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Magneto-ionics is an emerging field in materials science where voltage is used as an energy-efficient means to tune magnetic properties, such as magnetization, coercive field, or exchange bias, by voltage-driven ion transport. We first discuss the emergence of magneto-ionics in the last decade, its core aspects, and key avenues of research. We also highlight recent progress in materials and approaches made during the past few years. We then focus on the "structural-ion"approach as developed in our research group in which the mobile ions are already present in the target material and discuss its potential advantages and challenges. Particular emphasis is given to the energetic and structural benefits of using nitrogen as the mobile ion, as well as on the unique manner in which ionic motion occurs in CoN and FeN systems. Extensions into patterned systems and textures to generate imprinted magnetic structures are also presented. Finally, we comment on the prospects and future directions of magneto-ionics and its potential for practical realizations in emerging fields, such as neuromorphic computing, magnetic random-access memory, or micro- and nano-electromechanical systems.

Magneto-ionics, which deals with the change of magnetic properties through voltage-driven ion migration, is expected to be one of the emerging technologies to develop energy-efficient spintronics. While a precise modulation of magnetism is achieved when voltage is applied, much more uncontrolled is the spontaneous evolution of magneto-ionic systems upon removing the electric stimuli (i.e., post-stimulated behavior). Here, we demonstrate a voltage-controllable N ion accumulation effect at the outer surface of CoN films adjacent to a liquid electrolyte, which allows for the control of magneto-ionic properties both during and after voltage pulse actuation (i.e., stimulated and post-stimulated behavior, respectively). This effect, which takes place when the CoN film thickness is below 50 nm and the voltage pulse frequency is at least 100 Hz, is based on the trade-off between generation (voltage ON) and partial depletion (voltage OFF) of ferromagnetism in CoN by magneto-ionics. This novel effect may open opportunities for new neuromorphic computing functions, such as post-stimulated neural learning under deep sleep.

Voltage control of magnetism via electric-field-driven ion migration (magneto-ionics) has generated intense interest due to its potential to greatly reduce heat dissipation in a wide range of information technology devices, such as magnetic memories, spintronic systems or artificial neural networks. Among other effects, oxygen ion migration in transition-metal-oxide thin films can lead to the generation or full suppression of controlled amounts of ferromagnetism ('ON-OFF' magnetic transitions) in a non-volatile and fully reversible manner. However, oxygen magneto-ionic rates at room temperature are generally considered too slow for industrial applications. Here, we demonstrate that sub-second ON-OFF transitions in electrolyte-gated paramagnetic cobalt oxide films can be achieved by drastically reducing the film thickness from >200 nm down to 5 nm. Remarkably, cumulative magneto-ionic effects can be generated by applying voltage pulses at frequencies as high as 100 Hz. Neuromorphic-like dynamic effects occur at these frequencies, including potentiation (cumulative magnetization increase), depression (i.e., partial recovery of magnetization with time), threshold activation, and spike time-dependent magnetic plasticity (learning and forgetting capabilities), mimicking many of the biological synapse functions. The systems under investigation show features that could be useful for the design of artificial neural networks whose magnetic properties would be governed with voltage.

Magneto-ionics is an emerging actuation mechanism to control the magnetic properties of materials via voltage-driven ion motion. This effect largely relies on the strength and penetration of the induced electric field into the target material, the amount of generated ion transport pathways, and the ionic mobility inside the magnetic media. Optimizing all these factors in a simple way is a huge challenge, although highly desirable for technological applications. Here, we demonstrate that the introduction of suitable transition-metal elements to binary nitride compounds can drastically boost magneto-ionics. More specifically, we show that the attained magneto-ionic effects in CoN films (i.e., saturation magnetization, toggling speeds, and cyclability) can be drastically enhanced through 10% substitution of Co by Mn in the thin-film composition. Incorporation of Mn leads to transformation from nanocrystalline into amorphous-like structures, as well as from metallic to semiconducting behaviors, resulting in an increase of N-ion transport channels. Ab initio calculations reveal a lower energy barrier for CoMn-N compared to Co-N that provides a fundamental understanding of the crucial role of Mn addition in the voltage-driven magnetic effects. These results constitute an important step forward toward enhanced voltage control of magnetism via electric field-driven ion motion.

A comprehensive work in plasmonic chiral and achiral nanostructures is presented. The bottom-up fabricated structures range from simple building blocks such as metallic and dielectric disk dimers or rods, to more complex nanostructures built from these individual blocks, arranged in the plane (two dimensional structures) or out of plane (three dimensional structures). The fabrication process uses hole-mask colloidal lithography and multiaxial evaporation, a powerful and cost-effective fabrication technique able to obtain a variety of structures in large areas. Besides morphology changes, parameters such as the composition and the spatial arrangement of the building blocks which compose them have been varied to comprehend their role in the optical response of the structures. These parameters are tailored to obtain active chiral meta-structures with optimum properties. The fabricated structures are morphologically, optically and magneto-optically characterized in depth. A thorough and step-by-step experimental study to explain the chiro-optical response of complex chiral plasmonic nanostructures from the achiral responses of its constituents is shown. To fully characterize the physical properties hidden in the measurements of these structures, an alternative analysis of the Mueller matrix elements is developed. Using this method, the optical, chiro-optical and magnetooptical properties of these complex nano-systems are validated...

A comprehensive work in plasmonic chiral and achiral nanostructures is presented. The bottom-up fabricated structures range from simple building blocks such as metallic and dielectric disk dimers or rods, to more complex nanostructures built from these individual blocks, arranged in the plane (two dimensional structures) or out of plane (three dimensional structures). The fabrication process uses hole-mask colloidal lithography and multiaxial evaporation, a powerful and cost-effective fabrication technique able to obtain a variety of structures in large areas. Besides morphology changes, parameters such as the composition and the spatial arrangement of the building blocks which compose them have been varied to comprehend their role in the optical response of the structures. These parameters are tailored to obtain active chiral meta-structures with optimum properties. The fabricated structures are morphologically, optically and magneto-optically characterized in depth. A thorough and step-by-step experimental study to explain the chiro-optical response of complex chiral plasmonic nanostructures from the achiral responses of its constituents is shown. To fully characterize the physical properties hidden in the measurements of these structures, an alternative analysis of the Mueller matrix elements is developed. Using this method, the optical, chiro-optical and magnetooptical properties of these complex nano-systems are validated..., Se presenta un estudio experimental exhaustivo y detallado para entender la respuesta quiral de nanoestructuras plasmónicas quirales complejas a partir de las respuestas aquirales de sus bloques constituyentes. Esto conlleva diferentes estructuras fabricadas, desde simples nano-objetos como barras o dímeros de discos compuestos por metal o dieléctrico, hasta nanoestructuras más complejas ensambladas en el plano (estructuras bidimensionales) o apiladas perpendicularmente al plano (estructuras tridimensionales). En el proceso de fabricación se utiliza la litografía coloidal de máscara de agujeros y la evaporación multieje, una técnica de fabricación potente y económicamente asequible, capaz de obtener una gran variedad de estructuras en grandes áreas. Además de los cambios morfológicos, se han variado parámetros tales como la composición y la disposición espacial de las piezas constituyentes, a fin de estudiar su influencia en la respuesta óptica. Estos parámetros se optimizan para obtener metaestructuras quirales activas con las mejores propiedades. Posteriormente, las estructuras fabricadas se caracterizan morfológica, óptica y magneto ópticamente. Para identificar plenamente las propiedades físicas ocultas en las medidas de estas estructuras, se desarrolla un análisis alternativo de los elementos de la matriz de Mueller. Con este método, se validan las propiedades ópticas, quiroópticas y magnetoópticas de estos complejos nanosistemas...

The utility of electrical resistivity as an indicator of magnetoionic performance in stoichiometrically and structurally similar thin-film systems is demonstrated. A series of highly nanocrystalline cobalt nitride (Co-N) thin films (85 nm thick) with a broad range of electrical properties exhibit markedly different magnetoionic behaviors. Semiconducting, near stoichiometric CoN films show the best performance, better than their metallic and insulating counterparts. Resistivity reflects the interplay between atomic bonding, carrier localization, and structural defects, and in turn determines the strength and distribution of applied electric fields inside the actuated films. This fact, generally overlooked, reveals that resistivity can be used to quickly evaluate the potential of a system to exhibit optimal magnetoionic effects, while also opening interesting challenges.

Magneto-ionics allows for tunable control of magnetism by voltage-driven transport of ions, traditionally oxygen or lithium and, more recently, hydrogen, fluorine, or nitrogen. Here, magneto-ionic effects in single-layer iron nitride films are demonstrated, and their performance is evaluated at room temperature and compared with previously studied cobalt nitrides. Iron nitrides require increased activation energy and, under high bias, exhibit more modest rates of magneto-ionic motion than cobalt nitrides. Ab initio calculations reveal that, based on the atomic bonding strength, the critical field required to induce nitrogen-ion motion is higher in iron nitrides (≈6.6 V nm-1) than in cobalt nitrides (≈5.3 V nm-1). Nonetheless, under large bias (i.e., well above the magneto-ionic onset and, thus, when magneto-ionics is fully activated), iron nitride films exhibit enhanced coercivity and larger generated saturation magnetization, surpassing many of the features of cobalt nitrides. The microstructural effects responsible for these enhanced magneto-ionic effects are discussed. These results open up the potential integration of magneto-ionics in existing nitride semiconductor materials in view of advanced memory system architectures.

In this paper, the predictive power of diffracxtive magneto-optics concerning domain structure and reversal mechanisms in ordered arrays of magnetic elements is demonstrated. A simple theoretical model based on Fraunhoffer diffraction theory is used to predict the magnetisation reversal mechanisms in an array of magnetic elements. Different domain structures and simplified models (or educated guesses) of the associated reversal mechanisms produce marked differences in the spatial distributions of the magnetisation. These differences and the associated magnetisation distribution moments are experimentally accessible through conventional and diffractive magneto-optical Kerr effect measurements. The domain and magnetisation reversal predictions are corroborated with Magnetic Force Microscopy (MFM) measurements.

Magneto-ionics, understood as voltage-driven ion transport in magnetic materials, has largely relied on controlled migration of oxygen ions. Here, we demonstrate room-temperature voltage-driven nitrogen transport (i.e., nitrogen magneto-ionics) by electrolyte-gating of a CoN film. Nitrogen magneto-ionics in CoN is compared to oxygen magneto-ionics in Co3O4. Both materials are nanocrystalline (face-centered cubic structure) and show reversible voltage-driven ON-OFF ferromagnetism. In contrast to oxygen, nitrogen transport occurs uniformly creating a plane-wave-like migration front, without assistance of diffusion channels. Remarkably, nitrogen magneto-ionics requires lower threshold voltages and exhibits enhanced rates and cyclability. This is due to the lower activation energy for ion diffusion and the lower electronegativity of nitrogen compared to oxygen. These results may open new avenues in applications such as brain-inspired computing or iontronics in general.

Voltage control of magnetism through electric field-induced oxygen motion (magneto-ionics) could represent a significant breakthrough in the pursuit for new strategies to enhance energy efficiency in magnetically actuated devices. Boosting the induced changes in magnetization, magneto-ionic rates and cyclability continue to be key challenges to turn magneto-ionics into real applications. Here, it is demonstrated that room-temperature magneto-ionic effects in electrolyte-gated paramagnetic CoO films can be largely increased both in terms of generated magnetization (6 times larger) and speed (35 times faster) if the electric field is applied using an electrochemical capacitor configuration (utilizing an underlying conducting buffer layer) instead of placing the electric contacts at the side of the semiconductor (electric-double-layer transistor-like configuration). This is due to the greater uniformity and strength of the electric field in the capacitor design. These results are appealing to widen the use of ion migration in technological applications such as neuromorphic computing or iontronics in general.

Bismuth electroplating and fabrication efforts for its incorporation as an absorber in Superconducting Transition Edge X-ray Sensor devices are summarized. The bismuth films, a few microns thick, are grown using both aqueous and non-aqueous solutions. They are characterized by optical imaging, profilometry, X-ray diffraction, SEM, and by electrical transport (resistivity and Hall Effect) measurements from 30 to 310 K. While aqueous solutions produce an irregular rough and polycrystalline material, with very little chance to tune transport properties, films grown in non-aqueous solvents improve considerably homogeneity, surface roughness, and density. It is also demonstrated how the crystal texture of the films can be controlled with electroplating parameters. This allows orienting anisotropy axes directly during electroplating and thus tune electrical and thermal conductance depending on device requirements. This a significant step toward direct electrodeposition of Bi films with good transport properties, still a significant technological challenge. In order to have reasonable quantitative objectives for physical properties such as surface roughness, crystalline texture, electrical conductivity, etc. Bismuth films are also grown by Physical Vapor Deposition, characterized and compared with the electroplated.

Electron Paramagnetic Resonance (EPR) is used to study the transport properties in CaCu3Ti4O12 (CCTO) and its mixtures with TiO2. The temperature dependence of the peak to peak linewidth of the EPR spectra in the temperature range 430 ≤ T ≤ 660 K, is studied for the powders of CCTO, 5% CCTO + 95% TiO2 and 1% CCTO + 99% TiO2. All the samples show a behavior predicted by the Small Polaron Hopping model. This behavior is studied as a function of the TiO2 content, finding an activation energy that decreases with increasing TiO2 concentration. We found that two mechanisms govern the conductivity in these system one, intrinsic to the CCTO, via oxygen vacancies and the other one, which improve the conductivity, related to the polarizability of the Ti-ions at the surface of the grains and mediated by the oxygen vacancies at the surface of the CCTO.

We present a fabrication method for sub-micron lens-coupled lumped element kinetic inductance detectors (LEKIDs) for broadband sub-millimeter (sub-mm) and THz radiation detection. The LEKIDs are fabricated from very thin (12 nm) and narrow (200 nm) aluminum lines to match the impedance of the LEKID to the substrate impedance. The fabrication process is based on a combination of maskless laserwriter lithography and electron beam lithography, providing low (few microns) and high resolution (down to 200 nm) over large areas in a single process. Preliminary optical characterization shows that the fabricated LEKIDs are sensitive to 1.5 THz radiation.

In the field of energy harvesting, recycling the energy of mechanical vibrations from the environment becomes an attractive option to use in low power applications. With this goal, mechanisms of transduction have been developed with electrostatic, piezoelectric and electromagnetic principles of operation. In this work we present the analysis for an electromagnetic micro-generator using the linear generator approach combined with the dependence of the electromotive induced force (EMF) with geometrical parameters such as the radii of the coil and the magnet, the distance between them, and the >slope> of the coil, a parameter representing the reduction of radii as a function of each turn of the helicoidal winding. The latter feature introduces the possibility of pyramidal coils instead of solenoidal coils. We show that when the distance between the coil and the magnet is of the order of the radius of the magnet, the pyramidal geometry presents a higher induced electromotive force than a solenoidal coil of comparable dimensions. We also illustrate a method for the construction of a micro-electromagnetic generator which contemplates the fabrication of pyramidal coils with tridimensional electron beam lithography. Results for resonant structures and microcoils are discussed. The estimated induced EMF and power were calculated for a pyramidal high density coil generator, showing a better performance for the pyramidal when compared with the solenoidal geometry. © 2013 Elsevier B.V.

Recent experiments have reported an opposite sign of the differential surface stress produced on gold-coated cantilevers by a thiol-derivatized single-stranded DNA (SH-DNA) immobilization process. The sign of the surface stress depends on the method used to evaporate the gold thin film, being compressive (negative) or tensile (positive) for e-beam or resistively deposited gold, respectively. This study investigates the origin of this effect by means of a combination of x-ray diffraction and x-ray photoelectron spectroscopy. Both e-beam and resistively grown gold thin films are characterized to find the subtle differences responsible for this intriguing stress behaviour. Somewhat remarkably, these studies show a tight relation between the surface structure of the gold overlayer and the SH-DNA immobilization efficiency. The average grain size variation seems to correlate well with the differential surface stress triggered by the SH-DNA immobilization previously reported. These results suggest that the relation of the probe molecules with the surface structure must be considered to understand surface stress changes. © 2014 IOP Publishing Ltd.

A careful and wide comparison between Al and Ga as substitutional dopants in the ZnO wurtzite structure is presented. Both cations behave as n-type dopants and their inclusion improves the optical and electrical properties of the ZnO matrix, making it more transparent in the visible range and rising up its electrical conductivity. However, the same dopant/Zn ratio leads to a very different doping efficiency when comparing Al and Ga, being the Ga cation a more effective dopant of the ZnO film. The measured differences between Al- and Ga-doped films are explained with the hypothesis that different quantities of these dopant cations are able to enter substitutionally in the ZnO matrix. Ga cations seem to behave as perfect substitutional dopants, while Al cation might occupy either substitutional or interstitial sites. Moreover, the subsequent charge balance after doping appear to be related with the formation of different intrinsic defects that depends on the dopant cation. The knowledge of the doped-ZnO films microstructure is a crucial step to optimize the deposition of transparent conducting electrodes for solar cells, displays, and other photoelectronic devices.

A careful and wide comparison between Al and Ga as substitutional dopants in the ZnO wurtzite structure is presented. Both cations behave as n-type dopants and their inclusion improves the optical and electrical properties of the ZnO matrix, making it more transparent in the visible range and rising up its electrical conductivity. However, the same dopant/Zn ratio leads to a very different doping efficiency when comparing Al and Ga, being the Ga cation a more effective dopant of the ZnO film. The measured differences between Al- and Ga-doped films are explained with the hypothesis that different quantities of these dopant cations are able to enter substitutionally in the ZnO matrix. Ga cations seem to behave as perfect substitutional dopants, while Al cation might occupy either substitutional or interstitial sites. Moreover, the subsequent charge balance after doping appear to be related with the formation of different intrinsic defects that depends on the dopant cation. The knowledge of the doped-ZnO films microstructure is a crucial step to optimize the deposition of transparent conducting electrodes for solar cells, displays, and other photoelectronic devices. © 2013 AIP Publishing LLC.

An electric current through a transparent 80 nm ZnO thin film produces relevant changes on its Raman spectrum. The electrical current affects the vibrational modes of ZnO that are associated with intrinsic defects. A reversible Raman intensity decrease of the band centered at ∼578 cm -1 with the electrical current in the thin film is measured. The Raman peak decreases without a measurable energy shift, ruling out anharmornic contributions related to temperature increasing. The most probable mechanism is through a barrier height reduction with the applied electric field, producing a decrease of the charge at the crystallites interface or grain boundaries. The reported experiment is a direct proof of the significant role that intrinsic defects play on the electrical transport through ZnO thin films. © 2013 Elsevier B.V.

A detailed study of the effect of doping on the electrical transport characteristics of ZnO thin films is presented. This is performed for undoped ZnO and for ZnO doped with Al and Ga. The films have been characterized by conventional direct current, DC, Hall transport measurements, impedance spectroscopy, IS, and confocal Raman spectroscopy, CRS. The variation of the transport properties caused by doping is related to a variation of the built-in electric field at the grain boundaries. This variation is observed through a variation of confocal Raman spectroscopy localized bands that are related to structural ZnO defects. This technique allows one to measure the Raman spectra as electrical current is being fed to the film. The comparison between DC Hall transport measurements and impedance spectroscopy data confirms a semiconductor like behavior for ZnO and ZnO:Al and a metallic-like behavior of ZnO:Ga. This is due to the different amount of ions that enter substitutionally in the ZnO matrix and thus introduce carriers that compensate charge traps at the grain boundaries. The variation of this charge trap density and the associated electric field is probably the most relevant limiting factor of these films' transport properties. © 2013 Elsevier B.V. All rights reserved.

Aluminum nanocontact conductance histograms are studied experimentally from room temperature up to near the bulk melting point. The dominant stable configurations for this metal show a very early crossover from shell structures at low wire diameters to ionic subshell structures at larger diameters. At these larger radii, the favorable structures are temperature-independent and consistent with those expected for ionic subshell (faceted) formations in face-centered cubic geometries. When approaching the bulk melting temperature, these local stability structures become less pronounced as shown by the vanishing conductance histogram peak structure.

Molybdenum is a low Tc, type I superconductor whose fundamental properties are poorly known. Its importance as an essential constituent of new high performance radiation detectors, the so-called transition edge sensors (TESs) calls for better characterization of this superconductor, especially in thin film form. Here we report on a study of the basic superconducting features of Mo thin films as a function of their thickness. The resistivity is found to rise and the critical temperature decreases on decreasing film thickness, as expected. More relevant, the critical fields along and perpendicular to the film plane are markedly different, thickness dependent and much larger than the thermodynamic critical field of Mo bulk. These results are consistent with a picture of type II 2D superconducting films, and allow estimates of the fundamental superconducting lengths of Mo. The role of morphology in determining the 2D and type II character of the otherwise type I molybdenum is discussed. The possible consequences of this behaviour on the performance of radiation detectors are also addressed.

Elemento micromecánico magnetostrictivo resistente a la corrosión. Se describe un elemento micromecánico magnetostrictivo (1) resistente a la corrosión para microactuadores y microscopía de fuerzas atómicas en modos dinámicos y funcionamiento compatible con medios corrosivos, comprendiendo dicho elemento micromecánico magnetostrictivo (1) una pieza (2) de un material no magnetostrictivo recubierta, al menos por una primera cara, por al menos (3) de material magnetostrictivo resistente a la corrosión en medios líquidos, de manera que un campo magnético paralelo al plano de la lámina (3) magnetostrictiva provoca la deformación del elemento micromecánico magnetostrictivo (1).

Controlling electrochemical parameters (setting the bias voltage and monitoring the current) during immobilization of thiolated single stranded DNA (SH-ssDNA) on gold coated cantilevers allows tailoring the reaction kinetics. In particular, the immobilization time can be reduced more than 40 times when compared with conventional passive immobilization (cantilever electrically floated during the process). Besides the kinetics, the surface stress induced during the immobilization process can be also controlled and maximized. The combination of an electrochemically monitored immobilization with in-situ profile measurements provides a powerful tool to optimize DNA immobilization and increase the nanomechanical response.