Using state-of-the-art, aberration-corrected scanning transmission electron microscopy and electron energy loss spectroscopy with atomic-scale spatial resolution, experimental evidence for an intrinsic electronic reconstruction at the LAO/STO interface is shown. Simultaneous measurements of interfacial electron density and system polarization are crucial for establishing the highly debated origin of the 2D electron gas.

We show that the growth of the heterostructure LaGaO 3 / SrTiO 3 yields the formation of a highly conductive interface. Our samples were carefully analyzed by high resolution electron microscopy, in order to assess their crystal perfection and to evaluate the abruptness of the interface. Their carrier density and sheet resistance are compared to the case of LaAlO 3 / SrTiO 3 and a superconducting transition is found. The results open the route to widening the field of polar-nonpolar interfaces, pose some phenomenological constrains to their underlying physics and highlight the chance of tailoring their properties for future applications by adopting suitable polar materials.The quasi-two-dimensional electron gas ͑q2DEG͒ recently discovered at the LaAlO 3 ͑LAO͒ / SrTiO 3 ͑STO͒ interface 1 is presently envisaged as an ideal system for the realization of nanoscale oxide devices. 2 The electronic reconstruction model attributes the origin of the q2DEG to an electronic relaxation mechanism occurring at the interface between the ͑nominally͒ non-polar ͑001͒ STO substrate and the polar ͑001͒ LAO film. The wide band gap of LAO is considered as crucial in this approach, because it determines the capability of the polar film to transfer charges over the band gap of STO. Ideally, half an electron per areal unit cell ͑Ϸ3.3ϫ 10 14 cm −2 ͒ is expected to be transferred at the TiO 2 -LaO interface, partially filling the 3d Ti levels of the STO conduction band ͑CB͒. Alternatively, a possible active role of oxygen vacancies in STO near the interface was envisaged. 3 Actually, the transport properties of the heterostructure are affected both by oxygen pressure during growth 4,5 and by the application of an oxygen postanneal. 5 Finally, it was argued that a substantial La substitution for Sr during sample growth might drive the insulating surface of STO into a conductor. [6][7][8] Obviously, also LAO poses material issues. 9 In this context, we started the search of novel heterostructures based on a different overlayer. On this basis, we identified as a first test material LaGaO 3 ͑LGO͒, a polar, wide band gap, pseudocubic perovskite.Films of LAO and LGO were deposited on nominally TiO 2 terminated STO substrates, chemically treated in deionized water and buffered-HF. 10,11 The growth was performed by reflection high energy electron diffraction ͑RHEED͒ as-sisted pulsed laser deposition ͑KrF excimer laser, 248 nm͒ with a typical fluence of Ϸ1.5-2.5 J cm −2 at the target, a substrate temperature of 800°C and different oxygen pressures within the 10 −2 -10 −4 mbar range. 12 LAO films presented regular RHEED oscillations typical of layer-by-layer growth and a final pattern reminiscent of a single crystal surface, whereas LGO films showed damped and less regular oscillations, and a streaky 2D pattern at the end of the growth ͑Fig. 1͒.The atomic and electronic structures of LAO/STO and LGO/STO interfaces were investigated by high-resolution scanning transmission electron microscopy ͑STEM͒ and electron energy loss spectroscopy ͑EELS͒ measur...

The so-called "polar catastrophe," a sudden electronic reconstruction taking place to compensate for the interfacial ionic polar discontinuity, is currently considered as a likely factor to explain the surprising conductivity of the interface between the insulators LaAlO 3 and SrTiO 3 . We applied optical second harmonic generation, a technique that a priori can detect both mobile and localized interfacial electrons, to investigating the electronic polar reconstructions taking place at the interface. As the LaAlO 3 film thickness is increased, we identify two abrupt electronic rearrangements: the first takes place at a thickness of 3 unit cells, in the insulating state; the second occurs at a thickness of 4-6 unit cells, i.e., just above the threshold for which the samples become conducting. Two possible physical scenarios behind these observations are proposed. The first is based on an electronic transfer into localized electronic states at the interface that acts as a precursor of the conductivity onset. In the second scenario, the signal variations are attributed to the strong ionic relaxations taking place in the LaAlO 3 layer.

A major challenge for future spintronics is to develop suitable spin transport channels with long spin lifetime and propagation length. Graphene can meet these requirements, even at room temperature. On the other side, taking advantage of the fast motion of chiral textures, that is, Néel-type domain walls and magnetic skyrmions, can satisfy the demands for high-density data storage, low power consumption, and high processing speed. We have engineered epitaxial structures where an epitaxial ferromagnetic Co layer is sandwiched between an epitaxial Pt(111) buffer grown in turn onto MgO(111) substrates and a graphene layer. We provide evidence of a graphene-induced enhancement of the perpendicular magnetic anisotropy up to 4 nm thick Co films and of the existence of chiral left-handed Néel-type domain walls stabilized by the effective Dzyaloshinskii-Moriya interaction (DMI) in the stack. The experiments show evidence of a sizable DMI at the gr/Co interface, which is described in terms of a conduction electron mediated Rashba-DMI mechanism and points opposite to the spin orbit coupling-induced DMI at the Co/Pt interface. In addition, the presence of graphene results in (i) a surfactant action for the Co growth, producing an intercalated, flat, highly perfect face-centered cubic film, pseudomorphic with Pt and (ii) an efficient protection from oxidation. The magnetic chiral texture is stable at room temperature and grown on insulating substrate. Our findings open new routes to control chiral spin structures using interfacial engineering in graphene-based systems for future spin-orbitronics devices fully integrated on oxide substrates.

A site-dependent charge transfer to 7,7′,8,8′-tetracyanoquinodimethane (TCNQ) adsorbed on a single layer of periodically rippled graphene grown epitaxially on Ru(0001), identified by X-ray photoemission techniques, can be spatially resolved using Scanning Tunneling Microscopy, which can also detect the formation of magnetic moments. The molecules adsorbed on the lower part of the ripples are charged with electrons donated from the doped graphene overlayer and develop a magnetic moment, while those at the upper part of the ripples are neutral. On the other hand, TCNQ adsorbed on graphene on Ir(111) shows negligible charge transfer and no magnetic moment. These observations explain the spatially dependent longrange magnetic order observed recently for TCNQ on gr/Ru(0001).

The transport characterization in the dark and under light irradiation of three different interfaces-LaAlO3/SrTiO3, LaGaO3/SrTiO3, and the novel NdGaO3/ SrTiO3 heterostructure is reported. All of them share a perovskite structure, an insulating nature of the single building blocks, a polar/non-polar character, and a critical thickness of four unit cells for the onset of conductivity. The interface structure and charge confinement in NdGaO3/SrTiO3 are probed by atomic-scale-resolved electron energy loss spectroscopy showing that, similarly to LaAlO3/SrTiO3, extra electronic charge confined in a sheet of about 1.5 nm in thickness is present at the NdGaO3/SrTiO3 interface. Electric transport measurements performed in the dark and under radiation show remarkable similarities and provide evidence that the persistent perturbation induced by light is an intrinsic peculiar property of the three investigated oxide-based polar/non-polar interfaces. This sets a framework for understanding the previous contrasting results found in the literature about photoconductivity in LaAlO3/SrTiO3 and highlights the connection between the origin of persistent photoconductivity and the origin of conductivity itself. An improved understanding of the photoinduced metastable electron-hole pairs might allow light to be shed directly on the complex physics of this system and on the recently proposed perspectives of oxide interfaces for solar energy conversion

A detailed study of the angular dependence of the magnetization reversal in polycrystalline ferromagnetic ͑FM͒/antiferromagnetic Co/IrMn bilayers with noncollinear FM and unidirectional anisotropies shows a peculiar asymmetric magnetic behavior. The anisotropy configuration is set via a field cooling ͑FC͒ procedure with the magnetic field misaligned with respect to the easy magnetization direction of the FM layer. Different magnetization reversal modes are observed for either positive or negative angles with respect to the FC direction. The angular dependence of both coercivity and exchange bias also clearly displays the broken symmetry of the induced noncollinearity. Our findings are reproduced with a modified Stoner-Wohlfarth model including the induced anisotropy configuration. Our results highlight the importance of the relative angle between anisotropies in exchange bias systems, opening a new path for the tailoring of their magnetic properties. © 2009 American Institute of Physics. ͓doi:10.1063/1.3236768͔ Ferromagnetic/antiferromagnetic ͑FM/AFM͒ structures 1 are at the heart of today's spintronic devices, stabilizing the direction of FM reference layers, while taking advantage of the interfacial exchange interaction effects. 2 The most notable consequences of the FM/AFM exchange coupling are a shift in the hysteresis loop of the FM layer, called exchange bias 0 H E , an enhanced coercivity 0 H C , and an asymmetry in the magnetization reversal process. Experiments have shown that pinned ͑unpinned͒ uncompensated AFM spins at the interface are correlated with 0 H E ͑Ref. 3͒ ͑coercivity enhancement 4 ͒, and that the competition between anisotropies determines the asymmetric behavior of the magnetization reversal. 5,6 Different intrinsic parameters ͑e.g., materials, anisotropies, thicknesses, and shapes͒ 2 as well as extrinsic ones ͓e.g., field cooling ͑FC͒ procedures 7-10 ͔ have been explored to understand the exchange coupling phenomena in FM/AFM systems, aiming at improving the performance of magnetic devices. In general, the interfacial exchange coupling effects depend on the strength of the anisotropies 5 as well as their relative orientation, 6 exhibiting a complex phase diagram of different reversal modes. 5,6,[11][12][13][14][15] In fact, the relative orientation between the intrinsic FM anisotropy and the induced interfacial unidirectional anisotropy can be controlled by different FC procedures, varying both strength, 7,8 FC angle, 6,9,10 and/or interfacial magnetic frustration. 14,15 In this letter we present a detailed study on the magnetization reversal of FM/AFM systems with a noncollinear uniaxial, K U , and unidirectional, K E , anisotropy configuration. Our work reveals the importance of taking into account the misalignment between the K U direction and the direction of the applied field during the FC procedure in order to properly account for the asymmetry of the magnetization reversal and the angular dependences of 0 H C and 0 H E .The collinear and noncollinear relative orientation between t...

A vectorial magneto-optic Kerr effect (v-MOKE) setup with simultaneous and quantitative determination of the two in-plane magnetization components is described. The setup provides both polarization rotations and reflectivity changes at the same time for a given sample orientation with respect to a variable external magnetic field, as well as allowing full angular studies. A classical description based on the Jones formalism is used to calculate the setup's properties. The use of different incoming light polarizations and/or MOKE geometries, as well as the errors due to misalignment and solutions are discussed. To illustrate the capabilities of the setup a detailed study of a model four-fold anisotropy system is presented. Among others, the setup allows to study the angular dependence of the hysteresis phenomena, remanences, critical fields, and magnetization reversal processes, as well as the accurate determination of the easy and hard magnetization directions, domain wall orientations, and magnetic anisotropies.