Nicolas Reyren

Superconducting interfaces between insulating oxides.

At interfaces between complex oxides, electronic systems with unusual electronic properties can be generated. We report on superconductivity in the electron gas formed at the interface between two insulating dielectric perovskite oxides, LaAlO3 and SrTiO3. The behavior of the electron gas is that of a two-dimensional superconductor, confined to a thin sheet at the interface. The superconducting transition temperature of ≅ 200 millikelvin provides a strict upper limit to the thickness of the superconducting layer of ≅ 10 nanometers.

Electric field control of the LaAlO3/SrTiO3 interface ground state

Interfaces between complex oxides are emerging as one of the most interesting systems in condensed matter physics. In this special setting, in which translational symmetry is artificially broken, a variety of new and unusual electronic phases can be promoted. Theoretical studies predict complex phase diagrams and suggest the key role of the charge carrier density in determining the systems’ ground states. A particularly fascinating system is the conducting interface between the band insulators LaAlO3 and SrTiO3 (ref. 3). Recently two possible ground states have been experimentally identified: a magnetic state and a two-dimensional superconducting condensate. Here we use the electric field effect to explore the phase diagram of the system. The electrostatic tuning of the carrier density allows an on/off switching of superconductivity and drives a quantum phase transition between a two-dimensional superconducting state and an insulating state. Analyses of the magnetotransport properties in the insulating state are consistent with weak localization and do not provide evidence for magnetism. The electric field control of superconductivity demonstrated here opens the way to the development of new mesoscopic superconducting circuits.

Tunable Rashba spin-orbit interaction at oxide interfaces.

The quasi-two-dimensional electron gas found at the LaAlO{3}/SrTiO{3} interface offers exciting new functionalities, such as tunable superconductivity, and has been proposed as a new nanoelectronics fabrication platform. Here we lay out a new example of an electronic property arising from the interfacial breaking of inversion symmetry, namely, a large Rashba spin-orbit interaction, whose magnitude can be modulated by the application of an external electric field. By means of magnetotransport experiments we explore the evolution of the spin-orbit coupling across the phase diagram of the system. We uncover a steep rise in Rashba interaction occurring around the doping level where a quantum critical point separates the insulating and superconducting ground states of the system.

Two-dimensional electron gas with universal subbands at the surface of SrTiO3

As silicon is the basis of conventional electronics, so strontium titanate (SrTiO3) is the foundation of the emerging field of oxide electronics. SrTiO3 is the preferred template for the creation of exotic, two-dimensional (2D) phases of electron matter at oxide interfaces that have metal–insulator transitions, superconductivity or large negative magnetoresistance. However, the physical nature of the electronic structure underlying these 2D electron gases (2DEGs), which is crucial to understanding their remarkable properties, remains elusive. Here we show, using angle-resolved photoemission spectroscopy, that there is a highly metallic universal 2DEG at the vacuum-cleaved surface of SrTiO3 (including the non-doped insulating material) independently of bulk carrier densities over more than seven decades. This 2DEG is confined within a region of about five unit cells and has a sheet carrier density of ∼0.33 electrons per square lattice parameter. The electronic structure consists of multiple subbands of heavy and light electrons. The similarity of this 2DEG to those reported in SrTiO3-based heterostructures and field-effect transistors suggests that different forms of electron confinement at the surface of SrTiO3 lead to essentially the same 2DEG. Our discovery provides a model system for the study of the electronic structure of 2DEGs in SrTiO3-based devices and a novel means of generating 2DEGs at the surfaces of transition-metal oxides.

Additive interfacial chiral interaction in multilayers for stabilization of small individual skyrmions at room temperature

Facing the ever-growing demand for data storage will most probably require a new paradigm. Nanoscale magnetic skyrmions are anticipated to solve this issue as they are arguably the smallest spin textures in magnetic thin films in nature. We designed cobalt-based multilayered thin films in which the cobalt layer is sandwiched between two heavy metals and so provides additive interfacial Dzyaloshinskii-Moriya interactions (DMIs), which reach a value close to 2 mJ m(-2) in the case of the Ir|Co|Pt asymmetric multilayers. Using a magnetization-sensitive scanning X-ray transmission microscopy technique, we imaged small magnetic domains at very low fields in these multilayers. The study of their behaviour in a perpendicular magnetic field allows us to conclude that they are actually magnetic skyrmions stabilized by the large DMI. This discovery of stable sub-100 nm individual skyrmions at room temperature in a technologically relevant material opens the way for device applications in the near future.Facing the ever-growing demand for data storage will most probably require a new paradigm. Magnetic skyrmions are anticipated to solve this issue as they are arguably the smallest spin textures in magnetic thin films in nature. We designed cobalt-based multilayered thin films where the cobalt layer is sandwiched between two heavy metals providing additive interfacial Dzyaloshinskii-Moriya interactions, which reach about 2 mJ/m in the case of the Ir|Co|Pt multilayers. Using a magnetization-sensitive scanning x-ray transmission microscopy technique, we imaged magnetic bubble-like domains in these multilayers. The study of their behavior in magnetic field allows us to conclude that they are actually magnetic skyrmions stabilized by the Dzyaloshinskii-Moriya interaction. This discovery of stable skyrmions at room temperature in a technologically relevant material opens the way for device applications in a near future. A major societal challenge is related to the continually increasing quantity of information to process and store. The hard disk drives, in which information is encoded magnetically, allow nowadays the storage of zettabytes (10) of information, but this technology should soon reach its limits. An up-and-coming avenue has been opened by the discovery of magnetic skyrmions [1], i.e. spin windings that can be localized within a diameter of a few nanometers and can move like particles [2]. These magnetic solitons, remarkably robust against defects due to the topology of their magnetic texture [3], are promising for being the ultimate magnetic bits to carry and store information. The topology of the skyrmions also appears to further underlie other important features such as their current-induced motion induced by small dc currents that is crucial for real applications but also the existence of a specific component in Hall Effect [4-6] that can be used advantageously for an electrical read-out of the information carried by nano-scale skyrmions. We proposed recently that these skyrmions could be used in future storage devices and information processing [2]. The existence of skyrmion spin configuration has been predicted theoretically about thirty years ago [1] but it was only recently that skyrmion lattices have been observed in crystals with noncentrosymmetric lattices, e.g. B20 crystallographic structure in MnSi [7-9], FeCoSi [10] or FeGe [5] crystals. In 2011, skyrmions have also been identified in single ultrathin ferromagnetic films with out-of-plane magnetization (Fe and FePd) deposited on a heavy metal substrate such as Ir(1 1 1) [11, 12]. Thin magnetic films appear to be more compatible with technological developments, though the observation of skyrmions in thin films has been limited up to now to low temperature and also needs, in some cases, the presence of a large applied magnetic field [12]. The study of these new magnetic phases associated with chiral interactions has generated a very strong interest in the community of solid state physics and magnetic systems with reduced dimensions. The magnetic skyrmions are magnetic solitons that are, in most cases, induced by chiral interactions of the Dzyaloshinskii-Moriya (DM) type, which result from spin-orbit effects in the absence of inversion symmetry. The DM Interaction (DMI) between two neighboring atomic spins S1 and S2 can be expressed as: HDMI = D12·(S1×S2) where D12 is the DM vector (Fig. 1A). In this work, we will focus on ultrathin magnetic systems in which the DMI results from the breaking of the inversion symmetry at the interface of a magnetic layer and can be large at the interface with a heavy metal having a large spin-orbit coupling [11]. In such case, the DM vector D12 is perpendicular to the r(S1)−r(S2) line (r(·) being the position vector) and gives rise to cycloidal skyrmionic configurations, which have a given chirality, also called hedgehog skyrmion (see Fig. 1B). Our main goal is to extend the already-observed generation of interfaceinduced skyrmions from single magnetic films [11, 12] to multilayers by stacking layers of magnetic metals and nonmagnetic heavy metals (e.g. Pt and Ir) so as to induce DMI at all the magnetic interfaces (Fig.1A). The advantages of such innovative multilayered systems are twofold. Firstly we anticipate that thermal stability of skyrmions can be greatly improved, simply because of the increase of their magnetic volume, as it turns out for our samples that the same magnetic texture extends vertically throughout the multilayer. Secondly, the choice of two different heavy metals A and B sandwiching each magnetic layer potentially allows tuning the amplitude of interfacial chiral interaction, notably to increase it drastically if the two heavy materials induce interfacial chiral interactions of opposite symmetries and parallel D when they are on opposite sides of the magnetic layer. As we will see, our multilayers present bubble-like local configurations characterized by a reversed magnetization in the center [15]. The first important conclusion that can be draught is that the presence of these bubble-like configurations cannot be accounted for by dipolar interaction but only by the large DMI of our asymmetric multilayered systems. We will then show that the size of the bubble-like configurations and its field dependence is consistent with micromagnetic simulations of skyrmionic states induced by large DMI. This stabilization in multilayered films and at room temperature of individual magnetic skyrmions induced by chiral interactions represents the most important result of this work. Given the important features of magnetic skyrmions associated to their topological nature compared to other magnetic configurations (size, easier current-induced propagation, smaller sensitivity to defects, ...), these advances represent a definite breakthrough in the research on single skyrmion towards the potential development of skyrmion-based devices. Multilayers with additive chiral interaction at interfaces with heavy metal layers The prototype of the multilayered systems in which we aim to investigate the magnetic configuration is presented in Fig. 1A. The samples grown by sputtering deposition are stacks of 10 Ir|Co|Pt trilayers, each trilayer being composed of a 0.6 nm thick Co layer sandwiched between 1 nm of Ir and 1 nm of Pt: Pt10|Co0.6|Pt1|{Ir1|Co0.6|Pt1}10|Pt3. The choice of the two heavy materials i.e. Pt and Ir, has been guided by recent experiments of asymmetric domain wall propagation [16, 17] and recent ab initio predictions of opposite DMI for Co on Ir and Co on Pt [18], which corresponds to additive DMI at the two interfaces of the Co layers sandwiched between Ir and Pt. In addition to these Ir|Co|Pt asymmetric multilayers, we also prepared reference samples of Pt|Co|Pt with symmetric interfaces, in which one can expect a cancellation, at least partial, of the interfacial DMI [19]. Details about the growth conditions and the characterization of their magnetic properties are presented in the supplementary materials [14]. From SQUID measurements on our multilayers, we deduce a magnetization at saturation of 0.96 ± 0.10 MA/m (1.6 ± 0.2 MA/m) and an effective anisotropy of 0.17 ± 0.04 MJ/m (0.25 ± 0.07 MJ/m) for the Ir|Co|Pt (Pt|Co|Pt) system. These magnetic parameters have been used for micromagnetic simulations of their magnetic configuration [20, 21]. We present here simulations for the case of Co layers completely decoupled. The case taking into account the proximity-induced moment [22, 23] in Pt and Ir is detailed in the supplementary materials and lead to even larger estimation of the DMI amplitude [14]. Fig. 1. Interfacial Dzyaloshinskii-Moriya interaction (DMI) in asymmetric magnetic multilayers. (A) The DMI for two magnetic atoms close to an atom with large spin-orbit coupling in the Fert-Levy picture [13]. Zoom on a single trilayer composed of a magnetic layer (gray) sandwiched between two different heavy metals A (blue) and B (green) that induce the same chirality (same orientation of D) when A is below and B above the magnetic layer, and finally on an asymmetric multilayer made of several repetition of the trilayer. (B) Sketch of an isolated hedgehog skyrmion stabilized by interfacial chiral interaction in a magnetic thin film. (C-F) 800×800 nm out-of-plane magnetization (mz) map obtained by Scanning Transmission X-ray Microscopy (STXM) on a {Ir|Co|Pt}10 multilayer at room temperature for applied out-of-plane magnetic fields of 8 (C), 38 (D), 73 (E) and 83 mT (F). (G) Experimental dichroic signal through a magnetic circular domain (skyrmion) as observed at 22 mT (black dots). The corresponding STXM 360×360 nm image is in inset. The blue dashed curve is the magnetization profile of an ideal 30 nm-radius skyrmion and the red curve derives from the model described in the text. (H) Same type of data at 58 mT and corresponding simulation of 20 nm-radius skyrmion. The dichroic signal is not reversed due to the limited resolution of the STXM [14]. Magnetization mapping in asymetric multilayers: Bubbles or skyrmions? In order to map the distribution of the vertical component of the magnetization in our Ir|Co|Pt multilayers and to follow its evolution as a function of the external perpendicular field, we have performed Scanning Transmission X-ray Microscopy (STXM) experiments on samples grown on Si3N4 membranes by measuring the dichroic signal at Co L3-edge [14]. In Fig. 1C-F, we display a series of images obtained at different perpendicular field values. After saturation at large negative field, we observe a domain configuration (Fig. 1C) at low positive field that combines some worm-shape domains together with other domains having almost a circular shape. Note that the magnetic contrast that we detect by STXM is an averaged value of the magnetic configuration throughout the entire multilayer. When the field is increased to μ0H⊥ = 38 mT, the magnetic domains favored by the field extend (Fig. 1D). Before re

Spin Pumping and Inverse Spin Hall Effect in Platinum: The Essential Role of Spin-Memory Loss at Metallic Interfaces

Through combined ferromagnetic resonance, spin pumping, and inverse spin Hall effect experiments in Co|Pt bilayers and Co|Cu|Pt trilayers, we demonstrate consistent values of ℓsfPt=3.4±0.4  nm and θSHEPt=0.056±0.010 for the respective spin diffusion length and spin Hall angle for Pt. Our data and model emphasize the partial depolarization of the spin current at each interface due to spin-memory loss. Our model reconciles the previously published spin Hall angle values and explains the different scaling lengths for the ferromagnetic damping and the spin Hall effect induced voltage.

Two-dimensional quantum oscillations of the conductance at LaAlO3/SrTiO3 interfaces.

We report on a study of magnetotransport in LaAlO3 /SrTiO3 interfaces characterized by mobilities of the order of several thousands cm2/V s. We observe Shubnikov-de Haas oscillations whose period depends only on the perpendicular component of the magnetic field. This observation directly indicates the formation of a two-dimensional electron gas originating from quantum confinement at the interface. From the temperature dependence of the oscillation amplitude we extract an effective carrier mass m* ≃ 1.45 m(e). An electric field applied in the back-gate geometry increases the mobility, the carrier density, and the oscillation frequency.

Emergent phenomena induced by spin–orbit coupling at surfaces and interfaces

Spin–orbit coupling (SOC) describes the relativistic interaction between the spin and momentum degrees of freedom of electrons, and is central to the rich phenomena observed in condensed matter systems. In recent years, new phases of matter have emerged from the interplay between SOC and low dimensionality, such as chiral spin textures and spin-polarized surface and interface states. These low-dimensional SOC-based realizations are typically robust and can be exploited at room temperature. Here we discuss SOC as a means of producing such fundamentally new physical phenomena in thin films and heterostructures. We put into context the technological promise of these material classes for developing spin-based device applications at room temperature.

Magnetic skyrmions: advances in physics and potential applications

Magnetic skyrmions are small swirling topological defects in the magnetization texture stabilized by the protection due to their topology. In most cases they are induced by chiral interactions between atomic spins existing in non-centrosymmetric magnetic compounds or in thin films in which inversion symmetry is broken by the presence of an interface. The skyrmions can be extremely small with diameters in the nanometer range and, importantly, they behave as particles that can be moved, created or annihilated, making them suitable for abacus-type applications in information storage, logic or neuro-inspired technologies. Up to the last years skyrmions were observed only at low temperature (and in most cases under large applied fields) but important efforts of research has been recently devoted to find thin film and multilayered structures in which skyrmions are stabilized above room temperature and manipulated by current. This article focuses on these recent advances on the route to devices prototypes.

Influence of the growth conditions on the LaAIO3/SrTiO3 interface electronic properties

The effects of oxygen pressure during the growth of LaAlO3 on (001) SrTiO3, and of post-deposition annealing were investigated. While little influence on the structure was observed, the transport properties were found to depend on both growth pressure and annealing. For LaAlO3 layer thicknesses between 5 and 10 unit cells and growth pressures between 10− 4 and 10−2 mbar, the LaAlO3/SrTiO3 interfaces displayed similar metallic behavior with a sharp transition to a superconducting state. At an oxygen pressure of 10− 6 mbar oxygen vacancies were clearly introduced and extended deep into the SrTiO3 crystal. These vacancies could be removed by post-deposition annealing in 0.2 bar of O2 at ~530 °C. At a growth pressure of 10− 4 mbar, the electronic properties of samples with ultra-thin LaAlO3 layers (2 to 3 unit cells thick) were found to depend markedly on the post-annealing step.