Ivan K. Schuller

University of California, San Diego, San Diego, California, United States

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Publications (683)1838.31 Total impact

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    ABSTRACT: Using photoemission electron microscopy combined with x-ray magnetic circular dichroism we show that a progressive spatial confinement of a ferromagnet (FM), either through thickness variation or laterally via patterning, actively controls the domains of uncompensated spins in the antiferromagnet (AF) in exchange-biased systems. Direct observations of the spin structure in both sides of the FM/AF interface in a model system, Ni/FeF2, show that the spin structure is determined by the balance between the competing FM and AF magnetic energies. Coexistence of exchange bias domains, with opposite directions, can be established in Ni/FeF2 bilayers for Ni thicknesses below 10 nm. Patterning the Ni/FeF2 heterostructures with antidots destabilizes the FM state, enhancing the formation of opposite exchange bias domains below a critical antidot separation of the order of a few FeF2 crystal domains. The results suggest that dimensional confinement of the FM may be used to manipulate the AF spin structure in spintronic devices and ultrahigh-density information storage media. The underlying mechanism of the uncompensated AF domain formation in Ni/FeF2 may be generic to other magnetic systems with complex noncollinear FM/AF spin structures.
    No preview · Article · Nov 2015 · Physical Review B
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    ABSTRACT: Superconductivity and magnetism at intermediate (mesoscopic) length scales between atomic and bulk have a long history of interesting science. New science emerges due to the presence of multiple length scales, especially when these become comparable to relevant geometric sizes. New phenomena may appear due to topological interactions, geometric confinement, proximity between dissimilar materials, dimensional crossover, and collective effects induced by periodicity. In this review, we select a few, recent highlights that illustrate the type of novel science that can be accomplished in superconducting and magnetic structures. These materials can serve as model systems and provide new ideas, which can be extended to other systems such as ferroelectrics and multiferroics. We also highlight general open questions and new directions in which the field may move.
    No preview · Article · Nov 2015 · MRS Bulletin
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    E Navarro · C Monton · J Pereiro · Ali C Basaran · Ivan K Schuller
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    ABSTRACT: We present a comparative study of vortex pinning efficiency in superconducting (V) thin films grown on two similar triangular arrays of superconducting (Nb) and nonsuperconducting (Cu) nanodots. Resistance and magnetization anomalies at the same matching fields confirm the same pinning periodicity in both samples. However, we found two distinct features: First, the sample with superconducting dots shows stronger pinning that appears as sharper matching peaks in magnetization loops and shows higher critical current density and larger critical field at low temperatures. Second, an overall increase in the resistance of the V film with Nb nanodots is observed, while there is a crossover in the temperature dependence of the critical field and the critical current of both samples at T = 3 K. This crossover corresponds to the temperature when the superconducting coherence length of V thin film equals the edge-to-edge distance between nanodots. We argue that this change in superconducting properties is related to the change in the superconducting regime from pinning enhancement at low temperatures to a superconducting wire network at high temperatures.
    Full-text · Article · Oct 2015 · Physical Review B
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    ABSTRACT: The discovery of superconductivity in pnictides and iron chalcogenides inspires the search for new iron based superconducting phases. Iron-rich meteorites present a unique opportunity for this search, because they contain a broad range of compounds produced under extreme growth conditions. We investigated a natural iron sulfide based materials (Troilite) inclusion with its associated minerals in the iron meteorite Tlacotepec. Tlacotepec formed in an asteroidal core under high pressure and at high temperature over millions of years, while insoluble sulfur rich materials segregated into inclusions during cooling along with included minerals. The search for superconductivity in these heterogeneous materials requires a technique capable of detecting minute amounts of a superconducting phase embedded in a non-superconducting matrix. We used Magnetic Field Modulated Microwave Spectroscopy (MFMMS), a very sensitive, selective, and non-destructive technique, to search for superconductivity in heterogeneous systems. Here, we report the observation of an electro-magnetic phase transition at 117 K that causes a MFMMS-response typical of a superconductor. A pronounced and reproducible peak together with isothermal magnetic field sweeps prove the appearance of a new electromagnetic phase below 117 K. This is very similar to the characteristic response due to flux trapping in a granular superconductor with a short coherence length. Although the compound responsible for the peak in the MFMMS-spectra was not identified, it is possibly an iron sulfide based phase, or another material heterogeneously distributed over the inclusion.
    No preview · Article · Sep 2015
  • Siming Wang · Juan Gabriel Ramírez · Ivan K. Schuller
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    ABSTRACT: The resistance versus temperature across the metal-insulator transition (MIT) of V2O3 nanodevices exhibits multiple discontinuous jumps. The jump sizes range over three orders of magnitude in resistance and their distribution follows a power law, implying that the MIT of V2O3 occurs through avalanches. While the maximum jump size depends on the device size, the power law exponent for V2O3 is independent of device geometry and different than the one found earlier in VO2. A two-dimensional random percolation model exhibits a power law distribution different from the one found in V2O3. Instead, the model gives a similar exponent found in another vanadium oxide, VO2. Our results suggest that the MITs of VO2 and V2O3 are produced by different mechanisms.
    No preview · Article · Aug 2015 · Physical Review B
  • Juan Pereiro · Thomas Saerbeck · Ivan K Schuller
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    ABSTRACT: We investigated the superconducting properties of Nb/Mo superlattices (SLs). The structural changes as a function of Nb and Mo layer thickness allow us to investigate the effect of disorder on the superconducting properties in a controlled fashion. Systematic structural studies provide quantitative measures of disorder parameters, such as roughness, interdiffusion, and strain, which allow separating their effect on the individual superconducting layers. The Mo critical temperature does not change as the layer thickness decreases below its coherence length. Thus, the SL critical temperatures in the presence of disorder and proximity effects can be modeled by considering only the effects of the Nb mean free path and coherence length. With increasing layer thickness, the SL critical temperatures approach Nb bulk values. Contrary to expectations the T c of Mo remains below the Nb T c. We discuss the results using existing theories based on Coulomb repulsion or changes in the density of states at the Fermi surface as a function of disorder. Questions about current understanding of the effect of disorder on superconductivity arise from the results.
    No preview · Article · Aug 2015 · Superconductor Science and Technology
  • Ali C. Basaran · R. Morales · S. Guenon · Ivan K. Schuller
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    ABSTRACT: We developed a method to determine the magnetic helicity and to study reversal mechanisms in exchange biased nanostructures using Planar Hall Effect (PHE). As a test case, we use an in-depth helical spin configuration that occurs during magnetization reversal in exchange coupled Ni/FeF2 heterostructures. We show the way to induce and determine the sign of the helicity from PHE measurements on a lithographically patterned cross. The helicity sign can be controlled by the angle between the externally applied magnetic field and a well-defined unidirectional anisotropy axis. Furthermore, the PHE signal reveals complex reversal features due to small deviations of the local unidirectional anisotropy axes from the crystallographic easy axis. The simulations using an incomplete domain wall model are in excellent agreement with the experimental data. These studies show that helical spin formations in nanomagnetic systems can be studied using laboratory-based magnetotransport.
    No preview · Article · Jun 2015 · Applied Physics Letters
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    ABSTRACT: The observation of an anomalous quadrupolar signal in x-ray magnetic circular dichroism (XMCD) at the Fe $K$-edge of iron phthalocyanine (FePc) films is reported. All ground states previously suggested for FePc are incompatible with the experimental data. Based on ab initio molecular orbital multiplet calculations of the isolated FePc molecule, we propose a model for the magnetic ground state of the FePc film that explains the XMCD data and reproduces the observed values of the orbital moments in the perpendicular and planar directions.
    Full-text · Article · Jun 2015 · Physical Review B
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    ABSTRACT: The phase evolution and morphology of the solid state FeF2 conversion reaction with Li has been characterized using angle-resolved x-ray photoelectron spectroscopy (ARXPS). An epitaxial FeF2(110) film was grown on a MgF2(110) single crystal substrate and exposed to atomic lithium in an ultra-high vacuum chamber. A series of ARXPS spectra was taken after each Li exposure to obtain depth resolved chemical state information. The Li-Fe2 reaction initially proceeded in a layer-by-layer fashion to a depth of ~1.2 nm. Beyond this depth, the reaction front became non-planar, and regions of unreacted FeF2 were observed in the near-surface region. This reaction progression is consistent with molecular dynamics simulations. Additionally, the composition of the reacted layer was similar to that of electrochemically reacted FeF2 electrodes. An intermediary compound FexLi2-2xF2, attributed to iron substituted in the LiF lattice, has been identified using XPS. These measurements provide insight into the atomistics and phase evolution of high purity FeF2 conversion electrodes without contamination from electrolytes and binders, and the results partially explain the capacity losses observed in cycled FeF2 electrodes.
    Full-text · Article · May 2015 · Physical Chemistry Chemical Physics
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    ABSTRACT: Scanning tunneling microscopy and x-ray absorption spectroscopic results at the Fe K edge of Fe phthalocyanine (FePc) thin films grown on Au substrates, together with theoretical calculations, allow us to refine the structure of the film. In particular, we show that the columnar stacking of the FePc molecules is different from that found in bulk α and β phases. Moreover, the molecules do not lay parallel to the surface of the substrate. These structural findings are relevant to understand magnetism of FePc films.
    Full-text · Article · May 2015 · Journal of Applied Physics
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    ABSTRACT: In this paper we investigate the effect of disorder on highly correlated electron systems, which exhibit metal-insulator transition (MIT) and structural-phase transition (SPT). We show that the effect of ion irradiation is strikingly different between V2O3 and VO2, two otherwise similar materials. Upon irradiation, the MIT and SPT temperatures in V2O3 decrease drastically at low absolute dosages, much lower than for VO2. At a low threshold dose, the insulating state of V2O3 drastically collapses into a metallic state. Contrary to this, irradiation of VO2 leads to a much milder reduction of the MIT and SPT temperatures and to a weak, gradual decrease of the insulating state resistivity - not suppressed even at one order of magnitude higher doses than the V2O3 threshold. These major differences imply that the phase transition in V2O3 arises from global (rather than local as in VO2) physical mechanisms that are extremely sensitive to disorder. This shows that the MIT and SPT may have substantially different physical origins in different systems, with the consequent major implications for theoretical descriptions of the MIT in highly correlated electron systems.
    No preview · Article · May 2015 · Physical Review B
  • R Morales · Ali C Basaran · J E Villegas · D Navas · N Soriano · B Mora · C Redondo · X Batlle · Ivan K Schuller
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    ABSTRACT: The exchange bias of antiferromagnetic-ferromagnetic (AFM-FM) bilayers is found to be strongly dependent on the ferromagnetic spin configuration. The widely accepted inverse proportionality of the exchange bias field with the ferromagnetic thickness is broken in FM layers thinner than the FM correlation length. Moreover, an anomalous thermal dependence of both exchange bias field and coercivity is also found. A model based on springlike domain walls parallel to the AFM-FM interface quantitatively accounts for the experimental results and, in particular, for the deviation from the inverse proportionality law. These results reveal the active role the ferromagnetic spin structure plays in AFM-FM hybrids which leads to a new paradigm of the exchange bias phenomenon.
    No preview · Article · Mar 2015 · Physical Review Letters
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    S J Carreira · V Bekeris · Y J Rosen · C Monton · Ivan K Schuller
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    ABSTRACT: Vortex lattice dynamics has been studied in thin Nb superconducting films sputtered on top of a dense triangular array of V dots with and without an intermediate SiO2 insulating layer. While the insulating layer modifies only slightly the Nb film corrugation, it reduces superconducting commensurability effects (CE) substantially. This implies that superconducting commensurability is dominated by proximity effects. Moreover, the HC2 (T) phase diagram of the sample without an insulating layer shows a parabolic temperature dependence near TC and critical temperature oscillations with the periodicity of the matching field. Therefore, strong proximity effects locally suppress superconductivity leading to a superconducting mesh. When the proximity effect is decreased by an insulating layer, HC2(T) follows the expected linear T dependence.
    Preview · Article · Dec 2014 · Journal of Physics Conference Series
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    S Guénon · J G Ramírez · Ali C Basaran · J Wampler · M Thiemens · S Taylor · Ivan K Schuller
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    ABSTRACT: We have developed a very sensitive, highly selective, non-destructive technique for screening inhomogeneous materials for the presence of superconductivity. This technique, based on phase sensitive detection of microwave absorption is capable of detecting 10(-12) cc of a superconductor embedded in a non-superconducting, non-magnetic matrix. For the first time, we apply this technique to the search for superconductivity in extraterrestrial samples. We tested approximately 65 micrometeorites collected from the water well at the Amundsen-Scott South pole station and compared their spectra with those of eight reference materials. None of these micrometeorites contained superconducting compounds, but we saw the Verwey transition of magnetite in our microwave system. This demonstrates that we are able to detect electro-magnetic phase transitions in extraterrestrial materials at cryogenic temperatures.
    Full-text · Article · Dec 2014 · Scientific Reports
  • Russell Cowburn · Ivan K. Schuller · Ludwig Schultz · Bethanie Stadler
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    ABSTRACT: Most thin magnetic films have their magnetization in the plane of the film because of shape anisotropy. In recent years, there has been a resurgence of interest in thin magnetic films that exhibit a magnetization easy axis along the surface normal due to so-called perpendicular magnetic anisotropy (PMA). PMA has its origins in the symmetry breaking that occurs at surfaces and interfaces and can be strong enough to dominate the magnetic properties of some material systems. In this talk, I explain the physics of such materials and show how the magnetic properties associated with PMA are often very well suited to applications. I show three different examples of real and potential applications of PMA materials: ultralow power spin-transfer-torque magnetic RAM devices for low-energy computing, three-dimensional (3-D) magnetic logic structures, and a novel form of cancer therapy.
    No preview · Article · Dec 2014 · IEEE Transactions on Magnetics
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    ABSTRACT: Proximity effects and exchange coupling across interfaces of hybrid magnetic heterostructures present unique opportunities for functional material design. In this review, we present an overview of recent experiments on magnetic hybrid materials in which magnetism was controlled by proximity to an active material. In particular, we discuss interfacial strain coupling of ferromagnetic materials in contact with a material undergoing a structural deformation. Bilayers containing VO2 and V2O3 as active materials are shown to strongly affect the magnetization and coercivity of ferromagnetic materials due to stress anisotropy caused by a temperature-dependent structural displacement in the oxide. The possibilities of tuning the system by sample morphology and materials choice are discussed in detail. In addition, we highlight a length-scale competition between magnetic and structural domains which leads to a maximum change in the coercivity in a narrow temperature window of the vanadium oxide phase transition.
    Full-text · Article · Oct 2014 · Journal of Materials Research
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    ABSTRACT: Optical-pump terahertz-probe spectroscopy is used to investigate ultrafast far-infrared conductivity dynamics during the insulator-to-metal transition (IMT) in vanadium sesquioxide (V2O3). The resultant conductivity increase occurs on a tens of ps timescale, exhibiting a strong dependence on the initial temperature and fluence. We have identified a scaling of the conductivity dynamics upon renormalizing the time axis with a simple power law (alpha = 1/2) that depends solely on the initial, final, and conductivity onset temperatures. Qualitative and quantitative considerations indicate that the dynamics arise from nucleation and growth of the metallic phase which can be described by the Avrami model. We show that the temporal scaling arises from spatial scaling of the growth of the metallic volume fraction, highlighting the self-similar nature of the dynamics. Our results illustrate the important role played by mesoscopic effects in phase transition dynamics.
    Full-text · Article · Oct 2014 · Physical Review B
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    ABSTRACT: The resistivity of vanadium dioxide $({\mathrm{VO}}_{2})$ decreased by over one order of magnitude upon localized illumination with x rays at room temperature. Despite this reduction, the structure remained in the monoclinic phase and had no signature of the high-temperature tetragonal phase that is usually associated with the lower resistance. Once illumination ceased, relaxation to the insulating state took tens of hours near room temperature. However, a full recovery of the insulating state was achieved within minutes by thermal cycling. We show that this behavior is consistent with random local-potential fluctuations and random distribution of discrete recombination sites used to model residual photoconductivity.
    No preview · Article · Oct 2014 · Physical Review B
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    ABSTRACT: We present performance characteristics of nanoscaled cobalt phthalocyanine (CoPc)-based organic field-effect transistors (OFETs) as a function of channel length. We found a channel length range which maximizes the field effect mobility in a trade-off between the decrease in the number of organic grain boundaries and the increase of the electrode–organic contact region. Further reduction of channel length is limited by fringe currents, which lead to an increased off current and to a degradation of the sub-threshold slope. From this, we define an optimal channel length of 280 nm to 1 μm for applications in submicrometric CoPc-based OFETs. Our results are particularly relevant for the miniaturization of chemical sensing OFETs, where metal phthalocyanines have proven to be excellent candidates for the fabrication of the transistor channel.
    Full-text · Article · Oct 2014 · Physica Status Solidi (A) Applications and Materials
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    ABSTRACT: This article introduces magnetic field modulated microwave spectroscopy (MFMMS) as a unique and high-sensitivity technique for use in the search for new superconductors. MFMMS measures reflected microwave power as a function of temperature. The modulation induced by the external ac magnetic field enables the use of phase locked detection with the consequent sensitivity enhancement. The MFMMS signal across several prototypical structural, magnetic, and electronic transitions is investigated. A literature review on microwave absorption across superconducting transitions is included. We show that MFMMS can be used to detect superconducting transitions selectively with very high sensitivity.
    No preview · Article · Sep 2014 · Reports on Progress in Physics

Publication Stats

20k Citations
1,838.31 Total Impact Points


  • 1970-2015
    • University of California, San Diego
      • • Department of Physics
      • • Department of Chemistry and Biochemistry
      San Diego, California, United States
  • 2013
    • Universidad Politécnica de Madrid
      Madrid, Madrid, Spain
  • 2012
    • University of Tuebingen
      • Institute of Physical and Theoretical Chemistry
      Tübingen, Baden-Württemberg, Germany
    • University of São Paulo
      San Paulo, São Paulo, Brazil
    • CSU Mentor
      Long Beach, California, United States
  • 1977-2012
    • Argonne National Laboratory
      • Division of Materials Science
      Lemont, Illinois, United States
  • 2011
    • Université Paris-Sud 11
      Orsay, Île-de-France, France
  • 2009
    • California State University, Long Beach
      • Department of Physics & Astronomy
      Long Beach, California, United States
  • 2002-2009
    • University of Oviedo
      • Department of Physics
      Oviedo, Asturias, Spain
  • 2001
    • Pontifical Catholic University of Chile
      • Facultad de Física
      CiudadSantiago, Santiago Metropolitan, Chile
  • 1998
    • Centro Atómico Bariloche
      San Carlos de Bariloche, Río Negro, Argentina
  • 1990-1998
    • KU Leuven
      • Section of Nuclear and Radiation Physics (IKS)
      Leuven, VLG, Belgium
  • 1996
    • University of Porto
      Oporto, Porto, Portugal
  • 1995
    • Complutense University of Madrid
      Madrid, Madrid, Spain
  • 1991
    • University of Santiago, Chile
      CiudadSantiago, Santiago Metropolitan, Chile
  • 1988-1989
    • National University (California)
      San Diego, California, United States
  • 1987
    • Illinois Institute of Technology
      • Department of Physics
      Chicago, IL, United States
    • IIT Research Institute (IITRI)
      Chicago, Illinois, United States
  • 1975-1982
    • Northwestern University
      Evanston, Illinois, United States
  • 1977-1979
    • University of California, Los Angeles
      Los Angeles, California, United States