Superconducting imprint of magnetic textures in ferromagnets with perpendicular magnetic anisotropy.

Research on proximity effects in superconductor/ferromagnetic hybrids has most often focused on how superconducting properties are affected-and can be controlled-by the effects of the ferromagnet's exchange or magnetic fringe fields. The opposite, namely the possibility to craft, tailor and stabilize the magnetic texture in a ferromagnet by exploiting superconducting effects, has been more seldom explored. Here we show that the magnetic flux trapped in high-temperature superconducting YBa2Cu3O7-δ microstructures can be used to modify the magnetic reversal of a hard ferromagnet-a cobalt/platinum multilayer with perpendicular magnetic anisotropy-and to imprint unusual magnetic domain distributions in a controlled manner via the magnetic field history. The domain distributions imprinted in the superconducting state remain stable, in absence of an external magnetic field, even after increasing the temperature well above the superconducting critical temperature, at variance to what has been observed for soft ferromagnets with in-plane magnetic anisotropy. This opens the possibility of having non-trivial magnetic configuration textures at room temperature after being tailored below the superconducting transition temperature. The observed effects are well explained by micromagnetic simulations that demonstrate the role played by the magnetic field from the superconductor on the nucleation, propagation, and stabilization of magnetic domains.

Dimensionality-driven metal-insulator transition in spin-orbit-coupled IrO2.

A metal-insulator transition is observed in spin-orbit-coupled IrO2 thin films upon reduction of the film thickness. In the epitaxially grown samples, the critical thickness (t ∼ 1.5-2.2 nm) is found to depend on growth orientation (001), (100) or (110). Interestingly from the applied point of view, the insulating behavior is found even in polycrystalline ultrathin films. By analyzing the experimental electrical response with various theoretical models, we find good fits to the Efros-Shklovskii-VRH and the Arrhenius-type behaviors, which suggests an important role of electron correlations in determining the electrical properties of IrO2. Our magnetic measurements also point to a significant role of magnetic order. Altogether, our results would point to a mixed Slater- and Mott-type of insulator.

Controlled Sign Reversal of Electroresistance in Oxide Tunnel Junctions by Electrochemical-Ferroelectric Coupling.

The persistence of ferroelectricity in ultrathin layers relies critically on screening or compensation of polarization charges which otherwise destabilize the ferroelectric state. At surfaces, charged defects play a crucial role in the screening mechanism triggering novel mixed electrochemical-ferroelectric states. At interfaces, however, the coupling between ferroelectric and electrochemical states has remained unexplored. Here, we make use of the dynamic formation of the oxygen vacancy profile in the nanometer-thick barrier of a ferroelectric tunnel junction to demonstrate the interplay between electrochemical and ferroelectric degrees of freedom at an oxide interface. We fabricate ferroelectric tunnel junctions with a La_{0.7}Sr_{0.3}MnO_{3} bottom electrode and BaTiO_{3} ferroelectric barrier. We use poling strategies to promote the generation and transport of oxygen vacancies at the metallic top electrode. Generated oxygen vacancies control the stability of the ferroelectric polarization and modify its coercive fields. The ferroelectric polarization, in turn, controls the ionization of oxygen vacancies well above the limits of thermodynamic equilibrium, triggering the build up of a Schottky barrier at the interface which can be turned on and off with ferroelectric switching. This interplay between electronic and electrochemical degrees of freedom yields very large values of the electroresistance (more than 10^{6}% at low temperatures) and enables a controlled switching between clockwise and counterclockwise switching modes in the same junction (and consequently, a change of the sign of the electroresistance). The strong coupling found between electrochemical and electronic degrees of freedom sheds light on the growing debate between resistive and ferroelectric switching in ferroelectric tunnel junctions, and moreover, can be the source of novel concepts in memory devices and neuromorphic computing.

Ferroelectric Control of Interface Spin Filtering in Multiferroic Tunnel Junctions.

The electronic reconstruction occurring at oxide interfaces may be the source of interesting device concepts for future oxide electronics. Among oxide devices, multiferroic tunnel junctions are being actively investigated as they offer the possibility to modulate the junction current by independently controlling the switching of the magnetization of the electrodes and of the ferroelectric polarization of the barrier. In this Letter, we show that the spin reconstruction at the interfaces of a La_{0.7}Sr_{0.3}MnO_{3}/BaTiO_{3}/La_{0.7}Sr_{0.3}MnO_{3} multiferroic tunnel junction is the origin of a spin filtering functionality that can be turned on and off by reversing the ferroelectric polarization. The ferroelectrically controlled interface spin filter enables a giant electrical modulation of the tunneling magnetoresistance between values of 10% and 1000%, which could inspire device concepts in oxides-based low dissipation spintronics.

Paving the way to nanoionics: atomic origin of barriers for ionic transport through interfaces.

The blocking of ion transport at interfaces strongly limits the performance of electrochemical nanodevices for energy applications. The barrier is believed to arise from space-charge regions generated by mobile ions by analogy to semiconductor junctions. Here we show that something different is at play by studying ion transport in a bicrystal of yttria (9% mol) stabilized zirconia (YSZ), an emblematic oxide ion conductor. Aberration-corrected scanning transmission electron microscopy (STEM) provides structure and composition at atomic resolution, with the sensitivity to directly reveal the oxygen ion profile. We find that Y segregates to the grain boundary at Zr sites, together with a depletion of oxygen that is confined to a small length scale of around 0.5 nm. Contrary to the main thesis of the space-charge model, there exists no evidence of a long-range O vacancy depletion layer. Combining ion transport measurements across a single grain boundary by nanoscale electrochemical strain microscopy (ESM), broadband dielectric spectroscopy measurements, and density functional calculations, we show that grain-boundary-induced electronic states act as acceptors, resulting in a negatively charged core. Besides the possible effect of the modified chemical bonding, this negative charge gives rise to an additional barrier for ion transport at the grain boundary.

Proximity Driven Commensurate Pinning in YBa2Cu3O7 through All-Oxide Magnetic Nanostructures.

The design of artificial vortex pinning landscapes is a major goal toward large scale applications of cuprate superconductors. Although 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 , 439 - 443 and 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 , 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.

Insight into spin transport in oxide heterostructures from interface-resolved magnetic mapping.

At interfaces between complex oxides, electronic, orbital and magnetic reconstructions may produce states of matter absent from the materials involved, offering novel possibilities for electronic and spintronic devices. Here we show that magnetic reconstruction has a strong influence on the interfacial spin selectivity, a key parameter controlling spin transport in magnetic tunnel junctions. In epitaxial heterostructures combining layers of antiferromagnetic LaFeO(3) (LFO) and ferromagnetic La(0.7)Sr(0.3)MnO(3) (LSMO), we find that a net magnetic moment is induced in the first few unit planes of LFO near the interface with LSMO. Using X-ray photoemission electron microscopy, we show that the ferromagnetic domain structure of the manganite electrodes is imprinted into the antiferromagnetic tunnel barrier, endowing it with spin selectivity. Finally, we find that the spin arrangement resulting from coexisting ferromagnetic and antiferromagnetic interactions strongly influences the tunnel magnetoresistance of LSMO/LFO/LSMO junctions through competing spin-polarization and spin-filtering effects.

Electronic and magnetic reconstructions in La0.7Sr0.3MnO3/SrTiO3 heterostructures: a case of enhanced interlayer coupling controlled by the interface.

We report on the magnetic coupling of La0.7Sr0.3MnO3 layers through SrTiO3 spacers in La0.7Sr0.3MnO3/SrTiO3 epitaxial heterostructures. Combined aberration-corrected microscopy and electron-energy-loss spectroscopy evidence charge transfer to the empty conduction band of the titanate. Ti d electrons interact via superexchange with Mn, giving rise to a Ti magnetic moment as demonstrated by x-ray magnetic circular dichroism. This induced magnetic moment in the SrTiO3 controls the bulk magnetic and transport properties of the superlattices when the titanate layer thickness is below 1 nm.

Spin and orbital Ti magnetism at LaMnO3/SrTiO3 interfaces.

In systems with strong electron-lattice coupling, such as manganites, orbital degeneracy is lifted, causing a null expectation value of the orbital magnetic moment. Magnetic structure is thus determined by spin-spin superexchange. In titanates, however, with much smaller Jahn-Teller distortions, orbital degeneracy might allow non-zero values of the orbital magnetic moment, and novel forms of ferromagnetic superexchange interaction unique to t(2g) electron systems have been theoretically predicted, although their experimental observation has remained elusive. In this paper, we report a new kind of Ti(3+) ferromagnetism at LaMnO(3)/SrTiO(3) epitaxial interfaces. It results from charge transfer to the empty conduction band of the titanate and has spin and orbital contributions evidencing the role of orbital degeneracy. The possibility of tuning magnetic alignment (ferromagnetic or antiferromagnetic) of Ti and Mn moments by structural parameters is demonstrated. This result will provide important clues for understanding the effects of orbital degeneracy in superexchange coupling.

Colossal ionic conductivity at interfaces of epitaxial ZrO2:Y2O3/SrTiO3 heterostructures.

The search for electrolyte materials with high oxygen conductivities is a key step toward reducing the operation temperature of fuel cells, which is currently above 700 degrees C. We report a high lateral ionic conductivity, showing up to eight orders of magnitude enhancement near room temperature, in yttria-stabilized zirconia (YSZ)/strontium titanate epitaxial heterostructures. The enhancement of the conductivity is observed, along with a YSZ layer thickness-independent conductance, showing that it is an interface process. We propose that the atomic reconstruction at the interface between highly dissimilar structures (such as fluorite and perovskite) provides both a large number of carriers and a high-mobility plane, yielding colossal values of the ionic conductivity.

Ion dynamics under pressure in an ionic liquid.

We investigate ion dynamics under pressure in the ionic liquid 1-butyl-1-methylpyrrolidinium bis[oxalate]borate (BMP-BOB) by conductivity relaxation measurements in the temperature range 123-300 K and varying pressures from 0.1 MPa up to 0.5 GPa. We report on the influence of pressure on the relaxation times and on the spectral shape of the conductivity relaxation process. We also analyze the pressure dependence of the glass transition temperature and find that the dynamic response under pressure in this ionic liquid shows remarkable similarities to nonionic glass formers. The main relaxation process shows temperature-pressure superposition while a secondary relaxation process, very weakly depending on pressure, is observed. The spectral shape of the main relaxation broadens with increasing pressure or decreasing temperature, but is found to be the same when the relaxation time is the same, independently of the particular pressure and temperature values.

Electron microscopy and 3D reconstructions reveal that human ATM kinase uses an arm-like domain to clamp around double-stranded DNA.

The human tumor suppressor gene ataxia telangiectasia mutated (ATM) encodes a 3056 amino-acid protein kinase that regulates cell cycle checkpoints. ATM is defective in the neurodegenerative and cancer predisposition syndrome ataxia-telangiectasia. ATM protein kinase is activated by DNA damage and responds by phosphorylating downstream effectors involved in cell cycle arrest and DNA repair, such as p53, MDM2, CHEK2, BRCA1 and H2AX. ATM is probably a component of, or in close proximity to, the double-stranded DNA break-sensing machinery. We have observed purified human ATM protein, ATM-DNA and ATM-DNA-avidin bound complexes by single-particle electron microscopy and obtained three-dimensional reconstructions which show that ATM is composed of two main domains comprising a head and an arm. DNA binding to ATM induces a large conformational movement of the arm-like domain. Taken together, these three structures suggest that ATM is capable of interacting with DNA, using its arm to clamp around the double helix.