Interfacial Water Molecules as Agents for Phase Change Control and Proton Conductivity Enhancement in the Ammonium Vanadyl Tartrate System.

This study demonstrates the reversible structural transformation, single-crystal-to-single-crystal, of the ammonium vanadyl (L-tartrate) complex salt from the hydrate phase to the anhydrous phase. The transformation can be initiated by stimuli, such as temperature, humidity, or vacuum conditions. The hydrate and anhydrous phases exhibit a tetragonal structure (P41212), with marked differences in hydrogen bonding due to the presence or absence of one water molecule per asymmetric unit. The intricate relationship between crystal packing and intermolecular interactions in the hydrate phase was investigated by crystallographic charge density analysis revealing, at the molecular level, the reasons for the observed 5 orders of magnitude higher proton conductivity of the hydrate phase compared to that of the anhydrous phase. To gain further insight into the processes occurring at the surfaces of grain boundaries and the proton transfer mechanisms in this system, rehydration of the complex salt was carried out by using D2O instead of H2O and monitored by in situ ATR-FTIR spectroscopy. The results highlight the critical role of interfacial water molecules in driving structural transformations and influencing proton conductivity.

Strain-Driven Bidirectional Spin Orientation Control in Epitaxial High Entropy Oxide Films.

High entropy oxides (HEOs), based on the incorporation of multiple-principal cations into the crystal lattice, offer the possibility to explore previously inaccessible oxide compositions and unconventional properties. Here it is demonstrated that despite the chemical complexity of HEOs external stimuli, such as epitaxial strain, can selectively stabilize certain magneto-electronic states. Epitaxial (Co0.2 Cr0.2 Fe0.2 Mn0.2 Ni0.2 )3 O4 -HEO thin films are grown in three different strain states: tensile, compressive, and relaxed. A unique coexistence of rocksalt and spinel-HEO phases, which are fully coherent with no detectable chemical segregation, is revealed by transmission electron microscopy. This dual-phase coexistence appears as a universal phenomenon in (Co0.2 Cr0.2 Fe0.2 Mn0.2 Ni0.2 )3 O4 epitaxial films. Prominent changes in the magnetic anisotropy and domain structure highlight the strain-induced bidirectional control of magnetic properties in HEOs. When the films are relaxed, their magnetization behavior is isotropic, similar to that of bulk materials. However, under tensile strain, the hardness of the out-of-plane (OOP) axis increases significantly. On the other hand, compressive straining results in an easy OOP magnetization and a maze-like magnetic domain structure, indicating the perpendicular magnetic anisotropy. Generally, this study emphasizes the adaptability of the high entropy design strategy, which, when combined with coherent strain engineering, opens additional prospects for fine-tuning properties in oxides.

A New Class of Cluster-Matrix Nanocomposite Made of Fully Miscible Components.

Nanocomposite materials, consisting of two or more phases, at least one of which has a nanoscale dimension, play a distinctive role in materials science because of the multiple possibilities for tailoring their structural properties and, consequently, their functionalities. In addition to the challenges of controlling the size, size distribution, and volume fraction of nanometer phases, thermodynamic stability conditions limit the choice of constituent materials. This study goes beyond this limitation by showing the possibility of achieving nanocomposites from a bimetallic system, which exhibits complete miscibility under equilibrium conditions. A series of nanocomposite samples with different compositions are synthesized by the co-deposition of 2000-atom Ni-clusters and a flux of Cu-atoms using a novel cluster ion beam deposition system. The retention of the metastable nanostructure is ascertained from atom probe tomography (APT), magnetometry, and magnetotransport studies. APT confirms the presence of nanoscale regions with ≈100 at% Ni. Magnetometry and magnetotransport studies reveal superparamagnetic behavior and magnetoresistance stemming from the single-domain ferromagnetic Ni-clusters embedded in the Cu-matrix. Essentially, the magnetic properties of the nanocomposites can be tailored by the precise control of the Ni concentration. The initial results offer a promising direction for future research on nanocomposites consisting of fully miscible elements.

High Entropy Approach to Engineer Strongly Correlated Functionalities in Manganites.

Technologically relevant strongly correlated phenomena such as colossal magnetoresistance (CMR) and metal-insulator transitions (MIT) exhibited by perovskite manganites are driven and enhanced by the coexistence of multiple competing magneto-electronic phases. Such magneto-electronic inhomogeneity is governed by the intrinsic lattice-charge-spin-orbital correlations, which, in turn, are conventionally tailored in manganites via chemical substitution, charge doping, or strain engineering. Alternately, the recently discovered high entropy oxides (HEOs), owing to the presence of multiple-principal cations on a given sub-lattice, exhibit indications of an inherent magneto-electronic phase separation encapsulated in a single crystallographic phase. Here, the high entropy (HE) concept is combined with standard property control by hole doping in a series of single-phase orthorhombic HE-manganites (HE-Mn), (Gd0.25 La0.25 Nd0.25 Sm0.25 )1- x Srx MnO3 (x = 0-0.5). High-resolution transmission microscopy reveals hitherto-unknown lattice imperfections in HEOs: twins, stacking faults, and missing planes. Magnetometry and electrical measurements infer three distinct ground states-insulating antiferromagnetic, unpercolated metallic ferromagnetic, and long-range metallic ferromagnetic-coexisting or/and competing as a result of hole doping and multi-cation complexity. Consequently, CMR ≈1550% stemming from an MIT is observed in polycrystalline pellets, matching the best-known values for bulk conventional manganites. Hence, this initial case study highlights the potential for a synergetic development of strongly correlated oxides offered by the high entropy design approach.

A Unique Mechanochemical Redox Reaction Yielding Nanostructured Double Perovskite Sr2FeMoO6 With an Extraordinarily High Degree of Anti-Site Disorder.

Strontium ferromolybdate, Sr2FeMoO6, is an important member of the family of double perovskites with the possible technological applications in the field of spintronics and solid oxide fuel cells. Its preparation via a multi-step ceramic route or various wet chemistry-based routes is notoriously difficult. The present work demonstrates that Sr2FeMoO6 can be mechanosynthesized at ambient temperature in air directly from its precursors (SrO, α-Fe, MoO3) in the form of nanostructured powders, without the need for solvents and/or calcination under controlled oxygen fugacity. The mechanically induced evolution of the Sr2FeMoO6 phase and the far-from-equilibrium structural state of the reaction product are systematically monitored with XRD and a variety of spectroscopic techniques including Raman spectroscopy, 57Fe Mössbauer spectroscopy, and X-ray photoelectron spectroscopy. The unique extensive oxidation of iron species (Fe0 → Fe3+) with simultaneous reduction of Mo cations (Mo6+ → Mo5+), occuring during the mechanosynthesis of Sr2FeMoO6, is attributed to the mechanically triggered formation of tiny metallic iron nanoparticles in superparamagnetic state with a large reaction surface and a high oxidation affinity, whose steady presence in the reaction mixture of the milled educts initiates/promotes the swift redox reaction. High-resolution transmission electron microscopy observations reveal that the mechanosynthesized Sr2FeMoO6, even after its moderate thermal treatment at 923 K for 30 min in air, exhibits the nanostructured nature with the average particle size of 21(4) nm. At the short-range scale, the nanostructure of the as-prepared Sr2FeMoO6 is characterized by both, the strongly distorted geometry of the constituent FeO6 octahedra and the extraordinarily high degree of anti-site disorder. The degree of anti-site disorder ASD = 0.5, derived independently from the present experimental XRD, Mössbauer, and SQUID magnetization data, corresponds to the completely random distribution of Fe3+ and Mo5+ cations over the sites of octahedral coordination provided by the double perovskite structure. Moreover, the fully anti-site disordered Sr2FeMoO6 nanoparticles exhibit superparamagnetism with the blocking temperature T B = 240 K and the deteriorated effective magnetic moment µ = 0.055 µ B per formula unit.

Direct observation of elemental fluctuation and oxygen octahedral distortion-dependent charge distribution in high entropy oxides.

The enhanced compositional flexibility to incorporate multiple-principal cations in high entropy oxides (HEOs) offers the opportunity to expand boundaries for accessible compositions and unconventional properties in oxides. Attractive functionalities have been reported in some bulk HEOs, which are attributed to the long-range compositional homogeneity, lattice distortion, and local chemical bonding characteristics in materials. However, the intricate details of local composition fluctuation, metal-oxygen bond distortion and covalency are difficult to visualize experimentally, especially on the atomic scale. Here, we study the atomic structure-chemical bonding-property correlations in a series of perovskite-HEOs utilizing the recently developed four-dimensional scanning transmission electron microscopy techniques which enables to determine the structure, chemical bonding, electric field, and charge density on the atomic scale. The existence of compositional fluctuations along with significant composition-dependent distortion of metal-oxygen bonds is observed. Consequently, distinct variations of metal-oxygen bonding covalency are shown by the real-space charge-density distribution maps with sub-ångström resolution. The observed atomic features not only provide a realistic picture of the local physico-chemistry of chemically complex HEOs but can also be directly correlated to their distinctive magneto-electronic properties.

Creating a Ferromagnetic Ground State with Tc Above Room Temperature in a Paramagnetic Alloy through Non-Equilibrium Nanostructuring.

Materials with strong magnetostructural coupling have complex energy landscapes featuring multiple local ground states, thus making it possible to switch among distinct magnetic-electronic properties. However, these energy minima are rarely accessible by a mere application of an external stimuli to the system in equilibrium state. A ferromagnetic ground state, with Tc above room temperature, can be created in an initially paramagnetic alloy by nonequilibrium nanostructuring. By a dealloying process, bulk chemically disordered FeRh alloys are transformed into a nanoporous structure with the topology of a few nanometer-sized ligaments and nodes. Magnetometry and Mössbauer spectroscopy reveal the coexistence of two magnetic ground states, a conventional low-temperature spin-glass and a hitherto-unknown robust ferromagnetic phase. The emergence of the ferromagnetic phase is validated by density functional theory calculations showing that local tetragonal distortion induced by surface stress favors ferromagnetic ordering. The study provides a means for reaching conventionally inaccessible magnetic states, resulting in a complete on/off ferromagnetic-paramagnetic switching over a broad temperature range.

Structural and Magnetic Properties of BaFeO2.667 Synthesized by Oxidizing BaFeO2.5 Obtained via Nebulized Spray Pyrolysis.

A vacancy-ordered perovskite-type compound Ba3Fe3O8 (BaFeO2.667) was prepared by oxidizing BaFeO2.5 (P21/c) with the latter compound obtained by a spray pyrolysis technique. The structure of Ba3Fe3O8 was found to be isotypic to Ba3Fe3O7F (P21/m) and can be written as Ba3Fe3+2Fe4+1O8. Mössbauer spectroscopy and ab initio calculations were used to confirm mixed iron oxidation states, showing allocation of the tetravalent iron species on the tetrahedral site, and octahedral as well as square pyramidal coordination for the trivalent species within a G-type antiferromagnetic ordering. The uptake and release of oxygen were investigated over a broad temperature range from room temperature to 1100 °C under pure oxygen and ambient atmosphere via a combination of DTA/TG and variable temperature diffraction measurements. The compound exhibited a strong lattice enthalpy driven reduction to monoclinic and cubic BaFeO2.5 at elevated temperatures.

Magnetic properties of high entropy oxides.

High entropy oxides (HEOs) are single phase solid solutions consisting of five or more elements in equiatomic or near-equiatomic proportions incorporated into the cationic sub-lattice(s). The uniqueness of the HEOs lies in their extreme chemical complexity enveloped in a single crystallographic structure, which in many cases results in novel functionalities. From the local structure perspective, HEOs consist of an unusually large number of different metal-oxygen-metal couples. Consequently, magnetic correlations in HEOs that inherently depend on the coordination geometry, valence, spin state and type of the metal cations that are hybridized with the bridging oxygen, are naturally affected by an extreme diversity of neighboring ionic configurations. In these conditions, a complex magneto-electronic free-energy landscape in HEOs can be expected, potentially leading to stabilization of unconventional spin-electronic states. This Frontier article provides an overview of the unique magnetic features stemming from the extreme chemical disorder in HEOs along with the possible opportunities for further research and exploration of potential functionalities.

Magnetoelectric Tuning of Pinning-Type Permanent Magnets through Atomic-Scale Engineering of Grain Boundaries.

Pinning-type magnets with high coercivity at high temperatures are at the core of thriving clean-energy technologies. Among these, Sm2 Co17 -based magnets are excellent candidates owing to their high-temperature stability. However, despite intensive efforts to optimize the intragranular microstructure, the coercivity currently only reaches 20-30% of the theoretical limits. Here, the roles of the grain-interior nanostructure and the grain boundaries in controlling coercivity are disentangled by an emerging magnetoelectric approach. Through hydrogen charging/discharging by applying voltages of only ≈1 V, the coercivity is reversibly tuned by an unprecedented value of ≈1.3 T. In situ magneto-structural characterization and atomic-scale tracking of hydrogen atoms reveal that the segregation of hydrogen atoms at the grain boundaries, rather than the change of the crystal structure, dominates the reversible and substantial change of coercivity. Hydrogen reduces the local magnetocrystalline anisotropy and facilitates the magnetization reversal starting from the grain boundaries. This study opens a way to achieve the giant magnetoelectric effect in permanent magnets by engineering grain boundaries with hydrogen atoms. Furthermore, it reveals the so far neglected critical role of grain boundaries in the conventional magnetization-switching paradigm of pinning-type magnets, suggesting a critical reconsideration of engineering strategies to overcome the coercivity limits.

Magnetotransport Properties of Ferromagnetic Nanoparticles in a Semiconductor Matrix Studied by Precise Size-Selective Cluster Ion Beam Deposition.

The combination of magnetic and semiconducting properties in one material system has great potential for integration of emerging spintronics with conventional semiconductor technology. One standard route for the synthesis of magnetic semiconductors is doping of semiconductors with magnetic atoms. In many semiconductor-magnetic-dopant systems, the magnetic atoms form precipitates within the semiconducting matrix. An alternative and controlled way to realize such nanocomposite materials is the assembly by co-deposition of size-selected cluster ions and a semiconductor. Here we follow the latter approach to demonstrate that this fabrication route can be used to independently study the influence of cluster concentration and cluster size on magneto-transport properties. In this case we study Fe clusters composed of approximately 500 or 1000 atoms soft-landed into a thermally evaporated amorphous Ge matrix. The analysis of field and temperature dependent transport shows that tunneling processes affected by Coulomb blockade dominate at low temperatures. The nanocomposites show saturating tunneling magnetoresistance, additionally superimposed by at least one other effect not saturating upon the maximum applied field of 6 T. The nanocomposites' resistivity and the observed tunneling magnetoresistance depend exponentially on the average distance between cluster surfaces. On the contrary, there is no notable influence of the cluster size on the tunneling magnetoresistance.

Structural insights into metal-metalloid glasses from mass spectrometry.

Despite being studied for nearly 50 years, smallest chemically stable moieties in the metallic glass (MG) could not be found experimentally. Herein, we demonstrate a novel experimental approach based on electrochemical etching of amorphous alloys in inert solvent (acetonitrile) in the presence of a high voltage (1 kV) followed by detection of the ions using electrolytic spray ionization mass spectrometry (ESI MS). The experiment shows stable signals corresponding to Pd, PdSi and PdSi2 ions, which emerges due to the electrochemical etching of the Pd80Si20 metallic glass electrode. These fragments are observed from the controlled dissolution of the Pd80Si20 melt-spun ribbon (MSR) electrode. Annealed electrode releases different fragments in the same experimental condition. These specific species are expected to be the smallest and most stable chemical units from the metallic glass which survived the chemical dissolution and complexation (with acetonitrile) process. Theoretically, these units can be produced from the cluster based models for the MG. Similar treatment on Pd40Ni40P20 MSR resulted several complex peaks consisting of Pd, Ni and P in various combinations suggesting this can be adopted for any metal-metalloid glass.

Nanowire facilitated transfer of sensitive TEM samples in a FIB.

We introduce a facile approach to transfer thin films and other mechanically sensitive TEM samples inside a FIB with minimal introduction of stress and bending. The method is making use of a pre-synthetized flexible freestanding Ag nanowire attached to the tip of a typical tungsten micromanipulator inside the FIB. The main advantages of this approach are the significantly reduced stress-induced bending during transfer and attachment of the TEM sample, the very short time required to attach and cut the nanowire, the operation at very low dose and ion current, and only using the e-beam for Pt deposition during the transfer of sensitive TEM samples. This results in a reduced sample preparation time and reduced exposure to the ion beam or e-beam for Pt deposition during the sample preparation and thus also reduced contamination and beam damage. The method was applied to a number of thin films and different TEM samples in order to illustrate the advantageous benefits of the concept. In particular, the technique has been successfully tested for the transfer of a thin film onto a MEMS heating chip for in situ TEM experiments.

Lithium containing layered high entropy oxide structures.

Layered Delafossite-type Lix(M1M2M3M4M5…Mn)O2 materials, a new class of high-entropy oxides, were synthesized by nebulized spray pyrolysis and subsequent high-temperature annealing. Various metal species (M = Ni, Co, Mn, Al, Fe, Zn, Cr, Ti, Zr, Cu) could be incorporated into this structure type, and in most cases, single-phase oxides were obtained. Delafossite structures are well known and the related materials are used in different fields of application, especially in electrochemical energy storage (e.g., LiNixCoyMnzO2 [NCM]). The transfer of the high-entropy concept to this type of materials and the successful structural replication enabled the preparation of novel compounds with unprecedented properties. Here, we report on the characterization of a series of Delafossite-type high-entropy oxides by means of TEM, SEM, XPS, ICP-OES, Mössbauer spectroscopy, XRD including Rietveld refinement analysis, SAED and STEM mapping and discuss about the role of entropy stabilization. Our experimental data indicate the formation of uniform solid-solution structures with some Li/M mixing.

Giant voltage-induced modification of magnetism in micron-scale ferromagnetic metals by hydrogen charging.

Owing to electric-field screening, the modification of magnetic properties in ferromagnetic metals by applying small voltages is restricted to a few atomic layers at the surface of metals. Bulk metallic systems usually do not exhibit any magneto-electric effect. Here, we report that the magnetic properties of micron-scale ferromagnetic metals can be modulated substantially through electrochemically-controlled insertion and extraction of hydrogen atoms in metal structure. By applying voltages of only ~ 1 V, we show that the coercivity of micrometer-sized SmCo5, as a bulk model material, can be reversibly adjusted by ~ 1 T, two orders of magnitudes larger than previously reported. Moreover, voltage-assisted magnetization reversal is demonstrated at room temperature. Our study opens up a way to control the magnetic properties in ferromagnetic metals beyond the electric-field screening length, paving its way towards practical use in magneto-electric actuation and voltage-assisted magnetic storage.

Configurable Resistive Response in BaTiO3 Ferroelectric Memristors via Electron Beam Radiation.

Ferroelectric oxide memristors are currently in the highlights of a thriving area of research aiming at the development of nonvolatile, adaptive memories for applications in neuromorphic computing. However, to date a precise control of synapse-like functionalities by adjusting the interplay between ferroelectric polarization and resistive switching processes is still an ongoing challenge. Here, it is shown that by means of controlled electron beam radiation, a prototypical ferroelectric film of BaTiO3 can be turned into a memristor with multiple configurable resistance states. Ex situ and in situ analyses of current/voltage characteristics upon electron beam exposure confirm the quasi-continuous variation of BaTiO3 resistance up to two orders of magnitude under the typical experimental conditions employed in electron beam patterning and characterization techniques. These results demonstrate an unprecedented effective route to locally and scalably engineering multilevel ferroelectric memristors via application of moderate electron beam radiation.

Voltage-Control of Magnetism in All-Solid-State and Solid/Liquid Magnetoelectric Composites.

The control of magnetism by means of low-power electric fields, rather than dissipative flowing currents, has the potential to revolutionize conventional methods of data storage and processing, sensing, and actuation. A promising strategy relies on the utilization of magnetoelectric composites to finely tune the interplay between electric and magnetic degrees of freedom at the interface of two functional materials. Albeit early works predominantly focused on the magnetoelectric coupling at solid/solid interfaces; however, recently there has been an increased interest related to the opportunities offered by liquid-gating techniques. Here, a comparative overview on voltage control of magnetism in all-solid-state and solid/liquid composites is presented within the context of the principal coupling mediators, i.e., strain, charge carrier doping, and ionic intercalation. Further, an exhaustive and critical discussion is carried out, concerning the suitability of using the common definition of coupling coefficient α C = Δ M Δ E to compare the strength of the interaction between electricity and magnetism among different magnetoelectric systems.

Cluster-Assembled Nanocomposites: Functional Properties by Design.

The unique functional properties of nanocomposites meet many of the material requirements sought after in numerous applications of today's high-tech industry. This, in turn, inspires material scientists to devise new methods that can further expand the palette of available nanocomposites. Precise control over the chemistry, morphology, and microstructure of nanocomposites' constituents promises the eventual ability to design any composite material for any specific requirement. However, today's synthesis methods still lack the ability to simultaneously control all chemical, morphological, and microstructural features of nanocomposites in a one-step process. Here, an alternative approach to fabricate fully tailorable nanocomposites under well-defined conditions is described. In particular, this progress report focuses on the combination of cluster ion beam and thin-film deposition technologies to fabricate cluster-assembled nanocomposites via codeposition of cluster ions and matrix materials. Emphasis is given to the state-of-the-art cluster deposition system, designed and built by our research group, as well as to its unique abilities. Moreover, case studies on two cluster-assembled nanocomposite material systems (Fe/Agm and Fe/Crm ) prepared with this method are presented. Finally, an outlook on research directions for cluster-assembled nanocomposites is discussed.

Combination of pulsed laser ablation and inert gas condensation for the synthesis of nanostructured nanocrystalline, amorphous and composite materials.

A new instrument combining pulsed laser ablation and inert gas condensation for the production of nanopowders is presented. It is shown that various nanostructured materials, such as regular metallic, semiconducting, insulating materials, complex high entropy alloys, amorphous alloys, composites and oxides can be synthesized. The unique variability of the experimental set-up is possible due to the reproducible control of laser power (pulse energy and repetition rate), laser ablation pattern on the target, and experimental conditions during the inert gas condensation, all of which can be controlled and optimized independently. Microstructure analysis of the as-prepared composite and amorphous Ni60Nb40 nanopowders establishes the instrument's ability for the synthesis of materials with unique compositions and atomic structure. It is further shown that small variations of the synthesis parameters can influence materials properties of the final product, in terms of particle size, composition and properties. As an example, the laser power has been used to control the magnetic properties of amorphous Ni60Nb40 nanopowders. A few selected examples of the manifold possibilities of the new synthesis apparatus are presented in this report together with detailed structural characterization of the produced nanopowders.

Observation of electrochemically active Fe3+/Fe4+ in LiCo0.8Fe0.2MnO4 by in situ Mössbauer spectroscopy and X-ray absorption spectroscopy.

LiCo0.8Fe0.2MnO4 has been investigated as an active material for the positive electrode in lithium-ion batteries (LIBs) with a discharge potential of around 5 V (vs. Li+|Li). After synthesis by a Pechini based sol-gel route, the structural and morphological properties have been investigated by X-ray diffraction, scanning electron microscopy, 7Li MAS NMR spectroscopy, and 57Fe Mössbauer spectroscopy. With galvanostatic cycling, it was possible to obtain a specific discharge capacity of 117 mA h g-1, which is more than 80% of the theoretical capacity. The lithium extraction/insertion mechanism has been characterized by in situ synchrotron powder diffraction. The reversible oxidation process of Fe3+ to Fe4+ has been observed by in situ Mössbauer spectroscopy and in situ XAS measurements.