Physics and Engineering of Nanodevices

Group Leader: Sergio Valenzuela

Publications

2024

Spin-Orbit Torques and Magnetization Switching in (Bi,Sb)₂Te₃/Fe₃GeTe₂ Heterostructures Grown by Molecular Beam Epitaxy
Guillet, Thomas; Galceran, Regina; Sierra, Juan F; Belarre, Francisco J; Ballesteros, Belen; Costache, Marius V; Dosenovic, Djordje; Okuno, Hanako; Marty, Alain; Jamet, Matthieu; Bonell, Frederic; Valenzuela, Sergio O Nano Letters; 24 (3): 822 - 828. 2024. 10.1021/acs.nanolett.3c03291. IF: 9.600
Spintronics with two-dimensional materials and van der Waals heterostructures
Roche, Stephan; van Wees, Bart; Garello, Kevin; Valenzuela, Sergio O 2d Materials; 11 (4): 043001. 2024. 10.1088/2053-1583/ad64e2. IF: 4.500
Terahertz spin-light coupling in proximitized Dirac materials
Konstantin Denisov; I. V. Rozhansky; Sergio O. Valenzuela; Igor Žutić Physical Review. B./Physical Review. b; 109 (20) 2024. 10.1103/physrevb.109.l201406.
Unravelling the origin of thermal anisotropy in PdSe2
Xu, Kai; Armesto, Luis Martinez; Svetlik, Josef; Sierra, Juan F; Marinova, Vera; Dimitrov, Dimitre; Goni, Alejandro R; Krysztofik, Adam; Graczykowski, Bartlomiej; Rurali, Riccardo; Valenzuela, Sergio O; Reparaz, Juan Sebastian 2d Materials; 11 (4): 045006. 2024. 10.1088/2053-1583/ad64e3. IF: 4.500

2023

Artificial Graphene Spin Polarized Electrode for Magnetic Tunnel Junctions
Zatko, V; Galceran, R; Galbiati, M; Peiro, J; Godel, F; Kern, LM; Perconte, D; Ibrahim, F; Hallal, A; Chshiev, M; Martinez, B; Frontera, C; Balcells, L; Kidambi, PR; Robertson, J; Hofmann, S; Collin, S; Petroff, F; Martin, MB; Dlubak, B; Seneor, P Nano Letters; 23 (1): 34 - 41. 2023. 10.1021/acs.nanolett.2c03113. IF: 10.800
Efficient terahertz harmonic generation in topological metamaterials
Kovalev, S; Tielrooij, K; Ilyakov, I; Deinert, JC; de Oliveira, T; Ponomaryov, O; Principi, A; Reig, D; Block, A; Varghese, S; Schreyeck, S; Brunner, K; Karczewski, G; Carbonell, C; Valenzuela, S; Molenkamp, L; Kiessling, T; Astakhov, GV International Conference On Infrared, Millimeter, And Terahertz Waves, Irmmw-Thz; 2023. 10.1109/IRMMW-THz57677.2023.10298895.
Experimental Demonstration of a Magnetically Induced Warping Transition in a Topological Insulator Mediated by Rare-Earth Surface Dopants
Cano, BM; Ferreiros, Y; Pantaleon, PA; Dai, J; Tallarida, M; Figueroa, AI; Marinova, V; Garcia-Diez, K; Mugarza, A; Valenzuela, SO; Miranda, R; Camarero, J; Guinea, F; Silva-Guillen, JA; Valbuena, MA Nano Letters; 23 (13): 6249 - 6258. 2023. 10.1021/acs.nanolett.3c00587. IF: 10.800
Large-Area Synthesis of Ferromagnetic Fe5-xGeTe2/Graphene van der Waals Heterostructures with Curie Temperature above Room Temperature
Lv, H; da Silva, A; Figueroa, AI; Guillemard, C; Aguirre, IF; Camosi, L; Aballe, L; Valvidares, M; Valenzuela, SO; Schubert, J; Schmidbauer, M; Herfort, J; Hanke, M; Trampert, A; Engel-Herbert, R; Ramsteiner, M; Lopes, JMJ Small; 19 (39) 2023. 10.1002/smll.202302387. IF: 13.300

2022

Heat dissipation in few-layer MoS2and MoS2/hBN heterostructure
Arrighi A., Del Corro E., Urrios D.N., Costache M.V., Sierra J.F., Watanabe K., Taniguchi T., Garrido J.A., Valenzuela S.O., Sotomayor Torres C.M., Sledzinska M. 2D Materials; 9 (1, 015005) 2022. 10.1088/2053-1583/ac2e51. IF: 7.103

State-of-the-art fabrication and characterisation techniques have been employed to measure the thermal conductivity of suspended, single-crystalline MoS2 and MoS2/hBN heterostructures. Two-laser Raman scattering thermometry was used combined with real time measurements of the absorbed laser power. Measurements on MoS2 layers with thicknesses of 5 and 14 nm exhibit thermal conductivity in the range between 12 Wm-1 K-1 and 24 Wm-1 K-1. Additionally, after determining the thermal conductivity of the latter MoS2 sample, an hBN flake was transferred onto it and the effective thermal conductivity of the heterostructure was subsequently measured. Remarkably, despite that the thickness of the hBN layer was less than a hal of the thickness of the MoS2 layer, the heterostructure showed an almost eight-fold increase in the thermal conductivity, being able to dissipate more than ten times the laser power without any visible sign of damage. These results are consistent with a high thermal interface conductance G between MoS2 and hBN and an efficient in-plane heat spreading driven by hBN. Indeed, we estimate G ∼ 70 MW m-2 K-1 for hBN layer thermal conductivity of 450 Wm-1 K-1 which is significantly higher than previously reported values. Our work therefore demonstrates that the insertion of hBN layers in potential MoS2-based devices holds the promise for efficient thermal management. © 2021 IOP Publishing Ltd.

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Milliwatt terahertz harmonic generation from topological insulator metamaterials
Tielrooij KJ; Principi A; Reig DS; Block A; Varghese S; Schreyeck S; Brunner K; Karczewski G; Ilyakov I; Ponomaryov O; de Oliveira TVAG; Chen M; Deinert JC; Carbonell CG; Valenzuela SO; Molenkamp LW; Kiessling T; Astakhov GV; Kovalev S Light-Science & Applications; 11 (1) 2022. 10.1038/s41377-022-01008-y. IF: 20.257
Passivation of Bi2Te3 Topological Insulator by Transferred CVD-Graphene: Toward Intermixing-Free Interfaces
Galceran, R; Bonell, F; Camosi, L; Sauthier, G; Gebeyehu, ZM; Esplandiu, MJ; Arrighi, A; Aguirre, IF; Figueroa, AI; Sierra, JF; Valenzuela, SO Advanced Materials Interfaces; 9 (36) 2022. 10.1002/admi.202201997. IF: 6.389
Resolving spin currents and spin densities generated by charge-spin interconversion in systems with reduced crystal symmetry
Camosi L., Světl’ík J., Costache M.V., Savero Torres W., Fernández Aguirre I., Marinova V., Dimitrov D., Gospodinov M., Sierra J.F., Valenzuela S.O. 2D Materials; 9 (3, 035014) 2022. 10.1088/2053-1583/ac6fec.

The ability to control the generation of spins in arbitrary directions is a long-sought goal in spintronics. Charge to spin interconversion (CSI) phenomena depend strongly on symmetry. Systems with reduced crystal symmetry allow anisotropic CSI with unconventional components, where charge and spin currents and the spin polarization are not mutually perpendicular to each other. Here, we demonstrate experimentally that the CSI in graphene-WTe2 induces spins with components in all three spatial directions. By performing multi-terminal nonlocal spin precession experiments, with specific magnetic fields orientations, we discuss how to disentangle the CSI from the spin Hall and inverse spin galvanic effects. © 2022 IOP Publishing Ltd.

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Two-dimensional materials prospects for non-volatile spintronic memories
Yang H., Valenzuela S.O., Chshiev M., Couet S., Dieny B., Dlubak B., Fert A., Garello K., Jamet M., Jeong D.-E., Lee K., Lee T., Martin M.-B., Kar G.S., Sénéor P., Shin H.-J., Roche S. Nature; 606 (7915): 663 - 673. 2022. 10.1038/s41586-022-04768-0.

Non-volatile magnetic random-access memories (MRAMs), such as spin-transfer torque MRAM and next-generation spin–orbit torque MRAM, are emerging as key to enabling low-power technologies, which are expected to spread over large markets from embedded memories to the Internet of Things. Concurrently, the development and performances of devices based on two-dimensional van der Waals heterostructures bring ultracompact multilayer compounds with unprecedented material-engineering capabilities. Here we provide an overview of the current developments and challenges in regard to MRAM, and then outline the opportunities that can arise by incorporating two-dimensional material technologies. We highlight the fundamental properties of atomically smooth interfaces, the reduced material intermixing, the crystal symmetries and the proximity effects as the key drivers for possible disruptive improvements for MRAM at advanced technology nodes. © 2022, Springer Nature Limited.

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Unraveling Heat Transport and Dissipation in Suspended MoSe2 from Bulk to Monolayer
Saleta Reig D., Varghese S., Farris R., Block A., Mehew J.D., Hellman O., Woźniak P., Sledzinska M., El Sachat A., Chávez-Ángel E., Valenzuela S.O., van Hulst N.F., Ordejón P., Zanolli Z., Sotomayor Torres C.M., Verstraete M.J., Tielrooij K.-J. Advanced Materials; 34 (10, 2108352) 2022. 10.1002/adma.202108352. IF: 30.849

Understanding heat flow in layered transition metal dichalcogenide (TMD) crystals is crucial for applications exploiting these materials. Despite significant efforts, several basic thermal transport properties of TMDs are currently not well understood, in particular how transport is affected by material thickness and the material's environment. This combined experimental–theoretical study establishes a unifying physical picture of the intrinsic lattice thermal conductivity of the representative TMD MoSe2. Thermal conductivity measurements using Raman thermometry on a large set of clean, crystalline, suspended crystals with systematically varied thickness are combined with ab initio simulations with phonons at finite temperature. The results show that phonon dispersions and lifetimes change strongly with thickness, yet the thinnest TMD films exhibit an in-plane thermal conductivity that is only marginally smaller than that of bulk crystals. This is the result of compensating phonon contributions, in particular heat-carrying modes around ≈0.1 THz in (sub)nanometer thin films, with a surprisingly long mean free path of several micrometers. This behavior arises directly from the layered nature of the material. Furthermore, out-of-plane heat dissipation to air molecules is remarkably efficient, in particular for the thinnest crystals, increasing the apparent thermal conductivity of monolayer MoSe2 by an order of magnitude. These results are crucial for the design of (flexible) TMD-based (opto-)electronic applications. © 2022 The Authors. Advanced Materials published by Wiley-VCH GmbH

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2021

A valley of opportunities
Torres L.F., Valenzuela S.O. Physics World; 34 (11): 43 - 46. 2021. 10.1088/2058-7058/34/11/40. IF: 0.355

[No abstract available]

View abstract
Chiral anomaly trapped in Weyl metals: Nonequilibrium valley polarization at zero magnetic field
Perez-Piskunow P.M., Bovenzi N., Akhmerov A.R., Breitkreiz M. SciPost Physics; 11 (2, 046) 2021. 10.21468/SCIPOSTPHYS.11.2.046. IF: 6.093

In Weyl semimetals, the application of parallel electric and magnetic fields leads to valley polarization-an occupation disbalance of valleys of opposite chirality-a direct consequence of the chiral anomaly. In this work, we present numerical tools to explore such nonequilibrium effects in spatially confined three-dimensional systems with a variable disorder potential, giving exact solutions to leading order in the disorder potential and the applied electric field. Application to a Weyl-metal slab shows that valley polarization also occurs without an external magnetic field as an effect of chiral anomaly “trapping”: Spatial confinement produces chiral bulk states, which enable the valley polarization in a similar way as the chiral states induced by a magnetic field. Despite its finite-size origin, the valley polarization can persist up to macroscopic length scales if the disorder potential is sufficiently long ranged, so that direct inter-valley scattering is suppressed and the relaxation then goes via the Fermi-arc surface states. Copyright © Perez Piskunow et al.

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Control of spin-charge conversion in van der Waals heterostructures
Galceran R., Tian B., Li J., Bonell F., Jamet M., Vergnaud C., Marty A., García J.H., Sierra J.F., Costache M.V., Roche S., Valenzuela S.O., Manchon A., Zhang X., Schwingenschlögl U. APL Materials; 9 (10, 100901) 2021. 10.1063/5.0054865. IF: 5.096

The interconversion between spin and charge degrees of freedom offers incredible potential for spintronic devices, opening routes for spin injection, detection, and manipulation alternative to the use of ferromagnets. The understanding and control of such interconversion mechanisms, which rely on spin-orbit coupling, is therefore an exciting prospect. The emergence of van der Waals materials possessing large spin-orbit coupling (such as transition metal dichalcogenides or topological insulators) and/or recently discovered van der Waals layered ferromagnets further extends the possibility of spin-to-charge interconversion to ultrathin spintronic devices. Additionally, they offer abundant room for progress in discovering and analyzing novel spin-charge interconversion phenomena. Modifying the properties of van der Waals materials through proximity effects is an added degree of tunability also under exploration. This Perspective discusses the recent advances toward spin-to-charge interconversion in van der Waals materials. It highlights scientific developments which include techniques for large-scale growth, device physics, and theoretical aspects. © 2021 Author(s).

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Hinge Spin Polarization in Magnetic Topological Insulators Revealed by Resistance Switch
Perez-Piskunow P.M., Roche S. Physical Review Letters; 126 (16, 167701) 2021. 10.1103/PhysRevLett.126.167701. IF: 9.161

We report on the possibility of detecting hinge spin polarization in magnetic topological insulators by resistance measurements. By implementing a three-dimensional model of magnetic topological insulators into a multiterminal device with ferromagnetic contacts near the top surface, local spin features of the chiral edge modes are unveiled. We find local spin polarization at the hinges that inverts the sign between the top and bottom surfaces. At the opposite edge, the topological state with inverted spin polarization propagates in the reverse direction. A large resistance switch between forward and backward propagating states is obtained, driven by the matching between the spin polarized hinges and the ferromagnetic contacts. This feature is general to the ferromagnetic, antiferromagnetic, and canted antiferromagnetic phases, and enables the design of spin-sensitive devices, with the possibility of reversing the hinge spin polarization of the currents. © 2021 American Physical Society.

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Large-area van der Waals epitaxy and magnetic characterization of Fe3GeTe2films on graphene
Lopes J.M.J., Czubak D., Zallo E., Figueroa A.I., Guillemard C., Valvidares M., Rubio-Zuazo J., López-Sanchéz J., Valenzuela S.O., Hanke M., Ramsteiner M. 2D Materials; 8 (4, 041001) 2021. 10.1088/2053-1583/ac171d. IF: 7.103

Scalable fabrication of magnetic 2D materials and heterostructures constitutes a crucial step for scaling down current spintronic devices and the development of novel spintronic applications. Here, we report on van der Waals (vdW) epitaxy of the layered magnetic metal Fe3GeTe2 (FGT) - a 2D crystal with highly tunable properties and a high prospect for room temperature ferromagnetism (FM) - directly on graphene by employing molecular beam epitaxy. Morphological and structural characterization confirmed the realization of large-area, continuous FGT/graphene heterostructure films with stable interfaces and good crystalline quality. Furthermore, magneto-transport and x-ray magnetic circular dichroism investigations confirmed a robust out-of-plane FM in the layers, comparable to state-of-the-art exfoliated flakes from bulk crystals. These results are highly relevant for further research on wafer-scale growth of vdW heterostructures combining FGT with other layered crystals such as transition metal dichalcogenides for the realization of multifunctional, atomically thin devices. © 2021 The Author(s).

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Low-symmetry topological materials for large charge-to-spin interconversion: The case of transition metal dichalcogenide monolayers
Vila M., Hsu C.-H., Garcia J.H., Benítez L.A., Waintal X., Valenzuela S.O., Pereira V.M., Roche S. Physical Review Research; 3 (4, 043230) 2021. 10.1103/PhysRevResearch.3.043230. IF: 0.000

The spin polarization induced by the spin Hall effect (SHE) in thin films typically points out of the plane. This is rooted on the specific symmetries of traditionally studied systems, not in a fundamental constraint. Recently, experiments on few-layer MoTe2 and WTe2 showed that the reduced symmetry of these strong spin-orbit coupling materials enables a new form of canted spin Hall effect, characterized by concurrent in-plane and out-of-plane spin polarizations. Here, through quantum transport calculations on realistic device geometries, including disorder, we predict a very large gate-tunable SHE figure of merit λsθxy≈1-50 nm in MoTe2 and WTe2 monolayers that significantly exceeds values of conventional SHE materials. This stems from a concurrent long spin diffusion length (λs) and charge-to-spin interconversion efficiency as large as θxy≈80%, originating from momentum-invariant (persistent) spin textures together with large spin Berry curvature along the Fermi contour, respectively. Generalization to other materials and specific guidelines for unambiguous experimental confirmation are proposed, paving the way toward exploiting such phenomena in spintronic devices. These findings vividly emphasize how crystal symmetry and electronic topology can govern the intrinsic SHE and spin relaxation, and how they may be exploited to broaden the range and efficiency of spintronic materials and functionalities. © 2021 authors. Published by the American Physical Society.

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Self-organised stripe domains and elliptical skyrmion bubbles in ultra-thin epitaxial Au0.67Pt0.33/Co/W(110) films
Camosi L., Garcia J.P., Fruchart O., Pizzini S., Locatelli A., Menteş T.O., Genuzio F., Shaw J.M., Nembach H.T., Vogel J. New Journal of Physics; 23 (1, 013020) 2021. 10.1088/1367-2630/abdbe0. IF: 3.732

We studied the symmetry of magnetic properties and the resulting magnetic textures in ultra-thin epitaxial Au0.67Pt0.33/Co/W(110), a model system exhibiting perpendicular magnetic anisotropy and interface Dzyaloshinskii-Moriya interaction (DMI). As a peculiar feature, the C2v crystal symmetry induced by the Co/W interface results in an additional uniaxial in-plane magnetic anisotropy in the cobalt layer. Photo-emission electron microscopy with magnetic sensitivity reveals the formation of self-organised magnetic stripe domains oriented parallel to the hard in-plane magnetisation axis. We attribute this behavior to the lower domain wall energy when oriented along this axis, where both the DMI and the in-plane magnetic anisotropy favor a Néel domain wall configuration. The anisotropic domain wall energy also leads to the formation of elliptical skyrmion bubbles under a weak out-of-plane magnetic field. © 2021 The Author(s). Published by IOP Publishing Ltd on behalf of the Institute of Physics and Deutsche Physikalische Gesellschaft

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Valley-polarized quantum anomalous Hall phase in bilayer graphene with layer-dependent proximity effects
Vila M., Garcia J.H., Roche S. Physical Review B; 104 (16, A84) 2021. 10.1103/PhysRevB.104.L161113. IF: 4.036

Realizations of some topological phases in two-dimensional systems rely on the challenge of jointly incorporating spin-orbit and magnetic exchange interactions. Here, we predict the formation and control of a fully valley-polarized quantum anomalous Hall effect in bilayer graphene, by separately imprinting spin-orbit and magnetic proximity effects in different layers. This results in varying spin splittings for the conduction and valence bands, which gives rise to a topological gap at a single Dirac cone. The topological phase can be controlled by a gate voltage and switched between valleys by reversing the sign of the exchange interaction. By performing quantum transport calculations in disordered systems, the chirality and resilience of the valley-polarized edge state are demonstrated. Our findings provide a promising route to engineer a topological phase that could enable low-power electronic devices and valleytronic applications as well as putting forward layer-dependent proximity effects in bilayer graphene as a way to create versatile topological states of matter. © 2021 American Physical Society.

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Van der Waals heterostructures for spintronics and opto-spintronics
Sierra J.F., Fabian J., Kawakami R.K., Roche S., Valenzuela S.O. Nature Nanotechnology; 16 (8): 856 - 868. 2021. 10.1038/s41565-021-00936-x. IF: 39.213

The large variety of 2D materials and their co-integration in van der Waals heterostructures enable innovative device engineering. In addition, their atomically thin nature promotes the design of artificial materials by proximity effects that originate from short-range interactions. Such a designer approach is particularly compelling for spintronics, which typically harnesses functionalities from thin layers of magnetic and non-magnetic materials and the interfaces between them. Here we provide an overview of recent progress in 2D spintronics and opto-spintronics using van der Waals heterostructures. After an introduction to the forefront of spin transport research, we highlight the unique spin-related phenomena arising from spin–orbit and magnetic proximity effects. We further describe the ability to create multifunctional hybrid heterostructures based on van der Waals materials, combining spin, valley and excitonic degrees of freedom. We end with an outlook on perspectives and challenges for the design and production of ultracompact all-2D spin devices and their potential applications in conventional and quantum technologies. © 2021, Springer Nature Limited.

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2020

Absence of Magnetic Proximity Effect at the Interface of Bi2Se3 and (Bi,Sb)2Te3 with EuS
Figueroa A.I., Bonell F., Cuxart M.G., Valvidares M., Gargiani P., Van Der Laan G., Mugarza A., Valenzuela S.O. Physical Review Letters; 125 (22, 226801) 2020. 10.1103/PhysRevLett.125.226801. IF: 8.385

We performed X-ray magnetic circular dichroism (XMCD) measurements on heterostructures comprising topological insulators (TIs) of the (Bi,Sb)2(Se,Te)3 family and the magnetic insulator EuS. XMCD measurements allow us to investigate element-selective magnetic proximity effects at the very TI/EuS interface. A systematic analysis reveals that there is neither significant induced magnetism within the TI nor an enhancement of the Eu magnetic moment at such interface. The induced magnetic moments in Bi, Sb, Te, and Se sites are lower than the estimated detection limit of the XMCD measurements of ∼10-3 μB/at. © 2020 American Physical Society.

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Computation of topological phase diagram of disordered Pb1-xSnxTe using the kernel polynomial method
Dániel Varjas, Michel Fruchart, Anton R. Akhmerov, Pablo M. Perez-Piskunow Physical Review Research; 2 (13229) 2020. 10.1103/PhysRevResearch.2.013229 . IF: 0.000

We present an algorithm to determine topological invariants of inhomogeneous systems, such as alloys, disordered crystals, or amorphous systems. Based on the kernel polynomial method, our algorithm allows us to study samples with more than 107 degrees of freedom. Our method enables the study of large complex compounds, where disorder is inherent to the system. We use it to analyze Pb1−xSnxTe and tighten the critical concentration for the phase transition. Moreover, we obtain the topological phase diagram for related alloys in the family of three-dimensional mirror Chern insulators.

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Control of Spin-Orbit Torques by Interface Engineering in Topological Insulator Heterostructures
Bonell F., Goto M., Sauthier G., Sierra J.F., Figueroa A.I., Costache M.V., Miwa S., Suzuki Y., Valenzuela S.O. Nano Letters; 20 (8): 5893 - 5899. 2020. 10.1021/acs.nanolett.0c01850. IF: 11.238

(Bi1-xSbx)2Te3 topological insulators (TIs) are gathering increasing attention owing to their large charge-to-spin conversion efficiency and the ensuing spin-orbit torques (SOTs) that can be used to manipulate the magnetization of a ferromagnet (FM). The origin of the torques, however, remains elusive, while the implications of hybridized states and the strong material intermixing at the TI/FM interface are essentially unexplored. By combining interface chemical analysis and spin-transfer ferromagnetic resonance (ST-FMR) measurements, we demonstrate that intermixing plays a critical role in the generation of SOTs. By inserting a suitable normal metal spacer, material intermixing is reduced and the TI properties at the interface are largely improved, resulting in strong variations in the nature of the SOTs. A dramatic enhancement of a field-like torque, opposing and surpassing the Oersted-field torque, is observed, which can be attributed to the non-equilibrium spin density in Rashba-split surface bands and to the suppression of spin memory loss. These phenomena can play a relevant role at other interfaces, such as those comprising transition metal dichalcogenides. © 2020 American Chemical Society.

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Element- and Time-Resolved Measurements of Spin Dynamics Using X-ray Detected Ferromagnetic Resonance
Klewe C., Li Q., Yang M., N’Diaye A.T., Burn D.M., Hesjedal T., Figueroa A.I., Hwang C., Li J., Hicken R.J., Shafer P., Arenholz E., van der Laan G., Qiu Z. Synchrotron Radiation News; 33 (2): 12 - 19. 2020. 10.1080/08940886.2020.1725796. IF: 0.000

[No abstract available]

View abstract
Ferromagnetic Resonance Assisted Optomechanical Magnetometer
Colombano M.F., Arregui G., Bonell F., Capuj N.E., Chavez-Angel E., Pitanti A., Valenzuela S.O., Sotomayor-Torres C.M., Navarro-Urrios D., Costache M.V. Physical Review Letters; 125 (14, 147201) 2020. 10.1103/PhysRevLett.125.147201. IF: 8.385

The resonant enhancement of mechanical and optical interaction in optomechanical cavities enables their use as extremely sensitive displacement and force detectors. In this Letter, we demonstrate a hybrid magnetometer that exploits the coupling between the resonant excitation of spin waves in a ferromagnetic insulator and the resonant excitation of the breathing mechanical modes of a glass microsphere deposited on top. The interaction is mediated by magnetostriction in the ferromagnetic material and the consequent mechanical driving of the microsphere. The magnetometer response thus relies on the spectral overlap between the ferromagnetic resonance and the mechanical modes of the sphere, leading to a peak sensitivity of 850 pT Hz-1/2 at 206 MHz when the overlap is maximized. By externally tuning the ferromagnetic resonance frequency with a static magnetic field, we demonstrate sensitivity values at resonance around a few nT Hz-1/2 up to the gigahertz range. Our results show that our hybrid system can be used to build a high-speed sensor of oscillating magnetic fields. © 2020 American Physical Society.

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Magnetic order in 3D topological insulators-Wishful thinking or gateway to emergent quantumeffects?
Figueroa A.I., Hesjedal T., Steinke N.-J. Applied Physics Letters; 117 (15, 0027987) 2020. 10.1063/5.0027987. IF: 3.597

Three-dimensional topological insulators (TIs) are a perfectly tuned quantum-mechanical machinery in which counterpropagating and oppositely spin-polarized conduction channels balance each other on the surface of the material. This topological surface state crosses the bandgap of the TI and lives at the interface between the topological and a trivial material, such as vacuum. Despite its balanced perfection, it is rather useless for any practical applications. Instead, it takes the breaking of time-reversal symmetry (TRS) and the appearance of an exchange gap to unlock hidden quantum states. The quantum anomalous Hall effect, which has first been observed in Cr-doped (Sb,Bi)2Te3, is an example of such a state in which two edge channels are formed at zero field, crossing the magnetic exchange gap. The breaking of TRS can be achieved by magnetic doping of the TI with transition metal or rare earth ions, modulation doping to keep the electronically active channel impurity free, or proximity coupling to a magnetically ordered layer or substrate in heterostructures or superlattices. We review the challenges these approaches are facing in the famous 3D TI (Sb,Bi)2(Se,Te)3 family and try to answer the question whether these materials can live up to the hype surrounding them. © 2020 BMJ Publishing Group. All rights reserved.

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Magnetism, spin dynamics, and quantum transport in two-dimensional systems
Savero Torres W., Sierra J.F., Benítez L.A., Bonell F., García J.H., Roche S., Valenzuela S.O. MRS Bulletin; 45 (5): 357 - 365. 2020. 10.1557/mrs.2020.121. IF: 5.061

Two-dimensional (2D) quantum materials offer a unique platform to explore mesoscopic phenomena driven by interfacial and topological effects. Their tunable electric properties and bidimensional nature enable their integration into sophisticated heterostructures with engineered properties, resulting in the emergence of new exotic phenomena not accessible in other platforms. This has fostered many studies on 2D ferromagnetism, proximity-induced effects, and quantum transport, demonstrating their relevance for fundamental research and future device applications. Here, we review ongoing progress in this lively research field with special emphasis on spin-related phenomena. © Materials Research Society 2020.

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Molecular Approach for Engineering Interfacial Interactions in Magnetic/Topological Insulator Heterostructures
Cuxart M.G., Valbuena M.A., Robles R., Moreno C., Bonell F., Sauthier G., Imaz I., Xu H., Nistor C., Barla A., Gargiani P., Valvidares M., Maspoch D., Gambardella P., Valenzuela S.O., Mugarza A. ACS Nano; 14 (5): 6285 - 6294. 2020. 10.1021/acsnano.0c02498. IF: 14.588

Controlling interfacial interactions in magnetic/topological insulator heterostructures is a major challenge for the emergence of novel spin-dependent electronic phenomena. As for any rational design of heterostructures that rely on proximity effects, one should ideally retain the overall properties of each component while tuning interactions at the interface. However, in most inorganic interfaces, interactions are too strong, consequently perturbing, and even quenching, both the magnetic moment and the topological surface states at each side of the interface. Here, we show that these properties can be preserved using ligand chemistry to tune the interaction of magnetic ions with the surface states. By depositing Co-based porphyrin and phthalocyanine monolayers on the surface of Bi2Te3 thin films, robust interfaces are formed that preserve undoped topological surface states as well as the pristine magnetic moment of the divalent Co ions. The selected ligands allow us to tune the interfacial hybridization within this weak interaction regime. These results, which are in stark contrast with the observed suppression of the surface state at the first quintuple layer of Bi2Se3 induced by the interaction with Co phthalocyanines, demonstrate the capability of planar metal-organic molecules to span interactions from the strong to the weak limit. © 2020 American Chemical Society.

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Opportunities and challenges for spintronics in the microelectronics industry
Dieny B., Prejbeanu I.L., Garello K., Gambardella P., Freitas P., Lehndorff R., Raberg W., Ebels U., Demokritov S.O., Akerman J., Deac A., Pirro P., Adelmann C., Anane A., Chumak A.V., Hirohata A., Mangin S., Valenzuela S.O., Onbaşlı M.C., d’Aquino M., Prenat G., Finocchio G., Lopez-Diaz L., Chantrell R., Chubykalo-Fesenko O., Bortolotti P. Nature Electronics; 3 (8): 446 - 459. 2020. 10.1038/s41928-020-0461-5. IF: 27.500

Spintronic devices exploit the spin, as well as the charge, of electrons and could bring new capabilities to the microelectronics industry. However, in order for spintronic devices to meet the ever-increasing demands of the industry, innovation in terms of materials, processes and circuits are required. Here, we review recent developments in spintronics that could soon have an impact on the microelectronics and information technology industry. We highlight and explore four key areas: magnetic memories, magnetic sensors, radio-frequency and microwave devices, and logic and non-Boolean devices. We also discuss the challenges—at both the device and the system level—that need be addressed in order to integrate spintronic materials and functionalities into mainstream microelectronic platforms. © 2020, Springer Nature Limited.

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Preface
Juan F Sierra, Paolo Bondavalli Journal Of Physics-Materials; 3 (3, 30301) 2020. 10.1088/2515-7639/ab8187. IF: 0.000
The 2021 quantum materials roadmap
Giustino F., Lee J.H., Trier F., Bibes M., Winter S.M., Valentí R., Son Y.-W., Taillefer L., Heil C., Figueroa A.I., Plaçais B., Wu Q., Yazyev O.V., Bakkers E.P.A.M., Nygård J., Forn-Díaz P., de Franceschi S., McIver J.W., Foa Torres L.E.F., Low T., Kumar A., Galceran R., Valenzuela S.O., Costache M.V., Manchon A., Kim E.-A., Schleder G.R., Fazzio A., Roche S. JPhys Materials; 3 (4, 042006) 2020. 10.1088/2515-7639/abb74e. IF: 0.000

In recent years, the notion of ‘Quantum Materials’ has emerged as a powerful unifying concept across diverse fields of science and engineering, from condensed-matter and coldatom physics to materials science and quantum computing. Beyond traditional quantum materials such as unconventional superconductors, heavy fermions, and multiferroics, the field has significantly expanded to encompass topological quantum matter, two-dimensional materials and their van der Waals heterostructures, Moiré materials, Floquet time crystals, as well as materials and devices for quantum computation with Majorana fermions. In this Roadmap collection we aim to capture a snapshot of the most recent developments in the field, and to identify outstanding challenges and emerging opportunities. The format of the Roadmap, whereby experts in each discipline share their viewpoint and articulate their vision for quantum materials, reflects the dynamic and multifaceted nature of this research area, and is meant to encourage exchanges and discussions across traditional disciplinary boundaries. It is our hope that this collective vision will contribute to sparking new fascinating questions and activities at the intersection of materials science, condensed matter physics, device engineering, and quantum information, and to shaping a clearer landscape of quantum materials science as a new frontier of interdisciplinary scientific inquiry. We stress that this article is not meant to be a fully comprehensive review but rather an up-to-date snapshot of different areas of research on quantum materials with a minimal number of references focusing on the latest developments. © 2020 The Author(s). Published by IOP Publishing Ltd

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Tunable room-temperature spin galvanic and spin Hall effects in van der Waals heterostructures
Benítez L.A., Savero Torres W., Sierra J.F., Timmermans M., Garcia J.H., Roche S., Costache M.V., Valenzuela S.O. Nature Materials; 19 (2): 170 - 175. 2020. 10.1038/s41563-019-0575-1. IF: 38.663

Spin–orbit coupling stands as a powerful tool to interconvert charge and spin currents and to manipulate the magnetization of magnetic materials through spin-torque phenomena. However, despite the diversity of existing bulk materials and the recent advent of interfacial and low-dimensional effects, control of this interconversion at room temperature remains elusive. Here, we demonstrate strongly enhanced room-temperature spin-to-charge interconversion in graphene driven by the proximity of WS2. By performing spin precession experiments in appropriately designed Hall bars, we separate the contributions of the spin Hall and the spin galvanic effects. Remarkably, their corresponding conversion efficiencies can be tailored by electrostatic gating in magnitude and sign, peaking near the charge neutrality point with an equivalent magnitude that is comparable to the largest efficiencies reported to date. Such electric-field tunability provides a building block for spin generation free from magnetic materials and for ultra-compact magnetic memory technologies. © 2020, The Author(s), under exclusive licence to Springer Nature Limited.

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2019

A brief discussion of the magnetocaloric effect in thin films of manganite doped with chromium
Campillo G., Figueroa A.I., Arnache O., Osorio J., Marin Ramirez J.M., Fallarino L. Journal of Physics: Conference Series; 1247 (1, 012013) 2019. 10.1088/1742-6596/1247/1/012013.

In this work we report on the magnetocaloric effect of La2/3Ca1/3MnO3 (LCMO) and La2/3Ca1/3Mn0.94Cr0.06O3 (LCMCrO) manganite thin films grown by DC magnetron sputtering on LaAlO3 (100) substrates. X-ray diffraction shows that both doped and undoped films crystallize in the orthorhombic structure. Magnetic measurements show a decrease in both the Curie temperature, TC , and the saturation magnetization, MS , for the LCMCrO sample. The change in the magnetic entropy (ΔSm ) was extracted from hysteresis loops at different temperatures around the ferromagnetic to paramagnetic transition, displaying a maximum of entropy change (ΔSm ) max near TC in both films. Moreover, a shift in (ΔSm ) max toward temperatures above TC with increasing magnetic field and a broadening of the entropy change curve were observed. Results of refrigeration cooling power show a lower efficiency for LCMCrO. In order to obtain a local insight into the magnetic interactions of these films, measurements of X-ray absorption spectroscopy and X-ray magnetic circular dichroism (XMCD) were performed. XMCD suggests that an antiferromagnetic coupling between Mn4+ - Mn3+ is favored with Cr3+ incorporation, which reduces the Mn L 2,3 XMCD signal and results in a decrease of MS and (ΔSm ) max in LCMCrO films. © Published under licence by IOP Publishing Ltd.

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Cr2Te3 Thin Films for Integration in Magnetic Topological Insulator Heterostructures
Burn D.M., Duffy L.B., Fujita R., Zhang S.L., Figueroa A.I., Herrero-Martin J., van der Laan G., Hesjedal T. Scientific Reports; 9 (1, 10793) 2019. 10.1038/s41598-019-47265-7. IF: 4.011

Chromium telluride compounds are promising ferromagnets for proximity coupling to magnetic topological insulators (MTIs) of the Cr-doped (Bi,Sb)2(Se,Te)3 class of materials as they share the same elements, thus simplifying thin film growth, as well as due to their compatible crystal structure. Recently, it has been demonstrated that high quality (001)-oriented Cr2Te3 thin films with perpendicular magnetic anisotropy can be grown on c-plane sapphire substrate. Here, we present a magnetic and soft x-ray absorption spectroscopy study of the chemical and magnetic properties of Cr2Te3 thin films. X-ray magnetic circular dichroism (XMCD) measured at the Cr L2,3 edges gives information about the local electronic and magnetic structure of the Cr ions. We further demonstrate the overgrowth of Cr2Te3 (001) thin films by high-quality Cr-doped Sb2Te3 films. The magnetic properties of the layers have been characterized and our results provide a starting point for refining the physical models of the complex magnetic ordering in Cr2Te3 thin films, and their integration into advanced MTI heterostructures for quantum device applications. © 2019, The Author(s).

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Epitaxial graphene/silicon carbide intercalation: A minireview on graphene modulation and unique 2D materials
Briggs N., Gebeyehu Z.M., Vera A., Zhao T., Wang K., De La Fuente Duran A., Bersch B., Bowen T., Knappenberger K.L., Robinson J.A. Nanoscale; 11 (33): 15440 - 15447. 2019. 10.1039/c9nr03721g. IF: 6.970

Intercalation of atomic species through epitaxial graphene on silicon carbide began only a few years following its initial report in 2004. The impact of intercalation on the electronic properties of the graphene is well known; however, the intercalant itself can also exhibit intriguing properties not found in nature. This realization has inspired new interest in epitaxial graphene/silicon carbide (EG/SiC) intercalation, where the scope of the technique extends beyond modulation of graphene properties to the creation of new 2D forms of 3D materials. The mission of this minireview is to provide a concise introduction to EG/SiC intercalation and to demonstrate a simplified approach to EG/SiC intercalation. We summarize the primary techniques used to achieve and characterize EG/SiC intercalation, and show that thermal evaporation-based methods can effectively substitute for more complex synthesis techniques, enabling large-scale intercalation of non-refractory metals and compounds including two-dimensional silver (2D-Ag) and gallium nitride (2D-GaNx). © The Royal Society of Chemistry.

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Exploring the impact of core expansion on the vortex distribution in superconducting-normal-metal hybrid nanostructures
Panghotra R., Timmermans M., Xue C., Raes B., Moshchalkov V., Van De Vondel J. Physical Review B; 100 (5, 054519) 2019. 10.1103/PhysRevB.100.054519. IF: 3.736

The superconducting condensate in a bilayer normal-metal-superconducting film has a nonuniform dependence throughout the layer. For sufficiently thin normal metallic films, the superconducting correlations induced by the contacting superconducting layer result in the formation of a minigap and the characteristic length scales governing the response of the condensate will be different. In this work we use scanning tunneling spectroscopy to visualize the vortex states as a function of the applied magnetic field in a Au-MoGe nanostructure. By comparing the obtained zero-bias conductance maps with time-dependent Ginzburg-Landau simulations, we directly confirm that the observed vortex distributions can only be explained by taking into account the impact of the normal metallic layer. We illustrate this impact on the vortex state for two lithographically fabricated mesoscopic bilayer structures containing an identical antidot array but having different lateral sizes. © 2019 American Physical Society.

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Investigating the spin-orbit interaction in van der Waals heterostructures by means of the spin relaxation anisotropy
Benítez L.A., Sierra J.F., Savero Torres W., Timmermans M., Costache M.V., Valenzuela S.O. APL Materials; 7 (12, 120701) 2019. 10.1063/1.5124894. IF: 4.296

Graphene offers long spin propagation and, at the same time, a versatile platform to engineer its physical properties. Proximity-induced phenomena, taking advantage of materials with large spin-orbit coupling or that are magnetic, can be used to imprint graphene with large spin-orbit coupling and magnetic correlations. However, full understanding of the proximitized graphene and the consequences on the spin transport dynamics requires the development of unconventional experimental approaches. The investigation of the spin relaxation anisotropy, defined as the ratio of lifetimes for spins pointing out of and in the graphene plane, is an important step in this direction. This review discusses various methods for extracting the spin relaxation anisotropy in graphene-based devices. Within the experimental framework, current understanding on spin transport dynamics in single-layer and bilayer graphene is presented. Due to increasing interest, experimental results in graphene in proximity with high spin-orbit layered materials are also reviewed. © 2019 Author(s).

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Spin communication over 30 μm long channels of chemical vapor deposited graphene on SiO2
Gebeyehu Z.M., Parui S., Sierra J.F., Timmermans M., Esplandiu M.J., Brems S., Huyghebaert C., Garello K., Costache M.V., Valenzuela S.O. 2D Materials; 6 (3, 034003) 2019. 10.1088/2053-1583/ab1874. IF: 7.343

We demonstrate a high-yield fabrication of non-local spin valve devices with room-temperature spin lifetimes of up to 3 ns and spin relaxation lengths as long as 9 μm in platinum-based chemical vapor deposition (Pt-CVD) synthesized single-layer graphene on SiO2/Si substrates. The spin-lifetime systematically presents a marked minimum at the charge neutrality point, as typically observed in pristine exfoliated graphene. However, by studying the carrier density dependence beyond n ∼ 5 × 1012 cm-2, via electrostatic gating, it is found that the spin lifetime reaches a maximum and then starts decreasing, a behavior that is reminiscent of that predicted when the spin-relaxation is driven by spin-orbit interaction. The spin lifetimes and relaxation lengths compare well with state-of-the-art results using exfoliated graphene on SiO2/Si, being a factor two-to-three larger than the best values reported at room temperature using the same substrate. As a result, the spin signal can be readily measured across 30 μm long graphene channels. These observations indicate that Pt-CVD graphene is a promising material for large-scale spin-based logic-in-memory applications. © 2019 IOP Publishing Ltd.

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The phase diagram of 2D antiferromagnets
Valenzuela S.O., Roche S. Nature Nanotechnology; 14 (12): 1088 - 1089. 2019. 10.1038/s41565-019-0592-x. IF: 33.407

[No abstract available]

View abstract

2018

A barrier to spin filters
Valenzuela S.O., Roche S. Nature Electronics; 1 (6): 328 - 329. 2018. 10.1038/s41928-018-0089-x.

Electron tunnelling through a two-dimensional magnetic insulator is assisted by magnon inelastic processes that provide spin-filtering. © 2018 The Author(s).

View abstract
Bottom-up synthesis of multifunctional nanoporous graphene
Moreno C., Vilas-Varela M., Kretz B., Garcia-Lekue A., Costache M.V., Paradinas M., Panighel M., Ceballos G., Valenzuela S.O., Peña D., Mugarza A. Science; 360 (6385): 199 - 203. 2018. 10.1126/science.aar2009. IF: 41.058

Nanosize pores can turn semimetallic graphene into a semiconductor and, from being impermeable, into the most efficient molecular-sieve membrane. However, scaling the pores down to the nanometer, while fulfilling the tight structural constraints imposed by applications, represents an enormous challenge for present top-down strategies. Here we report a bottom-up method to synthesize nanoporous graphene comprising an ordered array of pores separated by ribbons, which can be tuned down to the 1-nanometer range. The size, density, morphology, and chemical composition of the pores are defined with atomic precision by the design of the molecular precursors. Our electronic characterization further reveals a highly anisotropic electronic structure, where orthogonal one-dimensional electronic bands with an energy gap of ∼1 electron volt coexist with confined pore states, making the nanoporous graphene a highly versatile semiconductor for simultaneous sieving and electrical sensing of molecular species. 2017 © The Authors

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Impact of the: In situ rise in hydrogen partial pressure on graphene shape evolution during CVD growth of graphene
Gebeyehu Z.M., Arrighi A., Costache M.V., Sotomayor-Torres C.M., Esplandiu M.J., Valenzuela S.O. RSC Advances; 8 (15): 8234 - 8239. 2018. 10.1039/c7ra13169k. IF: 2.936

Exposing graphene to a hydrogen post-etching process yields dendritic graphene shapes. Here, we demonstrate that similar dendritic structures can be achieved at long growth times without adding hydrogen externally. These shapes are not a result of a surface diffusion controlled growth but of the competing backward reaction (etching), which dominates the growth dynamics at long times due to an in situ rise in the hydrogen partial pressure. We have performed a systematic study on the growth of graphene as a function of time to identify the onset and gradual evolution of graphene shapes caused by etching and then demonstrated that the etching can be stopped by reducing the flow of hydrogen from the feed. In addition, we have found that the etching rate due to the in situ rise in hydrogen is strongly dependent on the confinement (geometrical confinement) of copper foil. Highly etched graphene with dendritic shapes was observed in unconfined copper foil regions while no etching was found in graphene grown in a confined reaction region. This highlights the effect of the dynamic reactant distribution in activating the in situ etching process during growth, which needs to be counteracted or controlled for large scale growth. © The Royal Society of Chemistry 2018.

View abstract
Strongly anisotropic spin relaxation in graphene-transition metal dichalcogenide heterostructures at room temperature
Benítez L.A., Sierra J.F., Savero Torres W., Arrighi A., Bonell F., Costache M.V., Valenzuela S.O. Nature Physics; 14 (3): 303 - 308. 2018. 10.1038/s41567-017-0019-2. IF: 22.727

A large enhancement in the spin-orbit coupling of graphene has been predicted when interfacing it with semiconducting transition metal dichalcogenides. Signatures of such an enhancement have been reported, but the nature of the spin relaxation in these systems remains unknown. Here, we unambiguously demonstrate anisotropic spin dynamics in bilayer heterostructures comprising graphene and tungsten or molybdenum disulphide (WS2, MoS2). We observe that the spin lifetime varies over one order of magnitude depending on the spin orientation, being largest when the spins point out of the graphene plane. This indicates that the strong spin-valley coupling in the transition metal dichalcogenide is imprinted in the bilayer and felt by the propagating spins. These findings provide a rich platform to explore coupled spin-valley phenomena and offer novel spin manipulation strategies based on spin relaxation anisotropy in two-dimensional materials. © 2017 The Author(s).

View abstract
Thermoelectric spin voltage in graphene
Sierra J.F., Neumann I., Cuppens J., Raes B., Costache M.V., Valenzuela S.O. Nature Nanotechnology; 13 (2): 107 - 111. 2018. 10.1038/s41565-017-0015-9. IF: 37.490

In recent years, new spin-dependent thermal effects have been discovered in ferromagnets, stimulating a growing interest in spin caloritronics, a field that exploits the interaction between spin and heat currents 1,2 . Amongst the most intriguing phenomena is the spin Seebeck effect 3-5, in which a thermal gradient gives rise to spin currents that are detected through the inverse spin Hall effect 6-8 . Non-magnetic materials such as graphene are also relevant for spin caloritronics, thanks to efficient spin transport 9-11, energy-dependent carrier mobility and unique density of states 12,13 . Here, we propose and demonstrate that a carrier thermal gradient in a graphene lateral spin valve can lead to a large increase of the spin voltage near to the graphene charge neutrality point. Such an increase results from a thermoelectric spin voltage, which is analogous to the voltage in a thermocouple and that can be enhanced by the presence of hot carriers generated by an applied current 14-17 . These results could prove crucial to drive graphene spintronic devices and, in particular, to sustain pure spin signals with thermal gradients and to tune the remote spin accumulation by varying the spin-injection bias. © 2017 The Author(s).

View abstract

2017

Growth of Twin-Free and Low-Doped Topological Insulators on BaF2(111)
Bonell F., Cuxart M.G., Song K., Robles R., Ordejón P., Roche S., Mugarza A., Valenzuela S.O. Crystal Growth and Design; 17 (9): 4655 - 4660. 2017. 10.1021/acs.cgd.7b00525. IF: 4.055

We demonstrate the growth of twin-free Bi2Te3 and Sb2Te3 topological insulators by molecular beam epitaxy and a sizable reduction of the twin density in Bi2Se3 on lattice-matched BaF2(111) substrates. Using X-ray diffraction, electron diffraction and atomic force microscopy, we systematically investigate the parameters influencing the formation of twin domains and the morphology of the films, and show that Se- and Te-based alloys differ by their growth mechanism. Optimum growth parameters are shown to result in intrinsically low-doped films, as probed by angle-resolved photoelectron spectroscopy. In contrast to previous approaches in which twin-free Bi2Se3 films are achieved by increasing the substrate roughness, the quality of our Bi2Te3 is superior on the flattest BaF2 substrates. This finding indicates that, during nucleation, the films not only interact with the topmost atomic substrate layer but also with buried layers that provide the necessary stacking information to promote a single twin, an observation that is supported by ab initio calculations. © 2017 American Chemical Society.

View abstract
Spin precession and spin Hall effect in monolayer graphene/Pt nanostructures
W Savero Torres, J F Sierra, L A Benítez, F Bonell, M V Costache, S O Valenzuela 2D Materials; 4 (4): 41008. 2017. 10.1088/2053-1583/aa8823. IF: 6.937

Spin Hall effects have surged as promising phenomena for spin logics operations without ferromagnets. 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 spin resistance of graphene but we also observe 100% spin absorption in Pt and find 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 spin-relaxation 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.

View abstract
Spin precession in anisotropic media
Raes B., Cummings A.W., Bonell F., Costache M.V., Sierra J.F., Roche S., Valenzuela S.O. Physical Review B; 95 (8, 085403) 2017. 10.1103/PhysRevB.95.085403. IF: 3.836

We generalize the diffusive model for spin injection and detection in nonlocal spin structures to account for spin precession under an applied magnetic field in an anisotropic medium, for which the spin lifetime is not unique and depends on the spin orientation. We demonstrate that the spin precession (Hanle) line shape is strongly dependent on the degree of anisotropy and on the orientation of the magnetic field. In particular, we show that the anisotropy of the spin lifetime can be extracted from the measured spin signal, after dephasing in an oblique magnetic field, by using an analytical formula with a single fitting parameter. Alternatively, after identifying the fingerprints associated with the anisotropy, we propose a simple scaling of the Hanle line shapes at specific magnetic field orientations that results in a universal curve only in the isotropic case. The deviation from the universal curve can be used as a complementary means of quantifying the anisotropy by direct comparison with the solution of our generalized model. Finally, we applied our model to graphene devices and find that the spin relaxation for graphene on silicon oxide is isotropic within our experimental resolution. © 2017 American Physical Society.

View abstract
The 2017 Magnetism Roadmap
Sander D., Valenzuela S.O., Makarov D., Marrows C.H., Fullerton E.E., Fischer P., McCord J., Vavassori P., Mangin S., Pirro P., Hillebrands B., Kent A.D., Jungwirth T., Gutfleisch O., Kim C.G., Berger A. Journal of Physics D: Applied Physics; 50 (36, 363001) 2017. 10.1088/1361-6463/aa81a1. IF: 2.588

Building upon the success and relevance of the 2014 Magnetism Roadmap, this 2017 Magnetism Roadmap edition follows a similar general layout, even if its focus is naturally shifted, and a different group of experts and, thus, viewpoints are being collected and presented. More importantly, key developments have changed the research landscape in very relevant ways, so that a novel view onto some of the most crucial developments is warranted, and thus, this 2017 Magnetism Roadmap article is a timely endeavour. The change in landscape is hereby not exclusively scientific, but also reflects the magnetism related industrial application portfolio. Specifically, Hard Disk Drive technology, which still dominates digital storage and will continue to do so for many years, if not decades, has now limited its footprint in the scientific and research community, whereas significantly growing interest in magnetism and magnetic materials in relation to energy applications is noticeable, and other technological fields are emerging as well. Also, more and more work is occurring in which complex topologies of magnetically ordered states are being explored, hereby aiming at a technological utilization of the very theoretical concepts that were recognised by the 2016 Nobel Prize in Physics. Given this somewhat shifted scenario, it seemed appropriate to select topics for this Roadmap article that represent the three core pillars of magnetism, namely magnetic materials, magnetic phenomena and associated characterization techniques, as well as applications of magnetism. While many of the contributions in this Roadmap have clearly overlapping relevance in all three fields, their relative focus is mostly associated to one of the three pillars. In this way, the interconnecting roles of having suitable magnetic materials, understanding (and being able to characterize) the underlying physics of their behaviour and utilizing them for applications and devices is well illustrated, thus giving an accurate snapshot of the world of magnetism in 2017. The article consists of 14 sections, each written by an expert in the field and addressing a specific subject on two pages. Evidently, the depth at which each contribution can describe the subject matter is limited and a full review of their statuses, advances, challenges and perspectives cannot be fully accomplished. Also, magnetism, as a vibrant research field, is too diverse, so that a number of areas will not be adequately represented here, leaving space for further Roadmap editions in the future. However, this 2017 Magnetism Roadmap article can provide a frame that will enable the reader to judge where each subject and magnetism research field stands overall today and which directions it might take in the foreseeable future. The first material focused pillar of the 2017 Magnetism Roadmap contains five articles, which address the questions of atomic scale confinement, 2D, curved and topological magnetic materials, as well as materials exhibiting unconventional magnetic phase transitions. The second pillar also has five contributions, which are devoted to advances in magnetic characterization, magneto-optics and magneto-plasmonics, ultrafast magnetization dynamics and magnonic transport. The final and application focused pillar has four contributions, which present non-volatile memory technology, antiferromagnetic spintronics, as well as magnet technology for energy and bio-related applications. As a whole, the 2017 Magnetism Roadmap article, just as with its 2014 predecessor, is intended to act as a reference point and guideline for emerging research directions in modern magnetism. © 2017 IOP Publishing Ltd.

View abstract

2016

Determination of the spin-lifetime anisotropy in graphene using oblique spin precession
Raes B., Scheerder J.E., Costache M.V., Bonell F., Sierra J.F., Cuppens J., Van De Vondel J., Valenzuela S.O. Nature Communications; 7 ( 11444) 2016. 10.1038/ncomms11444. IF: 11.329

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.

View abstract
Nanoscale thermal imaging of dissipation in quantum systems
Halbertal D., Cuppens J., Shalom M.B., Embon L., Shadmi N., Anahory Y., Naren H.R., Sarkar J., Uri A., Ronen Y., Myasoedov Y., Levitov L.S., Joselevich E., Geim A.K., Zeldov E. Nature; 539 (7629): 407 - 410. 2016. 10.1038/nature19843. IF: 38.138

Energy dissipation is a fundamental process governing the dynamics of physical, chemical and biological systems. It is also one of the main characteristics that distinguish quantum from classical phenomena. In particular, in condensed matter physics, scattering mechanisms, loss of quantum information or breakdown of topological protection are deeply rooted in the intricate details of how and where the dissipation occurs. Yet the microscopic behaviour of a system is usually not formulated in terms of dissipation because energy dissipation is not a readily measurable quantity on the micrometre scale. Although nanoscale thermometry has gained much recent interest, existing thermal imaging methods are not sensitive enough for the study of quantum systems and are also unsuitable for the low-temperature operation that is required. Here we report a nano-thermometer based on a superconducting quantum interference device with a diameter of less than 50 nanometres that resides at the apex of a sharp pipette: it provides scanning cryogenic thermal sensing that is four orders of magnitude more sensitive than previous devices-below 1 μKH . This non-contact, non-invasive thermometry allows thermal imaging of very low intensity, nanoscale energy dissipation down to the fundamental Landauer limit of 40 femtowatts for continuous readout of a single qubit at one gigahertz at 4.2 kelvin. These advances enable the observation of changes in dissipation due to single-electron charging of individual quantum dots in carbon nanotubes. They also reveal a dissipation mechanism attributable to resonant localized states in graphene encapsulated within hexagonal boron nitride, opening the door to direct thermal imaging of nanoscale dissipation processes in quantum matter. © 2016 Macmillan Publishers Limited.

View abstract
Spin Hall Effect and Origins of Nonlocal Resistance in Adatom-Decorated Graphene
Van Tuan D., Marmolejo-Tejada J.M., Waintal X., Nikolić B.K., Valenzuela S.O., Roche S. Physical Review Letters; 117 (17, 176602) 2016. 10.1103/PhysRevLett.117.176602. IF: 7.645

Recent experiments reporting an unexpectedly large spin Hall effect (SHE) in graphene decorated with adatoms have raised a fierce controversy. We apply numerically exact Kubo and Landauer-Büttiker formulas to realistic models of gold-decorated disordered graphene (including adatom clustering) to obtain the spin Hall conductivity and spin Hall angle, as well as the nonlocal resistance as a quantity accessible to experiments. Large spin Hall angles of ∼0.1 are obtained at zero temperature, but their dependence on adatom clustering differs from the predictions of semiclassical transport theories. Furthermore, we find multiple background contributions to the nonlocal resistance, some of which are unrelated to the SHE or any other spin-dependent origin, as well as a strong suppression of the SHE at room temperature. This motivates us to design a multiterminal graphene geometry which suppresses these background contributions and could, therefore, quantify the upper limit for spin-current generation in two-dimensional materials. © 2016 American Physical Society.

View abstract

2015

Graphene spintronics: The European Flagship perspective
Roche S., Åkerman J., Beschoten B., Charlier J.-C., Chshiev M., Dash S.P., Dlubak B., Fabian J., Fert A., Guimarães M., Guinea F., Grigorieva I., Schönenberger C., Seneor P., Stampfer C., Valenzuela S.O., Waintal X., Van Wees B. 2D Materials; 2 (3, 030202) 2015. 10.1088/2053-1583/2/3/030202. IF: 0.000

Wereview current challenges and perspectives in graphene spintronics, which is one of themost promising directions of innovation, given its room-temperature long-spin lifetimes and the ability of graphene to be easily interfaced with other classes ofmaterials (ferromagnets, magnetic insulators, semiconductors, oxides, etc), allowing proximity effects to be harvested. The general context of spintronics is first discussed togetherwith open issues and recent advances achieved by theGraphene SpintronicsWork Package consortiumwithin theGraphene Flagship project. Based on such progress, which establishes the state of the art, several novel opportunities for spinmanipulation such as the generation of pure spin current (through spinHall effect) and the control of magnetization through the spin torque phenomena appear on the horizon. Practical applications arewithin reach, but will require the demonstration of wafer-scale graphene device integration, and the realization of functional prototypes employed for determined applications such as magnetic sensors or nano-oscillators. This is a specially commissioned editorial from the Graphene Flagship Work Package on Spintronics. This editorial is part of the 2DMaterials focus collection on 'Progress on the science and applications of twodimensionalmaterials,' published in association with theGraphene Flagship. It provides an overviewof key recent advances of the spintronicswork package aswell as the mid-term objectives of the consortium. © 2015 IOP Publishing Ltd.

View abstract
Hot-Carrier Seebeck Effect: Diffusion and Remote Detection of Hot Carriers in Graphene
Sierra J.F., Neumann I., Costache M.V., Valenzuela S.O. Nano Letters; 15 (6): 4000 - 4005. 2015. 10.1021/acs.nanolett.5b00922. IF: 13.592

We investigate hot carrier propagation across graphene using an electrical nonlocal injection/detection method. The device consists of a monolayer graphene flake contacted by multiple metal leads. Using two remote leads for electrical heating, we generate a carrier temperature gradient that results in a measurable thermoelectric voltage VNL across the remaining (detector) leads. Due to the nonlocal character of the measurement, VNL is exclusively due to the Seebeck effect. Remarkably, a departure from the ordinary relationship between Joule power P and VNL, VNL ∼ P, becomes readily apparent at low temperatures, representing a fingerprint of hot-carrier dominated thermoelectricity. By studying VNL as a function of bias, we directly determine the carrier temperature and the characteristic cooling length for hot-carrier propagation, which are key parameters for a variety of new applications that rely on hot-carrier transport. (Figure Presented). © 2015 American Chemical Society.

View abstract
Spin Hall effects
Sinova J., Valenzuela S.O., Wunderlich J., Back C.H., Jungwirth T. Reviews of Modern Physics; 87 (4): 1213 - 1260. 2015. 10.1103/RevModPhys.87.1213. IF: 29.604

Spin Hall effects are a collection of relativistic spin-orbit coupling phenomena in which electrical currents can generate transverse spin currents and vice versa. Despite being observed only a decade ago, these effects are already ubiquitous within spintronics, as standard spin-current generators and detectors. Here the theoretical and experimental results that have established this subfield of spintronics are reviewed. The focus is on the results that have converged to give us the current understanding of the phenomena, which has evolved from a qualitative to a more quantitative measurement of spin currents and their associated spin accumulation. Within the experimental framework, optical-, transport-, and magnetization-dynamics-based measurements are reviewed and linked to both phenomenological and microscopic theories of the effect. Within the theoretical framework, the basic mechanisms in both the extrinsic and intrinsic regimes are reviewed, which are linked to the mechanisms present in their closely related phenomenon in ferromagnets, the anomalous Hall effect. Also reviewed is the connection to the phenomenological treatment based on spin-diffusion equations applicable to certain regimes, as well as the spin-pumping theory of spin generation used in many measurements of the spin Hall angle. A further connection to the spin-current-generating spin Hall effect to the inverse spin galvanic effect is given, in which an electrical current induces a nonequilibrium spin polarization. This effect often accompanies the spin Hall effect since they share common microscopic origins. Both can exhibit the same symmetries when present in structures comprising ferromagnetic and nonmagnetic layers through their induced current-driven spin torques or induced voltages. Although a short chronological overview of the evolution of the spin Hall effect field and the resolution of some early controversies is given, the main body of this review is structured from a pedagogical point of view, focusing on well-established and accepted physics. In such a young field, there remains much to be understood and explored, hence some of the future challenges and opportunities of this rapidly evolving area of spintronics are outlined. © 2015 American Physical Society.

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2014

Determination of the magnetic penetration depth in a superconducting Pb film
Brisbois, J.; Raes, B.; Van De Vondel, J.; Moshchalkov, V.V.; Silhanek, A.V. Journal of Applied Physics; 2014. 10.1063/1.4868298. IF: 2.185
Fingerprints of inelastic transport at the surface of the topological insulator Bi 2 Se 3: Role of electron-phonon coupling
Costache, M.V.; Neumann, I.; Sierra, J.F.; Marinova, V.; Gospodinov, M.M.; Roche, S.; Valenzuela, S.O. Physical Review Letters; 2014. 10.1103/PhysRevLett.112.086601. IF: 7.728
Graphene spintronics: Puzzling controversies and challenges for spin manipulation
Roche, S.; Valenzuela, S.O. Journal of Physics D - Applied Physics; 2014. 10.1088/0022-3727/47/9/094011. IF: 2.521
Observing vortex motion on NbSe2 with STM
Timmermans, M.; Samuely, T.; Raes, B.; Van De Vondel, J.; Moshchalkov, V.V. Physica C: Superconductivity and its Applications; 503: 154 - 157. 2014. 10.1016/j.physc.2014.05.003. IF: 1.110
Pseudospin-driven spin relaxation mechanism in graphene
Tuan, D.V.; Ortmann, F.; Soriano, D.; Valenzuela, S.O.; Roche, S. Nature Physics; 10 (11): 857 - 863. 2014. 10.1038/nphys3083. IF: 20.603

2013

Electrical detection of spin precession in freely suspended graphene spin valves on cross-linked poly(methyl methacrylate)
Neumann, I.; Van De Vondel, J.; Bridoux, G.; Costache, M.V.; Alzina, F.; Torres, C.M.S.; Valenzuela, S.O. Small; 9 (1): 156 - 160. 2013. 10.1002/smll.201201194. IF: 7.823
Enhanced spin accumulation at room temperature in graphene spin valves with amorphous carbon interfacial layers
Neumann, I.; Costache, M.V.; Bridoux, G.; Sierra, J.F.; Valenzuela, S.O. Applied Physics Letters; 2013. 10.1063/1.4820586. IF: 3.794

2012

Lateral metallic devices made by a multiangle shadow evaporation technique
Costache, M.V.; Bridoux, G.; Neumann, I.; Valenzuela, S.O. Journal of Vacuum Science and Technology B: Microelectronics and Nanometer Structures; 30: 4. 2012. .

2011

Enhanced spin signal in nonlocal devices based on a ferromagnetic CoFeAl alloy
Bridoux, G.; Costache, M. V.; Van de Vondel, J. ; Neumann, I.; Valenzuela, S. O. Applied Physics Letters; 2011. .
Generation of pure spin currents in a single electron transistor with a superconducting island
Costache, M.V.; Valenzuela, S.O. Proceedings of SPIE - The International Society for Optical Engineering; 2011. 10.1117/12.890231 .
Magnon-drag thermopile
Costache, M.V.; Bridoux, G.; Neumann, I.; Valenzuela, S.O. Nature Materials; 2011. .
Perpendicular switching of a single ferromagnetic layer induced by in-plane current injection
Miron I.M., Garello K., Gaudin G., Zermatten P.-J., Costache M.V., Auffret S., Bandiera S., Rodmacq B., Schuhl A., Gambardella P. Nature; 476 (7359): 189 - 193. 2011. 10.1038/nature10309.

Modern computing technology is based on writing, storing and retrieving information encoded as magnetic bits. Although the giant magnetoresistance effect has improved the electrical read out of memory elements, magnetic writing remains the object of major research efforts. Despite several reports of methods to reverse the polarity of nanosized magnets by means of local electric fields and currents, the simple reversal of a high-coercivity, single-layer ferromagnet remains a challenge. Materials with large coercivity and perpendicular magnetic anisotropy represent the mainstay of data storage media, owing to their ability to retain a stable magnetization state over long periods of time and their amenability to miniaturization. However, the same anisotropy properties that make a material attractive for storage also make it hard to write to. Here we demonstrate switching of a perpendicularly magnetized cobalt dot driven by in-plane current injection at room temperature. Our device is composed of a thin cobalt layer with strong perpendicular anisotropy and Rashba interaction induced by asymmetric platinum and AlO x interface layers. The effective switching field is orthogonal to the direction of the magnetization and to the Rashba field. The symmetry of the switching field is consistent with the spin accumulation induced by the Rashba interaction and the spin-dependent mobility observed in non-magnetic semiconductors, as well as with the torque induced by the spin Hall effect in the platinum layer. Our measurements indicate that the switching efficiency increases with the magnetic anisotropy of the cobalt layer and the oxidation of the aluminium layer, which is uppermost, suggesting that the Rashba interaction has a key role in the reversal mechanism. To prove the potential of in-plane current switching for spintronic applications, we construct a reprogrammable magnetic switch that can be integrated into non-volatile memory and logic architectures. This device is simple, scalable and compatible with present-day magnetic recording technology. © 2011 Macmillan Publishers Limited. All rights reserved.

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2010

All magnesium diboride Josephson junctions with MgO and native oxide barriers
Costache, M.V. ; Moodera, J.S. Applied Physics Letters; 2010. .
Experimental spin ratchet
Costache, M.V.; Valenzuela, S.O. SCIENCE; 330: 1645 - 1648. 2010. 10.1126/science.1196228.

2009

Large-amplitude driving of a superconducting artificial atom : IIInterferometry, cooling, and amplitude spectroscopy
Oliver, W.D.; Valenzuela, S.O. Quantum Information Processing; 8: 261 - 281. 2009. 10.1007/s11128-009-0108-y.
Nonlocal electronic spin detection, spin accumulation and the spin hall effect
Valenzuela, S.O. International Journal of Modern Physics B; 23: 2413 - 2438. 2009. 10.1142/S021797920905290X.
Pulse imaging and nonadiabatic control of solid-state artificial atoms
Bylander, J.; Rudner, M.S.; Shytov, A.V.; Valenzuela, S.O.; Berns, D.M.; Berggren, K.K.; Levitov, L.S.; Oliver, W.D. Physical Review B - Condensed Matter and Materials Physics; 80 2009. 10.1103/PhysRevB.80.220506.

2008

Amplitude spectroscopy of a solid-state artificial atom
D.M. Berns; M.S. Rudner; S.O. Valenzuela; K.K. Berggren; W.D. Oliver; L.S. Levitov; T.P. Orlando Nature; 455: 51 - 57. 2008. 10.1038/nature07262.
Microwave-induced cooling of a superconducting qubit
S.O. Valenzuela; W.D. Oliver; D.M. Berns; K.K. Berggren; L.S. Levitov; T.P. Orlando Science; 314 (5805): 1589 - 1592. 2008. 10.1126/science.1134008 .
Quantum Phase Tomography of a Strongly Driven Qubit
M.S. Rudner; A.V. Shytov; L.S. Levitov; D. M. Berns; W.D. Oliver; S.O. Valenzuela; T.P. Orlando Physical Review Letters; 101: 190502. 2008. 10.1103/PhysRevLett.101.190502.