Tag: 2019

B. Casals, S. van Dijken, G. Herranz, and E. K. H. Salje

Phys. Rev. Research 1, 032025(R) – Published 26 November 2019

DOI: 10.1103/PhysRevResearch.1.032025

Abstract

Ignasi Fina and Josep Fontcuberta

Journal of Physics D: Applied Physics, Volume 53, Number 2

DOI: 10.1088/1361-6463/ab4abd

Abstract

FeRh based alloys may display an uncommon transition from a ferromagnetic to an antiferromagnetic state upon cooling. The transition takes place roughly above room temperature and it can be sensitively modulated by composition and external parameters, including pressure and strain. Consequently, thin films of FeRh have received a great deal of attention for its potential applications in spintronics, antiferromagnetic spintronics and sensing. Interestingly, the extreme sensitivity of its properties to strain has created expectations for energy friendly voltage-control of the magnetic state of FeRh, with a number of potential applications at the horizon. Here, after summarizing the current understanding of strain effects on the magnetic properties of FeRh thin films, we review the achievements of exploiting piezoelectric substrates for in operando tuning of their magneto-electric properties. We conclude with a brief summary and an outlook for future initiatives.

The Nobel prize in Chemistry, awarded this year 2019 to J.B. Goodenough, has been longly waited by a large community of material’s scientist. This is a recognition to the “Oxide Science and Technology” and its impact on society. The brilliant mind of J. B.Goodenough allowed to rationalize the electric and magnetic properties of metal oxides, discovering the clues that govern these properties and providing the tools to understand and transform them into functional materials. It is an amazing coincidence, that on the 150 anniversary of the periodic table, the Nobel award in Chemistry has recognized the enormous knowledge of J. B. Goodenough and ability to combine chemical elements, in their most common oxide form, to create magnets and batteries that have changed our life. His books on “Les oxides des métaux de transition” and “Magnetism and Chemical bond” and his crucial intuition and perseverance on the role of oxides to store electric charge in batteries, constitute pillars of our culture that are going to last forever.

J.B. Goodenough hosted me as a young postdoc at Oxford University, when he was the head of Inorganic Laboratory. There, I enjoyed his exhilarating humanity, his respect for the young students and his deep knowledge and enthusiasm. The giant was convinced that there this a lot of randomness and fortune in decisions and life, but one need to be there. He tried to teach me, how to wear a tie (without much success), and he contaminated me with his passion the intricate “lego” world of oxides.

Latter, J. B. Goodenough became member of the International Advisory of the Material’s science Institutes of CSIC.

The warmest congratulations and beyond the Oslo party, keep the strength to blow the next coming 98 candles.

S. González-Casal, I. Fina, F. Sánchez, J. Fontcuberta

ACS Appl. Electron. Mater. 2019, 191937-1944

DOI:10.1021/acsaelm.9b00427

Abstract

Achieving large magnetoelectric coupling is of interest for memory and communication applications. In multiferroic hybrid structures (combining ferroelectric and magnetic materials) in the presence of a magnetoelectric coupling, the ferroelectric properties can be modulated by a magnetic field. This is called the direct magnetoelectric effect. Measuring the ferroelectric properties in multiferroic materials most commonly requires using metallic electrodes that sandwich the ferroelectric material. In the present work, we use the series resistance introduced by the metallic electrode (La2/3Sr1/3MnO3) to manipulate one relevant ferroelectric parameter, i.e., the coercive voltage, of an adjacent ferroelectric layer (BaTiO3) by a magnetic field. We will show that the variations are fully reversible and more apparent at high frequencies; thus, of particular interest for applications, where high commutation rates are required.

M. Qian,  I. Fina,  M. Cervo Sulzbach,  F. Sánchez,  J. Fontcuberta

Advanced Electronic Materials, Volume 5, Issue 3, 1800646 (2019)

https://doi.org/10.1002/aelm.201800646

Abstract

M. Mirjolet, H. B. Vasili, L. López-Conesa, S. Estradé, F. Peiró, J. Santiso, F. Sánchez, P. Machado, P. Gargiani, M. Valvidares, and J. Fontcuberta*

Adv. Funct. Mater. 2019, 1904238

DOI: 10.1002/adfm.201904238

ABSTRACT

Y. Chen, B. Casals, F. Sánchez, G. Herranz

ACS Appl. Electron. Mater. 1, 7, 1189-1197 (2019)

DOI: 10.1021/acsaelm.9b00183

ABSTRACT

Recently, inspired by neurobiological information processing, correlation-based learning has been expressed physically in nonbiological systems by exploiting the time causality of electric signals. Yet, the capability to learn from visual events requires extending these concepts to optical stimuli. Here we show a solid-state system that is sensitive to 100 ms-scale timing of pairs of light stimuli with complementary short/long visible wavelengths, causing asymmetric changes of photoconductance. This property endows optical signals with time causality, leading to wavelength-sensitive time correlations with time scales comparable with those of perceptual recognition. On the basis of these observations, we propose that complex information can be extracted from visual patterns imprinted as spatiotemporal modulations of persistent photoconductance. We suggest that this capability may stimulate neuromorphic hybrid electronic/photonic systems to construct biomimetic spatial memory and navigation maps inspired from neurobiology.

J. Fontcuberta, H. B. Vasili, J. Gàzquez and Dr. F. Casanova.

Adv. Mater. Interfaces, 6, 190047 (2019)

DOI: 10.1002/admi.2019004755

ABSTRACT

Spin currents have emerged as a new tool in spintronics, with promises of more efficient devices. A pure spin current can be generated in a nonmagnetic metallic (NM) layer by a charge current (spin Hall effect). When the NM layer is placed in contact with a magnetic material, a magnetoresistance (spin Hall magnetoresistance) develops in the former via the inverse spin Hall effect (ISHE). In other novel spin‐dependent phenomena, such as spin pumping or spin Seebeck effect, spin currents are generated by magnetic resonance or thermal gradients and detected via ISHE in a neighboring normal metal layer. All cases involve spin transport across interfaces between nonmagnetic metallic layers and magnetic materials; quite commonly, magnetic insulators. The structural, compositional, and electronic differences between these materials and their integration to form an interface, challenge the control and understanding of the spin transport across it, which is known to be sensitive to sub‐nanometric interface features. Here, the authors review the tremendous progress in material’s science achieved during the last few years and illustrate how the spin Hall magnetoresistance can be used as a probe for surface magnetism. The authors end with some views on concerted actions that may allow further progress.

S. Estandía, N. Dix, J. Gazquez, I. Fina, J. Lyu, M. F. Chisholm, J. Fontcuberta, and F. Sánchez

ACS Applied Electronic Materials, 1, 8, 1449-1457 (2019) 

DOI: 10.1021/acsaelm.9b00256

ABSTRACT

The critical impact of epitaxial stress on the stabilization of the ferroelectric orthorhombic phase of hafnia is proved. Epitaxial bilayers of Hf_0.5Zr_0.5O₂(HZO) and La_0.67Sr_0.33MnO₃ (LSMO) electrodes were grown on a set of single crystalline oxide (001)-oriented (cubic or pseudocubic setting) substrates with a lattice parameter in the 3.71–4.21 Å range. The lattice strain of the LSMO electrode, determined by the lattice mismatch with the substrate, is critical in the stabilization of the orthorhombic phase of HZO. On tensilely strained LSMO electrodes, most of the HZO film is orthorhombic, whereas the monoclinic phase is favored when LSMO is relaxed or compressively strained. Therefore, the HZO films on TbScO₃ and GdScO₃ substrates present substantially enhanced ferroelectric polarization in comparison to films on other substrates, including the commonly used SrTiO₃. The capability of having epitaxial doped HfO₂ films with controlled phase and polarization is of major interest for a better understanding of the ferroelectric properties and paves the way for fabrication of ferroelectric devices based on nanometric HfO₂ films.

Y. Chen, B. Casals, G. Herranz

ACS Appl. Electron. Mater. 1, 810-816 (2019)

10.1021/acsaelm.9b00127

ABSTRACT

We report on transport measurements under optical stimulation of persistent photoconductance (PPC) at the interface between amorphous LaAlO₃ films and SrTiO₃ crystals. The spectral response in the visible region was analyzed under varying illumination conditions and exposure times down to milliseconds. The PPC is plastically modulated by optical stimuli of varying strength and duration, demonstrating fine-tuned photoconductive responsivity over a diversity of cumulated timespans. Interestingly, under optimal conditions, the photoconductance is sensitive to intensity contrasts under conditions comparable to bright-sunlight environments. The prospects of exploiting photoconductance—including potential strategies to reach higher sensitivity—are discussed in the context of neuromorphic applications.

J. Lyu, I. Fina, R. Bachelet, G. Saint-Girons, S. Estandıa, J. Gazquez, J. Fontcuberta, and F. Sanchez

Appl. Phys. Lett. 114, 222901 (2019)

doi: 10.1063/1.5096002

ABSTRACT

Guest editors: M. Coll, J. Fontcuberta, N. Pryds and F. Miletto Granozio.

Applied Surface Science, 482, 1-93 (2019)

doi.org/10.1016/j.apsusc.2019.03.312

ABSTRACT

At the end of a rush lasting over half a century, in which CMOS technology has been experiencing a constant and breathtaking increase of device speed and density, Moore’s law is approaching the insurmountable barrier given by the ultimate atomic nature of matter. A major challenge for 21st century scientists is finding novel strategies, concepts and materials for replacing silicon-based CMOS semiconductor technologies and guaranteeing a continued and steady technological progress in next decades. Among the materials classes candidate to contribute to this momentous challenge, oxide films and heterostructures are a particularly appealing hunting ground. The vastity, intended in pure chemical terms, of this class of compounds, the complexity of their correlated behaviour, and the wealth of functional properties they display, has already made these systems the subject of choice, worldwide, of a strongly networked, dynamic and interdisciplinary research community.

Oxide science and technology has been the target of a wide four-year project, named Towards Oxide-Based Electronics (TO-BE), that has been recently running in Europe and has involved as participants several hundred scientists from 29 EU countries. In this review and perspective paper, published as a final deliverable of the TO-BE Action, the opportunities of oxides as future electronic materials for Information and Communication Technologies ICT and Energy are discussed. The paper is organized as a set of contributions, all selected and ordered as individual building blocks of a wider general scheme. After a brief preface by the editors and an introductory contribution, two sections follow. The first is mainly devoted to providing a perspective on the latest theoretical and experimental methods that are employed to investigate oxides and to produce oxide-based films, heterostructuresand devices. In the second, all contributions are dedicated to different specific fields of applications of oxide thin films and heterostructures, in sectors as data storage and computing, optics and plasmonics, magnonics, energy conversion and harvesting, and power electronics.

Nahuel Statuto, Christian Hahn, Joan Manel Hernàndez, Andrew D. Kent, and Ferran Macià

PHYSICAL REVIEW B 99, 174436 (2019)

DOI: 10.1103/PhysRevB.99.174436

ABSTRACT

Droplet solitons are large amplitude localized spin-wave excitations that can be created in magnetic thin films with uniaxial anisotropy by a spin-polarized current flowing through an electrical nanocontact. Here, we report a low-temperature (4 K) experimental study that shows there are multiple and, under certain conditions, combinations of droplet modes, each mode with a distinct high-frequency spin precession (tens of gigahertz). Low-frequency ($\lesssim 1\mathrm{GHz}$) voltage noise is used to assess the stability of droplet modes. It is found that droplets are stable only in a limited range of applied field and current, typically near the current and field at which they nucleate, in agreement with recent predictions. Applied fields in the film plane favor multiple droplet modes, whereas fields perpendicular to the film plane tend to stabilize a single droplet mode. Micromagnetic simulations are used to show that spatial variation in the energy landscape in the nanocontact region (e.g., spatial variation of magnetic anisotropy or magnetic field) can lead to quantized droplet modes and low-frequency mode modulation, characteristics observed in our experiments.

G. Singh, A. Jouan, G. Herranz, M. Scigaj, F. Sánchez, L. Benfatto, S. Caprara, M. Grilli, G. Saiz, F. Couëdo, C. Feuillet-Palma, J. Lesueur & N. Bergeal

Nature Materials, 18, 948 – 954 (2019) 

Doi: 10.1038/s41563-019-0354-z

ABSTRACT

In multi-orbital materials, superconductivity can exhibit several coupled condensates. In this context, quantum confinement in two-dimensional superconducting oxide interfaces offers new degrees of freedom to engineer the band structure and selectively control the occupancy of 3d orbitals by electrostatic doping. Here, we use resonant microwave transport to extract the superfluid stiffness of the (110)-oriented LaAlO3/SrTiO3 interface in the entire phase diagram. We provide evidence of a transition from single-condensate to two-condensate superconductivity driven by continuous and reversible electrostatic doping, which we relate to the Lifshitz transition between 3d bands based on numerical simulations of the quantum well. We find that the superconducting gap is suppressed while the second band is populated, challenging Bardeen–Cooper–Schrieffer theory. We ascribe this behaviour to the existence of superconducting order parameters with opposite signs in the two condensates due to repulsive coupling. Our findings offer an innovative perspective on the possibility to tune and control multiple-orbital physics in superconducting interfaces.

Roberta Ceravola, Carlos Frontera, Judith Oro ́-Solé, Ashley P. Black, Clemens Ritter,  Ignasi Mata, Elies Molins, Josep Fontcuberta* and  Amparo Fuertes

Chem.Commun. 55, 3105 (2019)

DOI: 10.1039/c8cc09845j

ABSTRACT

The topotactic nitridation of cation ordered, tetragonal Sr₂FeMoO₆ in NH₃ at moderate temperatures leads to cubic, Fmm double perovskite oxynitride Sr₂FeMoO_4.9N_1.1 where double-exchange interactions determine ferromagnetic order with T_C ≈ 100 K. Substitution of oxide by nitride induces bond asymmetries and local electronically driven structural distortions, which combined with Fermi level lowering restricts charge itinerancy 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.

Michael Foerster, Ignasi Fina, Simone Finizio, BlaiCasals, AniaMandziak, FrancoisFauthand Lucia Aballe

J. Phys.: Condens. Matter 31 084003 (2019)

DOI: 10.1088/1361-648X/aaf7ee

ABSTRACT

An odd-symmetry magnetic response of multiferroic composites comprising ultrathin Co layers on Pt electrodes on [Pb(Mg_0.33Nb_0.67)O₃]_(1−x)[PbTiO₃] _x (PMN-PT) (0 1 1) piezoelectric substrates is observed upon electrical poling of the PMN-PT substrates: the magnetic easy axis of the Co rotates by 90° in-plane between oppositely poled ferroelectric states, mimicking the signature of a surface polarization charge driven effect, which however can be excluded from the presence of the thick Pt interlayer. The origin for this unexpected behavior is as an odd symmetry piezoelectric response of the PMN-PT substrate, as indicated by x-ray diffraction with applied poling, in combination with conventional magnetoelastic coupling. Ferroelectric characterization reveals corresponding features, possibly related to an unswitchable polarization component.

Mathieu Mirjolet, Florencio Sánchez, and Josep Fontcuberta

Adv. Funct. Materials, 29, 1808432 (2019)

https://doi.org/10.1002/adfm.201808432

h

ABSTRACT

The urgent need for more performant transparent conducting electrodes is stimulating intensive research on oxide thin films based on early transition metals (e.g., V, Nb, Mo, etc.), where it is expected that the partially occupied (i.e., nd¹, nd²…) conduction band will give rise to metallic conductivity. Growing thin films of these oxides typically requires an extremely low oxygen pressure. However, in growth methods involving hyperthermal kinetics (such as pulsed laser deposition), this may have severe detrimental effects on the electrical and optical properties of the film. Here, it is shown that the use of a nonreactive gas during a pulsed laser deposition process allows epitaxial SrVO₃ films to be obtained with low room temperature resistivity (ρ ≈ 31 μΩ cm), large carrier mobility (μ ≈ 8.3 cm²V⁻¹ s⁻¹), and large residual resistivity ratio (RRR ≈ 11.5), while improving optical transparency in the visible range. It is argued that the success of this growth strategy relies on the modulation of energetics of plasma species and a concomitant reduction of defects in the films. These findings may find applications in other oxide‐based thin film technologies (i.e., ferroelectric tunnel memories, etc.) where growth‐induced point effects may compromise functionality.

Mengdi Qian,  Ignasi Fina,  Florencio Sánchez,  Josep Fontcuberta

Small 15, 1805042 (2019)

DOI: 10.1002/smll.201805042

ABSTRACT

Complementary resistive switching (CRS) devices are receiving attention because they can potentially solve the current‐sneak and current‐leakage problems of memory arrays based on resistive switching (RS) elements. It is shown here that a simple anti‐serial connection of two ferroelectric tunnel junctions, based on BaTiO₃, with symmetric top metallic electrodes and a common, floating bottom nanometric film electrode, constitute a CRS memory element. It allows nonvolatile storage of binary states (“1” = “HRS+LRS” and “0” = “LRS+HRS”), where HRS (LRS) indicate the high (low) resistance state of each ferroelectric tunnel junction. Remarkably, these states have an identical and large resistance in the remanent state, characteristic of CRS. Here, protocols for writing information are reported and it is shown that non‐destructive or destructive reading schemes can be chosen by selecting the appropriate reading voltage amplitude. Moreover, this dual‐tunnel device has a significantly lower power consumption than a single ferroelectric tunnel junction to perform writing/reading functions, as is experimentally demonstrated. These findings illustrate that the recent impressive development of ferroelectric tunnel junctions can be further exploited to contribute to solving critical bottlenecks in data storage and logic functions implemented using RS elements.

Jike Lyu, Ignasi Fina, Josep Fontcuberta, and Florencio Sanchez

ACS Appl. Mater. Interfaces, 11 (6), 6224 – 6229 (2019)

https://pubs.acs.org/doi/10.1021/acsami.8b18762

ABSTRACT

Epitaxial ferroelectric Hf_0.5Zr_0.5O₂ films have been successfully integrated in a capacitor heterostructure on Si(001). The orthorhombic Hf_0.5Zr_0.5O₂phase, [111] out-of-plane oriented, is stabilized in the films. The films present high remnant polarization P_r close to 20 μC/cm², rivaling with equivalent epitaxial films on single crystalline oxide substrates. Retention time is longer than 10 years for a writing field of around 5 MV/cm, and the capacitors show endurance up to 10⁹ cycles for a writing voltage of around 4 MV/cm. It is found that the formation of the orthorhombic ferroelectric phase depends critically on the bottom electrode, being achieved on La_0.67Sr_0.33MnO₃ but not on LaNiO₃. The demonstration of excellent ferroelectric properties in epitaxial films of Hf_0.5Zr_0.5O₂ on Si(001) is relevant toward fabrication of devices that require homogeneity in the nanometer scale, as well as for better understanding of the intrinsic properties of this promising ferroelectric oxide.

Jike Lyu, Ignasi Fina, Raul Solanas, Josep Fontcuberta, and Florencio Sanchez.

ACS Appl. Electron. Mater. 1 (2), 220 – 228 (2019)

https://pubs.acs.org/doi/10.1021/acsaelm.8b00065

ABSTRACT

The metastable orthorhombic phase of hafnia is generally obtained in polycrystalline films, whereas in epitaxial films, its formation has been much less investigated. We have grown Hf_0.5Zr_0.5O₂ films by pulsed laser deposition, and the growth window (temperature and oxygen pressure during deposition and film thickness) for epitaxial stabilization of the ferroelectric phase is mapped. The remnant ferroelectric polarization, up to ∼24 μC/cm², depends on the amount of orthorhombic phase and interplanar spacing and increases with temperature and pressure for a fixed film thickness. The leakage current decreases with an increase in thickness or temperature, or when decreasing oxygen pressure. The coercive electric field (E_C) depends on thickness (t) according to the E_C – t^–2/3 scaling, which is observed for the first time in ferroelectric hafnia, and the scaling extends to thicknesses down to around 5 nm. The proven ability to tailor the functional properties of high-quality epitaxial ferroelectric Hf_0.5Zr_0.5O₂ films paves the way toward understanding their ferroelectric properties and prototyping devices.

R. Cichelero, M. Kataja, M. Campoy-Quiles, and G. Herranz.

Optics Express, 26, Issue 26, 34842 – 34852 (2018)

https://doi.org/10.1364/OE.26.034842

ABSTRACT

Phase-matching conditions—used to bridge the wave vector mismatch between light and surface plasmon polaritons (SPPs)—have been exploited recently to enable nonreciprocal optical propagation and enhanced magneto-optic responses in magnetoplasmonic systems. Here we show that using diffraction in conjunction with plasmon excitations leads to a photonic system with a more versatile and flexible response. As a testbed, we analyzed diffracted magneto-optical effects in magnetoplasmonic gratings, where broken time-reversal symmetry induces frequency shifts in the energy and angular spectra of plasmon resonance. These result in exceptionally large responses in the diffracted magneto-optical effect. The concepts presented here can be used to develop non-reciprocal optical devices that exploit diffraction, in order to achieve tailored electromagnetic responses.

R. Cichelero, M.A. Oskuei, M. Kataja, S.M. Hamidi, G. Herranz.

Journal of Magnetism and Magnetic Materials 476, 54 – 58 (2019)

https://doi.org/10.1016/j.jmmm.2018.12.036

ABSTRACT

We investigate the transverse magneto-optic Kerr effect (TMOKE) of magnetoplasmonic crystals grown on top of commercial optical disks. From full angle-resolved scans we can identify Wood’s anomalies related to the excitation of plasmons of different orders. From these maps we also detect a wide range of wavelengths and angles of incidence for which the TMOKE signal is increased due to the interaction of light with surface propagating plasmons. Remarkably, conditions are established for unexpectedly large responses at quasi-normal incidence, where, by fundamental symmetry reasons, the intrinsic TMOKE should be vanishingly small. The key towards this unexpected outcome is to engineer the geometry of magnetoplasmonic crystals, so that first-order plasmon dispersion lines run up towards quasi-normal angles of incidence. These results provide general rules for magneto-optic enhancement and, in particular, show the potential of standard commercial disks as platforms for enhanced magneto-optic devices.

Michael Foerster, Nahuel Statuto, Blai Casals, Alberto Hernández-Mínguez, Simone Finizio, Ania Mandziak, Lucia Aballe, Joan Manel Hernàndez Ferràs and Ferran Macià

Journal of Synchrotron Radiation, 26, 184 – 193 (2019)

doi.org/10.1107/S1600577518

ABSTRACT

The quantification of surface acoustic waves (SAWs) in LiNbO₃ piezoelectric crystals by stroboscopic X-ray photoemission electron microscopy (XPEEM), with a temporal smearing below 80 ps and a spatial resolution below 100 nm, is reported. The contrast mechanism is the varying piezoelectric surface potential associated with the SAW phase. Thus, kinetic energy spectra of photoemitted secondary electrons measure directly the SAW electrical amplitude and allow for the quantification of the associated strain. The stroboscopic imaging combined with a deliberate detuning allows resolving and quantifying the respective standing and propagating components of SAWs from a superposition of waves. Furthermore, standing-wave components can also be imaged by low-energy electron microscopy (LEEM). Our method opens the door to studies that quantitatively correlate SAWs excitation with a variety of sample electronic, magnetic and chemical properties.