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Dynamic tuning of optical properties is technologically important for the development of future integrated electronic and photonic devices. Here, we demonstrate the electrically tunable permittivity in optical frequency in epitaxial BaTiO3 (BTO) thin films using spectroscopic ellipsometer measurements under DC bias condition. The lattice tetragonal distortion and domain switching of biased BTO is evident based on the increased anisotropy in permittivity in-plane and out-of-plane. Specifically, out-of-plane permittivity, arising from the c-domains, shows a continuous decrease with the applied voltage, relative to the in-plane permittivity, arising from the a-domains. The dielectric constant can also be reversibly adjusted through electric-reset cycles.

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.

The cubic perovskite SrMoO 3 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 SrMoO 3 thin films by hydrogen-free sputtering has hindered their practical use, especially due to the formation of thermodynamically favorable SrMoO 4 impurity. In this work, we developed a radio frequency sputtering technology enabling the reduction reaction and achieved conductive epitaxial SrMoO 3 films with pure phase from a SrMoO 4 target in a hydrogen-free, pure argon environment. We demonstrated the significance of controlling the target-to-substrate distance (TSD) on the synthesis of SrMoO 3 ; the film resistivity drastically changes from 1.46 × 10 5 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 SrMoO 3 with an epitaxial perovskite structure. The formation mechanism of the conductive single-phase SrMoO 3 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 SrMoO 3 films, while the high-resistive ones displayed semiconductor-like behavior. Our approach makes hydrogen-free, reliable and cost-efficient scalable deposition of SrMoO 3 films possible, which may open up promising prospects for a wide range of future applications of oxide materials., Competing Interests: No potential conflict of interest was reported by the author(s)., (© 2024 The Author(s). Published by National Institute for Materials Science in partnership with Taylor & Francis Group.)

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.

A series of experiments is carried out to identify the contribution of interface and bulk antiferromagnetic (AFM) spins to exchange bias (EB) in ultrathin epitaxial ferromagnetic (FM)/AFM bilayer samples. These are single‐crystalline AFM NixMn100−x and ferromagnetic Co layers on Cu3Au(001), deposited under ultrahigh vacuum conditions, in which structural or chemical defects are deliberately introduced by controlled Ar ion sputtering at the surface of the AFM layer or at a certain depth inside the AFM layer. Comparison of the magnetic properties measured by magneto‐optical Kerr effect for sputtered and nonsputtered parts of the same sample then allows a precise determination of the influence of sputtering on the AFM layer during the sample preparation, whereas all other parameters are kept identical. The results show that the creation of defects in the bulk of the AFM layer enhances the magnitude of EB and its blocking temperature, but not the creation of defects at the interface. It is also observed that the deeper the insertion of defects in the AFM layer, the higher the value of the EB field and the larger the coercivity, These findings are discussed as the effect of additional pinning centers in the bulk of the AFM layer. [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. Pulsed Laser Deposition is applied to grow phase pure and epitaxial Cr2AlC MAX phase thin films on MgO(111) and Al2O3(0001). Control of the sample stoichiometry is achieved by elemental targets. [ABSTRACT FROM AUTHOR]

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.

Rational control of local strain distributions and thus the functional properties of epitaxial thin films has been a long‐standing goal in the development of new physics and novel devices based on strain‐sensitive materials. Here, the fabrication of La2/3Sr1/3MnO3 (LSMO) films with strain fields arising from vertical epitaxial embedding of ultra‐small ZrO2 nanopillars, diameter 4.0 ± 0.6 nm, is reported. High quality films are obtained with average distance between adjacent nanopillars of 9.0 ± 0.3 nm for x = 0.2 in (LSMO)1−x:(ZrO2)x. The strain distribution of the vertical interface is analyzed in detail and the dominant state of the interfacial strain is verified. Remarkably, with increasing x, the Curie temperature TC and metal–insulator (MI) transition temperature TMI show a surprisingly large depression, revealing the significant tuning capability of the vertical tensile stress originating from the small‐size ZrO2 pillars. A systematic tunability of the low field magnetoresistance is also found. The field dependence of the magnetization exhibits both horizontal and vertical shifts. The exchange bias field HE increases with increasing x, while magnetization shift Mshift is unchanged. The results suggest the possibility of strain tuning through epitaxial nanostructures for multifunctional applications across many fields with appropriate selection of matrix and nanopillar materials. [ABSTRACT FROM AUTHOR]

Light is an effective tool to probe the polarization and domain distribution in ferroelectric materials passively, that is, non-invasively, for example, via optical second harmonic generation (SHG). With the emergence of oxide electronics, there is now a strong demand to expand the role of light toward active control of the polarization. In this work, optical control of the ferroelectric polarization is demonstrated in prototypical epitaxial PbZr x Ti 1-x O 3 (PZT)-based heterostructures. This is accomplished in three steps, using above-bandgap UV light, while tracking the response of the polarization with optical SHG. First, it is found that UV-light exposure induces a transient enhancement or suppression of the ferroelectric polarization in films with an upward- or downward-oriented polarization, respectively. This behavior is attributed to a modified charge screening driven by the separation of photoexcited charge carriers at the Schottky interface of the ferroelectric thin film. Second, by taking advantage of this optical handle on electrostatics, remanent optical poling from a pristine multi-domain into a single-domain configuration is accomplished. Third, via thermal annealing or engineered electrostatic boundary conditions, a complete reversibility of the optical poling is further achieved. Hence, this work paves the way for the all-optical control of the spontaneous polarization in ferroelectric thin films., (© 2024 The Authors. Advanced Materials published by Wiley‐VCH GmbH.)

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.

We have investigated the collective electronic and magnetic orderings of a series of La _1− _x Sr _x MnO _3 thin films grown epitaxially strained to (001) oriented strontium titanate substrates as a function of doping, x , for 0 ≤ x ≤ 0.4. We find that the ground states of these crystalline thin films are, in general, consistent with that observed in bulk crystals and thin film samples synthesized under a multitude of techniques. Our systematic study, however, reveal subtle features in the temperature dependent electronic transport and magnetization measurements, which presumably arise due to Jahn-Teller type distortions in the lattice for particular doping levels. For the parent compound LaMnO _3 ( x = 0), we report evidence of a strain-induced ferromagnetic ordering in contrast to the antiferromagnetic ground state found in bulk crystals.

The thickness dependence of crystal structure and electrical properties is studied for rutile‐type RuO2 (100) thin films. The island growth mode results in a monotonically varying lattice strain along the a‐axis from negative to positive with increasing thickness. Decrease in the thickness below 20 nm increases the resistivity drastically, resulting in insulating‐like behaviors. The metallic behaviors of the thicker films are well explained by the modified Bloch–Grüneisen function as in bulk RuO2, where the strain has a significant influence on the conduction parameters. The insulating‐like behaviors of the thinner films are assigned to 2D localization and Mott variable range hopping, reflecting the effects of the size and growth mode on electrical conduction. [ABSTRACT FROM AUTHOR]

Dynamic tuning of optical properties is technologically important for the development of future integrated electronic and photonic devices. Here, we demonstrate the electrically tunable permittivity in optical frequency in epitaxial BaTiO3 (BTO) thin films using spectroscopic ellipsometer measurements under DC bias condition. The lattice tetragonal distortion and domain switching of biased BTO is evident based on the increased anisotropy in permittivity in-plane and out-of-plane. Specifically, out-of-plane permittivity, arising from the c-domains, shows a continuous decrease with the applied voltage, relative to the in-plane permittivity, arising from the a-domains. The dielectric constant can also be reversibly adjusted through electric-reset cycles. The current study demonstrates the real-time electrically tunable dielectric permittivity in BaTiO3, probed using spectroscopic ellipsometer at optical frequencies, towards future tunable waveguides, optical components and devices. [ABSTRACT FROM AUTHOR]

Mixed-valance manganites with strong electron correlation exhibit strong potential for spintronics, where emergent magnetic behaviors, such as propagation of high-frequency spin waves and giant topological Hall Effects can be driven by their mesoscale spin textures. Here, we create magnetic vortex clusters with flux closure spin configurations in single-crystal La 0.67 Sr 0.33 MnO 3 wire. A distinctive transformation from out-of-plane domains to a vortex state is directly visualized using magnetic force microscopy at 4 K in wires when the width is below 1.0 μm. The phase-field modeling indicates that the inhomogeneous strain, accompanying with shape anisotropy, plays a key role for stabilizing the flux-closure spin structure. This work offers a new perspective for understanding and manipulating the non-trivial spin textures in strongly correlated systems. [ABSTRACT FROM AUTHOR]

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.

Several aspects such as the growth relation between the layers of the GaN/AlN/SiC heterostructure, the consistency of the interfaces, and elemental diffusion are achieved by High Resolution Transmission Electron Microscopy (HR-TEM). In addition, the dislocation densities together with the defect correlation lengths are investigated via High-Resolution X-ray Diffraction (HR-XRD) and the characteristic positron diffusion length is achieved by Doppler Broadening Spectroscopy (DBS). Moreover, a comparative analysis with our previous work (i.e., GaN/AlN/Si and GaN/AlN/Al2O3) has been carried out. Within the epitaxial GaN layer defined by the relationship F4¯3m (111) 3C-SiC || P63mc (0002) AlN || P63mc (0002) GaN, the total dislocation density has been assessed as being 1.47 × 1010 cm−2. Compared with previously investigated heterostructures (on Si and Al2O3 substrates), the obtained dislocation correlation lengths (Le = 171 nm and Ls =288 nm) and the mean distance between two dislocations (rd = 82 nm) are higher. This reveals an improved crystal quality of the GaN with SiC as a growth template. In addition, the DBS measurements upheld the aforementioned results with a higher effective positron diffusion length LeffGaN2 = 75 ± 20 nm for the GaN layer.

Spin-orbit coupling (SOC), the interaction between the electron spin and the orbital angular momentum, can unlock rich phenomena at interfaces, in particular interconverting spin and charge currents. Conventional heavy metals have been extensively explored due to their strong SOC of conduction electrons. However, spin-orbit effects in classes of materials such as epitaxial 5d-electron transition-metal complex oxides, which also host strong SOC, remain largely unreported. In addition to strong SOC, these complex oxides can also provide the additional tuning knob of epitaxy to control the electronic structure and the engineering of spin-to-charge conversion by crystalline symmetry. Here, we demonstrate room-temperature generation of spin-orbit torque on a ferromagnet with extremely high efficiency via the spin-Hall effect in epitaxial metastable perovskite SrIrO3.We first predict a large intrinsic spin-Hall conductivity in orthorhombic bulk SrIrO3 arising from the Berry curvature in the electronic band structure. By manipulating the intricate interplay between SOC and crystalline symmetry, we control the spin-Hall torque ratio by engineering the tilt of the corner-sharing oxygen octahedra in perovskite SrIrO3 through epitaxial strain. This allows the presence of an anisotropic spin-Hall effect due to a characteristic structural anisotropy in SrIrO3 with orthorhombic symmetry. Our experimental findings demonstrate the heteroepitaxial symmetry design approach to engineer spin-orbit effects. We therefore anticipate that these epitaxial 5d transition-metal oxide thin films can be an ideal building block for low-power spintronics. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Epitaxial growth of Ni-Mn-Ga film on Al 2 O 3 substrate using Magnetron sputtering. • Presence of martensitic variants with (220) and (022) crystallographic twins. • Co-existence of tetragonal and orthorhombic martensite phases. • Magnetic field induced reorientation effect. Abstract Epitaxial Ni-Mn-Ga thin films with a room temperature martensite phase were fabricated on single crystal Al 2 O 3 (11 2 ¯ 0) substrate using direct current (DC) magnetron sputtering. The X-ray pole figure measurements confirm the epitaxial relationship of Ni-Mn-Ga (101) 7M || Al 2 O 3 (11 2 ¯ 0) and Ni-Mn-Ga [ 10 1 ¯ ] 7M || Al 2 O 3 [ 1 1 ¯ 01 ] between the film and substrate. Transmission electron microscopy reveals the presence of (220) micro-twins at an angle of 62.5° and 56.3° with respect to (202) growth plane. Coexistence of non-modulated martensite phase (a NM = 5.45 Å and c NM = 6.56 Å) and seven modulated martensite phases ( a 14 M = 6.26 Å , b 14 M = 5.89 Å a n d c 14 M = 5.56 Å) has been observed in the film. The in-situ high-temperature X-ray diffraction indicates reversible thermal phase transformation in the film with a very low thermal hysteresis. The magnetic field induced reorientation (MIR) effect is displayed by the film. [ABSTRACT FROM AUTHOR]

Abstract Transition metal oxides show remarkably diverse quantum functional properties such as high temperature superconductivity, colossal magnetoresistance, multiferroicity, two-dimensional electron gas, and topological insulators. This diversity manifests as a result of many energy scales of similar magnitude competing rather than any particular one dominating in the system. In this regard, growth of atomically controlled epitaxial thin films and heterostructures would allow to control relevant energy scales by imposing various stimuli, such as reduction of dimensionality, introduction of interfaces, modification of the interfacial octahedral tilts, and symmetry breaking; in turn, modified functional properties or completely new phenomena may emerge in epitaxial thin films and heterostructures. Also of exceeding importance is the fact that atomically controlled epitaxial thin films and heterostructures of the multifunctional oxides offer promising potentials for next generation oxide electronics. In this short review, we collect representative examples of quantum correlated phenomena arising in epitaxial films and heterostructures of transition metal oxides and highlight some of the progresses achieved in thin film research of various functional oxides in the last couple of decades. [ABSTRACT FROM AUTHOR]

Perovskite (Pr0.002Sr0.998)(Ti0.98Al0.02)O3 epitaxial thin films were obtained and showed red luminescence under ultraviolet excitation. The films were grown on SrTiO3 (001) substrates by pulsed-laser deposition, and the crystallinity of the epitaxial growth was confirmed by X-ray diffraction and reflected high-energy electron diffraction. Red luminescence of the as-grown film showed a sharp emission peak centered at 615 nm. This emission line was assigned to the intra atomic transition from the 1D2 state to 3H4 in Pr3+ ions. The transparencies of the double-side-polished substrate with and without the film at around 550 nm were about 68% and 70%, respectively. The preparation of epitaxial thin films with a red luminescence and a high transparency may open up applications for light emitting devices. [ABSTRACT FROM AUTHOR]

We studied the symmetry of magnetic properties and the resulting magnetic textures in ultra-thin epitaxial Au _0.67 Pt _0.33 /Co/W(110), a model system exhibiting perpendicular magnetic anisotropy and interface Dzyaloshinskii–Moriya interaction (DMI). As a peculiar feature, the C _2v crystal symmetry induced by the Co/W interface results in an additional uniaxial in-plane magnetic anisotropy in the cobalt layer. Photo-emission electron microscopy with magnetic sensitivity reveals the formation of self-organised magnetic stripe domains oriented parallel to the hard in-plane magnetisation axis. We attribute this behavior to the lower domain wall energy when oriented along this axis, where both the DMI and the in-plane magnetic anisotropy favor a Néel domain wall configuration. The anisotropic domain wall energy also leads to the formation of elliptical skyrmion bubbles under a weak out-of-plane magnetic field.

Density functional theory (DFT) was used in the computation of thermodynamic terms relevant to the competition between epitaxial polytypes during nucleation of (Sr/Ba)MnO 3 on (100), (110), and (111) cubic (Sr/Ba)TiO 3 substrates. Values for volumetric formation energies, volumetric strain energies, and area-specific interface energies were computed for different polytypes (3 C , 4 H , and 2 H) and were incorporated in a standard (capillarity) model for epitaxial nucleation. Experimental orientation relationships (O R s) for SrMnO 3 were used in the construction of strained and interface cells for (Sr/Ba)MnO 3. For 3 C polytypes, the O R is simply cube-on-cube, or (111) [ 1 1 ̄ 0 ] 3 C , f i l m | | (111) [ 1 1 ̄ 0 ] 3 C , s u b , and is isostructural with cubic (Sr/Ba)TiO 3. For 4 H / 2 H polytypes, the orientation relationship is (001) [ 100 ] 4 H / 2 H , f i l m | | (111) [ 1 1 ̄ 0 ] 3 C , s u b , which can only be modeled with a coherent interface on the (111) substrate. Results indicate that 3 C SrMnO 3 has increased energies (becomes less stable) on moving from SrTiO 3 substrate orientations (100) to (110) to (111), consistent with experimental observations. For BaMnO 3 , similar trends are predicted, although no experimental data is available for comparison. We use the DFT results to discuss the different thermodynamic contributions to polytype stability, and assess the feasibility of stabilizing a 3 C BaMnO 3 film on (Sr/Ba)TiO 3 substrates. [Display omitted] • 3 C polytypes are surprisingly stable at monolayer thicknesses for many compositions. • 3 C polytype stability decreases in energy from (100) to (110) and (111) substrates. • (100) 3 C SrMnO 3 is more stable than 4 H SrMnO 3 at 1 nm on (100) SrTiO 3 substrate. • Hexagonal polytypes are more stable for other film/substrate combinations at 1 nm. [ABSTRACT FROM AUTHOR]

The present article evaluates, in qualitative and quantitative manners, the characteristics (i.e., thickness of layers, crystal structures, growth orientation, elemental diffusion depths, edge, and screw dislocation densities), within two GaN/AlN/Si heterostructures, that alter their efficiencies as positron moderators. The structure of the GaN film, AlN buffer layer, substrate, and their growth relationships were determined through high-resolution transmission electron microscopy (HR-TEM). Data resulting from high-resolution X-ray diffraction (HR-XRD) was mathematically modeled to extract dislocation densities and correlation lengths in the GaN film. Positron depth profiling was evaluated through an experimental Doppler broadening spectroscopy (DBS) study, in order to quantify the effective positron diffusion length. The differences in values for both edge ( ρ d e ) and screw ( ρ d s ) dislocation densities, and correlation lengths (Le, Ls) found in the 690 nm GaN film, were associated with the better effective positron diffusion length (Leff) of L eff GaN 2 = 43 ± 6 nm.

Graphical abstract Highlights • Size dependent metal-to-insulator transition in MoO 2 was unveiled by systematic study with various spectroscopic methods. • Electronic transport measurements showed similar metal-to-insulator transition by film thickness. • Surface oxidation in epitaxial thin films would be the main driver to trigger this transition. Abstract Interfaces in transition metal oxides play critical roles for tuning physical properties. In thin film form, multiple interfaces can be created in between a film and a substrate, in between a film and air, and within a thin film. The role of each interface has been rarely studied. In this research, we used MoO 2 as a model system to study the role of the oxidized layer at film-air interface in thickness-driven metal-insulator transition. The oxidized layer at the surface is likely to be the main cause in positive temperature coefficient of resistivity in MoO 2 thin films thinner than about 20 nm. To find the origin of this insulating behavior in electronic transport measurements, we used x-ray diffraction, density functional theory and various spectroscopic methods. We observed the formation of oxidized MoO 2+ x at the film-air interface and its thickness explain the peculiar insulating behavior in the thinner films and even nanoparticles from the literature. [ABSTRACT FROM AUTHOR]

Highlights • The L 1 0 Mn 53 Ga 47 islands epitaxially grown onto MgO(1 0 0) substrates were fabricated. • The order parameter is found to be 0.71–0.79. • The estimated Curie temperature is 600 K and consistent with reported bulk value. • The coercivity results from a domain wall pinning mechanism. • The n = 2.6 in K u (T)∝M s (T)n suggests a deviation from single-ion mechanism. Abstract Magnetic and structural properties of L 1 0 Mn 53 Ga 47 islands epitaxially grown onto MgO(1 0 0) substrates are discussed. The samples were fabricated by sputter-deposition at a substrate temperature T s during deposition. The samples deposited at T s above 500 °C were found to form the island structure of Volmer-Weber type. The average size of the islands fabricated at T s = 600 °C is around 200 nm in width and 60 nm in height. The XRD and TEM analyses indicate that those islands are of the L 1 0 phase with the 〈0 0 1〉 axis perpendicular to the substrate surface. The lattice constants a and c are 3.91 ± 0.01 Å and 3.65 ± 0.02 Å, respectively, which are larger by 0.8%, and smaller by 1.5% respectively, as compared to the bulk values of the same composition. The order parameter of these islands is found to increase from 0.7 to 0.8 for T s = 450–500 °C, and then remains nearly constant (≈0.8) for higher T s. The M-H hysteresis loops exhibit a high squareness for all the samples for T s = 450–600 °C. The saturation magnetization M s is about 220 and 350 emu/cm3 for T s = 450 and 600 °C, respectively. The out-of-plane coercivity H c is 5 and 13 kOe for T s = 450 and 600 °C, respectively. The temperature dependence of M s for the sample made at T s = 600 °C was measured and fitted to an empirical relation in order to estimate T c. The estimated T c around 600 K is in reasonable agreement with the reported value of bulk L 1 0 Mn 53 Ga 47. The uniaxial perpendicular magnetic anisotropy constant K u estimated by the out-of-plane torque curves is around 1.1 × 107 and 8.3 × 106 erg/cm3 at 5 and 300 K, respectively. The initial magnetization curves of these islands exhibit a typical pinning mode for the coercivity mechanism. A phenomenological discussion of the coercivity is presented on the basis of Kronmuller’s formula. It is found that the coefficient α and effective demagnetizing factor N eff in the formula by Kronmuller are approximately 0.38 and 12. For the magnetic anisotropy mechanism, the power law relation between K u (T) and M s (T) is examined. It is found that the exponent n for K u (T)∝M s (T)n varies from 2.3 to 3.5 for T from 350 to 5 K. However, the n in a whole temperature range from 5 to 350 K is 2.6. The n = 2.6 suggests a deviation from the single-ion mechanism, and is consistent with the theoretical predictions of Ga contribution. [ABSTRACT FROM AUTHOR]

Antiferroelectric PbZrO3 epitaxial thin film with thickness of 50 nm grown on the SrTiO3 substrate with SrRuO3 buffer layer was studied by Grazing incidence X-ray diffraction in a wide range of temperatures. Apart from the superstructure reflections characteristic for the antiferroelectric phase, we identify the reflections with pseudocubic coordinates of the form (H + 0.5, K + 0.5, L). This indicates the presence of structures having orthorhombic symmetry as an effect of anti-phase Pb atoms displacements, which are apparently similar to the ones previously proposed for the rhombohedral high-temperature phase in PbZrxTi1-xO3. For these structures only the domain orientations with displacements parallel to the surface are observed in contrast to the antiferroelectric domains where the parallel to surface Pb displacements are absent. The antiferroelectric phase coexists with the one characterized by (H + 0.5, K + 0.5, L) reflections in a broad temperature range with the maximal amount of the latter being present at T = 150 °C. [ABSTRACT FROM AUTHOR]

Epitaxial cobalt ferrite thin films with strong in-plane magnetic anisotropy have been grown on Si (001) substrates using a TiN buffer layer. The epitaxial films have been grown by ion beam sputtering using either metallic, CoFe 2 , or ceramic, CoFe 2 O 4 , targets. X-ray diffraction (XRD) and Rutherford spectrometry (RBS) in random and channeling configuration have been used to determine the epitaxial relationship CoFe 2 O 4 [100]/TiN [100]/Si [100]. Mössbauer spectroscopy, in combination with XRD and RBS, has been used to determine the composition and structure of the cobalt ferrite thin films. The TiN buffer layer induces a compressive strain in the cobalt ferrite thin films giving rise to an in-plane magnetic anisotropy. The degree of in-plane anisotropy depends on the lattice mismatch between CoFe 2 O 4 and TiN, which is larger for CoFe 2 O 4 thin films grown on the reactive sputtering process with ceramic targets. [ABSTRACT FROM AUTHOR]

In this work, a detailed investigation has been performed on hetero-epitaxial growth and microstructural evolution in highly oriented Ni–Mn–Ga (1 0 0) films grown on MgO (1 0 0) substrate using high-resolution X-ray diffraction and orientation imaging microscopy. Mosaicity of the films has been analysed in terms of tilt angle, twist angle, lateral and vertical coherence length and threading dislocation densities by performing rocking curve measurements and reciprocal space mapping. Density of edge dislocations is found to be an order of magnitude higher than the density of screw dislocations, irrespective of film thickness. X-ray pole figure measurements have revealed an orientation relationship of|| (1 0 0)MgO;|| [0 0 1]MgObetween the film and substrate. Microstructure predicted by X-ray diffraction is in agreement with that obtained from electron microscopy and atomic force microscopy. The evolution of microstructure in the film with increasing thickness has been explained vis-à-vis dislocation generation and growth mechanisms. Orientation imaging microscopy observations indicate evolutionary growth of film by overgrowth mechanism. Decrease in coercivity with film thickness has been explained as an interplay between stress field developed due to crystal defects and magnetic domain pinning due to surface roughness. [ABSTRACT FROM PUBLISHER]

Abstract: Purely mechanical strain‐tunable microwave magnetism device with lightweight, flexible, and wearable is crucial for passive sensing systems and spintronic devices (noncontact), such as flexible microwave detectors, flexible microwave signal processing devices, and wearable mechanics‐magnetic sensors. Here, a flexible microwave magnetic CuFe2O4 (CuFO) epitaxial thin film with tunable ferromagnetic resonance (FMR) spectra is demonstrated by purely mechanical strains, including tensile and compressive strains, on flexible fluorophlogopite (Mica) substrates. Tensile and compressive strains show remarkable tuning effects of up‐regulation and down‐regulation on in‐plane FMR resonance field (Hr), which can be used for flexible tunable resonators and filters. The out‐of‐plane FMR spectra can also be tuned by mechanical bending, including Hr and absorption peak. The change of out‐of‐plane FMR spectra has great potential for flexible mechanics‐magnetic deformation sensors. Furthermore, a superior microwave magnetic stability and mechanical antifatigue character are obtained in the CuFO/Mica thin films. These flexible epitaxial CuFO thin films with tunable microwave magnetism and excellent mechanical durability are promising for the applications in flexible spintronics, microwave detectors, and oscillators. [ABSTRACT FROM AUTHOR]

Hybrid magnetic heterostructures made of epitaxial manganite LaSr MnO thin film and an intermetallic superlattice (TeCo/FeCo) were prepared on orthorhombic NdGaO substrates and characterized by means of magneto-optical Kerr effect, magnetoresistance and ferromagnetic resonance. Experimental data show that magnetic interaction between LaSrMnO and (TeCo/FeCo) is of an antiferromagnetic nature. The features observed in magnetotransport characteristics are caused by the magnetization reversal at the interface between manganite thin film and intermetallic superlattice. [ABSTRACT FROM AUTHOR]

Phase transitions in correlated materials can be manipulated at the nanoscale to yield emergent functional properties, promising new paradigms for nanoelectronics and nanophotonics. Vanadium dioxide (VO2), an archetypal correlated material, exhibits a metal-insulator transition (MIT) above room temperature. At the thicknesses required for heterostructure applications, such as an optical modulator discussed here, the strain state of VO2 largely determines the MIT dynamics critical to the device performance. We develop an approach to control the MIT dynamics in epitaxial VO2 films by employing an intermediate template layer with large lattice mismatch to relieve the interfacial lattice constraints, contrary to conventional thin film epitaxy that favors lattice match between the substrate and the growing film. A combination of phase-field simulation, in situ real-time nanoscale imaging, and electrical measurements reveals robust undisturbed MIT dynamics even at preexisting structural domain boundaries and significantly sharpened MIT in the templated VO2 films. Utilizing the sharp MIT, we demonstrate a fast, electrically switchable optical waveguide. This study offers unconventional design principles for heteroepitaxial correlated materials, as well as novel insight into their nanoscale phase transitions. [ABSTRACT FROM AUTHOR]

The magnetic properties of epitaxial Fe/MgO structures on Si(100) substrates are investigated over an Fe thickness range of 2–20 nm. Superparamagnetic behavior is observed at an Fe thickness of 2 nm, indicating that no continuous Fe thin film is formed. At Fe thicknesses of 5 nm and higher, a continuous two-dimensional layered structure is formed, which has a dominant epitaxial relationship of Fe[010](100)//MgO [ 01 1 ¯ ] (100)//Si [ 01 1 ¯ ] (100) and a minor portion of Fe[010](100)//MgO[010](100)//Si [ 01 1 ¯ ] (100). This structural feature is echoed by a four-fold magnetic anisotropy in the film plane, and this tendency increases with increasing Fe thickness. The strength of the first-order cubic magnetocrystalline anisotropy, which can only be extracted from the structures, is in the range of 3.87×10 5 –4.04×10 5 erg/cm 3 , weaker than that of bulk Fe (4.8×10 5 erg/cm 3 ). [ABSTRACT FROM AUTHOR]

The magnetic and magnetodielectric behavior of epitaxial (001) transition metal oxide spinel iron vanadate (Fe1+xV2−xO4, FVO) thin films grown on (001) SrTiO3 substrates by pulsed laser deposition is reported. X‐ray absorption spectroscopy studies confirm the trivalent and nominally divalent nature of V and Fe ions, respectively, while an excess of Fe3+ ions is also found. Aberration‐corrected cross‐sectional scanning transmission electron microscopy analysis suggests that the excess Fe3+ may be accommodated through Fe3O4 stacking faults along the {111} planes. Temperature‐dependent X‐ray magnetic circular dichroism spectra reveal an anti‐parallel alignment of spins between the Fe2+ and V3+ cations. M–H loops acquired at 10 K, well below the noncollinear ferromagnetic phase transition temperature, show magnetic anisotropy and point to the existence of two distinct phases with different coercive fields. Visual evidence for magnetic domains and their switching is given by magnetic force microscopy, which finds a switching field of ≈3 T in agreement with the M–H loops. Magnetodielectric measurements for a Pd/FVO/Nb:STO heterostructure reveal a weak hysteresis in the magneto‐capacitance loops at 10 K, thus confirming magneto‐dielectric coupling. Detailed temperature and frequency‐dependent dielectric studies find that Maxwell‐Wagner‐type relaxation dominates capacitance behavior above ≈40 K due to a FVO/Nb:SrTiO3 Schottky interface. [ABSTRACT FROM AUTHOR]

[Display omitted] • Physical properties of epitaxial Fe 3- x Cr x O 4 thin films are controlled by stoichiometry. • Epitaxial thin films exhibit different cation site-occupation than their bulk counterparts. • Collinear spin structure promotes Curie temperatures above room temperature and higher magnetization. • Electron hopping is facilitated so that the electronic band gap is smaller than expected. Tuning magnetic and electronic transport properties in spinel oxides requires a faithful description between chemical composition and cation site-occupation. Here this challenge is addressed using Fe 3- x Cr x O 4 thin films grown by oxygen-assisted molecular-beam epitaxy within a wide range of composition (0.0 ≤ x ≤ 1.2). Spectroscopic measurements (e.g. , X-ray magnetic circular dichroism), refined by theoretical simulations (e.g. , crystal field multiplet), are performed to establish a quantitative link between chromium content, Fe2+/Fe3+ site-occupation and macroscopic physical properties of the layers. It is found that Fe 3- x Cr x O 4 thin films (i) delay the transition from inverse to normal spinel configuration with increasing chromium content and (ii) promote collinear spin structure, at odds with bulk material. As a result, strong antiferromagnetic interactions are preserved between spins in tetrahedral and octahedral spinel sublattices, so that chromium-rich thin films exhibit Curie temperatures above room temperature and higher magnetization. Electron hopping is also favored by this singular cation distribution and electronic band gap is smaller than expected for these thin films. The cation site-occupation is therefore a key feature to consider for applications of Fe 3- x Cr x O 4 thin films in spintronics and photocatalysis, as it enables manipulation of magnetic properties (Curie temperature and magnetization) and band gap engineering. [ABSTRACT FROM AUTHOR]

Spin-orbit torques generated by a spin current are key to magnetic switching in spintronic applications. The polarization of the spin current dictates the direction of switching required for energy-efficient devices. Conventionally, the polarizations of these spin currents are restricted to be along a certain direction due to the symmetry of the material allowing only for efficient in-plane magnetic switching. Unconventional spin-orbit torques arising from novel spin current polarizations, however, have the potential to switch other magnetization orientations such as perpendicular magnetic anisotropy, which is desired for higher density spintronic-based memory devices. Here, it is demonstrated that low crystalline symmetry is not required for unconventional spin-orbit torques and can be generated in a nonmagnetic high symmetry material, iridium dioxide (IrO 2 ), using epitaxial design. It is shown that by reducing the relative crystalline symmetry with respect to the growth direction large unconventional spin currents can be generated and hence spin-orbit torques. Furthermore, the spin polarizations detected in (001), (110), and (111) oriented IrO 2 thin films are compared to show which crystal symmetries restrict unconventional spin transport. Understanding and tuning unconventional spin transport generation in high symmetry materials can provide a new route towards energy-efficient magnetic switching in spintronic devices., (© 2023 The Authors. Advanced Materials published by Wiley-VCH GmbH.)

We report the epitaxial growth of Ni 2 MnGa thin films on MgO(100) substrate deposited at elevated temperature using direct current magnetron sputtering. The structural and texture analysis of as-deposited film reveal the (010) crystallographic orientation with seven modulated martensite phase and an epitaxial relationship of Ni 2 MnGa(010){101} || MgO(001){110} between the film and the substrate. The crystal quality of the films was analysed by performing symmetric and asymmetric rocking curve measurements and dislocation density for screw and edge dislocations is determined to be 6 × 10 9 cm −2 and 3.45 × 10 10 cm −2 , respectively. The presence of micro-twins at an angle of 45° with respect to growth direction and seven modulated nano-twins formed by adaptive modulation has been observed with transmission electron microscopy investigations. The magnetic measurements indicates magnetic field induced reorientation of martensitic variants, strong magneto-crystalline anisotropy and a very high saturation magnetization (I S ) of 61 emu g −1 in the film. [ABSTRACT FROM AUTHOR]

Thin films of Yb 3 + /Er 3 + codoped Pb(Zr,Ti)O 3 (PZT:Yb/Er) have been epitaxially grown on the SrTiO 3 buffered Si wafer by pulsed laser deposition. Strong upconversion photoluminescence was observed in the PZT:Yb/Er thin film. Using piezoresponse force microscopy, polar domains in the PZT:Yb/Er film can be reversibly switched with a phase change of 180°. Ferroelectric hysteresis loop shape with a well-saturated response was observed. The epitaxially grown lanthanide-doped PZT on silicon opens up a promising route to the integration of luminescent functional oxides on the silicon platform. [ABSTRACT FROM AUTHOR]