-
Interplay of nonrelativistic and relativistic spin splittings in altermagnets
Authors:
Daegeun Jo,
Peter M. Oppeneer,
Mohsen Yarmohammadi
Abstract:
Despite their compensated magnetic moments, altermagnets (AMs) exhibit nonrelativistic spin splitting (NRSS) that offers routes to spintronic functionality without relying on relativistic spin-orbit coupling. However, how NRSS influences relativistic phenomena remains largely unexplored. Here we show that NRSS plays a crucial role in reshaping the Rashba effect in AMs with broken inversion symmetr…
▽ More
Despite their compensated magnetic moments, altermagnets (AMs) exhibit nonrelativistic spin splitting (NRSS) that offers routes to spintronic functionality without relying on relativistic spin-orbit coupling. However, how NRSS influences relativistic phenomena remains largely unexplored. Here we show that NRSS plays a crucial role in reshaping the Rashba effect in AMs with broken inversion symmetry. Using a tight-binding model, we demonstrate that Rashba spin splitting, whose magnitude is typically limited by the strength of spin-orbit coupling, is governed by NRSS energy scales. The resulting Rashba bands combined with NRSS generate anomalously large charge-to-spin conversion with a Néel-vector-tunable spin polarization. First-principles calculations for the noncentrosymmetric AM GdAlSi corroborate the emergence of these effects in a real system. Our results reveal the interplay between nonrelativistic and relativistic effects in AMs and identify them as fertile platforms for spin-orbitronic applications.
△ Less
Submitted 20 August, 2026;
originally announced August 2026.
-
Spin-Orbital Hall Nano-Oscillators using PtCr/NiFe
Authors:
Utkarsh Shashank,
Akash Kumar,
Daegeun Jo,
Thi Ngoc Anh Nguyen,
Jong-Guk Choi,
Sambit Ghosh,
Michal Strach,
Lunjie Zeng,
Andrew B. Yankovich,
Roman Khymyn,
Ahmad A. Awad,
Eva Olsson,
Peter M. Oppeneer,
Johan Åkerman
Abstract:
The orbital Hall effect provides a promising route for generating angular-momentum currents beyond conventional spin Hall physics. PtCr alloys exhibit unusually large current-induced torques, but the contribution of orbital transport and the ability of these torques to sustain coherent nonlinear magnetization dynamics remain unresolved. Here we demonstrate spin-orbital Hall nano-oscillators by exp…
▽ More
The orbital Hall effect provides a promising route for generating angular-momentum currents beyond conventional spin Hall physics. PtCr alloys exhibit unusually large current-induced torques, but the contribution of orbital transport and the ability of these torques to sustain coherent nonlinear magnetization dynamics remain unresolved. Here we demonstrate spin-orbital Hall nano-oscillators by exploiting a homogeneous heavy-metal/light-metal alloy in which orbital Hall currents generated by Cr are converted by Pt into spin currents, producing giant spin-orbit torques. Using PtCr/NiFe heterostructures, the effective torque efficiency increases from ~0.14 in Pt/NiFe to ~0.40 in Pt0.38Cr0.62/NiFe despite substantial Pt dilution, enabling coherent auto-oscillations with the threshold current density reduced from ~ 1.07 x 10^12 to ~ 4.4 x 10^11 A m^-2. First-principles calculations show that Cr alloying suppresses the intrinsic spin Hall conductivity while enhancing the orbital Hall conductivity, and reproduce the observed torque enhancement only when orbital transport is included. Our combined experimental and first-principles results show that alloy engineering enables giant spin-orbit torques through an intrinsic orbital-mediated contribution, enabling coherent auto-oscillations without engineered multilayers and establishing a scalable materials platform for low-power nonlinear spintronic and orbitronic devices.
△ Less
Submitted 9 August, 2026;
originally announced August 2026.
-
Giant perpendicular Edelstein polarization in 2D compensated magnets via bichromatic Floquet driving
Authors:
Mohsen Yarmohammadi,
Daegeun Jo,
Marco Berritta,
Libor Šmejkal,
James K. Freericks,
Peter M. Oppeneer
Abstract:
While unconventional $p$-wave magnets can generate nonrelativistic Edelstein polarizations, spin-group symmetries strictly forbid these responses in unconventional magnets with higher-order harmonics, such as $d$-wave altermagnets. Here, we demonstrate that combining Rashba spin-orbit coupling with bichromatic Floquet driving activates giant perpendicular Edelstein polarizations (PEPs) across 2D a…
▽ More
While unconventional $p$-wave magnets can generate nonrelativistic Edelstein polarizations, spin-group symmetries strictly forbid these responses in unconventional magnets with higher-order harmonics, such as $d$-wave altermagnets. Here, we demonstrate that combining Rashba spin-orbit coupling with bichromatic Floquet driving activates giant perpendicular Edelstein polarizations (PEPs) across 2D altermagnets and broader classes of unconventional spin-polarized magnets -- a feat monochromatic driving cannot achieve. By dynamically breaking two-fold rotational symmetry, the two-frequency drive (including bilinear, bicircular, and circular-linear configurations) induces a stray-field-free in-plane Zeeman-like field that generates orbitally dominated PEPs (0.5--1.5 $μ_{\rm B}$). This massive response is governed by universal selection rules tied to the system's magnetic parity and the second beam's harmonics. These emergent PEPs provide a powerful mechanism for perpendicular memory writing.
△ Less
Submitted 30 June, 2026;
originally announced June 2026.
-
Theoretical study of orbital torque: Dependence on ferromagnet species and nonmagnetic layer thickness
Authors:
Daegeun Jo,
Peter M. Oppeneer
Abstract:
The manipulation of magnetization in ferromagnetic metals (FMs) through orbital torque (OT) has emerged as a promising route for energy-efficient magnetic devices without relying on heavy metals. While Ti and Cu are among the most extensively studied light nonmagnetic metals (NMs) for OT devices, theoretical calculations of the resulting torque have remained limited. Here, we present a systematic…
▽ More
The manipulation of magnetization in ferromagnetic metals (FMs) through orbital torque (OT) has emerged as a promising route for energy-efficient magnetic devices without relying on heavy metals. While Ti and Cu are among the most extensively studied light nonmagnetic metals (NMs) for OT devices, theoretical calculations of the resulting torque have remained limited. Here, we present a systematic and quantitative theoretical study of current-induced torques in Ti/FM and Cu/FM (FM = Co, Ni) bilayers using realistic tight-binding models derived from \textit{ab initio} electronic structures. We find that the torque in Ti/FM is larger for Ni than for Co, but this trend does not necessarily hold in Cu/FM, revealing that the FM dependence of OT is not universal but varies with the orbital current source. Moreover, the dependence of OT on NM thickness clearly indicates its NM bulk origin in both Ti- and Cu-based systems. Notwithstanding, the quantitative characteristics of OT cannot be explained by a simplified picture based on the individual bulk properties of the NM or FM layers. These results provide microscopic insight and practical guidance for designing light-metal-based orbitronic devices.
△ Less
Submitted 11 February, 2026; v1 submitted 14 November, 2025;
originally announced November 2025.
-
Electrotoroidicity: New Paradigm for Transverse Electromagnetic Responses
Authors:
Kai Du,
Daegeun Jo,
Xianghan Xu,
Fei-Ting Huang,
Ming-Hao Lee,
Ming-Wen Chu,
Kefeng Wang,
David Vanderbilt,
Hyun-Woo Lee,
Sang-Wook Cheong
Abstract:
The exploration of transverse electromagnetic responses in solids with broken spatial-inversion (I) and/or time-reversal (T) symmetries has unveiled numerous captivating phenomena, including the (anomalous) Hall effect, Faraday rotations, non-reciprocal directional dichroism, and off-diagonal linear magnetoelectricity, all within the framework of magnetotoroidicity. Here, we introduce a novel clas…
▽ More
The exploration of transverse electromagnetic responses in solids with broken spatial-inversion (I) and/or time-reversal (T) symmetries has unveiled numerous captivating phenomena, including the (anomalous) Hall effect, Faraday rotations, non-reciprocal directional dichroism, and off-diagonal linear magnetoelectricity, all within the framework of magnetotoroidicity. Here, we introduce a novel class of transverse electromagnetic responses originating from electrotoroidicity in ferro-rotational (FR) systems with preserved I and T symmetries, distinct from magnetotoroidicity. We discover a high-order off-diagonal magnetic susceptibility of FR domains and a reduced linear diagonal magnetic susceptibility at FR domain walls in doped ilmenite FeTiO3. The non-trivial "Hall-like" effect of the former corresponds to an anomalous transverse susceptibility in the presence of spontaneous electrotoroidal moments in FR materials. Our findings unveil an emergent type of transverse electromagnetic responses even in I and T symmetry-conserved conditions and illustrate new functionalities of abundant FR materials.
△ Less
Submitted 30 September, 2025;
originally announced October 2025.
-
Itinerant Orbital Hall Effect Mechanism Leading to Large Negative Orbital Torques from Light Metal Vanadium
Authors:
Nikhil Vijayan,
Durgesh Kumar,
Ao Du,
Mirco Sastges,
Lei Gao,
Zijie Xiao,
Dongwook Go,
José Omar Ledesma-Martin,
Hai I. Wang,
Daegeun Jo,
Peter M. Oppeneer,
Rahul Gupta,
Gerhard Jakob,
Sachin Krishnia,
Yuriy Mokrousov,
Mathias Kläui
Abstract:
The orbital Hall effect (OHE) has attracted significant attention for developing energy-efficient electronic devices. However, utilizing it in fast, low-power devices requires an enhanced understanding of underlying extrinsic and intrinsic contributions to OHE at timescales ranging from quasi-static to picoseconds. Here, we investigate OHE in light metal vanadium (V) using a combination of selecte…
▽ More
The orbital Hall effect (OHE) has attracted significant attention for developing energy-efficient electronic devices. However, utilizing it in fast, low-power devices requires an enhanced understanding of underlying extrinsic and intrinsic contributions to OHE at timescales ranging from quasi-static to picoseconds. Here, we investigate OHE in light metal vanadium (V) using a combination of selected measurement schemes, spanning the full frequency range. We observe a negative damping-like torque efficiency from V, opposite to conventional theoretical predictions, with a magnitude that depends on the adjacent ferromagnet, a dependence that indicates orbital effects. These results, with consistent torque efficiencies across all frequencies, corroborate a negative and intrinsic OHE in V with a large effective orbital Hall conductivity of $-(1.44 \pm 0.34)\,\frac{\hbar}{2e}\,\times 10^{5}\,Ω^{-1}\,\mathrm{m}^{-1}$ and a long orbital diffusion length of $(15.0 \pm 2.5)\,\mathrm{nm}$. To explain the observed OHE, we develop a theoretical model incorporating both local and itinerant circulation contributions to OHE. The model agrees excellently with the experimental results, demonstrating that itinerant contributions are essential for a complete physical understanding of intrinsic OHE. Our consistent experimental and theoretical data highlight the importance of itinerant contributions governing the fundamental understanding of intrinsic OHE and the large effects found open pathways for energy-efficient orbitronic devices.
△ Less
Submitted 17 December, 2025; v1 submitted 22 August, 2025;
originally announced August 2025.
-
Unconventional orbital currents and torques due to ferro-rotational orbital textures
Authors:
Daegeun Jo,
Peter M. Oppeneer
Abstract:
Orbital angular momentum transport has emerged as a promising route for manipulating magnetic devices, yet its generation has largely relied on the conventional orbital Hall effect. Here, we show that ferro-rotational order enables the electrical generation of unconventional orbital currents. These orbital currents represent the orbital counterparts of spin currents due to ferromagnetic order, but…
▽ More
Orbital angular momentum transport has emerged as a promising route for manipulating magnetic devices, yet its generation has largely relied on the conventional orbital Hall effect. Here, we show that ferro-rotational order enables the electrical generation of unconventional orbital currents. These orbital currents represent the orbital counterparts of spin currents due to ferromagnetic order, but arise from rotation-induced symmetry breaking rather than time-reversal symmetry breaking or spin-orbit coupling. Using tight-binding models, we identify the underlying intrinsic, nonrelativistic mechanism categorized as an electric hexadecapole moment and corroborate our findings with first-principles calculations for the ferro-rotational material TiAu$_4$. We further show that these rotation-induced orbital currents lead to surface orbital accumulation and unconventional orbital torque in a ferro-rotational/ferromagnetic metallic bilayer, allowing deterministic field-free switching. Our findings unveil a novel pathway for generating orbital currents beyond the conventional orbital Hall effect, broadening the landscape of orbitronics research to include novel ferroic materials and higher-order electric multipoles.
△ Less
Submitted 11 February, 2026; v1 submitted 7 May, 2025;
originally announced May 2025.
-
Weak Ferromagnetism in Altermagnets from Alternating $g$-Tensor Anisotropy
Authors:
Daegeun Jo,
Dongwook Go,
Yuriy Mokrousov,
Peter M. Oppeneer,
Sang-Wook Cheong,
Hyun-Woo Lee
Abstract:
Altermagnets are magnetic materials with antiferromagnetic spin ordering but exhibit ferromagnetic properties. Understanding the microscopic origin of the latter is a central problem. Ferromagnet-like properties such as the anomalous Hall effect are linked with weak ferromagnetism, whose microscopic origin in altermagnets remains unclear however. We show theoretically that the alternating $g$-tens…
▽ More
Altermagnets are magnetic materials with antiferromagnetic spin ordering but exhibit ferromagnetic properties. Understanding the microscopic origin of the latter is a central problem. Ferromagnet-like properties such as the anomalous Hall effect are linked with weak ferromagnetism, whose microscopic origin in altermagnets remains unclear however. We show theoretically that the alternating $g$-tensor anisotropy in altermagnets can induce weak ferromagnetism even when the Dzyaloshinskii-Moriya interaction is forbidden. We demonstrate this mechanism to explain weak ferromagnetism for both collinear and noncollinear spin altermagnets. Our findings provide new insights into the origin of weak ferromagnetism and suggest orbital-based ways for manipulating magnetic configurations in altermagnets.
△ Less
Submitted 18 March, 2025; v1 submitted 22 October, 2024;
originally announced October 2024.
-
Harnessing magnetic octupole Hall effect to induce torque in altermagnets
Authors:
Seungyun Han,
Daegeun Jo,
Insu Baek,
Peter M. Oppeneer,
Hyun-Woo Lee
Abstract:
d-wave altermagnets have magnetic octupoles as their order parameters [Phys. Rev. X 14, 011019 (2024)]. We theoretically show that magnetic octupoles injected from outside generate torque on the d-wave altermagnets. The injection can be achieved by the magnetic octupole Hall effect in an adjacent layer. We calculate the magnetic octupole Hall conductivity of the heavy metal Pt and find a sizable v…
▽ More
d-wave altermagnets have magnetic octupoles as their order parameters [Phys. Rev. X 14, 011019 (2024)]. We theoretically show that magnetic octupoles injected from outside generate torque on the d-wave altermagnets. The injection can be achieved by the magnetic octupole Hall effect in an adjacent layer. We calculate the magnetic octupole Hall conductivity of the heavy metal Pt and find a sizable value comparable to its spin Hall conductivity. Our work generalizes the spin Hall phenomenology (generation by heavy metals and detection by torque in ferromagnets) to the magnetic octupole Hall phenomenology (generation by heavy metals and detection by torque in altermagnets), which can be utilized to electrically control magnetic configurations of altermagnets.
△ Less
Submitted 22 September, 2024;
originally announced September 2024.
-
Direct observation of nanometer-scale orbital angular momentum accumulation
Authors:
Juan Carlos Idrobo,
Ján Rusz,
Gopal Datt,
Daegeun Jo,
Sanaz Alikhah,
David Muradas,
Ulrich Noumbe,
M. Venkata Kamalakar,
Peter M. Oppeneer
Abstract:
Conversion of charge to orbital angular momentum through the orbital Hall effect (OHE) holds transformative potential for the development of orbital-based electronics, however, it is challenging to directly observe the electrically generated orbital accumulation. Here, we detect the OHE by directly quantifying the orbital accumulation along the edges of a titanium thin film using a scanning transm…
▽ More
Conversion of charge to orbital angular momentum through the orbital Hall effect (OHE) holds transformative potential for the development of orbital-based electronics, however, it is challenging to directly observe the electrically generated orbital accumulation. Here, we detect the OHE by directly quantifying the orbital accumulation along the edges of a titanium thin film using a scanning transmission electron microscope. We measure the Ti L-edge using electron energy-loss spectroscopy with nanometer resolution and find a sizable orbital accumulation at the sample's outer perimeters, consistent with all signatures expected for the OHE, and determine an orbital diffusion length $\ell_o \approx 7.3$ nm. Our data points to a surprising dependence of the orbital diffusion length on the nano-structural morphology.
△ Less
Submitted 14 March, 2024;
originally announced March 2024.
-
Oxide layer dependent orbital torque efficiency in ferromagnet/Cu/Oxide heterostructures
Authors:
Junyeon Kim,
Jun Uzuhashi,
Masafumi Horio,
Tomoaki Senoo,
Dongwook Go,
Daegeun Jo,
Toshihide Sumi,
Tetsuya Wada,
Iwao Matsuda,
Tadakatsu Ohkubo,
Seiji Mitani,
Hyun-Woo Lee,
YoshiChika Otani
Abstract:
The utilization of orbital transport provides a versatile and efficient spin manipulation mechanism. As interest in orbital-mediated spin manipulation grows, we face a new issue to identify the underlying physics that determines the efficiency of orbital torque (OT). In this study, we systematically investigate the variation of OT governed by orbital Rashba-Edelstein effect at the Cu/Oxide interfa…
▽ More
The utilization of orbital transport provides a versatile and efficient spin manipulation mechanism. As interest in orbital-mediated spin manipulation grows, we face a new issue to identify the underlying physics that determines the efficiency of orbital torque (OT). In this study, we systematically investigate the variation of OT governed by orbital Rashba-Edelstein effect at the Cu/Oxide interface, as we change the Oxide material. We find that OT varies by a factor of ~2, depending on the Oxide. Our results suggest that the active electronic interatomic interaction (hopping) between Cu and oxygen atom is critical in determining OT. This also gives us an idea of what type of material factors is critical in forming a chiral orbital Rashba texture at the Cu/Oxide interface.
△ Less
Submitted 19 July, 2023;
originally announced July 2023.
-
Observation of long-range orbital transport and giant orbital torque
Authors:
Hiroki Hayashi,
Daegeun Jo,
Dongwook Go,
Tenghua Gao,
Satoshi Haku,
Yuriy Mokrousov,
Hyun-Woo Lee,
Kazuya Ando
Abstract:
Modern spintronics relies on the generation of spin currents through spin-orbit coupling. The spin-current generation has been believed to be triggered by current-induced orbital dynamics, which governs the angular momentum transfer from the lattice to the electrons in solids. The fundamental role of the orbital response in the angular momentum dynamics suggests the importance of the orbital count…
▽ More
Modern spintronics relies on the generation of spin currents through spin-orbit coupling. The spin-current generation has been believed to be triggered by current-induced orbital dynamics, which governs the angular momentum transfer from the lattice to the electrons in solids. The fundamental role of the orbital response in the angular momentum dynamics suggests the importance of the orbital counterpart of spin currents: orbital currents. However, evidence for its existence has been elusive. Here, we demonstrate the generation of giant orbital currents and uncover fundamental features of the orbital response. We experimentally and theoretically show that orbital currents propagate over longer distances than spin currents by more than an order of magnitude in a ferromagnet and nonmagnets. Furthermore, we find that the orbital current enables electric manipulation of magnetization with efficiencies significantly higher than the spin counterpart. These findings open the door to orbitronics that exploits orbital transport and spin-orbital coupled dynamics in solid-state devices.
△ Less
Submitted 6 February, 2023; v1 submitted 28 February, 2022;
originally announced February 2022.
-
Observation of the orbital Hall effect in a light metal Ti
Authors:
Young-Gwan Choi,
Daegeun Jo,
Kyung-Hun Ko,
Dongwook Go,
Kyung-Han Kim,
Hee Gyum Park,
Changyoung Kim,
Byoung-Chul Min,
Gyung-Min Choi,
Hyun-Woo Lee
Abstract:
The orbital angular momentum is a core ingredient of orbital magnetism, spin Hall effect, giant Rashba spin splitting, orbital Edelstein effect, and spin-orbit torque. However, its experimental detection is tricky. In particular, direct detection of the orbital Hall effect remains elusive despite its importance for electrical control of magnetic nanodevices. Here we report the direct observation o…
▽ More
The orbital angular momentum is a core ingredient of orbital magnetism, spin Hall effect, giant Rashba spin splitting, orbital Edelstein effect, and spin-orbit torque. However, its experimental detection is tricky. In particular, direct detection of the orbital Hall effect remains elusive despite its importance for electrical control of magnetic nanodevices. Here we report the direct observation of the orbital Hall effect in a light metal Ti. The Kerr rotation by the accumulated orbital magnetic moment is measured at Ti surfaces, whose result agrees with theoretical calculations semiquantitatively and is supported by the orbital torque measurement in Ti-based magnetic heterostructures. The results confirm the electron orbital angular momentum as an essential dynamic degree of freedom, which may provide a novel mechanism for the electric control of magnetism. The results may also deepen the understanding of spin, valley, phonon, and magnon dynamics coupled with orbital dynamics.
△ Less
Submitted 30 September, 2021;
originally announced September 2021.
-
Gigantic current control of coercive field and magnetic memory based on nm-thin ferromagnetic van der Waals Fe3GeTe2
Authors:
Kaixuan Zhang,
Seungyun Han,
Youjin Lee,
Matthew J. Coak,
Junghyun Kim,
Inho Hwang,
Suhan Son,
Jeacheol Shin,
Mijin Lim,
Daegeun Jo,
Kyoo Kim,
Dohun Kim,
Hyun-Woo Lee,
Je-Geun Park
Abstract:
Controlling magnetic states by a small current is essential for the next-generation of energy-efficient spintronic devices. However, it invariably requires considerable energy to change a magnetic ground state of intrinsically quantum nature governed by fundamental Hamiltonian, once stabilized below a phase transition temperature. We report that surprisingly an in-plane current can tune the magnet…
▽ More
Controlling magnetic states by a small current is essential for the next-generation of energy-efficient spintronic devices. However, it invariably requires considerable energy to change a magnetic ground state of intrinsically quantum nature governed by fundamental Hamiltonian, once stabilized below a phase transition temperature. We report that surprisingly an in-plane current can tune the magnetic state of nm-thin van der Waals ferromagnet Fe3GeTe2 from a hard magnetic state to a soft magnetic state. It is the direct demonstration of the current-induced substantial reduction of the coercive field. This surprising finding is possible because the in-plane current produces a highly unusual type of gigantic spin-orbit torque for Fe3GeTe2. And we further demonstrate a working model of a new nonvolatile magnetic memory based on the principle of our discovery in Fe3GeTe2, controlled by a tiny current. Our findings open up a new window of exciting opportunities for magnetic van der Waals materials with potentially huge impacts on the future development of spintronic and magnetic memory.
△ Less
Submitted 7 January, 2025; v1 submitted 27 August, 2021;
originally announced August 2021.
-
Orbitronics: Orbital Currents in Solids
Authors:
Dongwook Go,
Daegeun Jo,
Hyun-Woo Lee,
Mathias Kläui,
Yuriy Mokrousov
Abstract:
In solids, electronic Bloch states are formed by atomic orbitals. While it is natural to expect that orbital composition and information about Bloch states can be manipulated and transported, in analogy to the spin degree of freedom extensively studied in past decades, it has been assumed that orbital quenching by the crystal field prevents significant dynamics of orbital degrees of freedom. Howev…
▽ More
In solids, electronic Bloch states are formed by atomic orbitals. While it is natural to expect that orbital composition and information about Bloch states can be manipulated and transported, in analogy to the spin degree of freedom extensively studied in past decades, it has been assumed that orbital quenching by the crystal field prevents significant dynamics of orbital degrees of freedom. However, recent studies reveal that an orbital current, given by the flow of electrons with a finite orbital angular momentum, can be electrically generated and transported in wide classes of materials despite the effect of orbital quenching in the ground state. Orbital currents also play a fundamental role in the mechanisms of other transport phenomena such as spin Hall effect and valley Hall effect. Most importantly, it has been proposed that orbital currents can be used to induce magnetization dynamics, which is one of the most pivotal and explored aspects of magnetism. Here, we give an overview of recent progress and the current status of research on orbital currents. We review proposed physical mechanisms for generating orbital currents and discuss candidate materials where orbital currents are manifest. We review recent experiments on orbital current generation and transport and discuss various experimental methods to quantify this elusive object at the heart of $orbitronics$ $-$ an area which exploits the orbital degree of freedom as an information carrier in solid-state devices.
△ Less
Submitted 18 July, 2021;
originally announced July 2021.
-
Long-Range Orbital Magnetoelectric Torque in Ferromagnets
Authors:
Dongwook Go,
Daegeun Jo,
Kyoung-Whan Kim,
Soogil Lee,
Min-Gu Kang,
Byong-Guk Park,
Stefan Blügel,
Hyun-Woo Lee,
Yuriy Mokrousov
Abstract:
While it is often assumed that the orbital response is suppressed and short-ranged due to strong crystal field potential and orbital quenching, we show that the orbital magnetoelectric response can be remarkably long-ranged in ferromagnets. In a bilayer consisting of a nonmagnet and a ferromagnet, spin injection from the interface results in spin accumulation and torque in the ferromagnet, which r…
▽ More
While it is often assumed that the orbital response is suppressed and short-ranged due to strong crystal field potential and orbital quenching, we show that the orbital magnetoelectric response can be remarkably long-ranged in ferromagnets. In a bilayer consisting of a nonmagnet and a ferromagnet, spin injection from the interface results in spin accumulation and torque in the ferromagnet, which rapidly oscillate and decay by spin dephasing. In contrast, we find that even when an external electric field is applied only on the nonmagnet, we find substantially long-ranged orbital magnetoelectric response in the FM, which can go far beyond the spin dephasing length. This unusual feature is attributed to nearly degenerate orbital characters imposed by the crystal symmetry, which form hotspots for the intrinsic orbital response. Because only the states near the hotspots contribute dominantly, the induced orbital angular momentum does not exhibit destructive interference among states with different momentum as in the case of the spin dephasing. This gives rise to a distinct type of orbital torque on the magnetization, increasing with the thickness of the ferromagnet. Such behavior may serve as critical long-sought evidence of orbital transport to be directly tested in experiments. Our findings open the possibility of using long-range orbital magnetoelectric effect in orbitronic device applications.
△ Less
Submitted 16 May, 2022; v1 submitted 15 June, 2021;
originally announced June 2021.
-
Orbital Rashba effect in surface oxidized Cu film
Authors:
Dongwook Go,
Daegeun Jo,
Tenghua Gao,
Kazuya Ando,
Stefan Blügel,
Hyun-Woo Lee,
Yuriy Mokrousov
Abstract:
Recent experimental observation of unexpectedly large current-induced spin-orbit torque in surface oxidized Cu on top of a ferromagnet suggested a possible role of the orbital Rashba effect (ORE). With this motivation, we investigate the ORE from first principles by considering an oxygen monolayer on top of a Cu(111) film. We show that surface oxidization of Cu film leads to gigantic enhancement o…
▽ More
Recent experimental observation of unexpectedly large current-induced spin-orbit torque in surface oxidized Cu on top of a ferromagnet suggested a possible role of the orbital Rashba effect (ORE). With this motivation, we investigate the ORE from first principles by considering an oxygen monolayer on top of a Cu(111) film. We show that surface oxidization of Cu film leads to gigantic enhancement of the ORE for states near the Fermi surface. The resulting chiral orbital texture in the momentum space is exceptionally strong, reaching $\sim 0.5\hbar$ in magnitude. We find that resonant hybridization between O $p$-states and Cu $d$-states is responsible for the emergence of the ORE. We demonstrate that application of an external electric field generates huge orbital Hall current, which is an order of magnitude larger than the spin Hall current found in heavy metals. This implies that "orbital torque" mechanism may be significant in surface oxidized Cu/ferromagnet structures. It also encourages experimental verification of the orbital texture in surface oxidized Cu through optical measurements such as angle-resolved photoemission spectroscopy.
△ Less
Submitted 17 November, 2020;
originally announced November 2020.
-
Non-trivial charge-to-spin conversion in ferromagnetic metal/Cu/Al2O3 by orbital transport
Authors:
Junyeon Kim,
Dongwook Go,
Hanshen Tsai,
Daegeun Jo,
Kouta Kondou,
Hyun-Woo Lee,
YoshiChika Otani
Abstract:
Efficient spin/charge interconversion is desired to develop innovative spin-based devices. So far, the interconversion has been performed by using heavy atomic elements, strong spin-orbit interaction of which realizes the interconversion through the spin Hall effect and the Edelstein effect. We demonstrate highly efficient charge-to-spin conversion in a ferromagnetic metal/Cu/Al2O3 trilayers, whic…
▽ More
Efficient spin/charge interconversion is desired to develop innovative spin-based devices. So far, the interconversion has been performed by using heavy atomic elements, strong spin-orbit interaction of which realizes the interconversion through the spin Hall effect and the Edelstein effect. We demonstrate highly efficient charge-to-spin conversion in a ferromagnetic metal/Cu/Al2O3 trilayers, which do not contain any heavy element. The resulting spin torque efficiency is higher than those of conventional spin Hall and Rashba systems consisting of heavy elements such as Pt and Bi. Our experimental results qualitatively deviate from typical behaviors arising from spin transport. However, they are surprisingly consistent with the behaviors arising from the orbital transport. Our results thus demonstrate a new direction for efficient charge-to-spin conversion through the orbital transport.
△ Less
Submitted 25 February, 2020; v1 submitted 3 February, 2020;
originally announced February 2020.
-
Gigantic intrinsic orbital Hall effects in weakly spin-orbit coupled metals
Authors:
Daegeun Jo,
Dongwook Go,
Hyun-Woo Lee
Abstract:
A recent paper [Go $\textit{et al}$., Phys. Rev. Lett. $\textbf{121}$, 086602 (2018)] proposed that the intrinsic orbital Hall effect (OHE) can emerge from momentum-space orbital texture in centrosymmetric materials. In searching for real materials with strong OHE, we investigate the intrinsic OHE in metals with small spin-orbit coupling (SOC) in face-centered cubic and body-centered cubic structu…
▽ More
A recent paper [Go $\textit{et al}$., Phys. Rev. Lett. $\textbf{121}$, 086602 (2018)] proposed that the intrinsic orbital Hall effect (OHE) can emerge from momentum-space orbital texture in centrosymmetric materials. In searching for real materials with strong OHE, we investigate the intrinsic OHE in metals with small spin-orbit coupling (SOC) in face-centered cubic and body-centered cubic structures (Li, Al, V, Cr, Mn, Ni, and Cu). We find that orbital Hall conductivities (OHCs) in these materials are gigantic $\sim 10^3-10^4\ (\hbar/e)(Ω\cdot\mathrm{cm})^{-1}$, which are comparable or larger than spin Hall conductivity (SHC) of Pt. Although SHCs in these materials are smaller than OHCs due to small SOC, we found that SHCs are still sizable and the spin Hall angles may be of the order of 0.1. We discuss implications on recent spin-charge interconversion experiments on materials having small SOC.
△ Less
Submitted 15 November, 2018; v1 submitted 16 August, 2018;
originally announced August 2018.
-
Intrinsic Spin and Orbital Hall Effects from Orbital Texture
Authors:
Dongwook Go,
Daegeun Jo,
Changyoung Kim,
Hyun-Woo Lee
Abstract:
We show theoretically that both intrinsic spin Hall effect (SHE) and orbital Hall effect (OHE) can arise in centrosymmetric systems through momentum-space orbital texture, which is ubiquitous even in centrosymmetric systems unlike spin texture. OHE occurs even without spin-orbit coupling (SOC) and is converted into SHE through SOC. The resulting spin Hall conductivity is large (comparable to that…
▽ More
We show theoretically that both intrinsic spin Hall effect (SHE) and orbital Hall effect (OHE) can arise in centrosymmetric systems through momentum-space orbital texture, which is ubiquitous even in centrosymmetric systems unlike spin texture. OHE occurs even without spin-orbit coupling (SOC) and is converted into SHE through SOC. The resulting spin Hall conductivity is large (comparable to that of Pt) but depends on the SOC strength in a nonmonotonic way. This mechanism is stable against orbital quenching. This work suggests a path for an ongoing search for materials with stronger SHE. It also calls for experimental efforts to probe orbital degrees of freedom in OHE and SHE. Possible ways for experimental detection are briefly discussed.
△ Less
Submitted 12 July, 2018; v1 submitted 5 April, 2018;
originally announced April 2018.
-
Synchronous imaging for rapid visualization of complex vibration profiles in electromechanical microresonators
Authors:
Yoav Linzon,
Daniel J. Joe,
Slava Krylov,
Bojan Ilic,
Juraj Topolancik,
Jeevak M. Parpia,
Halrod G. Craighead
Abstract:
Synchronous imaging is used in dynamic space-domain vibration profile studies of capacitively driven, thin n+ doped poly-silicon microbridges oscillating at rf frequencies. Fast and high-resolution actuation profile measurements of micromachined resonators are useful when significant device nonlinearities are present. For example, bridges under compressive stress near the critical Euler value ofte…
▽ More
Synchronous imaging is used in dynamic space-domain vibration profile studies of capacitively driven, thin n+ doped poly-silicon microbridges oscillating at rf frequencies. Fast and high-resolution actuation profile measurements of micromachined resonators are useful when significant device nonlinearities are present. For example, bridges under compressive stress near the critical Euler value often reveal complex dynamics stemming from a state close to the onset of buckling. This leads to enhanced sensitivity of the vibration modes to external conditions, such as pressure, temperatures, and chemical composition, the global behavior of which is conveniently evaluated using synchronous imaging combined with spectral measurements. We performed an experimental study of the effects of high drive amplitude and ambient pressure on the resonant vibration profiles in electrically-driven microbridges near critical buckling. Numerical analysis of electrostatically driven post-buckled microbridges supports the richness of complex vibration dynamics that are possible in such micro-electromechanical devices.
△ Less
Submitted 31 October, 2011;
originally announced October 2011.