J. Santamaria

Complutense University of Madrid, Madrid, Madrid, Spain

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Publications (258)635.1 Total impact

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    ABSTRACT: We report on the fabrication and magnetotransport characterization of hybrid graphene-based nanodevices with epitaxial nanopatterned La0.7Sr0.3MnO3 manganite electrodes grown on SrTiO3 (100). The few-layer graphene was deposited onto the predefined manganite nanowires by using a mechanical transfer technique. These nanodevices exhibit resistive switching and hysteretic transport as measured by current-voltage curves. The resistance can be reversibly switched between high and low states, yielding a consistent non-volatile memory response. The effect is discussed in terms of changes in the concentration of oxygen vacancies at the space charge region of the Schottky barriers building at the contacts.
    Applied Physics Letters 03/2014; 104. · 3.79 Impact Factor
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    ABSTRACT: We report a strong effect of interface-induced magnetization on the transport properties of magnetic tunnel junctions consisting of ferromagnetic manganite La0.7Ca0.3MnO3 and insulating cuprate PrBa2Cu3O7. Contrary to the typically observed steady increase of the tunnel magnetoresistance with decreasing temperature, this system exhibits a sudden anomalous decrease at low temperatures. Interestingly, this anomalous behavior can be attributed to the competition between the positive spin polarization of the manganite contacts and the negative spin-filter effect from the interface-induced Cu magnetization.
    Physical Review Letters 12/2013; 111(24):247203. · 7.94 Impact Factor
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    ABSTRACT: The magnetism of La_{0.7}Ca_{0.3}MnO_{3} (LCMO) epitaxial thin films grown on SrTiO_{3} (STO) and BaTiO_{3} (BTO) substrates is studied using polarized neutron reflectometry (PNR) and ferromagnetic resonance (FMR) techniques. In LCMO/BTO, PNR reveals a strongly suppressed magnetization of 300 kA/m, equivalent to a magnetic moment of 2 μB/Mn, throughout the LCMO layer, amounting to half the expected value. The largest suppression occurs near the interface with BTO, with magnetization values of 50 kA/m, equivalent to 0.3 μB/Mn. FMR is observable at 8.9 GHz only around the [110] crystallographic direction in thin LCMO/BTO. The resonance barely shifts as the applied field is rotated away from [110]. The FMR results are analyzed in terms of magnetoelastic anisotropy and compared to LCMO/STO grown under the same conditions. A two-layer magnetization model is proposed, based on strong out-of-plane anisotropy near the BTO interface and shown to qualitatively explain the main characteristics of the FMR results.
    Physical Review B 10/2013; 88(13). · 3.77 Impact Factor
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    ABSTRACT: Interfaces between correlated oxides are attracting great interest. Electron correlations give rise to novel forms of couplings between electronic ground states at both sides of the interface. The bonding discontinuity at the interface between magnetic and nonmagnetic oxides is at the origin of a form of low dimensional magnetism in the otherwise nonmagnetic material. Its origin is the splitting of its bands due to the hybridization with the exchange split bands of the magnetic material. This induced magnetism could find interesting functionalities in devices with operation controlled by the interface such as tunnel or field effect devices of interest in spintronics.
    International Journal of Modern Physics B 07/2013; 27(19):30013-. · 0.36 Impact Factor
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    ABSTRACT: We examine the effect of interface disorder in suppressing superconductivity in coherently grown ultrathin YBa2Cu3O7 (YBCO) layers on SrTiO3 (STO) in YBCO/STO superlattices. The termination plane of the STO is TiO2 and the CuO chains are missing at the interface. Disorder (steps) at the STO interface cause alterations of the stacking sequence of the intracell YBCO atomic layers. Stacking faults give rise to antiphase boundaries which break the continuity of the CuO2 planes and depress superconductivity. We show that superconductivity is directly controlled by interface disorder outlining the importance of pair breaking and localization by disorder in ultrathin layers.
    Physical review. B, Condensed matter 06/2013; 87(24).
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    ABSTRACT: In this paper, the structural and electronic properties of heterostructures involving Sm2CuO4 (SCO) and LaFeO3 (LFO) will be studied. SCO is a Mott insulator that becomes superconducting upon replacing Sm3+ ions with Ce4+ to form Sm2−xCexCuO4 , an electron doped HTSC. LFO is also a Mott insulator with its chemical potential being approximately 1 eV above the chemical potential of SCO. The gap in both of these materials is of the charge-transfer (CT) type, i.e. the energy difference Δ between the O–2p band and the upper Hubbard band is smaller than the energy difference U between the lower and upper Hubbard bands. The transfer of charge involving CT Mott insulators provides an interesting framework, which to our knowledge has not been examined before. Moreover, LFO is a good candidate to transfer electrons from its O 2p valence band to the CuO 2 d-like upper Hubbard band of the SCO. Since La 1−x Sr x FeO 3 remains insulating and antiferromagnetic even if doped with holes up to x < 0.7, this state should not be detrimental to a possible superconducting state if an interface with SCO is made. Here, it will be shown that it is indeed possible to obtain a doped n-type material at the SCO/LFO interface of two CT insulators, which displays metallic behavior in the temperature range of 150–300 K.
    Advanced Materials 03/2013; 25:1468. · 14.83 Impact Factor
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    ABSTRACT: Functional properties of magnetic tunnel junction can be enhanced by employing a ferroelectric material as the barrier layer. We report on La0.7Sr0.3MnO3(LSMO)/BaTiO3(BTO)/LSMO magnetic tunnel junctions(MTJ) with BTO ferroelectric tunnel barrier. Switching BTO ferroelectric polarization influences the tunneling magnetoresistance (TMR) achieving two different r resistance states for each magnetic state (parallel or antiparallel) of the magnetization of the electrodes . The voltage dependence of the differential conductance obtained from IV curves displays oscillations whose period depends on the BTO electric polarization. This unusual behavior could be related to the presence of an induced magnetic moment in BTO ferroelectric barrier detected by XMCD measurements. These results reveal that spin polarization, and its tunneling conductance can be electrically tuned through reversal of the ferroelectric polarization of the barrier.
    03/2013;
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    ABSTRACT: Interfaces between the ferromagnetic (FM) and ferroelectric (FE) oxides may host nanoscale multiferroic phases with strong magnetoelectric coupling, which can be potentially utilized for energy-efficient spintronics. In this work, we have investigated the magnetic properties of the interface between ferromagnetic La0.7Sr0.3MnO3 (LSMO) and ferroelectric BaTiO3 (BTO) via X-ray resonant magnetic scattering (XRMS) and X-ray magnetic circular dichroism (XMCD) on a series of 10 nm LSMO / t BTO bilayers, with t = 1.2, 2.4 and 4.8 nm, respectively. Additionally, we have studied a LSMO/BTO/LSMO trilayer. Interestingly, we have observed magnetic dichroism from Ti ions between 30 K and 210 K, which closely tracks the Mn's dichroism during the magnetization reversal. In contrast, no Ti magnetization has been observed in a single-layer BTO film on a SrTiO3 substrate. These results suggest that there are Ti^3+ ions that reside at the LSMO/BTO interfaces and the interfacial Mn and Ti moments are exchange coupled.
    03/2013;
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    ABSTRACT: Interfaces alter the subtle balance among different degrees of freedom responsible for exotic phenomena in complex oxides, such as cuprate-manganite interfaces. We study these interfaces by means of scanning transmission electron microscopy and theoretical calculations. Microscopy and EEL spectroscopy indicate that the interfaces are sharp, and the chemical profile is symmetric with two equivalent interfaces. Spectroscopy also allows us to establish an oxidation state profile with sub-nanometer resolution. We find an anomalous charge redistribution: a non-monotonic behavior of the occupancy of d orbitals in the manganite layers as a function of distance to the interface. Relying on model calculations, we establish that this profile is a result of the competition between standard charge transfer tendencies involving materials with different chemical potentials and strong bonding effects across the interface. The competition can be tuned by different factors (temperature, doping, magnetic fields...). As examples, we report different charge distributions as a function of doping of the manganite layers. ACKNOWLEDGEMENTS ORNL:U.S. DOE-BES, Material Sciences and Engineering Division & ORNL's ShaRE. UCM:Juan de la Cierva, Ramon y Cajal, & ERC Starting Investigator Award programs.
    03/2013;
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    ABSTRACT: We report on large magnetocapacitance (MC) effects in LaMnO3/SrTiO3 multilayer structures. Frequency, temperature and magnetic field dependent dielectric spectroscopy was employed using in-plane and out-of-plane measurement set-ups to investigate multilayers of LaMnO3 (15 u.c.) and SrTiO3 (2 u.c.) with a repetition rate of 8: (LMO 15/STO 2)8. Such multilayer structures have been identified previously to display an electron transfer across the epitaxial interface from LMO to STO, orbital reconstruction and a considerable Ti^3+ magnetic moment near the LMO/STO interface [Garcia-Barriocanal et al. Adv. Mater. 22 (2010) p.627]. We demonstrate moderate in-plane magnetocapacitance (MC) of up to -5 % associated with an intrinsic magneto-electric coupling (MEC) effect originating from magnetic STO layers. Massive out-of-plane MC of up to -52 % was ascribed to current path changes due to magnetically active STO pin-holes and current path meandering.
    03/2013;
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    ABSTRACT: Using X-ray absorption spectroscopy and electron energy loss spectroscopy with atomic-scale spatial resolution, experimental evidence for charge transfer at the interface between the Mott insulators Sm(2) CuO(4) and LaFeO(3) is obtained. As a consequence of the charge transfer, the Sm(2) CuO(4) is doped with electrons and thus epitaxial Sm(2) CuO(4) /LaFeO(3) heterostructures become metallic.
    Advanced Materials 01/2013; 25(10):1468. · 14.83 Impact Factor
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    ABSTRACT: The breakdown of the lattice translational invariance symmetry that occurs at complex oxide interfaces may profoundly modify their electronic structure, leading to interfacial states with properties drastically different from those of the superlattice individual components. The appearance of a conducting two dimensional (2D) electron gas at the interface between two insulating oxides and induced magnetism in a non-magnetic material are just two among many fascinating examples. [ 1 8 ] One of the key factors underlying novel properties is the modifi cation of the doping and orbital occupancy near those interfaces, which may result from charge transfer processes. [ 3 , 9 11 ] If materials used in heterostructures have different work functions, a non-equilibrium situation will be created at the interface and charge will be transferred until the chemical potential levels off. [ 12 ] The use of such phenomena to modify doping in heterostructures has been proposed theoretically as a new route to avoid the quenched disorder that inevitably accompanies the chemical doping. At the interface between a Mott insulating parent compound of the high critical temperature superconductor (HTSC) family and a suitable material that would act as the charge donor, electron doped phases could be stabilized which would eventually turn metallic and perhaps superconducting. [ 12 , 13 ] Such charge transfer processes have been observed at interfaces involving copper oxides such as La 0.7 Ca 0.3 MnO 3 / YBa 2 Cu 3 O 7 , [ 14 ] La 2 x Sr x CuO 4 /La 2 CuO 4 [ 15 ] and SrTi 1 x Nb x O 3 / Sm 2 CuO 4 . [ 16 ] While a novel 2D superconducting state was found at the La 2-x Sr x CuO 4 /La 2 CuO 4 interface, [ 17 ] the effect of doping by charge transfer could not be examined in the other two cases due to the detrimental effect on the YBa 2 Cu 3 O 7 superconductivity of the spin polarized electrons from La 0.7 Ca 0.3 MnO 3 in one case and due to the conducting nature of the SrTi 1 x Nb x O 3 in the other case, which obscures changes in the conducting properties of the interface layer. In this paper, the structural and
    Advanced Materials 01/2013; 25(10). · 14.83 Impact Factor
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    ABSTRACT: Optical and electrical properties of samples with Li1 + xCrxTi2 − x(PO4)3 composition (x = 0, 0.05 and 0.1) prepared by a low temperature sol-gel method have been investigated. XRD and Raman spectroscopy measurements respectively reveal the rhombohedral-Nasicon structure of the grown samples and a disorder of Li+ ions with increasing chromium content. Electrical properties were studied by impedance spectroscopy and both dc and grain boundary ionic conductivity increase with Cr content. A high ionic conductivity value of 1.4·10− 4 S cm− 1 at room temperature and an activation energy of 0.31 eV have been obtained for x = 0.1, which are comparable to those measured in the best ionic conductors reported to date. Photoluminescence and cathodoluminescence spectra indicate that self-trapped excitons and oxygen defects related to the TiO6 octahedra structural units are responsible for the observed luminescence in the visible spectral range, explaining the possible electronic origin of a residual conductivity measured in all the samples. Cr incorporation gives rise to infrared emission bands, attributed to different Cr3 + and Ti3 + intraionic transitions, that may be related to the better grain connectivity achieved by Cr doping.
    Solid State Ionics 01/2013; 241:36–45. · 2.05 Impact Factor
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    ABSTRACT: The existence of coherent magnetic correlations in the normal phase of cuprate high-temperature superconductors has proven difficult to measure directly. Here we report on a study of ferromagnetic-superconductor bilayers of La2/3Ca1/3MnO3/YBa2Cu3O7 (LCMO/YBCO) with varying YBCO layer thicknesses. Using x-ray magnetic circular dichroism, we demonstrate that the ferromagnetic layer induces a Cu magnetic moment in the adjacent high-temperature superconductor. For thin samples, this moment exists at all temperatures below the Curie temperature of the LCMO layer. However, for a YBCO layer thicker than 12 unit cells, the Cu moment is suppressed for temperatures above the superconducting transition, suggesting this to be a direct measurement of magnetic coherence in the normal state of a superconducting oxide.
    Physical Review Letters 10/2012; 109(13). · 7.94 Impact Factor
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    ABSTRACT: The existence of coherent magnetic correlations in the normal phase of cuprate high-temperature superconductors has proven difficult to measure directly. Here we report on a study of ferromagnetic-superconductor bilayers of La_{2/3}Ca_{1/3}MnO_{3}/YBa_{2}Cu_{3}O_{7} (LCMO/YBCO) with varying YBCO layer thicknesses. Using x-ray magnetic circular dichroism, we demonstrate that the ferromagnetic layer induces a Cu magnetic moment in the adjacent high-temperature superconductor. For thin samples, this moment exists at all temperatures below the Curie temperature of the LCMO layer. However, for a YBCO layer thicker than 12 unit cells, the Cu moment is suppressed for temperatures above the superconducting transition, suggesting this to be a direct measurement of magnetic coherence in the normal state of a superconducting oxide.
    Physical Review Letters 09/2012; 109(13):137005. · 7.94 Impact Factor
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    ABSTRACT: An unusual conducting surface state can be produced in SrTiO(3) substrates by irradiation with Argon ions from a plasma source, at low energy and high doses. The effects of irradiation are analyzed here by atomic force microscopy (AFM) and aberration corrected scanning transmission electron microscopy (STEM) combined with electron energy loss spectroscopy (EELS). Depth sensitive studies demonstrate the existence of a heavily damaged surface layer and an oxygen vacancy rich layer immediately underneath, both induced during the irradiation process. We find a clear dependence of the Ti oxidation state with the depth, with a very intense Ti(3+) component near the surface. Oxygen vacancies act as n-type doping by releasing electrons into the lattice and producing an insulator-to-metal transition, which explains the unusual metallic behavior of these samples.
    Ultramicroscopy 07/2012; · 2.47 Impact Factor
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    ABSTRACT: Conventional superconductivity is incompatible with ferromagnetism, because the magnetic exchange field tends to spin-polarize electrons and breaks apart the opposite-spin singlet Cooper pairs. Yet, the possibility of a long-range penetration of superconducting correlations into strong ferromagnets has been evinced by experiments that found Josephson coupling between superconducting electrodes separated afar by a ferromagnetic spacer. This is considered a proof of the emergence at the superconductor/ferromagnetic (S/F) interfaces of equal-spin triplet pairing, which is immune to the exchange field and can therefore propagate over long distances into the F (ref. ). This effect bears much fundamental interest and potential for spintronic applications. However, a spectroscopic signature of the underlying microscopic mechanisms has remained elusive. Here we do show this type of evidence, notably in a S/F system for which the possible appearance of equal-spin triplet pairing is controversial: heterostructures that combine a half-metallic F (La0.7Ca0.3MnO3) with a d-wave S (YBa2Cu3O7). We found quasiparticle and electron interference effects in the conductance across the S/F interfaces that directly demonstrate the long-range propagation across La0.7Ca0.3MnO3 of superconducting correlations, and imply the occurrence of unconventional equal-spin Andreev reflection. This allows for an understanding of the unusual proximity behaviour observed in this type of heterostructures.
    Nature Physics 07/2012; 8(7):539-543. · 19.35 Impact Factor
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    ABSTRACT: We report on the electrical transport properties of all-oxide La0.7Ca0.3MnO3/SrTiO3:Nb heterojunctions with lateral size of just a few micrometers. The use of lithography techniques to pattern manganite pillars ensures perpendicular transport and allows exploration of the microscopic conduction mechanism through the interface. From the analysis of the current-voltage characteristics in the temperature range 20–280 K we find a Schottky-like behavior that can be described by a mechanism of thermally assisted tunneling if a temperature-dependent value of the dielectric permittivity of SrTiO3:Nb (NSTO) is considered. We determine the Schottky energy barrier at the interface, qVB=1.10±0.02 eV, which is found to be temperature independent, and a value of ξ=17±2 meV for the energy of the Fermi level in NSTO with respect to the bottom of its conduction band.
    Physical Review B 06/2012; 85(24). · 3.77 Impact Factor

Publication Stats

373 Citations
1k Downloads
635.10 Total Impact Points

Institutions

  • 1985–2013
    • Complutense University of Madrid
      • • Department of Applied Physics III (Electricity and Electronic)
      • • Facultad de Ciencias Físicas
      Madrid, Madrid, Spain
  • 2011
    • Los Alamos National Laboratory
      Los Alamos, California, United States
  • 2007
    • University of Quindio
      Armenia, Quindío, Colombia
  • 1996–2006
    • University of California, San Diego
      • Department of Physics
      San Diego, CA, United States
  • 2005
    • Instituto de Ciencia de Materiales de Madrid
      Madrid, Madrid, Spain
  • 2001
    • Universidad del Valle (Colombia)
      • Departamento de Física
      Cali, Departamento del Valle del Cauca, Colombia
  • 1998–2001
    • University Carlos III de Madrid
      • High Technical College
      Getafe, Madrid, Spain
    • University of London
      Londinium, England, United Kingdom
  • 1982
    • The International Society for Optics and Photonics
      International Falls, Minnesota, United States