Unraveling Ferroelectric Polarization and Ionic Contributions to Electroresistance in Epitaxial Hf0.5Zr0.5O2 Tunnel Junctions

M. Cervo Sulzbach, S. Estandía, X. Long, J. Lyu, N. Dix, J. Gàzquez, M. F. Chisholm, F. Sánchez, I. Fina,* and J. Fontcuberta*

Adv. Electron. Mater. 2019, 1900852

DOI: 10.1002/aelm.201900852

Abstract

Tunnel devices based on ferroelectric Hf0.5Zr0.5O2 (HZO) barriers hold great promises for emerging data storage and computing technologies. The resistance state of the device can be changed through use of a suitable writing voltage. However, the microscopic mechanisms leading to the resistance change are an intricate interplay between ferroelectric polarization controlled barrier properties and defect‐related transport mechanisms. The fundamental role of the microstructure of HZO films determining the balance between those contributions is demonstrated. The HZO film presents coherent or incoherent grain boundaries, associated to the coexistance of monoclinic and orthorhombic phases, which are dictated by the mismatch with the substrates for epitaxial growth. These grain boundaries are the toggle that allows to obtain either large (up to ≈450%) and fully reversible genuine polarization controlled electroresistance when only the orthorhombic phase is present or an irreversible and extremely large (≈103–105%) electroresistance when both phases coexist.

 

Strain and voltage control of magnetic and electric properties of FeRh films

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.

 

Direct Reversible Magnetoelectric Coupling in a Ferroelectric/Ferromagnetic Structure Controlled by Series Resistance Engineering

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.

 

Synergetic Electronic and Ionic Contributions to Electroresistance in Ferroelectric Capacitors

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

Advanced use of ferroelectric capacitors in data storage and computing relies on the control of their electrical resistance (electroresistance, ER) by the change of the electrostatic potential profile across the capacitor occurring upon electric field–driven polarization switching. Here it is reported the observation that BaTiO3‐based capacitors, sandwiched between Pt and La2/3Sr1/3MnO3 electrodes, display a large ER, whose magnitude (near 104% at room temperature) and sign (ER > 0, ER < 0) are determined by the writing pulse duration and temperature. Temperature‐dependent measurements have been instrumental to obtain evidence of the presence of a thermally activated process coexisting with the electronic changes produced by ferroelectric polarization switching, both contributing to ER. Detailed analysis allows concluding that the thermally activated process can be attributed to field‐assisted ionic motion. It is argued that the relative balance between purely electronic and ionic diffusion processes modulate the height of the interfacial Schottky barriers and, consequently, are responsible for the observed variations of magnitude and sign of ER.

 

Independent Tuning of Optical Transparency Window and Electrical Properties of Epitaxial SrVO3 Thin Films by Substrate Mismatch

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

Transparent metallic oxides are pivotal materials in information technology, photovoltaics, or even in architecture. They display the rare combination of metallicity and transparency in the visible range because of weak interband photon absorption and weak screening of free carriers to impinging light. However, the workhorse of current technology, indium tin oxide (ITO), is facing severe limitations and alternative approaches are needed. AMO3 perovskites, M being a nd1 transition metal, and A an alkaline earth, have a genuine metallic character and, in contrast to conventional metals, the electron–electron correlations within the nd1 band enhance the carriers effective mass (m*) and bring the transparency window limit (marked by the plasma frequency, ωp*) down to the infrared. Here, it is shown that epitaxial strain and carrier concentration allow fine tuning of optical properties (ωp*) of SrVO3 films by modulating m* due to strain‐induced selective symmetry breaking of 3d‐t2g(xyyzxz) orbitals. Interestingly, the DC electrical properties can be varied by a large extent depending on growth conditions whereas the optical transparency window in the visible is basically preserved. These observations suggest that the harsh conditions required to grow optimal SrVO3 films may not be a bottleneck for their future application.

Solid-State Synapses Modulated by Wavelength-Sensitive Temporal Correlations in Optic Sensory Inputs

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.

On the role of interfaces on spin transport in magnetic insulator/normal metal heterostructures

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.

 

Engineering Ferroelectric Hf0.5Zr0.5O2 Thin Films by Epitaxial Stress

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 Hf0.5Zr0.5O2(HZO) and La0.67Sr0.33MnO3 (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.

 

Plasticity of Persistent Photoconductance of Amorphous LaAlO3/SrTiO3 Interfaces under Varying Illumination Conditions

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.

Enhanced ferroelectricity in epitaxial Hf0.5Zr0.5O2 thin films integrated with Si(001) using SrTiO3 templates

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

 
SrTiO3 templates have been used to integrate epitaxial bilayers of ferroelectric Hf0.5Zr0.5O2and La2/3Sr1/3MnO3 bottom electrodes on Si(001). The Hf0.5Zr0.5O2 films show enhanced properties in comparison to equivalent films on SrTiO3(001) single crystalline substrates. The films, thinner than 10 nm, have a very high remnant polarization of 34 μC/cm2. Hf0.5Zr0.5O2capacitors at an operating voltage of 4 V present a long retention time well beyond 10 years and high endurance against fatigue up to 109 cycles. The robust ferroelectric properties displayed by the epitaxial Hf0.5Zr0.5O2 films on Si(001) using SrTiO3 templates pave the way for the monolithic integration on silicon of emerging memory devices based on epitaxial HfO2.
Financial support from the Spanish Ministry of Economy, Competitiveness and Universities, through the “Severo Ochoa” Programme for Centres of Excellence in R&D (No. SEV-2015-0496) and the MAT2017-85232-R (AEI/FEDER, EU) and MAT2015-73839-JIN projects and from Generalitat de Catalunya (2017 SGR 1377) is acknowledged. I.F. acknowledges Ramón y Cajal Contract No. RYC-2017-22531. J.L. is financially supported by the China Scholarship Council (CSC) with No. 201506080019. S.E. acknowledges the Spanish Ministry of Economy, Competitiveness and Universities for his Ph.D. Contract (No. SEV-2015-0496-16-3) and its cofunding by the ESF. The work of J.L. and S.E. has been done as a part of their Ph.D. program in Materials Science at Universitat Autònoma de Barcelona. INL authors acknowledge the financial support from the European Commission through the Project TIPS (No. H2020-ICT-02-2014-1-644453) and from the French national research agency (ANR) through the projects DIAMWAFEL (No. ANR-15-CE08-0034), LILIT (No. ANR-16-CE24-0022), and MITO (No. ANR-17-CE05-0018). They are also grateful to the joint laboratory INL-RIBER and P. Regreny, C. Botella, and J. B. Goure for the MBE technical support on the Nanolyon technological platform.

Towards oxide electronics: A roadmap

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.

Multiple magnetic droplet soliton modes

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 (1GHz) 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.

Gap suppression at a Lifshitz transition in a multi-condensate superconductor


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.

Topochemical nitridation of Sr2FeMoO6

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, Fm[3 with combining macron]m double perovskite oxynitride Sr2FeMoO4.9N1.1 where 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 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.

Disclosing odd symmetry, strain driven magnetic response of Co on Pt/PMN-PT (011)

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(Mg0.33Nb0.67)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.

High Carrier Mobility, Electrical Conductivity, and Optical Transmittance in Epitaxial SrVO3 Thin Films

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.

Complementary Resistive Switching Using Metal–Ferroelectric–Metal Tunnel Junctions

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.

Epitaxial Integration on Si(001) of Ferroelectric Hf0.5Zr0.5O2 Capacitors with High Retention and Endurance

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 Hf0.5Zr0.5O2 films have been successfully integrated in a capacitor heterostructure on Si(001). The orthorhombic Hf0.5Zr0.5O2phase, [111] out-of-plane oriented, is stabilized in the films. The films present high remnant polarization Pr 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 La0.67Sr0.33MnO3 but not on LaNiO3. The demonstration of excellent ferroelectric properties in epitaxial films of Hf0.5Zr0.5O2 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.

The Growth Window of Ferroelectric Epitaxial Hf0.5Zr0.5O2 Thin Films

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 Hf0.5Zr0.5O2 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 (EC) depends on thickness (t) according to the EC – 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 Hf0.5Zr0.5O2 films paves the way toward understanding their ferroelectric properties and prototyping devices.

Non-reciprocal diffraction in magnetoplasmonic gratings

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.

Unexpected large transverse magneto-optic Kerr effect at quasi-normal incidence in magnetoplasmonic crystals

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.

Quantification of propagating and standing surface acoustic waves by stroboscopic X-ray photoemission electron microscopy

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.

Asymmetric Resistive Switching Dynamics in BaTiO3 Tunnel Junctions

M. Qian, I. Fina, F. Sánchez, and J. Fontcuberta

Adv. Electron. Mater. 5, 1800407, 2019

DOI: 10.1002/aelm.201800407

ABSTRACT

The resistive switching associated with polarization reversal is studied in detail in ferroelectric BaTiO3 tunnel junctions, with focus on the dynamics of the ferroelectric domain switching. It is observed that the transition between the high‐resistance state (HRS) and the low‐resistance state (LRS) is largely asymmetric being smooth from LRS to HRS, but proceeds via avalanches in the HRS‐to‐LRS transitions. It is shown that this distinct behavior is related to the presence of an imprint field in the junction and has important consequences on the junction’s performance.

Giant topological Hall effect in correlated oxide thin films

L Vistoli, W Wang, A Sander, Q Zhu, B Casals, R Cichelero, A Barthélémy, S Fusil, G Herranz, S Valencia, R Abrudan, E Weschke, K Nakazawa, H Kohno, J Santamaria, W Wu, V Garcia & M Bibes 

Nature Physics 15, 67 – 72 (2019)

https://doi.org/10.1038/s41567-018-0307-5

ABSTRACT

Strong electronic correlations can produce remarkable phenomena such as metal–insulator transitions and greatly enhance superconductivity, thermoelectricity or optical nonlinearity. In correlated systems, spatially varying charge textures also amplify magnetoelectric effects or electroresistance in mesostructures. However, how spatially varying spin textures may influence electron transport in the presence of correlations remains unclear. Here we demonstrate a very large topological Hall effect (THE) in thin films of a lightly electron-doped charge-transfer insulator, (Ca,Ce)MnO3. Magnetic force microscopy reveals the presence of magnetic bubbles, whose density as a function of magnetic field peaks near the THE maximum. The THE critically depends on carrier concentration and diverges at low doping, near the metal–insulator transition. We discuss the strong amplification of the THE by correlation effects and give perspectives for its non-volatile control by electric fields.

The 2016 oxide electronic materials and oxide interfaces roadmap.

M Lorenz, M S Ramachandra Rao, T Venkatesan, E Fortunato, P Barquinha, R Branquinho, D Salgueiro, R Martins, E Carlos, A Liu, F K Shan, M Grundmann, H Boschker, J Mukherjee, M Priyadarshini,N DasGupta, D J Rogers, F H Teherani, E V Sandana, P Bove, K Rietwyk, A Zaban, A Veziridis,A Weidenkaff, M Muralidhar, M Murakami, S Abel, J Fompeyrine, J Zuniga-Perez, R Ramesh,N A Spaldin, S Ostanin, V Borisov, I Mertig, V Lazenka, G Srinivasan, W Prellier, M Uchida,M Kawasaki, R Pentcheva, P Gegenwart, F Miletto Granozio, J Fontcuberta and N Pryds.

J. Phys. D: Appl. Phys. 49 (2016) 433001 (53pp). DOI: 10.1088/0022-3727/49/43/433001

Giant Optical Polarization Rotation Induced by Spin-Orbit Coupling in Polarons

In this work, published in Physical Review Letters, we demonstrate that the polarization of light is changed dramatically when its energy resonates with the polaron-hopping energy of self-trapped electrons.  In the presence of spin-orbit coupling, the self-trapped hopping electron may reverse its spin producing a large gyrotropic effect, which is observed close to room temperature. The effect is observed in La23Ca13MnO3 thin films and, plausibly, should be present in other manganites.

Since straining this kind of materials can change the polaron concentration, our finding suggests a route to mechanical or electro-mechanical control of the magneto-optical effect, paving  the way to unconventional magnetoelectric effects.

This work has been highlighted with a Synopsis on the Physics website:

http://physics.aps.org/synopsis-for/10.1103/PhysRevLett.117.026401

 

 

The Shortening of MWNT-SPION Hybrids by Steam Treatment Improves Their Magnetic Resonance Imaging Properties In Vitro and In Vivo.

Laura Cabana, Maxime Bourgognon, Julie T.-W. Wang, Andrea Protti, Rebecca Klippstein, Rafael T. M. de Rosales, Ajay M. Shah, Josep Fontcuberta, Ester Tobías-Rossell, Jane K. Sosabowski, Khuloud T. Al-Jamal,* and Gerard Tobias*

Small 2016, 12, No. 21, 2893–2905. DOI: 10.1002/smll.201502721

large roomtemperature

Large Room-Temperature Electroresistance in Dual-Modulated Ferroelectric Tunnel Barriers

Greta Radaelli , Diego Gutiérrez , Florencio Sánchez , Riccardo Bertacco , Massimiliano Stengel , and Josep Fontcuberta
Adv. Mater. 2015, DOI: 10.1002/adma.201405117


Pt/BaTiO3/La0.7Sr0.3MnO3 tunnel junctions.

It is demonstrated that reversing the polarization direction of a ferroelectric barrier in a tunnel junction leads to a change of junction conductance and capacitance, with concomitant variations on the barrier height and effective thickness, both contributing to produce larger electroresistance

Electrons moving in 2D: symmetry matters

The development of today’s electronics is reaching fundamental limits and novel concepts and materials are researched to achieve ever faster and more efficient electronic devices. In this regard, oxides are well positioned, as they display a fascinating complexity of electronic phases –not present in conventional semiconductors–that may open new pathways.

The interface between LaAlO3 and SrTiO3 is the oxide quantum well (QW) par excellence, where superconductivity and magnetism emerge. In LaAlO3/SrTiO3 QWs electrons move in two dimensions forming a two-dimensional electron gas (2DEG, Box a). However, not all the electrons reside in the same QW subband (Box b) and, indeed, they behave differently depending on which orbital are they occupying. We demonstrate that the symmetry of the conduction band inside the QWs can be selected, so that the properties of electrons vary largely. In particular, we show that in the superconductive state Cooper pairs do not bind in the same way either when we change the number of electrons in the 2DEG by electric fields (Box a) or when the direction of confinement is changed (Box b). We also show that choosing the confinement plane allows manipulating the spin-dependent transport in these QWs.

Our work opens a groundbreaking route to manipulate selectively electrons in two dimensions. See more @ G. Herranz at al., Engineering two-dimensional superconductivity and Rashba spin–orbit coupling in LaAlO3/SrTiO3 quantum wells by selective orbital occupancy, Nature Communications 2014, doi:10.1038/ncomms7028. 

Imagen1

Magneto-optic responses boosted by plasmons

Imagen1

 

OndřejVlašín, OanaPascu, Anna Roig, and GervasiHerranz

Phys. Rev. Applied 2, 054003 (2014)  DOI: http://dx.doi.org/10.1103/PhysRevApplied.2.054003

 


We show that very diluted dispersions of ferromagnetic metal nanoparticles in diamagnetic hosts show a large, linear response under an applied magnetic field, which is of interest for emerging applications in sensing, integrated optical communications, and magneto-optical current transformers and transducers.  We present also a theory that describes accurately these results and that can be further extended to describe the optical properties of e.g., metal inclusions in polymers or glasses.

prl113

Two-Dimensional Electron Gases at LaAlO3/SrTiO3 Interfaces: Orbital Symmetry and Hierarchy Engineered by Crystal Orientation

prl113

 

D. Pesquera, M. Scigaj, P. Gargiani, A. Barla, J. Herrero-Martín, E. Pellegrin, S. M. Valvidares, J. Gázquez, M. Varela, N. Dix, J. Fontcuberta, F. Sánchez, and G. Herranz

Phys. Rev. Lett. 113, 156802 (2014), DOI: http://dx.doi.org/10.1103/PhysRevLett.113.156802

 


Quantum wells at the LaAlO3/SrTiO3 interface with tunable electron states

Quantum wells with d-electrons at the LaAlO3(LAO)/SrTiO3(STO) interface exhibit physical properties, such as superconductivity or magnetism, unseen in conventional semiconductors (Si, Ge, GaAs, …). We have demonstrated that the symmetry of the conduction band inside the QWs can be selected at whish. This opens up novel perspectives to manipulate the electronic properties of QWs at the LAO/STO interface.

apl2014

Spin Hall magnetoresistance at Pt/CoFe2O4 interfaces and texture effects

apl2014

 

MirenIsasa, Amilcar Bedoya-Pinto, Saül Vélez, Federico Golmar, Florencio Sánchez, Luis E. Hueso, Josep Fontcuberta, and Fèlix Casanova
Applied Physics Letters 105, 142402 (2014) 


 SPINTRONICS WITH NON-MAGNETIC MATERIALS. The neighborhood   makes all difference

It has been discovered that  electric resistance of a nonmagnetic metal (Pt in our case) is depending on  magnetization state of neighboring insulating ferromagnetic layer, namely on the atomic planes forming the interface and their crystallographic orientation.

Electric transport through nanometric CoFe2O4 thin films investigated by conducting atomic force microscopy

M. Foerster, D.F. Gutierrez, J. M. Rebled, E. Arbelo, F. Rigato, M. Jourdan, F. Peiró, J. Fontcuberta

J. Appl. Phys. 111, 013904 (2012)

http://dx.doi.org/10.1063/1.3672839

A systematic study of electric transport through thin (2–8 nm) CoFe2O4 films deposited on epitaxial

SrRuO3 bottom electrodes was performed by conducting atomic force microscopy (CAFM).

Experimental procedures to investigate transport through thin insulating films by CAFM are critically

revised, and the potential of CoFe2O4 films for the use as spin-filtering barriers is assessed. It is concluded that, at room-temperature, a non-tunnel channel significantly contributes to the electric transport, thuslimiting the spin-filtering efficiency.