2020
Giant voltage-induced modification of magnetism in micron-scale ferromagnetic metals by hydrogen charging
Xinglong Ye, Harish K. Singh, Hongbin Zhang, Holger Gesswein, Mohammed Reda Chellali, Ralf Witte, Alan Molinari, Konstantin Skokov, Oliver Gutfleisch, Horst Hahn, Robert Kruk
Nature Communications 11 (4849), 1-8, 2020
Abstract
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
Alan Molinari, Ralf Witte, Krishna Kanth Neelisetty, Saleh Gorji, Christian Kuebel, Ingo Muench, Franziska Woehler, Lothar Hahn, Stefan Hengsbach, Klaus Bade, Horst Hahn Robert Kruk
Advanced Materials 32 (12), 1907541, 2020
Abstract
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.
2019
Voltage-Control of Magnetism in All-Solid-State and Solid/Liquid Magnetoelectric Composites
A Molinari, H Hahn, R Kruk
Advanced Materials 31 (26), 1806662, 2019
Abstract
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 to compare the strength of the interaction between electricity and magnetism among different magnetoelectric systems.
Electron beam effects on oxide thin films: structure and electrical property correlations
Krishna Kanth Neelisetty, Xiaoke Mu, Sebastian Gutsch, Alexander Vahl, Alan Molinari, Falk von Seggern, Mirko Hansen, Torsten Scherer, Margit Zacharias, Lorenz Kienle, VS Kiran Chakravadhanula and Christian Kuebel
Microscopy and Microanalysis 25 (3), 592-600, 2019
Abstract
In situ transmission electron microscope (TEM) characterization techniques provide valuable information on structure-property correlations to understand the behavior of materials at the nanoscale. However, understanding nanoscale structures and their interaction with the electron beam is pivotal for the reliable interpretation of in situ/ex situ TEM studies. Here, we report that oxides commonly used in nanoelectronic applications, such as transistor gate oxides or memristive devices, are prone to electron beam induced damage that causes small structural changes even under very low dose conditions, eventually changing their electrical properties as examined via in situ measurements. In this work, silicon, titanium, and niobium oxide thin films are used for in situ TEM electrical characterization studies. The electron beam induced reduction of the oxides turns these insulators into conductors. The conductivity change is reversible by exposure to air, supporting the idea of electron beam reduction of oxides as primary damage mechanism. Through these measurements we propose a limit for the critical dose to be considered for in situ scanning electron microscopy and TEM characterization studies.
2018
Anion Doping of Ferromagnetic Thin Films of La0. 74Sr0. 26MnO3-delta
P. A. Sukkurji, A. Molinari, C. Reitz, R. Witte, C. Kuebel, V. S. K. Chakravadhanula, R. Kruk, O. Clemens,
Materials 11, 1204 (2018),
Abstract
Chemical doping via insertion of ions into the lattice of a host material is a key strategy to flexibly manipulate functionalities of materials. In this work, we present a novel case study on the topotactic insertion of fluoride ions into oxygen-deficient ferromagnetic thin films of La0.74Sr0.26MnO3-delta (LSMO) epitaxially grown onto single-crystal SrTiO3 (STO) substrates. The effect of fluorination on the film structure, composition, and magnetic properties is compared with the case of oxygen-deficient and fully-oxidized LSMO films. Although incorporation of F- anions does not significantly alter the volume of the LSMO unit cell, a strong impact on the magnetic characteristics, including a remarkable suppression of Curie temperature and saturation magnetization accompanied by an increase in magnetic coercivity, was found. The change in magnetic properties can be ascribed to the disruption of the ferromagnetic exchange interactions along Mn-anion-Mn chains driven by F- doping into the LSMO lattice. Our results indicate that F- doping is a powerful means to effectively modify the magnetic functional properties of perovskite manganites.
Magnetoelectric coupling at the La1-xSrxMnO3/ionic liquid interface
A Molinari
2018
Abstract
One of the major quests in today's microelectronic era is the development of novel low-power magnetic devices for a variety of applications spanning from memory storage and processing to transduction and sensing. Control of magnetism by means of an electric field, based on the phenomenon of magnetoelectric (ME) effect, may be the key alternative to conventional electronics relying on dissipative electrical currents.
In the last years various strategies to interconnect electric and magnetic degrees of freedom have been put to test. A promising approach to straightforwardly and precisely master ME coupling is via charge carrier doping of a magnetic material using an external voltage. This can be realized, akin to the working principle of the field effect transistor, by gating a magnetic electrode with an electrically-polarizable solid (e.g. a dielectric or a ferroelectric) or a liquid electrolyte.
This dissertation reports on the investigation of ME coupling at solid/liquid interfaces in a prototypical system consisting of a La1-xSrxMnO3 (LSMO) magnetic electrode electrically charged with an ionic liquid (IL) electrolyte. LSMO - a magnetic perovskite manganite - belongs to the celebrated class of strongly-correlated oxides featuring multiple magnetic states, which directly depend on the oxidation state of the magnetically-coupled manganese ions (Mn3+/4+). Upon voltage-driven charge doping the Mn oxidation state is altered, which, in turn, allows to control the balance between double-exchange and superexchange magnetic interactions in LSMO. Furthermore, LSMO possesses a para/ferromagnetic transition slightly above room temperature, which makes it a promising candidate in the perspective of potential applications.
Epitaxial thin (about 13 nm) and ultrathin (about 3 nm) LSMO films were grown onto single-crystalline SrTiO3 substrates via Large-Distance Magnetron Sputtering (LDMS). This deposition technique demonstrated to be an ideal tool for fabrication of LSMO films with highest quality in terms of crystallinity, surface smoothness and magnetic properties.
The interfacial ME coupling was investigated by combining in situ Superconducting Quantum Interference Device (SQUID) magnetometry and Cyclic Voltammetry (CV). This experimental configuration allowed to concurrently extract quantitative information about surface charge density and magnetization as a function of different applied voltages and temperatures.
The analysis of the interfacial charging/discharging processes revealed that the accumulation/depletion of charge carriers is not only driven by electrostatic (electric double layer capacitance) but also electrochemical (redox pseudocapacitance) doping. The presence of both charging mechanisms indicated that the LSMO/IL system behaves as an archetypal hybrid supercapacitor.
Large values of surface charge density up to about 300 microC/cm2 enabled to robustly and flexibly control the magnetic response of LSMO. In case of LSMO thin films with a thickness of about 13 nm a relative magnetic change DeltaM/M of up to 33% was reached, whereas for thinner LSMO films of about 3 nm, featuring an enhanced surface-to-volume ratio, ferromagnetism (FM) could be completely suppressed and restored at will. Together with the significant magnitude of the magnetic tuning effect, IL gating provided an outstanding level of reversibility upon cycling, low energy consumption and remarkable switching speed. Additionally, the magnetic signal could be manipulated in-phase and/or anti-phase with respect to the surface charge modulation by appropriately adjusting the applied voltage.
The observed interfacial ME coupling can be qualitatively explained with the major features of the bulk magnetoelectronic phase diagram of LSMO. However, in this study a more precise and consistent microscopic model is proposed on the basis of the quantitative values of the ME coupling coefficient |alpha| = |Delta M/Delta Q| = 3 muB/h+ and the phenomenon of magnetic phase separation. In such scenario competing FM and non-FM domains expand or shrink at the expense of each other upon voltage-induced charge doping.
On the whole, this work intends to elucidate the physico-chemical mechanisms behind the ME effect at solid/liquid interfaces with the aim of fostering further studies in the yet unexplored area of ME supercapacitors.
Voltage-Controlled On/Off Switching of Ferromagnetism in Manganite Supercapacitors
A Molinari, H Hahn, R Kruk
Advanced Materials 30 (1), 1703908, 2018
Abstract
The ever-growing technological demand for more advanced microelectronic and spintronic devices keeps catalyzing the idea of controlling magnetism with an electric field. Although voltage-driven on/off switching of magnetization is already established in some magnetoelectric (ME) systems, often the coupling between magnetic and electric order parameters lacks an adequate reversibility, energy efficiency, working temperature, or switching speed. Here, the ME performance of a manganite supercapacitor composed of a ferromagnetic, spin-polarized ultrathin film of La0.74Sr0.26MnO3 (LSMO) electrically charged with an ionic liquid electrolyte is investigated. Fully reversible, rapid, on/off switching of ferromagnetism in LSMO is demonstrated in combination with a shift in Curie temperature of up to 26 K and a giant ME coupling coefficient of alpha about 226 Oe V^-1. The application of voltages of only about 2 V results in ultralow energy consumptions of about 90 microJ cm^-2. This work provides a step forward toward low-power, high-endurance electrical switching of magnetism for the development of high-performance ME spintronics.
Proton Conduction in Grain-Boundary-Free Oxygen-Deficient BaFeO2.5+delta Thin Films
A. Benes, A. Molinari, R. Witte, R. Kruk, J. Broetz, R. Chellali, H. Hahn, O. Clemens,
Materials 11, 52 (2018),
Abstract
Reduction of the operating temperature to an intermediate temperature range between 350 C and 600 C is a necessity for Solid Oxide Fuel/Electrolysis Cells (SOFC/SOECs). In this respect the application of proton-conducting oxides has become a broad area of research. Materials that can conduct protons and electrons at the same time, to be used as electrode catalysts on the air electrode, are especially rare. In this article we report on the proton conduction in expitaxially grown BaFeO2.5+delta (BFO) thin films deposited by pulsed laser deposition on Nb:SrTiO3 substrates. By using Electrochemical Impedance Spectroscopy (EIS) measurements under different wet and dry atmospheres, the bulk proton conductivity of BFO (between 200 C and 300 C) could be estimated for the first time (3.6 x 10^-6 S cm^-1 at 300 C). The influence of oxidizing measurement atmosphere and hydration revealed a strong dependence of the conductivity, most notably at temperatures above 300 C, which is in good agreement with the hydration behavior of BaFeO2.5 reported previously.
2017
Hybrid supercapacitors for reversible control of magnetism
A. Molinari, P. M. Leufke, C. Reitz, S. Dasgupta, R. Witte, R. Kruk, H. Hahn
Nature Communications 8, 15339 (2017),
Abstract
Electric field tuning of magnetism is one of the most intensely pursued research topics of recent times aiming at the development of new-generation low-power spintronics and microelectronics. However, a reversible magnetoelectric effect with an on/off ratio suitable for easy and precise device operation is yet to be achieved. Here we propose a novel route to robustly tune magnetism via the charging/discharging processes of hybrid supercapacitors, which involve electrostatic (electric-double-layer capacitance) and electrochemical (pseudocapacitance) doping. We use both charging mechanisms occurring at the La0.74Sr0.26MnO3/ionic liquid interface to control the balance between ferromagnetic and non-ferromagnetic phases of La1-xSrxMnO3 to an unprecedented extent. A magnetic modulation of up to 33% is reached above room temperature when applying an external potential of only about 2.0 V. Our case study intends to draw attention to new, reversible physico-chemical phenomena in the rather unexplored area of magnetoelectric supercapacitors.
Structure and conductivity of epitaxial thin films of barium ferrite and its hydrated form BaFeO2.5-x+delta(OH)2x
P. A. Sukkurji, A. Molinari, A. Benes, C. Loho, V. S. K. Chakravadhanula, S. K. Garlapati, O. Clemens,
Journal of Physics D: Applied Physics 50, 115302 (2017),
Abstract
Barium ferrite and its hydrated form (BaFeO2.5-x+delta(OH)2x, BFO) is an interesting cathode material for protonic ceramic fuel cells (PCFC) due to its potential to be both, conducting for electrons and protons. We report on the fabrication of almost epitaxially grown thin films (22 nm) of barium ferrite BaFeO~2.5 (BFO) on Nb-doped SrTiO3 substrates via pulsed laser deposition (PLD), followed by treatment under inert, and subsequently wet inert atmospheres to induce water (respectively proton) incorporation. Microstructure, chemical composition and conducting properties are investigated for the BFO films and their hydrated forms, highlighting the influence of hydration on the conductivity characteristics between ~200 - 290 K. We find that water incorporation gives a strong enhancement of the conductivity to ~10^-9 S cm^-1 compared to argon annealed films, inducing electronic and protonic charge carriers at the same time. In comparison to bulk powders, proton conductivity is found to be strongly suppressed in such thin hydrated BFO films, pointing towards the influence of strain on the conductivity, which is evaluated based on a detailed investigation by high-resolution transmission electron microscopy.
Epitaxial strain-engineered self-assembly of magnetic nanostructures in FeRh thin films
R. Witte, R. Kruk, A. Molinari, D. Wang, S. Schlabach, R. A. Brand, V. Provenzano, H. Hahn,
Journal of Physics D: Applied Physics 50, 025007 (2017),
Abstract
In this paper we introduce an innovative bottom-up approach for engineering self-assembled magnetic nanostructures using epitaxial strain-induced twinning and phase separation. X-ray diffraction, 57Fe Moessbauer spectroscopy, scanning tunneling microscopy, and transmission electron microscopy show that epitaxial films of a near-equiatomic FeRh alloy respond to the applied epitaxial strain by laterally splitting into two structural phases on the nanometer length scale. Most importantly, these two structural phases differ with respect to their magnetic properties, one being paramagnetic and the other ferromagnetic, thus leading to the formation of a patterned magnetic nanostructure. It is argued that the phase separation directly results from the different strain-dependence of the total energy of the two competing phases. This straightforward relation directly enables further tailoring and optimization of the nanostructures' properties.
2016
Interface and strain effects on the H-sorption thermodynamics of size-selected Mg nanodots
A. Molinari, F. D' Amico, M. Calizzi, Y. Zheng, C. Boelsma, L. Mooij, Y. Lei, H. Hahn, B. Dam, L. Pasquini,
International Journal of Hydrogen Energy 41, 9841 (2016)
Abstract
This work deals with the thermodynamics of hydride formation in 3-D nanoconfined Mg. Two ensembles of nearly monodisperse Mg nanodots (NDs) with different diameters (60 and 320 nm), were grown by the template nanopatterning method, using ultra-thin alumina membranes (UTAMs) with ordered porosity as evaporation masks. Multilayer NDs consisting of 30 nm Mg, 5 nm Ti and 5 nm Pd were deposited on UTAM-coated glass substrates by molecular beam epitaxy. The lateral surface of the NDs is constituted by native MgO. The morphology of the NDs was characterized by field emission scanning electron microscopy and atomic force microscopy. Hydride formation and decomposition was studied at low temperature (363-393 K) by means of optical hydrogenography. Compared to bulk Mg, the plateau pressure for hydrogen absorption in NDs exhibits an upward shift, which is larger for small NDs. Differently, the desorption plateau pressure is almost the same for the two NDs size and is lower than for bulk Mg. These hydrogen sorption features are discussed in the frame of a model that takes into account both interface energy and elastic strain energy in the constrained nanodots. The onset of plastic deformation, marked by a high pressure hysteresis between hydrogen absorption and desorption isotherms, limits the extent of hydride destabilization that can be achieved by elastic strain engineering.
2015
Isotropic microscale mechanical properties of coral skeletons
L. Pasquini, A. Molinari, P. Fantazzini, Y. Dauphen, J-P. Cuif, O. Levy, Z. Dubinsky, E. Caroselli, F. Prada, S. Goffredo, M. Di Giosia, M. Reggi, G. Falini
Journal of the Royal Society Interface 12 (2015)
Abstract
Scleractinian corals are a major source of biogenic calcium carbonate, yet the relationship between their skeletal microstructure and mechanical properties has been scarcely studied. In this work, the skeletons of two coral species: solitary Balanophyllia europaea and colonial Stylophora pistillata, were investigated by nanoindentation. The hardness HIT and Young's modulus EIT were determined from the analysis of several load-depth data on two perpendicular sections of the skeletons: longitudinal (parallel to the main growth axis) and transverse. Within the experimental and statistical uncertainty, the average values of the mechanical parameters are independent on the section's orientation. The hydration state of the skeletons did not affect the mechanical properties. The measured values, EIT in the 76 - 77 GPa range, and HIT in the 4.9 - 5.1 GPa range, are close to the ones expected for polycrystalline pure aragonite. Notably, a small difference in HIT is observed between the species. Different from corals, single-crystal aragonite and the nacreous layer of the seashell Atrina rigida exhibit clearly orientation-dependent mechanical properties. The homogeneous and isotropic mechanical behaviour of the coral skeletons at the microscale is correlated with the microstructure, observed by electron microscopy and atomic force microscopy, and with the X-ray diffraction patterns of the longitudinal and transverse sections.
2014
The power of in situ pulsed laser deposition synchrotron characterization for the detection of domain formation during growth of Ba0.5Sr0.5TiO3 on MgO
S. Bauer, S. Lazarev, A. Molinari, A. Breitenstein, P. Leufke, R. Kruk, H. Hahn, T. Baumbach,
Journal of Synchrotron Radiation 21, 386 (2014)
Abstract
A highly sophisticated pulsed laser deposition (PLD) chamber has recently been installed at the NANO beamline at the synchrotron facility ANKA (Karlsruhe, Germany), which allows for comprehensive studies on the PLD growth process of dielectric, ferroelectric and ferromagnetic thin films in epitaxial oxide heterostructures or even multilayer systems by combining in situ reflective high-energy diffraction with the in situ synchrotron high-resolution X-ray diffraction and surface diffraction methods. The modularity of the in situ PLD chamber offers the opportunity to explore the microstructure of the grown thin films as a function of the substrate temperature, gas pressure, laser fluence and target-substrate separation distance. Ba0.5Sr0.5TiO3 grown on MgO represents the first system that is grown in this in situ PLD chamber and studied by in situ X-ray reflectivity, in situ two-dimensional reciprocal space mapping of symmetric X-ray diffraction and acquisition of time-resolved diffraction profiles during the ablation process. In situ PLD synchrotron investigation has revealed the occurrence of structural distortion as well as domain formation and misfit dislocation which all depend strongly on the film thickness. The microstructure transformation has been accurately detected with a time resolution of 1 s. The acquisition of two-dimensional reciprocal space maps during the PLD growth has the advantage of simultaneously monitoring the changes of the crystalline structure as well as the formation of defects. The stability of the morphology during the PLD growth is demonstrated to be remarkably affected by the film thickness. A critical thickness for the domain formation in Ba0.5Sr0.5TiO3 grown on MgO could be determined from the acquisition of time-resolved diffraction profiles during the PLD growth. A splitting of the diffraction peak into two distinguishable peaks has revealed a morphology change due to modification of the internal strain during growth.
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