An updated state-of-the art in complex oxide science

Título: An updated state-of-the art in complex oxide science
Subtítulo: Oxide thin films and related interfaces display a plethora of functionalities and emerging properties that call for a pivotal role in electronics beyond Moore’s dictate.
Authors:

Towards Oxide Electronics: a Roadmap
Mariona Coll, Josep Fontcuberta et al.
Appl. Surf. Science
482, 1, 2019
DOI: 10.1016/j.apsusc.2019.03.312

Figure: Towards Oxide Electronics: a Roadmap is a compendium of perspective papers on the state and opportunities of oxides in electronics in five specific fields: data storage and computing, quantum materials and emergent phenomena, spintronics, energy harvesting and power electronics. 

''Towards Oxide Electronics: a Roadmap", elaborated within the framework of the TO-BE COST Action, addresses crucial questions related to development and implementation of these materials in emerging technologies. Namely: What is the future role of oxide thin film in modern technologies? How far are oxides from taking this role? Which are the competing technologies? Which are the hurdles presently preventing the diffusion of marketable applications? Which are the chances these hurdles can be overcome? Which are the advancement in synthesis and characterization methods that can help us facing them?

We focused explicitly on thin-film-based applications, and when possible on epitaxial films. Most of the applications presented are expected to impact the fields of ICT and Energy. In times in which mankind is facing the extreme ambition of artificially emulating the computing power of a human brain, exceeding the exaFLOP range and corresponding, with the present energy-hungry electronic technologies, to a power consumption in the several GW range, the two fields are related as never before. Selected topics of this Roadmap include: (a) Insights from theory and modelling, (b) advanced growth, nanofabrication and characterization techniques and (c) applications on: data storage and computing, optics and plasmonics, magnonics, energy conversion and harvesting, and power electronics. This Roadmap is a valuable tool as a collective self-analysis of the oxide electronics community, providing an updated picture of the state-of-the-art in the field, and as a dissemination tool, helpful to whoever is willing to spread knowledge about oxide science and to attract towards this exciting field of research new young scientists and further public and private resources.

The paper contains 28 contributions by about 56 authors and its elaboration has been chaired by Mariona Coll and Josep Fontcuberta from ICMAB, together with Fabio Miletto Granozio and Nini Pryds.

Cation arrangement on perovskites films

Título: Cation arrangement on perovskites films
Authors:

Spontaneous cationic ordering in chemical solution-grown La2CoMnO6 double perovskite thin films
Hailin Wang, Jaume Gazquez, Carlos Frontera, Matthew F. Chisholm, Alberto Pomar, Benjamin Martinez, Narcis Mestres
NPG Asia Materials 11:44, 2019
DOI: 10.1038/s41427-019-0144-8

Figure: From left to right, Z-contrast image of the LCMO film, ADF image, atomic maps of La M, Co L, and Mn L absorption edges and RGB map produced by overlaying the Co (in red) and Mn (in green) elemental maps. The spectrum image was acquired along the [110]-zone axis for visualizing the B-site ordering.

Double perovskite oxides are of interest because of their physical properties; however, these properties are strongly dependent on the ordered arrangement of cations in the double perovskite structure. We show that the slow growth rates close to thermodynamic equilibrium conditions in chemical solution methods are advantageous promoting spontaneous B-site cationic ordering.

Combining old and modern tools to predict how materials respond to external stimuli

Título: Combining old and modern tools to predict how materials respond to external stimuli
Subtítulo: Combining the “long-wave method”—a mainstay of condensed-matter theory since the 1950s—with modern electronic-structure techniques allows for highly accurate predictions of physical responses of crystals to nonhomogenous external perturbations.
Authors:

First-principles theory of spatial dispersion: Dynamical quadrupoles and flexoelectricity
Miquel Royo and Massimiliano Stengel
Physical Review X 9, 2, 021050-22, 2019.
DOI: 10.1103/PhysRevX.9.021050

Figure: Charge density response to the displacement of the two dissimilar atoms in the silicon lattice. The density distribution clearly shows a quadrupolar, i.e. induced by spatial-dispersion effects, character. 

 

In materials science and engineering, scientists often assume that crystals respond locally to an externally applied perturbation such as a strain or an electromagnetic field. Microscopically, however, the effects of the perturbation always propagate over a neighborhood around the point of application. At the macroscopic level, this means that the material response depends on gradients of the applied field, which is known as spatial dispersion. While these effects are generally small, they have attracted increasing interest in the past few years. Notable examples are flexoelectricity, the electrical voltage generated by a flexural deformation, and natural optical activity, the rotation of transmitted light polarization by some crystals. Here, we establish a general and efficient quantum-mechanical formalism to address this broad class of problems.

Density-functional perturbation theory (DFPT) is nowadays the state-of-the-art method to accurately calculate from first principles how materials respond to external stimuli. Our new approach consists of incorporating the long-wave method, a mainstay of condensed-matter theory since the early days of quantum mechanics, into the modern tools of DFPT. This allows one to access a broad range of spatial-dispersion properties at a surprisingly small computational cost and with unprecedented accuracy. We demonstrate our method, which we have implemented in a publicly distributed package (abinit), by calculating the flexoelectric tensor and the “dynamical quadrupoles” (i.e., the quadrupolar moment of the charge-density response to an atomic displacement) of several materials. We obtain excellent agreement with earlier studies, whenever available.

Our “long-wave DFPT” significantly extends the scopes and capabilities of perturbative electronic-structure approaches and opens the door to the systematic exploration of a vast range of gradient-related physical properties.

Electron microscopy reveales rotational polarization domains

Título: Electron microscopy reveales rotational polarization domains
Authors:

Rotational Polarization Nanotopologies in BaTiO3/SrTiO3 Superlattices
Saúl Estandía, Florencio Sánchez, Matthew F. Chisholm, Jaume Gázquez
Nanoscale 11, 21275-21283, 2019
DOI: 10.1039/c9nr08050c

Figure: Scanning transmission electron microscopy HAADF image (left panel) of a 6x (10 u.c. BaTiO3)/(10 u.c. SrTiO3) superlattice and corresponding dipoles map (right panel). Marked regions are zoomed in the bottom images (without angle colour map).

Rotational polarization topologies at the nanoscale have been observed by means of scanning transmission electron microscopy in BaTiO3/SrTiO3 superlattices grown on cubic SrTiO3(001). The transition from a highly homogeneous polarization state to rotational nanodomains is achieved by controlling the superlattice period while maintaining compressive epitaxial strain. The nanodomains prove that nominal tetragonal structure of BaTiO3 allows rotational polar textures.

Ferroelectric capacitors of epitaxial complex oxide on silicon

Título: Ferroelectric capacitors of epitaxial complex oxide on silicon
Authors:

Epitaxial Integration on Si(001) of Ferroelectric Hf0.5Zr0.5O2 Capacitors with High Retention and Endurance
Jike Lyu, Ignasi Fina, Josep Fontcuberta, Florencio Sánchez
ACS Applied Materials & Interfaces 11, 6, 6224–6229, 2019
DOI: 10.1021/acsami.8b18762

Figure: Sketch of the ferroelectric capacitor (left) and polarization retention after poling with voltage pulses of indicated amplitude (right).

Ferroelectric capacitors with epitaxial Hf0.5Zr0.5O2, integrated on Si(001) are reported for the first time, showing a remnant polarization around 20 μC/cm2, and high endurance and retention. The low films roughness and their improved insulating properties makes epitaxial Hf0.5Zr0.5O2 ideal for being integrated in ferroelectric-based devices.

First observation of a single-condensate to two-condensate superconductive transition

Título: First observation of a single-condensate to two-condensate superconductive transition
Authors:

Gap suppression at a Lifshitz transition in a multi-condensate superconductor
Gyanendra Singh, Alexis Jouan, Gervasi Herranz, Mateusz Scigaj, Florencio Sánchez, Lara Benfatto, Sergio Caprara, Marco Grilli, Guilhem Saiz, François Couëdo, Cheryl Feuillet-Palma, Jérôme Lesueur, Nicolas Bergeal
Nature Materials 18, 948–954, 2019
DOI: 10.1038/s41563-019-0354-z

Figure: Superconducting phase diagram. We plot the gap energies of the two condensates Δ1(0) and Δ2(0) (left axis) and γ coefficient (right axis), which accounts for the weight of each condensate in the LaAlO3/SrTiO3 quantum well. These magnitudes are plotted as a function of the voltage gate (VG) and are superimposed on the sheet resistance color map.

We report an unprecedented observation of a transition between single-condensate to two-condensate superconductivity. The experiments were done using resonant microwave transport carried out in the Laboratoire de Physique et d'Etude des Matériaux, ESPCI Paris, enabling continuous and reversible transitions between the two condensate regimes via electrostatic gating of LaAlO3/SrTiO3 quantum wells.

Improved resistive switching memory devices

Título: Improved resistive switching memory devices
Subtítulo: A non-volatile memory element in which data is stored in a single high resistance state is built based on back-to-back series connection of two ferroelectric tunnel devices.
Authors:

Complementary resistive switching using metal-ferroelectric-metal tunnel junctions
Mengdi Qian, Ignasi Fina, Milena C. Sulzbach, Florencio Sánchez, Josep Fontcuberta
Small 15, 1805042, 2019
DOI: 10.1002/smll.201805042

Figure: Intensity-Voltage (I-V) characteristics of back-to-back series connection of ferroelectric resistive switching device. Top-left: sketch of the used back-to-back series connection. The device resistance at remanence is always high, but the two ferroelectric devices connected show different polarization up-down or down-up states corresponding to different logic states.

Resistive switching elements are attracting a lot of interest, because these are building blocks for artificial intelligence computing. In resistive switching devices a switch from a high resistance to a low resistance and vice versa is produced depending on the prepoling applied voltage. Intermediate resistance states can be also set while sweeping prepoling applied voltage. Ferroelectric tunnel junctions are resistive switching devices, where resistance is modulated by ferroelectric polarization. Ferroelectric tunnel junctions are interesting because they can show improved power consumption, retention and reliability. In a resistive switching memory device, arrays of resistive switching elements are fabricated. However, these arrays present the sneak current problem. The sneak problem results from the fact that reading currents addressed to a high resistance state leak through memory elements in the low resistance state. If the resistive switching memory device is fabricated based on a ferroelectric material the same problem holds. In the highlighted work, it is shown that two different logic states can be stored in a series back-to-back connection of ferroelectric tunnel junctions performing the same high resistance. The two logic states correspond to up-down or down-up polarization state of the two ferroelectric junctions. Therefore, as all the junctions are in high resistance state, the sneak currents are avoided and the device scale-up is possible. Moreover, authors demonstrate that the engineered device shows less power consumption than resistive switching elements based on single ferroelectric tunnel junction.

Magnetic properties of oxide thin films deposited by Polymer-Assisted-Deposition

Título: Magnetic properties of oxide thin films deposited by Polymer-Assisted-Deposition
Subtítulo:

High microstructural quality of La0.92MnO3 thin films grown by Polymer-Assisted-Deposition technique, suitable or spintronic applications

Authors:

Dynamic magnetic properties and spin pumping in polymer-assisted-deposited La0.92MnO3 thin films
Hailin Wang, Alberto Pomar, Sergi Martín-Rio, Carlos Frontera, Narcis Mestres, Benjamín Martıínez
J. Mater. Chem. C 7, 12633-12640, 2019
DOI: 10.1039/C9TC04008K

Figure: Magnetic damping is enhanced in La0.92MnO3 thin films grown by Polymer-Assisted Deposition by spin pumping in a Pt capping layer. Inset: Ferromagnetic resonance spectra as a function of applied field for the two main in-plane orientations taken at 100K and 9GHz.

The growth of thin films by chemical methods has been a hot topic in the recent years as they offer appealing advantages over vacuum techniques as stoichiometric versatility and low-cost scalability. Among them, Polymer-Assisted-Deposition (PAD) is particularly attractive as it relies in environmentally friendly water-based solutions. However, serious concerns have been raised on the control of their interfacial quality limiting their use in emergent applications relaying in flat sharp interfaces, for example, for spintronics.

It is shown that La0.92MnO3 (LMO) thin films grown by PAD are of high microstructural quality with low magnetic damping, thus suitable for spintronic applications. Ferromagnetic resonance measurements in LMO/Pt bilayers gives clear indications of injection of pure spin currents into the Pt layer by spin pumping. This transfer of spin angular momentum through the interface between the ferromagnetic layer (LMO) and Pt layer is evidenced by an increase of magnetic damping. These results are of strong interest since they demonstrate that PAD technique allows obtaining complex oxide thin films of high microstructural quality suitable for spintronic applications.

We also present a deep study of the temperature dependence of the magnetodynamic properties of LMO thin films prepared by PAD showing that microstructural strain release from rhombohedral bulk phase results in an in-plane four-fold anisotropy with [110] as easy axis.

Our results demonstrate that LMO films grown by PAD may be used as efficient spin source systems in heterostructures for spintronic devices.

Nonpolar/polar magnetic switch in a strong ferromagnet

Título: Nonpolar/polar magnetic switch in a strong ferromagnet
Subtítulo: We unveil a nonpolar/polar transition in a strong ferromagnetic perovskite associated to the concurrence of two non-polar magnetic distortions. Several switching mechanisms are proposed based on magnetic trilinear coupling.
Authors:

Magnetic inversion symmetry breaking and spin reorientation in Tb2MnNiO6: a polar strong ferromagnet 
Jose Luis García-Muñoz, Javier Blasco, Xiaodong Zhang, Oscar Fabelo
Physical Review B 99, 184444, 9pp, 2019
DOI: 10.1103/PhysRevB.99.184444

Figure: (left) Magnetic irreps and isotropy subgroups: successive activation of nonpolar magnetic modes at the A and B sites that produce the P21' polar phase, allowing a trilinear coupling with the polarization in the free energy. (right) Changes in the magnetic order at A and B sites during the FM3 (P21'/c') « FM4 (P21') transformation showing the ferromagnetic spin reorientation. Electric and magnetic mechanisms to switch the polar/non-polar (FE/anti-FE) transformation between P21'/c' and P21' states.   

 

  

Unlike proper ferroelectrics, in improper ferroelectrics the polarization does not arises from the condensation of a polar lattice distortion associated to a zone-center instability. Understanding the interplay between magnetism and ferroelectricity in improper multiferroics is of interest in fundamental and applied research. At present, there is a good number of examples of improper ferroelectrics induced by antiferromagnetic order. This order is compatible with antitranslations that can suppress inversion centers or screw axis and favor symmetry breaks into polar symmetries.

Generally, cooperative distortions at the point k=0 (zone center) are more favorable to ferromagnetic configurations. In practice, ferromagnetic improper multiferroics are much more rare than antiferromagnets, and in most cases are simply weak-ferromagnets. However strong ferromagnetic multiferroics can have important advantages in magnetoelectric devices.

We completed a comprehensive study on four successive magnetic phases driven by zone-center modes in Tb2MnNiO6 ferromagnet. The interactions between A (Tb) and B (metals) spin subsystems stabilizes a ferromagnetic ground state with polar symmetry which makes this oxide potentially multiferroic with polarization by magnetic trilinear coupling. Its macroscopic magnetization is large and not related to a weak-ferromagnetic component from Dzyaloshinskii–Moriya terms.   

Symmetry analysis of neutron data (ILL, Grenoble) unveils that the nonpolar/polar transition occurs due to the concurrence of two different non-polar magnetic distortions. The concurrence of these non-polar distortions produces an overall polar symmetry, in which the loss of inversion center splits the orbits of Tb and O sites. In addition, thanks to a severe spin reorientation of the ferromagnetic axis across the polar/non polar transition we anticipate that in this material the direction of the magnetization can be used as a lever to switch the polar/non-polar (ferroelectric/antiferroelectric) transformation. Likewise, the control of the magnetization direction would be possible by electrical fields.

Observation of a giant topological Hal effect (THE) in an oxide material

Título: Observation of a giant topological Hal effect (THE) in an oxide material
Authors:

Giant topological Hall effect in correlated oxide thin films
Lorenzo Vistoli, Wenbo Wang, Anke Sander, Qiuxiang Zhu, Blai Casals, Rafael Cichelero, Agnès Barthélémy, Stéphane Fusil, Gervasi Herranz, Sergio Valencia, Radu Abrudan, Eugen Weschke, Kazuki Nakazawa, Hiroshi Kohno, Jacobo Santamaria, Weida Wu, Vincent Garcia, Manuel Bibes 
Nature Physics 15, 67–72, 2019
DOI: 10.1038/s41567-018-0307-5

Figure: Topological Hall effect (THE) in (Ca,Ce)MnO3. (a) Hall effect at different temperatures. The data are shifted vertically for clarity. (b) Decomposition of the Hall effect into anomalous Hall effect (AHE) and topological Hall effect (THE) using magneto-optical Kerr ellipticity at 15 K.

A topological Hall effect (THE) arises from the interaction of electrons with topological spin distributions, which are robust against fluctuations and noise. ICMAB researchers contributed by detecting THE from magneto-optic spectroscopy. The giant THE observed in (Ca,Ce)MnO3 is dependent on carrier concentration, which can be controlled by electric fields, opening new prospects for applications in electronics.

Probing time correlations with quantum wells

Título: Probing time correlations with quantum wells
Subtítulo:

The photoresponse of LaAlO3/SrTiO3 quantum wells is sensitive to time order of optical pulses and replicates the synaptic plasticity observed in neurobiological systems.

Authors:

Plasticity of Persistent Photoconductance of Amorphous LaAlO3/SrTiO3 Interfaces under Varying Illumination Conditions
Yu Chen, Blai Casals, Gervasi Herranz
ACS Appl. Electron. Mater. 2019, 1, 6, 810–816, 2019
DOI: 10.1021/acsaelm.9b00127

Solid-State Synapses Modulated by Wavelength-Sensitive Temporal Correlations in Optic Sensory Inputs
Yu Chen, Blai Casals, Florencio Sanchez, Gervasi Herranz
ACS Appl. Electron. Mater. 2019, 1, 7, 1189–1197, 2019
DOI: 10.1021/acsaelm.9b00183

Figure: Probing time correlations with quantum wells. We have uncovered photoresponsive quantum wells that mimic spike-timing dependent plasticity using optical pulses as stimuli. The conductance is sensitive to the time order of optical inputs of different wavelengths (see bottom panel, where blue and red arrows indicate different sequences of blue and red pulses). Therefore, these quantum wells can be used as optical synapses in neuromorphic devices.

Spike-timing dependent plasticity (STDP) is a fundamental concept in neurobiology. Briefly, the relative timing of two neuron spikes reinforces or weakens the synapse between them, so that events caused by another should trigger spikes in a particular time order. A reversal of this order causes the weakening of the synapse, so it penalizes the correlation. This process allows the brain to establish causal correlations from the environment and it is widely used in computational neuroscience. Recently, Yu Chen et al. (ACS Appl. Electron. Mater. 2019, 1, 6, 810–816 (2019)) have found that the photoconductive properties of quantum wells at the LaAlO3/SrTiO3 interface mimic STDP, using time correlations in optical pulses. More specifically, the conductance of the quantum well is increased or decreased depending on the relative timing of optical pulses of short- (blue) and long- (red) wavelengths (Figure). Remarkably, the conductance changes plastically in proportion to the intensity of the optical stimulus, in a way that is reminiscent of synaptic plasticity found in neurobiological systems (Yu Chen, ACS Appl. Electron. Mater. 2019, 1, 6, 810–816 (2019)). The sensitivity of the conductance to these correlations opens up fascinating perspectives on the use of optical synapses for neuromorphic devices based on these photoconductive systems.

Resistive switching in semimetallic thin films

Título: Resistive switching in semimetallic thin films
Subtítulo: The study of different thickness SrIO3 thin films shows different behaviors, from insulating to semimetallic, in resistive switching memory devices
Authors:

Resistive Switching in Semimetallic SrIrO3 Thin Films
Víctor Fuentes, Borislav Vasić, Zorica Konstantinović, Benjamín Martínez, Lluís Balcells, Alberto Pomar
ACS Appl. Electron. Mater. 1, 1981−1988, 2019
DOI: 10.1021/acsaelm.9b00519

Figure: Left: Disorder and spatial localization due to thickness reduction allow generating a metal-insulator transition in semimetallic SrIrO3 thin films. Below about 3nm films are insulating with hysteretic I-V curves indicative of resistive switching behavior at room temperature. Right: Current maps (a and c) allow demonstrating the writing/erasing processes required for the implementation of Re-RAMs. Corresponding topography images (b, d).

Iridates are of strong interest because large spin-orbit coupling (SOC) is expected in these materials, thus the interplay between magnetic and electronic properties will be strongly reinforced opening the access to new magneto-electronic devices.

In the search for a new generation of faster and more energy efficient electronic devices, the use of reversible resistive switching (RS) phenomena has been proposed as a very appealing solution for the development of non-volatile memory devices. Here we address the analysis of resistive switching processes in SrIrO3 thin films by means of local Intensity-Voltage (I-V) curve measurements and current mapping, by using conductive atomic force microscopy (C-AFM).

While SrIrO3 exhibits semimetallic character, in thin films, an Anderson-type metal-insulator transition (MIT) triggered by disorder and spatial localization due to film thickness reduction is observed, and their influence on the resistive switching behaviour is analysed. For thin enough films (below ~3nm) samples are insulating with hysteretic I-V curves indicative of reversible resistive switching behaviour between two states of clearly different resistance at room temperature. A sharp transition into a low resistance state (LRS), i.e an abrupt increase of the current intensity, is detected above a well-defined threshold voltage indicative of localization of charge carriers. On the other hand, thicker samples exhibit a semimetallic character and I-V curves show progressive changes of the local resistance without a clearly defined threshold voltage, thus evidencing the absence of a MI transition with a well-defined resistance jump between the different resistance states.

Self-assembly of magnetic chains from nanoparticles

Título: Self-assembly of magnetic chains from nanoparticles
Authors:

Spontaneous in-flight assembly of magnetic nanoparticles into macroscopic chains
Lluis Balcells, Igor Stankovic, Zorica Konstantinovic,  Aanchal Alagh, Victor Fuentes, Laura López-Mir, Judit Oró, Narcis Mestres, Carlos García , Alberto Pomar, Benjamin Martínez
Nanoscale 11, 14194, 2019
DOI: 10.1039/c9nr02314c

Figure: Spontaneous self-assembly of magnetic nanoparticles into macroscopic chains by using a combination of magnetron sputtering and gas aggregation techniques

We have pushed the capabilities of the cluster gun technique beyond single particle fabrication as magnetic nanoparticles spontaneously self-assembled into macroscopic chains in a controlled and reproducible way. The low-kinetic energy inside a magnetron sputtering vacuum chamber and strong dipolar magnetic interaction are responsible for particles’ agglomeration at very low volume fractions.

Soft litography microstructuring of epitaxial quartz on silicon

Título: Soft litography microstructuring of epitaxial quartz on silicon
Authors:

Tailoring the crystal growth of quartz on silicon for patterning epitaxial piezoelectric films
Qianzhe Zhang, David Sanchez-Fuentes, Andrés Gomez, Rudy Desgarceaux, Benoit Charlot, Jaume Gazquez, Adrian Carretero-Genevrier, Martí Gich
Nanoscale Advances 1, 3741-3752, 2019
DOI: 10.1039/c9na00388f

Figure: The systematic study of chemical solution deposition parameters has made possible finding the optimal conditions to prepare structured epitaxial quartz films on silicon by soft lithography.

We gained control over epitaxial growth of quartz on silicon by chemical solution deposition to demonstrate the structuring of those films by soft lithography. The key aspects to obtain the desired microstructure were the surfactant, the concentration of melting agent, the deposition of several layers and relative humidity.

Synchrotron X-rays reveals the structural symmetry from bulk to film

Título: Synchrotron X-rays reveals the structural symmetry from bulk to film
Authors:

Double-cell superstructure and vacancy ordering in tensile-strained metallic thin films of Pr0.50Ca0.50CoO3 on LaAlO3
Jessica Padilla-Pantoja, Xavier Torrelles, Jaume Gazquez, Juan Rubio-Zuazo, Javier Blasco, Javier Herrero-Martín, Jose Luis García-Muñoz
Physical Review Materials 3, 104407, 9pp, 2019
DOI: 10.1103/PhysRevMaterials.3.104407

Figure: (top) Selected synchrotron X-ray diffraction scans at RT from PCCO thin film, showing a double-cell in the film due to the (1/2, 1/2, 1/2) superstructure. (H,K,L) reflections are expressed in the LAO basis. (bottom) High-resolution Z-contrast image of the PCCO/LAO interface. d1 and d2 signal different (Pr/Ca) - (Pr/Ca) distances. Yellow circles in the image Fourier transform mark two superlattice peaks. The arrows mark the O-deficient planes. Scale bar 10 nm. Histogram of the out-of-plane distance values (Δz) between lanthanide neighbors in successive layers.

Pr0.5Ca0.5CoO3 presents in bulk form a singular valence, spin-state and metal-insulator transition. We report a reduction of the structural symmetry from Pnma(bulk) to P212121 (film) revealed by synchrotron X-rays. In contrast to the general tendency reported for strained ferromagnetic Co perovskite films, we show that unexpectedly a nominal tensile strain can also be compatible with the presence of alternating O vacancy planes parallel to the interface.

Topotactic nitridation of transition metal double perovskites

Título: Topotactic nitridation of transition metal double perovskites
Authors:

Topochemical nitridation of Sr2FeMoO6
Roberta Ceravola, Carlos Frontera, Judith Oró-Solé, Ashley P.Black, C.Ritter, Ignasi Mata, Elies Molins, Josep Fontcuberta and Amparo Fuertes
Chemical Communications 55, 3105-3108, 2019
DOI: 10.1039/C8CC09845J

Figure: (a) Schematic band filing of Sr2FeMoO6. Fermi level is nearby the bottom of Fe-3dt2g↓-Mo-(4d,5s)↓ bands. (b) In the oxide FeO6 and MoO6 octahedra are regular (top sketch) and the free carriers in the conduction band (bottom sketch) promote long range ferromagnetic order. (c) Schematic band filing of Sr2FeMoO4.9N1.1. (d) Localized states formed around defect-related potential wells (e.g.: nitride sites, Jahn-Teller Fe4+ ions). (e) Magnetoresistance (MR=[r(H)-r(0)]/r(0)) of the oxynitride sample at different temperatures; Inset: logarithm of the resistance (at zero field) as a function of T-1/4.

The topochemical nitridation of cation ordered, tetragonal Sr2FeMoO6 in NH3 at moderate temperatures leads to the cubic, Fm-3m double perovskite oxynitride Sr2FeMoO4.9N1.1where double-exchange interactions determine ferromagnetic order with Tc ≈ 100 K.  Substitution of oxide by nitride induces bond asymmetries and local electronically-driven structural distortions, which combined with Fermi level lowering restrict charge itineracy to confined regions and preclude spontaneous long-range magnetic order. Under a magnetic field, ferromagnetic correlations expand, favoring charge delocalization and a negative magnetoresistance is observed.

Transparent and highly conducting metallic oxides

Título: Transparent and highly conducting metallic oxides
Subtítulo: Metals are known to be bright by reflecting light. Many applications however, require transparent rather than reflecting metals. Transition metal oxides offer a solution to this dilemma
Authors:

High Carrier Mobility, Electrical Conductivity, and Optical Transmittance in Epitaxial SrVO3 Thin Films
Mathieu Mirjolet, Florencio Sánchez, Josep Fontcuberta
Adv. Funct. Mater. 29, 1808432, 2019
DOI: 10.1002/adfm.201808432

Independent Tuning of Optical Transparency Window and Electrical Properties of Epitaxial SrVO3 Thin Films by Substrate Mismatch
Mathieu Mirjolet, Hari Babu Vasili, LLuís López-Conesa, Sònia Estradé, Francesca Peiró, José Santiso, Florencio Sánchez, Pamela Machado, Pierluigi Gargiani, Manuel Valvidares, Josep Fontcuberta
Adv. Funct. Mater. 29, 1904238, 2019
DOI: 10.1002/adfm.201904238

Figure: Plasma energy of SrVO3 thin films (left axis) and room-temperature electrical resistivity (right axis) grown on different substrates, imposing tensile or compressive stress on SrVO3, as indicated.

Information technologies require new materials with high electrical conductivity and optical transparency to face scarcity of critical materials and improve performances. Oxide thin films based on early transition metals (e.g. V, Nb, Mo, etc.), the partially occupation of ndxorbitals (i.e. nd1, nd2...) gives rise to metallic conductivity. Transparency at visible range requires that the plasm edge to be at red or infrared. The narrow conducting 3d band of SrVO3 is expected to enhance the carrier effective mass, thus lowering the plasma frequency below the visible as required. However, growing thin films of these oxides typically requires an extremely low oxygen pressure, that compromises point defect concentration in the film and thus challenges obtaining high conducting materials as required. Using pulsed laser deposition (PLD) as a tool to grow epitaxial thin films of SrVO3, we have discovered that the use of a non-reactive gas controls the ablation plume expansion and allows to obtain SrVO3 films with low room-temperature resistivity (r ≈ 31 mW cm) and large carrier mobility (m ≈ 8.3 cm2 V-1 s-1) and residual resistivity ratio (RRR ≈ 11.5), which are record figures of merit for PLD grown films, while preserving the plasma frequency at infrared and improving optical transparency in the visible range.

Moreover, we show that by exploiting the strain caused by the substrates on the structure and microstructure of the films, the carrier concentration and the effective mass of carriers can be modulated while preserving the optical transmittance window. As indicated by linear X-ray dichroism experiments performed at Boreas Beamline at ALBA synchrotron, the electronic bandwidth and orbital occupation appear to rule the observed effects.

 

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