Publications
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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]
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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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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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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.