Enhanced magnetoresistance by monoatomic roughness in epitaxial Fe/MgO/Fe tunnel junctions

Under the terms of the Creative Commons Attribution License 3.0 (CC-BY).-- et al.Interfacial effects on spin and symmetry filtering in single-crystal Fe(001)/MgO/Fe magnetic tunnel junctions are investigated with the insertion of a Fe monoatomic step at the bottom MgO interface. After annealing, the atomically flat bottom electrode is covered by a fractional part of a Fe monoatomic layer resulting in two-dimensional Fe islands that are separated for low coverages and percolated above around half a monolayer. The magnetotransport properties of the junctions are studied as a function of this Fe sublayer coverage that is varied from 0 to 1 monolayer. Surprisingly, the magnetoresistance ratio exhibits a maximum for a coverage around half a monolayer. Tunneling spectroscopy experiments performed at low temperature allow relating this result to the decrease of the contribution of the interfacial resonance state to the conductance of the junction.C.T. acknowledges the following projects: SPINCHAT (ANR-07-BLAN-341), POS CCE ID.574, code SMIS-CSNR 12467, and the Exploratory Research Project, “SPINTAIL” PN-II-ID-PCE-2012-4-0315, No. 23/29.08.2013.Peer Reviewe

Magnetic proximity effects in topological insulator/magnetic insulator heterostructures

Resumen del póster presentado al 1st Workshop Spain-Taiwan: "2D Materials and Interfaces for Spintronics", celebrado en Barcelona (España) del 23 al 25 de octubre de 2017.Topological insulators (TIs) are novel class of materials with promising electronic properties that could allow for dissipationless carrier transport in future electronic and spintronic devices through their spinmomentum locked surface states. Combination of TIs with magnetic materials has become one the main strategies for their practical use due to the emergence of novel magneto-electric effects. Recent reports on TI thin films in direct contact with magnetic insulators (MI) have revealed interesting phenomena at the TI/MI interface. However, the mechanisms for magnetic proximity and electronic transport at these interfaces are not completely understood and therefore not well controlled. Investigation of these effects would yield to the observation of exotic magneto-electric phenomena, such as the quantum anomalous Hall effect, as well as incorporation of TIs into future devices. Thus, we have grown Bi2Te3/EuS heterostructures using molecular beam epitaxy with the aim of studying magnetic proximity effects. In this work, we will show results of structural, magnetic and magneto-electrical properties of these heterostructures using a variety of characterization techniques.Peer reviewe

Thermoelectric properties of topological insulators

Resumen del póster presentado al 1st Workshop Spain-Taiwan: "2D Materials and Interfaces for Spintronics", celebrado en Barcelona (España) del 23 al 25 de octubre de 2017.Topological Insulators (TIs) are some of the best thermoelectric materials. They are made of heavy elements that give a low phonon thermal conductivity and a small bandgap. Bi2Te3 is one of the best with a large figure of merit ZT. In this work, we take into account the Dirac states of TIs in the study case of thermoelectric properties. This can lead to improve the power factor. Different alloys of (Bi1-xSbx)2Te3 have been grown by MBE on a (111) Barium Fluoride substrate. Usual XPS measurements were made in order to see the different concentration. In addition, ARPES and Hall measurements combined to define the surface states. Finally, thermoelectric devices were designed by lithography in the objective to measure the Seebeck coefficient.Peer reviewe

Spin precession and spin Hall effect in monolayer graphene/Pt nanostructures

Resumen del póster presentado al 1st Workshop Spain-Taiwan: "2D Materials and Interfaces for Spintronics", celebrado en Barcelona (España) del 23 al 25 de octubre de 2017.Spin Hall effects have surged as promising phenomena for spin logics operations without ferromagnets. So far, the exploration of the SHE using nonlocal (NL) measurements has been performed in metals using Pt as the large SOC material. These studies have led to a better comprehension of the phenomena and more accurate determinations and control of the spinto-charge conversion efficiency, which is quantified by the spin Hall angle (αSHE). However, the magnitude of the detected electric signals at room temperature in metallic systems has been so far underwhelming. Here, we demonstrate a two-order of magnitude enhancement of the signal in monolayer graphene/Pt devices when compared to their fully metallic counterparts. The enhancement stems in part from efficient spin injection and the large resistivity of graphene but we also observe 100% spin absorption in Pt and an unusually large effective spin Hall angle of up to 0.15. The large spin-to-charge conversion allows us to characterise spin precession in graphene under the presence of a magnetic field. Furthermore, by developing an analytical model based on the 1D diffusive spin-transport, we demonstrate that the effective spinrelaxation time in graphene can be accurately determined using the (inverse) spin Hall effect as a means of detection. This is a necessary step to gather full understanding of the consequences of spin absorption in spin Hall devices, which is known to suppress effective spin lifetimes in both metallic and graphene systems.Peer reviewe

Determination of the spin-lifetime anisotropy in graphene using oblique spin precession

We determine the spin-lifetime anisotropy of spin-polarized carriers in graphene. In contrast to prior approaches, our method does not require large out-of-plane magnetic fields and thus it is reliable for both low-and high-carrier densities. We first determine the in-plane spin lifetime by conventional spin precession measurements with magnetic fields perpendicular to the graphene plane. Then, to evaluate the out-of-plane spin lifetime, we implement spin precession measurements under oblique magnetic fields that generate an out-of-plane spin population. We find that the spin-lifetime anisotropy of graphene on silicon oxide is independent of carrier density and temperature down to 150 K, and much weaker than previously reported. Indeed, within the experimental uncertainty, the spin relaxation is isotropic. Altogether with the gate dependence of the spin lifetime, this indicates that the spin relaxation is driven by magnetic impurities or random spin-orbit or gauge fields.This research was partially supported by the European Research Council under Grant Agreement No. 308023 SPINBOUND, by the Spanish Ministry of Economy and Competitiveness, MINECO (under Contract No. MAT2013-46785-P and Severo Ochoa No. SEV-2013-0295), and by the Secretariat for Universities and Research, Knowledge Department of the Generalitat de Catalunya. M.V.C., J.F.S. and J.C. acknowledge support from the Ramón y Cajal, Juan de la Cierva and Beatriu de Pinós programs, respectively. F.B. acknowledges funding from the People Programme (Marie Curie Actions) of the European Union’s Seventh Framework Programme FP7/2007-2013/ under REA Grant Agreement No. 624897. J.E.S. and J.V.d.V. acknowledge funding from the Methusalem Funding of the Flemish Government and the Research Foundation-Flanders (FWO).Peer Reviewe

Spin relaxation anisotropy in graphene

Resumen del trabajo presentado a la 6th edition of Graphene Conference series, the largest European Event in Graphene and 2D Materials, celebrada en Genova (Italia) del 19 al 22 de abril de 2016In recent years, graphene-based spintronics have shown impressive progress. Spin relaxation lengths in graphene have been observed to be larger than several tens of micrometers and are already within the required range for technological applications. This has been accomplished by a steady improvement of the quality of graphene and of the interfaces with contacting materials. However, the microscopic mechanisms that determine the spin lifetime, and spin relaxation length, are still under heated debate. This lack of understanding hampers graphene spintronics in reaching its full potential, as for applications it is desirable to achieve full control of the spin dynamics. The spin relaxation anisotropy, which can be quantified by the ratio between the spin lifetimes for perpendicular and parallel spin components to the graphene plane, is a key property that can provide information on the microscopic mechanisms that is not accessible by other means. This is so because the anisotropy is determined by the preferential direction of the spin-orbit fields that may cause the spin relaxation. Despite such inherent interest, measurements of the spin lifetime anisotropy are scarce and limited to large carrier densities. Here, we demonstrate a conceptually new approach that overcomes this limitation. The concept relies on spin precession measurements under oblique magnetic fields that generate an out-of-plane spin population, which is further used to evaluate the out-of-plane spin lifetime. Our experiments demonstrate that the spin relaxation anisotropy of graphene on silicon oxide is independent of carrier density and temperature, and much lower than previously reported; indeed, within the experimental uncertainty, the spin relaxation is isotropic. Together with the gate dependence of the spin lifetime, this indicates that the spin relaxation is driven either by magnetic impurities or by randomly oriented spin-orbit fields, relative to the spin. These findings open the way for systematic anisotropy studies with tailored impurities and on different substrates, which are crucial to find a route to manipulate the spin lifetime in graphene and as such has important implications for both fundamental science and technological applications.Peer reviewe

Measurement of spin-orbit torques from Rashba and topological surface states

Resumen del trabajo presentado al 1st Workshop Spain-Taiwan: "2D Materials and Interfaces for Spintronics", celebrado en Barcelona (España) del 23 al 25 de octubre de 2017.-- et al.The Dirac bands formed at the interface between a three-dimensional topological insulator (TI) and a trivial insulator are characterized by their spin-helicity, arising from the combination of a large spin-orbit coupling and time reversal invariance. Spin-momentum locking potentially makes TIs extremely efficient charge-to-spin converters, able to exert a large spin-orbit torque (SOT) on an adjacent ferromagnet (FM). SOT-induced ferromagnetic resonance (SOT-FMR) gives access to the nature and magnitude of the torques. So far, only few studies have focused on SOT-FMR in TI/FM multilayers, and large spin torques have been reported. However, little is known about TI/metal interfaces. Charge doping together with a strong hybridization between topological bands and metallic states is suspected to deeply weaken the spin-momentum locking, which questions the origin of the large SOT measured. We will present measurements of the non-equilibrium SOTs by current-induced ferromagnetic resonance on TI/NM/FM multilayers embedded in coplanar waveguides. A systematic combination of spin transport and photoemission shows that the chemical and electronic properties of the TI/Py interface can be tuned by using various normal metal spacers (NM=Ag,Al,Te), which results in a strong modulation of the SOTs. Details about the fabrication of single-crystal TI films by molecular beam epitaxy and their characterization will also be presented. We will show that crystal twin defects can be fully suppressed by using lattice-matched substrates and proper growth conditions, which is associated with a low doping of the free surface. Band bending effects at interfaces and their compensation by alloying will also be addressed.Peer reviewe

Magnetic anisotropy and electronic structure of molecular networks on topological insulators

Resumen del póster presentado al 1st Workshop Spain-Taiwan: "2D Materials and Interfaces for Spintronics", celebrado en Barcelona (España) del 23 al 25 de octubre de 2017.In topological insulators (TIs), long-range interactions between magnetic dopants mediated by the topological surface states can give rise to exotic phenomena such as the quantum anomalous Hall effect (QAHE) and dissipationless spin polarized edge currents. Such interactions, however, only occur under specific conditions of magnetic moment, anisotropy, and exchange coupling with the surrounding electron bath. Here we propose the use of metalorganic networks as an alternative strategy for tuning the electronic and magnetic interaction between the magnetic impurity and the substrate. The electronic and magnetic properties of TI surfaces modified by Brominated Cobalt tetraphenyl-porphyrin (4Br-CoTPP) networks are studied by combining Angle Resolved Photoemission Spectroscopy (ARPES) with X-Ray Magnetic Circular Dichroism (XMCD) measurements. ARPES measurements reveal that the topological surface state (TSS) is not quenched upon adsorption of slightly more than 1 ML 4Br-CoTPP in contrast with previous ARPES in Cobalt phthalocyanine/TI interfaces. The weaker interaction between the molecular layer and the substrate is confirmed by XMCD measurements where a strong in-plane magnetic moment is preserved. This ligand-mediated tuneability of interfacial interactions, together with the well-known capability of controlling magnetic anisotropy, makes the use of metalorganic molecules a promising route for engineering the electronic and magnetic properties of hybrid TI-based heterostructures.Peer reviewe

Magnetic dopants on the surface of 2D heavy metal alloys and topological insulators

Resumen del trabajo presentado al 1st Workshop Spain-Taiwan: "2D Materials and Interfaces for Spintronics", celebrado en Barcelona (España) del 23 al 25 de octubre de 2017.-- et al.Spin-Orbit Coupling (SOC) is at the origin of important new phenomena in materials such as heavy metal surface alloys with strong Rashba interaction and topological insulators (TIs), both with evident and promising applications in spintronics. Of special relevance are the emergent phenomena derived from the magnetic interactions in ferromagnetic (FM)/SOC interfaces. The Rashba and topological surface states can be used efficiently for reversing the magnetization of FM thin films by charge currents via spin transfer torque and for spin to charge conversion as a result of an inverse Edelstein effect. Besides, in topological insulators (TIs), long-range interactions between magnetic dopants mediated by the topological surface states can give rise to exotic phenomena such as the quantum anomalous Hall effect and dissipationless spin polarized edge currents. Despite their experimental demonstrations, an atomistic understanding of these phenomena is still missing. In this work we present a systematic experimental investigation on the magnetic properties in model FM/SOC interfaces. In a first step we will use 3d transition metals (TM) adatoms as magnetic dopants on the substitutional BiAg2 surface alloy grown on Ag(111). Through the controlled deposition of a series of TM impurities at cryogenic temperatures we systematically study different magnetic configurations. The influence of the SOC interaction, impurity adsorption sites and surface structure on the magnetic anisotropy is analyzed correlating experimental (XMCD) and theoretical (DFT) data. On the other side, we try to overcome the experimental difficulties of controlling the magnetic ground state and anisotropy of 3d transition metal impurities by using metalorganic molecular networks (porphyrin based) as an alternative strategy for tuning the electronic and magnetic interaction between the dopant and the TI substrate. Here, depending on the transition metal ion, the ligand field related to the organic molecular structure and the molecular charge, the magnetic moment and anisotropy can be consequently manipulated.Peer reviewe