains the subject of intense debate due to a limited understanding of the atomic-scale dynamics which are extremely challenging to detect and measure. Here, we observe directly the polarisation switching pathways by visualising oxygen site dynamics in ZrO2 and Hf0.5Zr0.5O2 freestanding membranes using an advanced atomic-column imaging technique-optimum bright-field scanning transmission electron microscopy. We observe that the 180° and 90° polarisation pathways involve different nonpolar intermediate states with distinct spatial scales. Coupled with density functional theory, we also reveal how different cation species in fluorite oxides impact the accessible polarisation switching pathways. Our atomic-level insights into the polarisation switching dynamics open new avenues for the advanced engineering of fluorite ferroelectric materials and resulting memory devices.

Abstract We synthesized epitaxial films of Ba-substituted brownmillerite strontium cobaltite, (Sr 1− y Ba y )CoO 2.5 (Ba-SCO) by pulsed laser deposition, and investigated how Ba-substitution-induced structural changes affect protonation and the resulting properties of the films. Ba substitution significantly affects electrochemical protonation in SrCoO 2.5 . Increasing Ba substitution up to y = 0.3 drives a structural phase transition from the brownmillerite structure with oxygen vacancy ordering to a perovskite structure with a disordered arrangement of oxygen vacancies. Furthermore, Ba substitution promotes protonation in the brownmillerite-structured Ba-SCO films, reaching a maximum concentration of 1.3 H per formula unit at y = 0.2, whereas the H concentration in the perovskite-structured films remains low. Based on structural and electrical characterization of the as-grown and protonated Ba-SCO films, we show that Ba substitution mainly expands the CoO 6 octahedral layer and possibly modifies the stability of bridging oxygen sites and the electronic structure of the brownmillerite lattice, thereby promoting hydrogen (or proton) incorporation. Our results imply that maintaining oxygen-vacancy ordering while tuning the local lattice and electronic structures through A-site substitution can be a useful design strategy for developing proton-containing oxides.

Yttrium iron garnet (YIG) is known for its extremely low magnetic damping, making it a key material for magnon-based spintronic devices. Although perpendicular magnetic anisotropy (PMA) has been reported in rare-earth-substituted or Bi-doped YIG thin films, its realization in pure YIG remains limited. In this work, we demonstrate that YIG thin films grown on the substituted gadolinium gallium garnet substrates exhibit PMA and a low damping constant. X-ray diffraction measurements revealed an in-plane tensile strain relative to bulk YIG, suggesting that the strain-induced lattice distortion plays an important role in the emergence of PMA. Spin-wave propagation with a forward-volume configuration is investigated using two separated coplanar waveguides and a network analyzer. The measured propagating spin-wave spectra show a coherent propagation over several micrometers with a group velocity of approximately 0.2 km/s. Furthermore, in a heterostructure with a heavy metal layer of Pt, current-induced spin-orbit torque successfully switches the perpendicular magnetization. These results establish sputter-deposited YIG thin films with perpendicular magnetization as a promising platform for studying low-damping spin-wave transport and current-driven magnetization dynamics.

Controlling proton accumulation and migration in oxides is important for understanding and tuning their functional properties. In this work, we demonstrate that electrochemical hydrogenation and proton diffusion in rutile-structured VO2 epitaxial films with (001) orientation can be modulated by capping them with epitaxial TiO2 layers. Quantitative evaluation of proton concentration using elastic recoil detection analysis (ERDA) revealed that when protons were electrochemically injected into TiO2-capped VO2 films in transistor structures with Nafion membrane gate layers at 100 degrees C, the injected protons penetrated the TiO2 cap layer and accumulated in the VO2 films, regardless of the TiO2 layer thickness. In contrast, after the gate voltage was turned off, the accumulated protons in TiO2/VO2 heterostructures with a 15 nm-thick TiO2 cap layer remained confined within the VO2 layers even at 100 degrees C, whereas protons in heterostructures with thinner TiO2 cap layers migrated through the capping layers and were desorbed from the heterostructures. These results highlight the critical role of chemical potential mismatch at the TiO2/VO2 interface in governing proton accumulation and migration in VO2. They further indicate that engineering interfacial chemical potential mismatch provides an effective strategy for controlling proton transport in oxide heterostructures.

Ferroelectric hafnia-based compounds, known for exhibiting strong ferroelectricity in films of sub-5 nm thickness, hold significant potential for being integrated into complementary metal-oxide-semiconductor devices. Due to the polymorphic nature of hafnia, their ferroelectric properties can be modulated through various mechanisms, including defects, strain, and electrochemical states. In this study, we fabricated ultrathin freestanding hafnia membranes, free from substrate and electrode-capping effects, to explore the relationship between their intrinsic ferroelectricity and surface electrochemical state by modulating humidity conditions during scanning probe microscopy measurements. Our results demonstrate enhanced ferroelectricity in hafnia under low-humidity conditions without requiring a wake-up process. This enhancement is attributed to reduced adsorption of water molecules on the membrane surface, which helps preserve oxygen vacancies that stabilize the ferroelectric phase in hafnia under an applied electric field. These findings suggest that beyond electrical control via field-cycling-induced phase transitions, electrochemical modulation through humidity provides an effective approach for tuning the ferroelectric properties of hafnia-based compounds, optimizing their performance in flexible nanoelectronics applications.

Electrochemical protonation, which can be realized in an electric-field-effect transistor with a gate layer consisting of a proton-conducting electrolyte Nafion membrane, offers a simple way of electrically tuning the physical properties of materials. In this study, we grew (010)-oriented epitaxial films of brownmillerite-structured SrCoO2.5 on various substrates whose lattice mismatch against SCO ranged from 0% to -2.9% by pulsed laser deposition and investigated the effect of substrate-induced strain on their protonation in field effect transistor structures with gate layers consisting of Nafion membranes. We found that the H concentration of the SCO films that were fully compressive-strained by up to 1.3% was similar to 1.7 and that it was almost independent of the magnitude of the substrate-induced strain. We also found that the H content of the strain-partially-relaxed film with a residual compressive strain of 1.3% was lower, similar to 1.3. These results indicate that lattice deformations arising from substrate-induced strain have insignificant effects on protonation, while lattice defects and dislocations introduced upon strain relaxation, which hinders proton diffusion in the films' lattices, dominantly affect protonation in SrCoO2.5 films.

We demonstrate the epitaxial growth of NiAs-type CrSb thin films on LaAlO3(110) and Al2O3(1120) substrates via magnetron cosputtering, aiming to establish a platform for altermagnetism studies. Two distinct orientations-CrSb(1100) and CrSb(1120)-were realized by optimizing lattice matching and applying a W(110) buffer layer when necessary. X-ray diffraction and scanning tunneling electron microscopy (STEM) analyses confirm high crystallinity, with temperature and preannealing conditions found to critically affect film quality and phase formation. A coexistence of (1100) and (0001) phases was observed at elevated temperatures, accompanied by improved mosaicity. Nanobeam diffraction reveals local grain structures, while EDS confirms the near-stoichiometric composition. The hysteresis loop measurements reveal weak ferromagnetic-like signals, likely originating from interfacial defects, while no anomalous signals are observed. The minimal buffer layer requirement on insulating substrates makes the films compatible with advanced characterizations such as magnetotransport and ARPES. We believe our results offer new insight into the growth behavior of CrSb and provide a foundation for future experimental exploration of the altermagnetic materials.

We investigated the dependence of the electro-optic (EO) properties of (111)-epitaxial Hf0.5Zr0.5O2 (HZO) thin films on their thickness in the range of 3–30 nm. HZO films were deposited on (La, Sr)MnO3-bufferred SrTiO3(001) substrates using pulsed laser deposition. Both the ferroelectric orthorhombic and paraelectric monoclinic phases were found to coexist in the 30 nm thick film, and the fraction of the former phase increased with the decrease in thickness down to 5 nm. Although the effective EO coefficient, reff, remained almost unchanged down to 10 nm and decreased with a further decrease in thickness, the evident EO response was observed down to 3 nm, which agrees with the fact that HZO films can maintain ferroelectric properties down to a few monolayers in thickness. The small reff in the HZO films with thickness below 5 nm was attributed to the stabilization of the ferroelectric rhombohedral-like phase.

All-optical switching (AOS) involves manipulating magnetization using only a pulsed laser, presenting a promising approach for next-generation magnetic recording devices. NiCo2O4 (NCO) thin films, a rare-earth-free ferrimagnetic oxide, exhibit a high Curie temperature and strong perpendicular magnetic anisotropy. This study demonstrates AOS in NCO thin films at room temperature using long-duration laser pulses and high repetition rates. Unlike previous findings, the AOS phenomena we report here are helicity-dependent and observable with an ultrashort pulsed laser. Consequently, two distinct types of AOS can be observed in a single NCO thin film, contingent on the characteristics of the laser pulses and temperature.

Fabricating and investigating freestanding membranes of materials are key approaches for exploring the intrinsic properties of those materials, even in their metastable phases stabilized by external factors. In this study, we fabricated freestanding ZrO2 membranes with metastable crystal structures by exfoliating ZrO2 (ZO) epitaxial layers from ZrO2/La0.7Sr0.3MnO3/SrTiO3 heterostructures. By combining the results of first-principles calculations, we found that with increasing thickness up to 15 nm, ZO membranes undergo structural relaxation from the metastable tetragonal structure to a metastable orthorhombic one. In addition, the dielectric constant of the tetragonal ZO is susceptible to the out-of-plane lattice constant of the ZO layer. The change in out-of-plane interplanar distance, which primarily stems from the strain release associated with the exfoliation of ZO layers, leads to an increase in the dielectric constant (to similar to 25). ZO membranes with high dielectric constants could be utilized in energy storage capacitors.

Electrochemically inserting and extracting hydrogen into and from solids are promising ways to explore materials’ phases and properties. However, it is still challenging to identify the structural factors that promote hydrogen insertion and extraction and to develop materials whose functional properties can be largely modulated by inserting and extracting hydrogen through solid-state reactions at room temperature. In this study, guided by theoretical calculations on the energies of oxygen reduction and hydrogen insertion reactions with oxygen-deficient perovskite oxides, we demonstrated that the oxygen vacancy ordering in Sr(Fe1−xCox)Oy (SFCO) epitaxial films can be stabilized by increasing the Co content (x ≥ 0.3) and revealed that it plays a key role in promoting proton accommodation into the SFCO lattice. We also show that the electrical resistance of SFCO films can be reversibly modulated by electrochemical proton insertion and extraction, and the modulation exceeds three orders of magnitude for Sr(Fe0.5Co0.5)O2.5 epitaxial films. Our results provide guidelines for controlling material properties through the insertion and extraction of hydrogen and for designing and exploring hydrogen-insertion materials. Hydrogen insertion and extraction in solids is a way to explore materials’ phases and properties. Here authors investigate the stabilisation of oxygen vacancy ordering in strontium iron cobalt oxides epitaxial films and their impact on proton accommodation and electrical resistance modulation.

We epitaxially grew rutile-structured VO _2 films with various out-of-plane lattice constants on (001) TiO _2 substrates by pulsed laser deposition and investigated their protonation by electrochemically injecting protons to the films in transistor structures with gate layers of proton conducting Nafion membranes. We found that VO _2 films with out-of-plane lattice expansion are less protonated. On the basis of the experimental results, we discuss the correlation between the out-of-plane lattice expansion and protonation of (001) VO _2 epitaxial films and highlight that reducing lattice defects is key to promoting the protonation of VO _2 films.

While oxide heterostructures with emergent properties have garnered a great amount of research interest, heterostructures consisting of oxides with metastable phases have barely been investigated. Here, we fabricated epitaxial bilayer structures based on ZrO2 (ZO) and Hf0.5Zr0.5O2 (HZO) whose single layers, respectively, possess non-polar and polar metastable structures and investigated their structural and ferroelectric properties. We found that the ZO/HZO heterostructures exhibit ferroelectric polarizations comparable to those of HZO films. The bilayer is found to consist of polar orthorhombic lattices with Moir & eacute; superlattices, highlighting the key role of the ZO/HZO interfaces in stabilizing the ferroelectricity in the bilayer structures. Furthermore, we found that both the dielectric constant and polarization are increased by reducing the thickness of HZO and ZO layers in the stacked structures and enhancing the interfacial effects.

Two-dimensional freestanding membranes of materials, which can be transferred onto and make interfaces with any material, have attracted attention in the search for functional properties that can be utilized for next-generation nanoscale devices. We fabricated stable 1-nm-thick hafnia membranes exhibiting the metastable rhombohedral structure and out-of-plane ferroelectric polarizations as large as 13 μC/cm2. We also found that the rhombohedral phase transforms into another metastable orthorhombic phase without the ferroelectricity deteriorating as the thickness increases. Our results reveal the key role of the rhombohedral phase in the scale-free ferroelectricity in hafnia and also provide critical insights into the formation mechanism and phase stability of the metastable hafnia. Moreover, ultrathin hafnia membranes enable heterointerfaces and devices to be fabricated from structurally dissimilar materials beyond structural constrictions in conventional film-growth techniques.

Electrochemical protonation provides ways to control physical properties and even explore unprecedented phases of solid-state materials. While how proton accumulation changes materials' properties is investigated, how protonation of solids can be controlled and promoted remains an enigmatic puzzle. In the work reported here, the influence of electrochemical proton injection duration (t(Vg)) is investigated on the protonation of SrCoO2.5 (SCO) films in electric-field-effect transistor structures with gate layers of the proton-conducting electrolyte Nafion. The proton concentration accumulated in SCO films varies depending on the duration of the proton injection. When protons are injected in a relatively short t(Vg) (<= 600 s), the hydrogen concentration accumulated in SCO film increases with increasing t(Vg), reaching the maximum proton concentration of approximate to 1.9 per formula unit of SCO for the t(Vg) = 600 s case. On the other hand, when t(Vg) is longer than 900 s, the proton concentration decreases with t(Vg), implying the occurrence of counterreactions that extract protons from protonated SCO and oxidize the channel. These observations indicate that protons accumulated at the Nafion/SCO interface play a role in the protonation of SCO films and that suppressing the interfacial proton accumulation is the key to maximizing the proton concentration accumulated in SCO films.

Fully oxygenated perovskite BaFeO3 containing unusually high-valence Fe4+ shows three crystal polymorphs with the same chemical composition. The 3C-type BaFeO3 has a simple cubic perovskite structure consisting of corner-sharing FeO6 octahedra, while the 6H- and 12R-type BaFeO3 have hexagonal perovskite structures consisting of both corner-sharing and face-sharing FeO6 octahedra. The compounds readily release oxygen into the air to reduce the high-valence state of the Fe ions, but the oxygen release behaviors strongly depend on the crystal structure. The 3C-type BaFeO3 releases oxygen topotactically from the corner-shared sites of the FeO6 octahedra at a temperature as low as 130 degrees C. In contrast, the 6H- and 12R-type BaFeO3 preferentially release oxygen from the face-shared sites above 320 and 460 degrees C, respectively, although they include the corner-shared sites in the crystal structures. The resultant oxygen-deficient 3C-type BaFeO2.5 does not incorporate back oxygen in air, whereas the 12R-type hexagonal structure shows completely reversible oxygen release and incorporation in air. Once the 12R-type structure is established, unusually high-valence states such as Fe4+ can be stabilized without extreme conditions.