Publications
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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 .
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]
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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
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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.
