Paul Noël

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Noël

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Paul

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0000-0002-8240-4206

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Publications 1 - 10 of 16

Rashba-like physics in condensed matter
Bihlmayer, Gustav; Noël, Paul; Vyalikh, Denis V.; et al. (2022)
Nature Reviews Physics
Spin-orbit coupling induces a unique form of Zeeman interaction in momentum space in materials that lack inversion symmetry: the electron's spin is locked on an effective magnetic field that is odd in momentum. The resulting interconnection between the electron's momentum and its spin leads to various effects such as electric dipole spin resonance, anisotropic spin relaxation and the Aharonov-Casher effect, but also to electrically driven and optically driven spin galvanic effects. Over the past 15 years, the emergence of topological materials has widened this research field by introducing complex forms of spin textures and orbital hybridization. The vast field of Rashba-like physics is now blooming, with great attention paid to non-equilibrium mechanisms such as spin-to-charge conversion, but also to nonlinear transport effects. This Review aims to offer an overview of recent progress in the development of condensed matter research that exploits the unique properties of spin-orbit coupling in non-centrosymmetric heterostructures.
Nonlinear Longitudinal and Transverse Magnetoresistances due to Current-Induced Magnon Creation-Annihilation Processes
Noël, Paul; Schlitz, Richard; Karadža, Emir; et al. (2025)
Physical Review Letters
Charge-spin conversion phenomena, such as the spin Hall effect, allow for the excitation of magnons in a magnetic layer by passing an electric current in an adjacent nonmagnetic conductor. We demonstrate that this current-induced modification of the magnon density generates an additional nonlinear longitudinal and transverse magnetoresistance for every magnetoresistance that depends on the magnetization. Using harmonic measurements, we evidence that these magnon creation-annihilation magnetoresistances dominate the second harmonic longitudinal and transverse resistance of thin Y3Fe5O12/Pt bilayers. Our results apply to both insulating and metallic magnetic layers, elucidating the dependence of the magnetoresistance on applied current and magnetic field for a broad variety of systems excited by spin currents.
Estimation of spin-orbit torques in the presence of current-induced magnon creation and annihilation
Noël, Paul; Karadža, Emir; Schlitz, Richard; et al. (2025)
Physical Review B
We present a comprehensive set of harmonic resistance measurements of the dampinglike (DL) and fieldlike (FL) torques in Pt/CoFeB, Pt/Co, W/CoFeB, W/Co, and YIG/Pt bilayers complemented by measurements of the DL torque using the magneto-optical Kerr effect and calibrated by nitrogen vacancy magnetometry on the same devices. The magnon creation-annihilation magnetoresistances depend strongly on temperature and on the magnetic and transport properties of each bilayer, affecting the estimate of both the DL and FL torque. The DL torque, the most important parameter for applications, is overestimated by a factor of 2 in W/CoFeB and by one order of magnitude in YIG/Pt when not accounting for the magnonic contribution to the planar Hall resistance. We further show that the magnonic contribution can be quantified by combining measurements of the nonlinear longitudinal and transverse magnetoresistances, thus providing a reliable method to measure the spin-orbit torques in different material systems.
Generation, transmission, and conversion of orbital torque by an antiferromagnetic insulator
Ding, Shilei; Noël, Paul; Krishnaswamy , Gunasheel Kauwtilyaa; et al. (2025)
Nature Communications
Electrical control of magnetization in nanoscale devices can be significantly improved through the efficient generation of orbital currents and their conversion into spin currents. In nonmagnetic/ferromagnetic bilayers, this conversion produces a torque on the magnetization, enabling magnetization switching and dynamic manipulation. While previous studies focus on metallic ferromagnets, we demonstrate a large orbital torque and enhanced orbital-to-spin conversion by an antiferromagnetic insulating CoO layer. Measurements in CuOx/CoO/Co trilayers show that inserting CoO reverses the torque’s sign and triples its magnitude compared to CuOx/Co. This behaviour stems from the inverted oxygen gradient at the CuOx/CoO interface and CoO’s high orbital multiplicity, which favours the transmission of orbital momenta and efficient orbital-to-spin conversion. At low temperatures, the onset of antiferromagnetic order induces a further many-fold increase of the torque, which we attribute to the efficient excitation and propagation of spin-orbit excitons induced by magnetic coupling. Comparative measurements of CuOx/NiO/Co and CuOx/MnO/Co trilayers show that the torque efficiency scales with the orbital momentum of the Co²⁺, Ni²⁺, and Mn²⁺ ions in the antiferromagnet. These results reveal that antiferromagnetic insulators like CoO provide highly effective orbital-to-spin transduction, combining orbital torque and exchange bias functionalities to improve the performance of spintronic devices.
Mitigation of Gilbert Damping in the CoFe/CuOₓ Orbital Torque System
Ding, Shilei; Wang, Hanchen; Legrand, William; et al. (2024)
Nano Letters
Charge-spin interconversion processes underpin the generation of spin–orbit torques in magnetic/nonmagnetic bilayers. However, efficient sources of spin currents such as 5d metals are also efficient spin sinks, resulting in a large increase of magnetic damping. Here we show that a partially oxidized 3d metal can generate a strong orbital torque without a significant increase in damping. Measurements of the torque efficiency ξ and Gilbert damping α in CoFe/CuOₓ and CoFe/Pt indicate that ξ is comparable in the two systems. The increase in damping relative to a single CoFe layer is Δα < 0.002 in CoFe/CuOₓ and Δα ≈ 0.005–0.02 in CoFe/Pt, depending on CoFe thickness. We ascribe the nonreciprocal relationship between Δα and ξ in CoFe/CuOₓ to the small orbital–spin current ratio generated by magnetic resonance in CoFe and the lack of an efficient spin sink in CuOₓ. Our findings provide new perspectives on the efficient excitation of magnetization dynamics via the orbital torque.
Spin-orbit torques and magnetization switching in Gd/Fe multilayers generated by current injection in NiCu alloys
Nasr, Federica; Binda, Federico; Lambert, Charles-Henri; et al. (2023)
Applied Physics Letters
Light transition metals have recently emerged as a sustainable material class for efficient spin–charge interconversion. We report measurements of current-induced spin–orbit torques generated by Ni₁-ₓCuₓ alloys in perpendicularly magnetized ferrimagnetic Gd/Fe multilayers. We show that the spin–orbit torque efficiency of Ni₁-ₓCuₓ increases with the Ni/Cu atomic ratio, reaching values comparable to those of Pt for Ni₅₅Cu₄₅. Furthermore, we demonstrate magnetization switching of a 20-nm-thick Gd/Fe multilayer with a threshold current that decreases with increasing Ni concentration, similar to the spin–orbit torque efficiency. Our findings show that Ni₁-ₓ Cuₓ- based magnetic heterostructures allow for efficient control of the magnetization by electric currents.
Nonlinear longitudinal and transverse magnetoresistances due to current-induced magnon creation-annihilation processes
Noël, Paul; Schlitz, Richard; Karadža, Emir; et al. (2024)
arXiv
Charge-spin conversion phenomena such as the spin Hall effect allow for the excitation of magnons in a magnetic layer by passing an electric current in an adjacent nonmagnetic conductor. We demonstrate that this current-induced modification of the magnon density generates an additional nonlinear longitudinal and transverse magnetoresistance for every magnetoresistance that depends on the magnetization. Using harmonic measurements, we evidence that these magnon creation-annihilation magnetoresistances dominate the second harmonic longitudinal and transverse resistance of thin Y3Fe5O12/Pt bilayers. Our results apply to both insulating and metallic magnetic layers, elucidating the dependence of the magnetoresistance on applied current and magnetic field for a broad variety of systems excited by spin currents.
Non-volatile electric control of spin-orbit torques in an oxide two-dimensional electron gas
Grezes, Cécile; Kandazoglou, Aurélie; Cosset-Cheneau, Maxen; et al. (2023)
Nature Communications
Spin-orbit torques (SOTs) have opened a novel way to manipulate the magnetization using in-plane current, with a great potential for the development of fast and low power information technologies. It has been recently shown that two-dimensional electron gases (2DEGs) appearing at oxide interfaces provide a highly efficient spin-to-charge current interconversion. The ability to manipulate 2DEGs using gate voltages could offer a degree of freedom lacking in the classical ferromagnetic/spin Hall effect bilayers for spin-orbitronics, in which the sign and amplitude of SOTs at a given current are fixed by the stack structure. Here, we report the non-volatile electric-field control of SOTs in an oxide-based Rashba-Edelstein 2DEG. We demonstrate that the 2DEG is controlled using a back-gate electric-field, providing two remanent and switchable states, with a large resistance contrast of 1064%. The SOTs can then be controlled electrically in a non-volatile way, both in amplitude and in sign. This achievement in a 2DEG-CoFeB/MgO heterostructures with large perpendicular magnetization further validates the compatibility of oxide 2DEGs for magnetic tunnel junction integration, paving the way to the advent of electrically reconfigurable SOT MRAMS circuits, SOT oscillators, skyrmion and domain-wall-based devices, and magnonic circuits.
Spin-Orbit Readout Using Thin Films of Topological Insulator Sb2Te3 Deposited by Industrial Magnetron Sputtering
Teresi, Salvatore; Sebe, Nicolas; Patterson, Jessy; et al. (2023)
Advanced Functional Materials
Driving a spin-logic circuit requires the production of a large output signal by spin-charge interconversion in spin-orbit readout devices. This should be possible by using topological insulators, which are known for their high spin-charge interconversion efficiency. However, high-quality topological insulators have so far only been obtained on a small scale, or with large scale deposition techniques that are not compatible with conventional industrial deposition processes. The nanopatterning and electrical spin injection into these materials have also proven difficult due to their fragile structure and low spin conductance. The fabrication of a spin-orbit readout device from the topological insulator Sb2Te3 deposited by large-scale industrial magnetron sputtering on SiO2 is presented. Despite a modification of the Sb2Te3 layer structural properties during the device nanofabrication, a sizeable output voltage is measured that can be unambiguously ascribed to a spin-charge interconversion process. The results pave the way for the integration of layered van der Waals materials in spin-logic devices.
Unidirectional orbital magnetoresistance in light-metal-ferromagnet bilayers
Ding, Shilei; Noël, Paul; Krishnaswamy, Gunasheel Kauwtilyaa; et al. (2022)
Physical Review Research
We report the observation of a unidirectional magnetoresistance (UMR) that originates from the nonequilib-rium orbital momentum induced by an electric current in a naturally oxidized Cu/Co bilayer. The orbital UMR scales with the torque efficiency due to the orbital Rashba-Edelstein effect upon changing the Co thickness and temperature, reflecting their common origin. We attribute the UMR to orbital-dependent electron scattering and orbital to spin conversion in the ferromagnetic layer. In contrast to the spin current induced UMR, the magnon contribution to the orbital UMR is absent in thin Co layers, which we ascribe to the lack of coupling between low-energy magnons and orbital current. The magnon contribution to the UMR emerges in Co layers thicker than about 5 nm, which is comparable to the orbital to spin conversion length. Our results provide insight into orbital-to spin-momentum transfer processes relevant for the optimization of spintronic devices based on light metals and orbital transport.

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