Nanostructured thin films of the La0.7Sr0.3MnO3 manganite obtained using the extraction pyrolytic method

The extraction pyrolytic method is used to fabricate thin (100–300 nm) films of the lanthanum manganites La_0.7Sr_0.3MnO₃ on fused silica substrates. The films are deposited on the substrate using the alternate sessions of the centrifuging of solution and pyrolysis. The annealing of thin films at temperatures of greater than 650°C yields the single-phase La_0.7Sr_0.3MnO₃ material. It is demonstrated that the annealing temperature substantially affects the magnetic properties of the resulting films: the films exhibit the properties of spin glasses and ferromagnetic properties at temperatures of less than 700°C and greater than 700°C, respectively.     

Controllable nanostructures on La0.7Sr0.3MnO3 thin film surfaces formed by scanning tunneling microscopy

Nanoscale structuring on La_0.7Sr_0.3MnO₃ (LSMO) thin film surfaces has been performed by scanning tunneling microscopy (STM) under ambient conditions. From line etching experiments we found that the line-depth increases in a stepwise fashion with increasing bias voltage. It also increases with decreasing scan speed and increasing scan repetition. We observed that the line-depth is an integral multiple of the LSMO out-of-plane lattice constant about 0.4 nm. Lateral structure with minimum feature size of 1 nm is possible to obtain. In addition, a four-level inverse-pyramid structure has been created on LSMO thin film surfaces. Our work shows the feasibility of using STM to fabricate controllable and complex nanostructures in LSMO thin film.     

Large,Temperature‐Tunable Low‐Field Magnetoresistance in La0.7Sr0.3MnO3:NiO Nanocomposite Films Modulated by Microstructures

Magnetic properties and low‐field magnetoresistance (LFMR) in La_0.7Sr_0.3MnO₃ (LSMO):NiO nanocomposite films grown on SrTiO₃ (001) substrates, which are shown to be tunable with different microstructures, are investigated. The LSMO:NiO nanocomposite films with NiO volume ratio of 50% have a checkerboard‐like structure and show a large LFMR in a temperature range from 200 to 300 K (≈17% at 250 K with a magnetic field of 1 T). As the NiO volume ratio is increased to 70%, a nano‐columnar structure formed in the films. Their LFMR is significantly enhanced at a wide temperature range of 10–210 K. The highest value of LFMR with 41% is achieved at 10 K in a magnetic field of 1 T. The enhanced LFMR can be considered to result from the electron scattering at the ferromagnetic LSMO/NiO interfaces and magnetic tunnel junctions (MTJs) of LSMO/NiO/LSMO at the nanometer scale. These results demonstrate that large and tunable LFMR from low temperature to room temperature can be realized by controlling the microstructures in the epitaxial La_0.7Sr_0.3MnO₃:NiO nano­composite thin films, which will be expected to be applied in the devices using for a wide temperature range.     

Tunable Low‐Field Magnetoresistance in (La0.7Sr0.3MnO3)0.5:(ZnO)0.5 Self‐Assembled Vertically Aligned Nanocomposite Thin Films

Tunable and enhanced low‐field magnetoresistance (LFMR) is observed in epitaxial (La_0.7Sr_0.3MnO₃)_0.5:(ZnO)_0.5 (LSMO:ZnO) self‐assembled vertically aligned nanocomposite (VAN) thin films, which have been grown on SrTiO₃ (001) substrates by pulsed laser deposition (PLD). The enhanced LFMR properties of the VAN films reach values as high as 17.5% at 40 K and 30% at 154 K. They can be attributed to the spin‐polarized tunneling across the artificial vertical grain boundaries (GBs) introduced by the secondary ZnO nanocolumns and the enhancement of spin fluctuation depression at the spin‐disordered phase boundary regions. More interestingly, the vertical residual strain and the LFMR peak position of the VAN films can be systematically tuned by changing the deposition frequency. The tunability of the physical properties is associated with the vertical phase boundaries that change as a function of the deposition frequency. The results suggest that the tunable artificial vertical GB and spin‐disordered phase boundary in the unique VAN system with vertical ferromagnetic‐insulating‐ferromagnetic (FM‐I‐FM) structure provides a viable route to manipulate the low‐field magnetotransport properties in VAN films with favorable epitaxial quality.     

Effects of the incorporation of Fe on the electromagnetic and microwave absorption performance of La0.7Sr0.3MnO3±δ

The pure perovskite structure La_0.7Sr_0.3Mn_1−χFe_χO_3±δ (χ=0, 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, and 0.35) powders were fabricated by the traditional solid state reaction method. Effects of the incorporation of Fe into La_0.7Sr_0.3MnO_3±δ on the permittivity and permeability were examined, and the microwave absorption performance was also investigated. The electromagnetic loss was notably enhanced, and the microwave absorption performance was improved after Fe doping. The absorbing peak shifted to the lower frequency after Fe doping. The 20 at% Fe-doped La_0.7Sr_0.3MnO_3±δ powders had the best microwave absorption property. The maximum reflection loss was −27.67 dB at 10.97 GHz, and the −6 dB absorbing bandwidth was 6.81 GHz with a matching thickness of 2 mm.     

Ferroelectric Self‐Poling,Switching, and Monoclinic Domain Configuration in BiFeO3 Thin Films

Self‐poling of ferroelectric films, i.e., a preferred, uniform direction of the ferroelectric polarization in as‐grown samples is often observed yet poorly understood despite its importance for device applications. The multiferroic perovskite BiFeO₃, which crystallizes in two distinct structural polymorphs depending on applied epitaxial strain, is well known to exhibit self‐poling. This study investigates the effect of self‐poling on the monoclinic domain configuration and the switching properties of the two polymorphs of BiFeO₃ (R′ and T′) in thin films grown on LaAlO₃ substrates with slightly different La_0.3Sr_0.7MnO₃ buffer layers. This study shows that the polarization state formed during the growth acts as “imprint” on the polarization and that switching the polarization away from this self‐poled direction can only be done at the expense of the sample's monoclinic domain configuration. The observed reduction of the monoclinic domain size is largely reversible; hence, the domain size is restored when the polarization is switched back to its original orientation. This is a direct consequence of the growth taking place in the polar phase (below T_c). Switching the polarization away from the preferred configuration, in which defects and domain patterns synergistically minimize the system's energy, leads to a domain state with smaller (and more highly strained and distorted) monoclinic domains.     

Tailoring of LaxSr1‐xCoyFe1‐yO3‐δ Nanostructure by Pulsed Laser Deposition

Pulsed Laser Deposition (PLD) was used to prepare thin films with the nominal composition La_0.58Sr_0.4Co_0.2Fe_0.8O_3‐δ (LSCF). The thin film microstructure was investigated as a function of PLD deposition parameters such as: substrate temperature, ambient gas pressure, target‐to‐substrate distance, laser fluence and frequency. It was found that the ambient gas pressure and the substrate temperature are the key PLD process parameters determining the thin film micro‐ and nanostructure. A map of the LSCF film nanostructures is presented as a function of substrate temperature (25–700 °C) and oxygen background pressure (0.013–0.4 mbar), with film structures ranging from fully dense to highly porous. Fully crystalline, dense, and crack‐free LSCF films with a thickness of 300 nm were obtained at an oxygen pressure lower than 0.13 mbar at a temperature of 600 °C. The obtained knowledge on the structure allows for tailoring of perovskite thin film nanostructure, e.g., for solid oxide fuel cell cathodes. A simple geometrical model is proposed, allowing estimation of the catalytic active surface area of the prepared thin films. It is shown that voids at columnar grain boundaries can result in an increase of the surface area by approximately 25 times, when compared to dense flat films.     

Modulation of Spin Dynamics via Voltage Control of Spin‐Lattice Coupling in Multiferroics

Motivated by the most recent progresses in both magnonics (spin dynamics) and multiferroics fields, this work aims at magnonics manipulation by the magnetoelectric coupling effect. Here, voltage control of magnonics, particularly the surface spin waves, is achieved in La_0.7Sr_0.3MnO₃/0.7Pb(Mg_1/3Nb_2/3)O₃‐0.3PbTiO₃ multiferroic heterostructures. With the electron spin resonance method, a large 135 Oe shift of surface spin wave resonance (≈7 times greater than conventional voltage‐induced ferromagnetic resonance shift of 20 Oe) is determined. A model of the spin‐lattice coupling effect, i.e., varying exchange stiffness due to voltage‐induced anisotropic lattice changes, has been established to explain experiment results with good agreement. Additionally, an “on” and “off” spin wave state switch near the critical angle upon applying a voltage is created. The modulation of spin dynamics by spin‐lattice coupling effect provides a platform for realizing energy‐efficient, tunable magnonics devices.     

Strain‐Mediated Inverse Photoresistivity in SrRuO3/La0.7Sr0.3MnO3 Superlattices

In the pursuit of novel functionalities by utilizing the lattice degree of freedom in complex oxide heterostructure, the control mechanism through direct strain manipulation across the interfaces is still under development, especially with various stimuli, such as electric field, magnetic field, light, etc. In this study, the superlattices consisting of colossal‐magnetoresistive manganites La_0.7Sr_0.3MnO₃ (LSMO) and photostrictive SrRuO₃ (SRO) have been designed to investigate the light‐dependent controllability of lattice order in the corresponding functionalities and rich interface physics. Two substrates, SrTiO₃ (STO) and LaAlO₃ (LAO), have been employed to provide the different strain environments to the superlattice system, in which the LSMO sublayers exhibit different orbital occupations. Subsequently, by introducing light, we can modulate the strain state and orbital preference of LSMO sublayers through light‐induced expansion of SRO sublayers, leading to surprisingly opposite changes in photoresistivity. The observed photoresistivity decreases in the superlattice grown on STO substrate while increases in the superlattice grown on LAO substrate under light illumination. This work has presented a model system that demonstrates the manipulation of orbital–lattice coupling and the resultant functionalities in artificial oxide superlattices via light stimulus.     

Spontaneous Outcropping of Self‐Assembled Insulating Nanodots in Solution‐Derived Metallic Ferromagnetic La0.7Sr0.3MnO3 Films

A new mechanism is proposed for the generation of self‐assembled nanodots at the surface of a film based on spontaneous outcropping of the secondary phase of a nanocomposite epitaxial film. Epitaxial self‐assembled Sr–La oxide insulating nanodots are formed through this mechanism at the surface of an epitaxial metallic ferromagnetic La_0.7Sr_0.3MnO₃ (LSMO) film grown on SrTiO₃ from chemical solutions. TEM analysis reveals that, underneath the La–Sr oxide (LSO) nanodots, the film switches from the compressive out‐of‐plane stress component to a tensile one. It is shown that the size and concentration of the nanodots can be tuned by means of growth kinetics and through modification of the La excess in the precursor chemical solution. The driving force for the nanodot formation can be attributed to a cooperative effect involving the minimization of the elastic strain energy and a thermodynamic instability of the LSMO phase against the formation of a Ruddelsden–Popper phase Sr₃Mn₄O₇ embedded in the film, and LSO surface nanodots. The mechanism can be described as a generalization of the classical Stranski–Krastanov growth mode involving phase separation. LSO islands induce an isotropic strain to the LSMO film underneath the island which decreases the magnetoelastic contribution to the magnetic anisotropy.     

Low‐Field Magnetoresistance in La0.67Sr0.33MnO3:ZnO Composite Film

A nanocomposite film of La_0.67Sr_0.33MnO₃ (LSMO):ZnO is synthesized by depositing LSMO solution on a vertical array of ZnO nanorods grown on (0001) Al₂O₃ substrate. The magnetic behavior of the composite film differs from that of a pure LSMO film, possibly due to smaller grain size in the composite, small amount of Zn doping, or the presence of nonmagnetic ZnO phase near the LSMO grain boundaries. Magnetotransport measurements show that the low‐field magnetoresistance (LFMR) of the nanocomposite film is significantly enhanced as compared to that observed for pure LSMO film. The highest value of the LFMR of the nanocomposite film at 10 K is –23.9% with a magnetic field of 0.5 T applied parallel to the current.     

Structural Characterization and Magnetic and Electrical Properties of La0.7Ca0.3MnO3–La1.5Sr0.5NiO4 Nanocomposites

Nanocomposite samples of (1 ? x)La_0.7Ca_0.3MnO₃ + xLa_1.5Sr_0.5NiO₄ (x = 0 to 0.3) were synthesized by a combination of the mechanical milling and solid-state reaction methods. X-ray diffraction analyses and magnetic measurements indicated that no reaction occurred between La_0.7Ca_0.3MnO₃ (LCMO) and La_1.5Sr_0.5NiO₄ (LSNO). The Curie temperature (T _C) was almost independent of x, while the metal–insulator transition temperature (T _MI) shifted from 251 K for x = 0.0 to 65 K for x = 0.2. The samples with x ≥ 0.25 exhibited insulating behavior in the temperature range from 30 K to 300 K. Addition of LSNO substantially increased the resistivity of the composites. This is attributed to enhanced magnetic disorder at LCMO grain boundaries due to the addition of LSNO. The temperature dependence of the resistivity, ρ(T), could be described by the phenomenological percolation model of phase segregation. Fitting the experimental ρ(T) data in the temperature range of 30 K to 300 K indicated that the activation energy of the composites increases as a function of the LSNO doping concentration (x).     

On the Role of the Sr3−xCaxAl2O6 Sacrificial Layer Composition in Epitaxial La0.7Sr0.3MnO3 Membranes

The possibility to fabricate freestanding single crystal complex oxide films has raised enormous interest to be integrated in next-generation electronic devices envisaging distinct and novel properties that can deliver unprecedented performance improvement compared to traditional semiconductors. The use of the water-soluble Sr₃Al₂O₆ (SAO) sacrificial layer to detach the complex oxide film from the growth substrate has significantly expanded the complex oxide perovskite membranes library. Nonetheless, the extreme water sensitivity of SAO hinders its manipulation in ambient conditions and restricts the deposition approaches to those using high vacuum. This study presents a pioneering study on the role of Ca-substitution in solution processed SAO (Sr_3−xCa_xAl₂O₆ with x ⩽ 3) identifying a noticeable improvement on surface film crystallinity preserving a smooth surface morphology while favoring the manipulation in a less-restricted ambient conditions. Then, the study focuses on the effect of the sacrificial composition on the subsequent ex situ deposition of La_0.7Sr_0.3MnO₃ (LSMO) by pulsed laser deposition, to obtain epitaxial films with a variable degree of strain. Finally, epitaxial and strain-free LSMO membranes with metal-insulator transition at 290 K are delivered. This study offers a hybrid and versatile approach to prepare and easily manipulate crystalline perovskite oxide membranes by facilitating ex situ growth on SAO-based sacrificial layer.     

The Misfit Dislocation Core Phase in Complex Oxide Heteroepitaxy

Misfit dislocations form self‐organized nanoscale linear defects exhibiting their own distinct structural, chemical, and physical properties which, particularly in complex oxides, hold a strong potential for the development of nanodevices. However, the transformation of such defects from passive into potentially active functional elements necessitates a deep understanding of the particular mechanisms governing their formation. Here, different atomic resolution imaging and spectroscopic techniques are combined to determine the complex structure of misfit dislocations in the perovskite type La_0.67Sr_0.33MnO₃/LaAlO₃ heteroepitaxial system. It is found that while the position of the film–substrate interface is blurred by cation intermixing, oxygen vacancies selectively accumulate at the tensile region of the dislocation strain field. Such accumulation of vacancies is accompanied by the reduction of manganese cations in the same area, inducing chemical expansion effects, which partly accommodate the dislocation strain. The formation of oxygen vacancies is only partially electrically compensated and results in a positive net charge q ≈ +0.3 ± 0.1 localized in the tensile region of the dislocation, while the compressive region remains neutral. The results highlight a prototypical core model for perovskite‐based heteroepitaxial systems and offer insights for predictive manipulation of misfit dislocation properties.     

Fabrication and properties of multilayer Sr0.7Ba0.3TiO3 ceramic varistors

The fabrication and properties of the multilayer Sr_0.7Ba_0.3TiO₃ ceramic varistors made from low-fire Sr_0.7Ba_0.3TiO₃-based ceramic materials doped with small amounts of additives such as Nb₂O₅ and Li₂CO₃ are described. After being sintered at 1100° C in a reducing atmosphere and annealed at 900° C in air, the varistors exhibited the following characteristics: varistor voltage V_1mA, 10 v; nonlinear coefficientα, 15. The varistor voltage was almost unchanged against a supplied surge energy of 300 J/cm³. The varistors had excellent capacitor characteristics. The capacitance and loss factor were 80 nF and 2%, respectively.     

Engineering Large Anisotropic Magnetoresistance in La0.7Sr0.3MnO3 Films at Room Temperature

The magnetoresistance (MR) effect is widely used in technologies that pervade the world, from magnetic reading heads to sensors. Diverse contributions to MR, such as anisotropic, giant, tunnel, colossal, and spin‐Hall, are revealed in materials depending on the specific system and measuring configuration. Half‐metallic manganites hold promise for spintronic applications but the complexity of competing interactions has not permitted the understanding and control of their magnetotransport properties to enable the realization of their technological potential. This study reports on the ability to induce a dominant switchable magnetoresistance in La_0.7Sr_0.3MnO₃ epitaxial films at room temperature (RT). By engineering an extrinsic magnetic anisotropy, a large enhancement of anisotropic magnetoresistance (AMR) is achieved which at RT leads to signal changes much larger than the other contributions such as the colossal magnetoresistance. The dominant extrinsic AMR exhibits large variation in the resistance in low field region, showing high sensitivity to applied low magnetic fields. These findings have a strong impact on the real applications of manganite‐based devices for the high‐resolution low field magnetic sensors or spintronics.     

Patterning of Epitaxial Perovskites from Micro and Nano Molded Stencil Masks

A process is developed that combines soft lithographic molding with pulsed laser deposition (PLD) to make heteroepitaxial patterns of functional perovskite oxide materials. Micro‐ and nanostructures of sacrificial ZnO are made by micro molding in capillaries (MiMiC) and nano transfer molding, respectively, and used to screen the single crystalline substrates during subsequent PLD. ZnO is used because of its compatibility with the high temperatures reached during PLD and because of the ease of its removal after use by benefiting from its amphoteric nature. Sub‐micrometer sized lines of La_0.67Sr_0.33MnO₃ are made by the transfer molding approach, preserving the anisotropic features expected for a fully oriented thin film and taking account for the magnetostatic contribution from the line shapes. Different patterns of SrRuO₃ are made with lateral dimensions of a few micrometers having individual features for which electrical isolation is illustrated. The bottom‐up soft lithographic methods can be compliantly utilized for making epitaxial structures of various shapes and sizes in the μm down to the nm range, and offer unique opportunities for fundamental studies as well as for realizing technological applications.     

Growth of single unit-cell superconducting La2−xSrxCuO4 films

We have developed an approach to grow high quality ultra-thin films of La_2−xSr_xCuO₄ with molecular beam epitaxy, by adding a homoepitaxial buffer layer in order to minimize the degradation of the film structure at the interface. The advantage of this method is to enable a further reduction of the minimal thickness of a superconducting La_1.9Sr_0.1CuO₄ film. The main result of our work is that a single unit cell (only two copper oxide planes) grown on a SrLaAlO₄ substrate exhibits a superconducting transition at 12.5 K (zero resistance) and an in-plane magnetic penetration depth λ_ab(0)=535 nm.     

Microstructure and Superconductivity of La1.85Sr0.15CuO4 Nanowire Arrays

Superconducting La_1.85Sr_0.15CuO₄ nanowire arrays are successfully synthesized through a sol–gel method combined with porous alumina as a morphology‐directing hard template for the first time. The morphology, structure, and composition of the as‐prepared nanowire arrays are characterized by field‐emission scanning electron microscopy, transmission electron microscopy, high‐resolution transmission electron microscopy, X‐ray diffraction, and energy‐dispersive X‐ray spectroscopy. These experimental results indicate that the nanowires are well crystallized with an approximately uniform diameter of about 30 nm. The superconducting transition temperature T_c (ca. 30 K) of the annealed nanowires is lower than that in bulk La_1.85Sr_0.15CuO₄. It is suggested that this superconductivity suppression is derived from the weakening of in‐plane hybridization in the nanowire system.     

Template‐Assisted Synthesis of Metallic 1T′‐Sn0.3W0.7S2 Nanosheets for Hydrogen Evolution Reaction

Crystal phase control still remains a challenge for the precise synthesis of 2D layered metal dichalcogenide (LMD) materials. The T′ phase structure has profound influences on enhancing electrical conductivity, increasing active sites, and improving intrinsic catalytic activity, which are urgently needed for enhancing hydrogen evolution reaction (HER) activity. Theoretical calculations suggest that metastable T′ phase 2D Sn_1?xW_xS₂ alloys can be formed by combining W with 1T tin disulfide (SnS₂) as a template to achieve a semiconductor‐to‐metallic transition. Herein, 2D Sn_1?xW_xS₂ alloys with varying x are prepared by adjusting the molar ratio of reactants via hydrothermal synthesis, among which Sn_0.3W_0.7S₂ displays a maximum of concentration of 81% in the metallic phase and features a distorted octahedral‐coordinated metastable 1T′ phase structure. The obtained 1T′‐Sn_0.3W_0.7S₂ has high intrinsic electrical conductivity, lattice distortion, and defects, showing a prominently improved HER catalytic performance. Metallic Sn_0.3W_0.7S₂ coupled with carbon black exhibits at least a 215‐fold improvement compared to pristine SnS₂. It has excellent long‐term durability and HER activity. This work reveals a general phase transition strategy by using T phase materials as templates and merging heteroatoms to achieve synthetic metastable phase 2D LMDs that have a significantly improved HER catalytic performance.