M. García-Hernández

Complutense University of Madrid, Madrid, Madrid, Spain

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Publications (260)799.03 Total impact

  • Mar Garcia-Hernandez · Jonathan Coleman
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    ABSTRACT: Driving the superlative properties of mechanically exfoliated graphene to real world applications requires a large effort to develop synthetic routes providing cost effective high quality materials. It can also be agreed, that when it comes to reality, one should not refer just to Graphene but 'Graphenes' as each synthesis method renders a material characterized by different properties. Recently, Graphene and other 2D materials scalable synthesis methods have provided improved materials at highly competitive costs. However, a long way is ahead to approach the properties of mechanically exfoliated materials. Also, as a scalable method succeeds and it is indeed upscaled for industrial production, new characterization protocols and metrics have to be devised to enable efficient on line quality control of the produced materials. Significant advances can be reported recently in the synthesis of high quality graphene although a shift towards other 2D materials research is clearly observed. An overview of the progress made by several groups in WP ' Materials' of the Graphene Flagship is given.
    No preview · Article · Jan 2016 · 2D Materials
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    ABSTRACT: The layered rare-earth diantimonides RSb2 are anisotropic metals with generally low electronic densities whose properties can be modified by substituting the rare earth. LaSb2 is a nonmagnetic metal with a low residual resistivity presenting a low-temperature magnetoresistance that does not saturate with the magnetic field. It has been proposed that the latter can be associated to a charge density wave (CDW), but no CDW has yet been found. Here we find a kink in the resistivity above room temperature in LaSb2 (at 355 K) and show that the kink becomes much more pronounced with substitution of La by Ce along the La1−xCexSb2 series. We find signatures of a CDW in x-ray scattering, specific heat, and scanning tunneling microscopy (STM) experiments in particular for x≈0.5. We observe a distortion of rare-earth–Sb bonds lying in-plane of the tetragonal crystal using x-ray scattering, an anomaly in the specific heat at the same temperature as the kink in resistivity and charge modulations in STM. We conclude that LaSb2 has a CDW which is stabilized in the La1−xCexSb2 series due to substitutional disorder.
    No preview · Article · Dec 2015 · Physical Review B
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    ABSTRACT: We study the origin of the magnetoelectric coupling in manganite films on ferroelectric substrates. We find large magnetoelectric coupling in La0.7Ca0.3MnO3/BaTiO3 ultra-thin films in experiments based on the converse magnetoelectric effect. The magnetization changes by around 30-40% upon applying electric fields on the order of 1 kV/cm to the BaTiO3 substrate, corresponding to magnetoelectric coupling constants on the order of α = (2-5)·10-7 s/m. Magnetic anisotropy is also affected by the electric field induced strain, resulting in a considerable reduction of coercive fields. We compare the magnetoelectric effect in pre-poled and unpoled BaTiO3 substrates. Polarized neutron reflectometry reveals a two-layer behavior with a depressed magnetic layer of around 30 Å at the interface. Magnetic force microscopy (MFM) shows a granular magnetic structure of the La0.7Ca0.3MnO3. The magnetic granularity of the La0.7Ca0.3MnO3 film and the robust magnetoelastic coupling at the La0.7Ca0.3MnO3/BaTiO3 interface are at the origin of the large magnetoelectric coupling, which is enhanced by phase separation in the manganite.
    Full-text · Article · Dec 2015 · Scientific Reports
  • V Fuente · L Rufo · B.H. Juárez · N Menéndez · M García-Hernández · E Salas-Colera · A Espinosa
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    ABSTRACT: We report a detailed work of composition and location of naturally formed iron biominerals in plant cells tissues grown in iron rich environments as Imperata cylindrica. This perennial grass grows on the Tinto River banks (Iberian Pyritic Belt) in an extreme acidic ecosystem (pH∼2.3) with high concentration of dissolved iron, sulphate and heavy metals. Iron biominerals were found at the cellular level in tissues of root, stem and leaf both in collected and laboratory-cultivated plants. Iron accumulated in this plant as a mix of iron compounds (mainly as jarosite, ferrihydrite, hematite and spinel phases) was characterized by X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS), Mössbauer spectroscopy (MS), magnetometry (SQUID), electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDX; TEM-EDX; HRSTEM). A low fraction of phosphorous was detected in this iron hypperaccumulator plant. Root and rhizomes tissues present a high proportion of ferromagnetic iron oxide compounds. Iron oxides-rich zones are localized in electron dense intra and inter-cellular aggregates that appear as dark deposits covering the inner membrane and organelles of the cell. This study aims to contribute to a better understanding of the mechanisms of accumulation, transport, distribution of iron in Imperata cylindrica.
    No preview · Article · Nov 2015 · Journal of Structural Biology
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    ABSTRACT: The design of artificial vortex pinning landscapes is a major goal towards large scale applications of cuprate superconductors. While disordered nanometric inclusions have shown to modify their vortex phase diagram and to produce enhancements of the critical current (MacManus-Driscoll, J. L.; Foltyn, S. R.; Jia, Q. X.; Wang, H.; Serquis, a; Civale, L.; Maiorov, B.; Hawley, M. E.; Maley, M. P.; Peterson, D. E. Nat. Mater. 2004, 3 (7), 439-443; Yamada, Y.; Takahashi, K.; Kobayashi, H.; Konishi, M.; Watanabe, T.; Ibi, a.; Muroga, T.; Miyata, S.; Kato, T.; Hirayama, T.; Shiohara, Y. Appl. Phys. Lett. 2005, 87 (13), 1-3), the effect of ordered oxide nanostructures remains essentially unexplored. This is due to the very small nanostructure size imposed by the short coherence length, and to the technological difficulties in the nanofabrication process. Yet, the novel phenomena occurring at oxide interfaces open a wide spectrum of technological opportunities to interplay with the superconductivity in cuprates. Here we show that the unusual long range suppression of the superconductivity occurring at the interface between manganites and cuprates affects vortex nucleation and provides a novel vortex pinning mechanism. In particular, we show evidence of commensurate pinning in YBCO films with ordered arrays of LCMO ferromagnetic nanodots. Vortex pinning results from the proximity induced reduction of the condensation energy at the vicinity of the magnetic nanodots, and yields an enhanced friction between the nanodot array and the moving vortex lattice in the liquid phase. This result shows that all-oxide ordered nanostructures constitute a powerful, new route for the artificial manipulation of vortex matter in cuprates.
    Full-text · Article · Oct 2015 · Nano Letters
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    ABSTRACT: A route for fabricating structurally perfect cobalt ferrite magnetic nanostructures is demonstrated. Ultrathin islands of up to 100 μm(2) with atomically flat surfaces and free from antiphase boundaries are developed. The extremely low defect concentration leads to a robust magnetic order, even for thicknesses below 1 nm, and exceptionally large magnetic domains. This approach allows the evaluation of the influence of specific extrinsic effects on domain wall pinning.
    No preview · Article · Aug 2015 · Advanced Materials
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    ABSTRACT: We present very low temperature scanning tunneling microscopy (STM) experiments on single crystalline samples of the superconductor $\beta$-Bi$_2$Pd. We find a single fully isotropic superconducting gap. However, the magnetic field dependence of the intervortex density of states is higher than the one expected in a single gap superconductor, and the hexagonal vortex lattice is locked to the square atomic lattice. Such increase in the intervortex density of states and vortex lattice locking have been found in superconductors with multiple superconducting gaps and anisotropic Fermi surfaces. We compare the upper critical field $H_{c2}(T)$ obtained in our sample with previous measurements and explain available data within multiband supercondutivity. We propose that $\beta$-Bi$_2$Pd is a single gap multiband superconductor. We anticipate that single gap multiband superconductivity can occur in other compounds with complex Fermi surfaces.
    Full-text · Article · Jun 2015 · Physical Review B
  • M. Smari · R. Hamouda · I. Walha · E. Dhahri · F.J. Mompeán · Mar García-Hernández
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    ABSTRACT: La0.5Ca0.5MnO3 and La0.5Ca0.4Ag0.1MnO3 ceramics have been prepared by sol-gel methods and its magnetic properties, and electronic transport properties have been experimentally investigated. X-ray diffraction data reveal that all samples crystallize in a orthorhombic structure with Pnma space group. Magnetic measurements of La0.5Ca0.5MnO3 show multiple magnetic transitions, a paramagnetic to ferromagnetic and a ferromagnetic to charge-ordered antiferromagnetic. Two metal-semiconductor transitions (Tρ1 and Tρ2) are observed in the electrical resistivity. The electrical resistivity decreases when an external magnetic field is applied rendering a magnetoresistive effect of the order of 75% at the charge-ordered temperature. All the samples clearly reveal the unusual low temperature resistivity minimum, presumably due to the combined effect of weak localization, electron-electron and electron-phonon scattering. The present results are discussed and possible explanations were given based on the related theory and previous reported results.
    No preview · Article · May 2015 · Journal of Alloys and Compounds
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    ABSTRACT: A cheap and green technology based on electro-erosion, performed at room conditions and scalable to large area (cm2) is demonstrated for structuring graphene and graphene oxide films on any substrate. This one-step technique based on electrical discharges produced by a direct current voltage source competes favorably with laser patterning. The threshold voltage for complete graphene elimination as determined by Raman mapping is around 20 V. At low relative humidity conditions (30%) the transformation to graphene oxide is also detected for operating voltages above 40 V, the oxidation being probably mediated by the residual adsorbed water at the surface. We also show the close correlation of atomic force microscopy (AFM) phase images with the modified graphene characteristics while AFM topographic images are dominated by extrinsic aspects. The use of a spring probe relaxes the requirement of precision for the tip-sample distance and sample flatness to around 10 μm which is very convenient for large scale applications. Furthermore, this technique allows the formation of high quality nanographene, with size around 18 nm, and graphene micro-ribbon lattices in a very fast way and very well defined edges, with dimensions down to one μm width and mm length, very promising for terahertz graphene plasmonic applications.
    No preview · Article · Apr 2015 · Carbon
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    ABSTRACT: The Bi0.5Pb0.5CrO3 double perovskite is prepared by high pressure reaction of stoichiometric amounts of PbO, Bi2O3, Cr2O3, and CrO2 (Pt capsule in a belt-type press, 70 kbar, 1000 °C, 15 min).
    Full-text · Article · Apr 2015 · ChemInform
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    ABSTRACT: We have determined why hydrothermal synthesis does not turn out to be an adequate method to obtain nanoparticles in the Sr-Mn-O system. At high KOH concentration, the appropriate one to obtain nanoparticles, Sr is partially incorporated inside an intermediate (K, Sr)xMnO2•nH2O layered phase that prevents the 4H-nanoparticles formation. Numerous experiments have been performed by screening the parameters involved in such process (precursors, pH, temperature, etc...) in order to understand their influence in the shape, size and purity of the 4H-final product. Analysing the experimental results by means of XRD, TEM, TGA and EDS among others, some peculiarities of the synthetic procedure have been detected; the appearance of side phases, SrMn3O6-x nanoribbons or the above mentioned birnessite-type (K, Sr)xMnO2•nH2O phase, and the insertion of K within the 4H-phase prepared at high KOH concentration. Magnetic features are also discussed.
    Full-text · Article · Mar 2015 · Crystal Growth & Design
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    ABSTRACT: Magnetic nanobeads are synthesized by co-precipitation of hollow iron oxide nanoparticles and an amphiphilic polymer. The resulting nanobeads can be tuned in diameter and nanoparticle content. X-ray Absorption Near-Edge Structure (XANES) spectroscopy and superconducting quantum interferometer device (SQUID) characterization of the nanobeads reveal that they exhibit an increased effective magnetic anisotropy compared to the individual nanoparticles, despite no structural changes of the particles had occurred during the embedding process into the polymer. The spin-spin relaxation times of the pristine hollow nanoparticles and of the final magnetic nanobeads reveal a high R2 relaxivity of 206 s-1 mM-1 for the magnetic nanobeads. This result should enable their application as negative contrast enhancing agents in magnetic resonance imaging.
    Full-text · Article · Feb 2015 · The Journal of Physical Chemistry C
  • A. Perez-Rivero · J. Ricote · I. Bretos · M. García-Hernández · M. L. Calzada · R. Jiménez
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    ABSTRACT: The study of the functional properties of the multilayer composite films of ferroelectric (Bi0.5Na0.5)(0.945)Ba0.055TiO3 (BNBT) and multiferroic BiFeO3 (BF) obtained by chemical solution deposition has been performed in this work. First, it has been observed a significant improvement of the remnant polarization of the multilayer composite with respect to the single phase film of BNBT: P-r = 32 mu C x cm(-2), which is comparable with the values reported for bulk BNBT. This is a consequence of the stabilization of the ferroelectric domains of BNBT, although the relaxor character is still present. The leakage currents, which prevent the full exploitation of the properties of BF films, are reduced thanks to the combination with insulator BNBT layers. Besides, the magnetic behavior shows values of remnant magnetization at room temperature that makes this multilayer composites promising for multiferroic applications. (C) 2015 AIP Publishing LLC.
    No preview · Article · Feb 2015 · Journal of Applied Physics
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    ABSTRACT: We describe a three axis vector magnet system for cryogenic scanning probe microscopy measurements. We discuss the magnet support system and the power supply, consisting of a compact three way 100 A current source. We obtain tilted magnetic fields in all directions with maximum value of 5T along z-axis and of 1.2T for XY-plane magnetic fields. We describe a scanning tunneling microscopy-spectroscopy (STM-STS) set-up, operating in a dilution refrigerator, which includes a new high voltage ultralow noise piezodrive electronics and discuss the noise level due to vibrations. STM images and STS maps show atomic resolution and the tilted vortex lattice at 150 mK in the superconductor β-Bi2Pd. We observe a strongly elongated hexagonal lattice, which corresponds to the projection of the tilted hexagonal vortex lattice on the surface. We also discuss Magnetic Force Microscopy images in a variable temperature insert.
    No preview · Article · Feb 2015 · Review of Scientific Instruments
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    ABSTRACT: We have synthesized a new Bi0.5Pb0.5CrO3 perovskite phase by means of a high pressure reaction at 70 kbar and 1000 °C. The distorted orthorhombic perovskite structure can be indexed in the space group Pnma with lattice parameters a = 5.4768(1)Å, b = 7.7450(2) Å, and c = 5.4574(1) Å at room temperature, but undergoes a structural phase transition and enters in to a P21/m monoclinic distorted perovskite phase below 150 K with a = 5.4173(2), b = 7.7286(4)and c = 5.4930(3). The structural transition is coincident with the onset of magnetic interactions. At lower temperatures a weak ferromagnetic structure is evident related to antiferromagnetic Cr-spin canting and a spin-glass transition is observed at ≈ 40 K. The semiconducting-type electrical resistivity is relatively low, associated with Cr3+/Cr4+ electron hopping, and shows considerable magneto-resistance (up to 15%). Due to the low resistivity the dielectric permittivity εr could be determined only below T < 80 K to be ≈ 300 and did not show any strong temperature-dependence.Ferroelectricity was not detected in the T-range investigated and no magnetocapacitance effects were observed.
    Full-text · Article · Feb 2015 · Journal of Solid State Chemistry
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    ABSTRACT: New magnetic bio-hybrid matrices for potential application in drug delivery are developed from the assembly of the biopolymer alginate and magnetic graphite nanoparticles. Ibuprofen (IBU) intercalated in a Mg–Al layered double hydroxide (LDH) was chosen as a model drug delivery system (DDS) to be incorporated as third component of the magnetic bionanocomposite DDS. For comparative purposes DDS based on the incorporation of pure IBU in the magnetic bio-hybrid matrices were also studied. All the resulting magnetic bionanocomposites were processed as beads and films and characterized by different techniques with the aim to elucidate the role of the magnetic graphite on the systems, as well as that of the inorganic brucite-like layers in the drug-loaded LDH. In this way, the influence of both inorganic components on the mechanical properties, the water uptake ability, and the kinetics of the drug release from these magnetic systems were determined. In addition, the possibility of modulating the levels of IBU release by stimulating the bionanocomposites with an external magnetic field was also evaluated in in vitro assays.
    Full-text · Article · Jan 2015 · International Journal of Pharmaceutics
  • S. Fernández · J.D. Santos · C. Munuera · M. García-Hernández · F.B. Naranjo
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    ABSTRACT: The 0.4 μm-thick Aluminum-doped Zinc Oxide (ZnO:Al) films were deposited at 100 °C on polyethylene terephthalate (PET) substrates by Radio Frequency (RF) magnetron sputtering. Prior to the AZO deposition, an Argon plasma treatment on the substrate surface was carried out by applying a RF bias power on it without intentional heating. The parameters varied in the etching process were the plasma etching time from 0 to 360 s, the RF bias power from 50 to 250 W, and the gas flux from 3 to 5 sccm. The effect of the substrate surface treatment on the mechanical stability, crystallinity and the optoelectronic properties of ZnO:Al thin films were evaluated. The results showed physically stable ZnO:Al films with good adherence to the substrate using appropriated plasma treatment parameters. This fact was attributed to physico/chemical PET surface modifications on the first few molecular layers after the plasma irradiation. The performance of flexible solar devices fabricated on optimized ZnO:Al thin films with adequate adhesion and optoelectronic properties was analysed.
    No preview · Article · Nov 2014 · Solar Energy Materials and Solar Cells
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    D. Llamosa · M. Ruano · L. Martínez · A. Mayoral · E. Roman · M. García-Hernández · Y. Huttel
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    ABSTRACT: Complex Core@Shell and Core@Shell@Shell nanoparticles are systems that combine the functionalities of the inner core and outer shell materials together with new physico-chemical properties originated by their low (nano) dimensionality. Such nanoparticles are of primer importance in the fast growing nanotechnology as building blocks for more sophisticated systems and a plethora of applications. Here it is shown that, although conceptually simple, a modified gas aggregation approach allows the one-step generation of well-controlled complex nanoparticles. In particular it is demonstrated that the atoms of the core and shell of the nanoparticles can be easily inverted, avoiding intrinsic constraints of chemical methods.
    Full-text · Article · Nov 2014 · Nanoscale
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    ABSTRACT: Polydopamine is nowadays widely used for coating of various materials, including magnetic nanoparticles, and is easily obtained by polymerization of dopamine. Herein a facile and versatile one step synthesis of magnetite nanoparticles coated with polydopamine is presented. Magnetite nanoparticles are synthesized electrochemically by electrooxidation of iron in aqueous medium, which involves oxidative conditions and alkaline pH. These conditions favor the self-polymerization of dopamine that adheres at the surface of the magnetic nanoparticles in a simultaneous process. It is showed that by varying the synthetic approach, for instance the amount of dopamine added to the reaction medium, the size of the magnetite nanoparticles as well as the polydopamine coating can be controlled as demonstrated by thermogravimetric analysis (TGA), infrared spectroscopy (FTIR), X-ray diffraction (XRD) and transmission electron microscopy (TEM). In addition, the magnetic measurements of the different obtained materials were carried out showing a variety of magnetic behaviors depending on the synthetic procedure.
    No preview · Article · Sep 2014 · RSC Advances
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    ABSTRACT: We present the science and technology roadmap (STR) for graphene, related two-dimensional (2d) crystals, and hybrid systems, targeting an evolution in technology, with impacts and benefits reaching into most areas of society. The roadmap was developed within the framework of the European Graphene Flagship and outlines the main targets and research areas as best understood at the start of this ambitious project. In this document we provide an overview of the key aspects of graphene and related materials (GRMs), ranging from fundamental research challenges to a variety of applications in a large number of sectors, highlithing the roadmap to take GRMs from a state of raw potential to a point where they might revolutionize multiple industries: from flexible, wearable and transparent electronics to high performance computing and spintronics.
    Full-text · Article · Sep 2014 · Nanoscale

Publication Stats

3k Citations
799.03 Total Impact Points

Institutions

  • 2009-2015
    • Complutense University of Madrid
      • Department of Inorganic Chemistry and Bioorganic
      Madrid, Madrid, Spain
  • 1998-2015
    • Spanish National Research Council
      • • Instituto de Ciencia de Materiales de Madrid
      • • Institute for the Structure of Matter
      Madrid, Madrid, Spain
    • Universidad del País Vasco / Euskal Herriko Unibertsitatea
      Leioa, Basque Country, Spain
  • 1997-2015
    • Instituto de Ciencia de Materiales de Madrid
      Madrid, Madrid, Spain
  • 1992-2015
    • Universidad Autónoma de Madrid
      • Department of Condensed Matter Physics
      Madrid, Madrid, Spain
  • 2008
    • University of Santiago de Compostela
      • Department of Applied Physics
      Santiago, Galicia, Spain
  • 1989-1998
    • Instituto de Estructura de la Materia
      Madrid, Madrid, Spain