arXiv subjects

Hyun-Woo Lee

Publications and source records attributed to Hyun-Woo Lee.

At least 37 records · Page 2Linked to original sources

arXiv2025

Orbital Pumping Incorporating Both Orbital Angular Momentum and Position

We develop a theory of adiabatic orbital pumping, highlighting qualitative differences from spin pumping. An oscillating magnetic field pumps not only orbital angular momentum current but also orbital angular position current. The latter, which has no spin counterpart, underscores the incompleteness of existing orbital torque theories. Importantly, both types of orbital currents can be detected as transverse electric voltages, which contain considerable second harmonic components unlike in spin pumping. Moreover, orbital currents can be pumped by lattice dynamics that carry phonon angular momentum, implying that orbital currents can, in turn, induce phonon angular momentum. Our work open up new possibilities for generating orbital currents and provides a broader understanding of the interplay between spin, orbital, and phonon dynamics.

cond-mat.mes-hall↗

arXiv2025

Novel magnetic-field-free switching behavior in vdW-magnet/oxide heterostructure

Magnetization switching by charge current without a magnetic field is essential for device applications and information technology. It generally requires a current-induced out-of-plane spin polarization beyond the capability of conventional ferromagnet/heavy-metal systems, where the current-induced spin polarization aligns in-plane orthogonal to the in-plane charge current and out-of-plane spin current. Here, we demonstrate a new approach for magnetic-field-free switching by fabricating a van-der-Waals magnet and oxide Fe3GeTe2/SrTiO3 heterostructure. This new magnetic-field-free switching is possible because the current-driven accumulated spins at the Rashba interface precess around an emergent interface magnetism, eventually producing an ultimate out-of-plane spin polarization. This interpretation is further confirmed by the switching polarity change controlled by the in-plane initialization magnetic fields with clear hysteresis. We successfully combined van-der-Waals magnet and oxide for the first time, especially taking advantage of spin-orbit torque on the SrTiO3 oxide. This allows us to establish a new way of magnetic field-free switching. Our work demonstrates an unusual perpendicular switching application of large spin Hall angle materials and precession of accumulated spins, and in doing so, opens up a new field and opportunities for van-der-Waals magnets and oxide spintronics.

cond-mat.mtrl-sci↗

arXiv2025

Highly efficient nonvolatile magnetization switching and multi-level states by current in single van der Waals topological ferromagnet Fe3GeTe2

Robust multi-level spin memory with the ability to write information electrically is a long-sought capability in spintronics, with great promise for applications. Here we achieve nonvolatile and highly energy-efficient magnetization switching in a single-material device formed of van-der-Waals topological ferromagnet Fe3GeTe2, whose magnetic information can be readily controlled by a tiny current. Furthermore, the switching current density and power dissipation are about 400 and 4000 times smaller than those of the existing spin-orbit-torque magnetic random access memory based on conventional magnet/heavy-metal systems. Most importantly, we also demonstrate multi-level states, switched by electrical current, which can dramatically enhance the information capacity density and reduce computing costs. Thus, our observations combine both high energy efficiency and large information capacity density in one device, showcasing the potential applications of the emerging field of van-der-Waals magnets in the field of spin memory and spintronics.

cond-mat.mtrl-sci↗

arXiv2025

Gigantic current control of coercive field and magnetic memory based on nm-thin ferromagnetic van der Waals Fe3GeTe2

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.

cond-mat.mtrl-sci↗

arXiv2024

Orbital Edelstein effect of electronic itinerant orbital motion at edges

In the study of orbital angular momentum (OAM), the focus has been predominantly on the intra-atomic contribution. However, recent research has begun to shift towards exploring the inter-atomic contribution to OAM dynamics. In this paper, we investigate the orbital Edelstein effect (OEE) arising from the inter-atomic OAM at the edges. We explore the OAM texture within edge states and unveil the OAM accumulation at the edges using several lattice models based on the $s$ orbital. By comparing slabs with differently shaped edges, we not only clarify the role of electron wiggling motion in shaping OAM texture but also highlight the absence of bulk-boundary correspondence in the accumulation process. The topological insulator and higher-order topological insulator models further confirm these findings and provide evidence for the relationship between the higher-order topology and the OEE. Our study advances the comprehension of orbital physics and extends its scope to higher-order topological insulators.

cond-mat.mes-hall↗

arXiv2024

Harnessing magnetic octupole Hall effect to induce torque in altermagnets

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.

cond-mat.mes-hall↗

arXiv2024

Dyakonov-Perel-like Orbital and Spin Relaxations in Centrosymmetric Systems

The Dyakonov-Perel (DP) mechanism of spin relaxation has long been considered irrelevant in centrosymmetric systems since it was developed originally for non-centrosymmetric ones. We investigate whether this conventional understanding extends to the realm of orbital relaxation, which has recently attracted significant attention. Surprisingly, we find that orbital relaxation in centrosymmetric systems exhibits the DP-like behavior in the weak scattering regime. Moreover, the DP-like orbital relaxation can make the spin relaxation in centrosymmetric systems DP-like through the spin-orbit coupling. We also find that the DP-like orbital and spin relaxations are anisotropic even in materials with high crystal symmetry (such as face-centered cubic structure) and may depend on the orbital and spin nature of electron wavefunctions.

cond-mat.mes-hall↗

arXiv2024

First-principles calculation of orbital Hall effect by Wannier interpolation: Role of orbital dependence of the anomalous position

The position operator in a Bloch representation acquires a gauge correction in the momentum space on top of the canonical position, which is called the anomalous position. We show that the anomalous position is generally orbital-dependent and thus plays a crucial role in the description of the intrinsic orbital Hall effect in terms of Wannier basis. We demonstrate this from the first-principles calculation of orbital Hall conductivities of transition metals by Wannier interpolation. Our results show that consistent treatment of the velocity operator by adding the additional term originating from the anomalous position predicts the orbital Hall conductivities different from those obtained by considering only the group velocity. We find the difference is crucial in several metals. For example, we predict the negative sign of the orbital Hall conductivities for elements in the groups X and XI such as Cu, Ag, Au, and Pd, for which the previous studies predicted the positive sign. Our work suggests the importance of consistently describing the spatial dependence of basis functions by first-principles methods as it is fundamentally missing in the tight-binding approximation.

cond-mat.mes-hall↗

arXiv2024

Nonlinear Orbital and Spin Edelstein Effect in Centrosymmetric Metals

Nonlinear spintronics combines nonlinear dynamics with spintronics, opening up new possibilities beyond linear responses. A recent theoretical work [Xiao et al., Phys. Rev. Lett. 130, 166302 (2023)] predicts the nonlinear generation of spin density [nonlinear spin Edelstein effect (NSEE)] in centrosymmetric metals based on symmetry analysis combined with first principle calculation. However, its microscopic mechanism is limited to a specific set of materials with local inversion symmetry breaking and is not applicable to general materials. This paper focuses on the fundamental role of orbital degrees of freedom for the nonlinear generation in centrosymmetric systems. Using a combination of tight-binding model and density functional theory calculations, we demonstrate that nonlinear orbital density can arise independently of spin-orbit coupling. In contrast, spin density follows through spin-orbit coupling. We further elucidate the microscopic mechanism responsible for this phenomenon, which involves the NSEE induced by electric-field-induced orbital Rashba texture. In addition, we also explore the potential applications of the nonlinear orbital and spin Edelstein effect for field-free switching of magnetization.

cond-mat.mtrl-sci↗

arXiv2023

Cavity magnonics with easy-axis ferromagnet: Critically enhanced magnon squeezing and light-matter interaction

Generating and probing the magnon squeezing is an important challenge in the field of quantum magnonics. In this work, we propose a cavity magnonics setup with an easy-axis ferromagnet to address this challenge. To this end, we first establish a mechanism for the generation of magnon squeezing in the easy-axis ferromagnet and show that the magnon squeezing can be critically enhanced by tuning an external magnetic field near the Ising phase transition point. When the magnet is coupled to the cavity field, the effective cavity-magnon interaction becomes proportional to the magnon squeezing, allowing one to enhance the cavity-magnon coupling strength using a static field. We demonstrate that the magnon squeezing can be probed by measuring the frequency shift of the cavity field. Moreover, a magnonic superradiant phase transition can be observed in our setup by tuning the static magnetic field, overcoming the challenge that the magnetic interaction between the cavity and the magnet is typically too weak to drive the superradiant transition. Our work paves the way to develop unique capabilities of cavity magnonics that goes beyond the conventional cavity QED physics by harnessing the intrinsic property of a magnet.

cond-mat.mes-hall↗

arXiv2023

Topological magnon-photon interaction for cavity magnonics

The study of cavity magnonics and topological insulators has made significant advances over the past decade, however the possibility of combining the two fields is still unexplored. Here, we explore such connection by investigating hybrid cavity systems that incorporate both a ferromagnet and a topological insulator. We find that electrons in the topological surface state efficiently mediate the effective electric dipole coupling between the spin of the ferromagnet and the electric field of the cavity, in contrast with the conventional cavity magnonics theory based on magnetic dipole coupling. We refer to this coupling as topological magnon-photon interaction, estimating it one order of magnitude stronger than the conventional magnon-photon coupling, and showing that its sign can be manipulated. We discuss the potential of our proposed device to allow for scaling down and controlling the cavity system using electronics. Our results provide solid ground for exploring the functionalities enabled by merging cavity magnonics with topological insulators.

cond-mat.mes-hall↗

arXiv2023

Oxide layer dependent orbital torque efficiency in ferromagnet/Cu/Oxide heterostructures

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.

cond-mat.mtrl-sci↗

arXiv2023

Non-divergent Chiral Charge Pumping in Weyl Semimetal

Recent studies suggest that the nonlinear transport properties in Weyl semimetal may be a measurable consequence of its chiral anomaly. Nonlinear responses in transport are estimated to be substantial, because in real materials such as TaAs or Bi$_{1-x}$Sb$_x$, the Fermi level resides near the Weyl nodes where the chiral charge pumping is said to diverge. However, this work presents semiclassical Boltzmann analysis that indicates that the chiral charge pumping is non-divergent even at the zero-temperature limit. We demonstrate that the divergence in common semiclassical calculation scheme is not a problem of the scheme itself, but occurs because a commonly-used approximation of the change in particle number breaks down near the Weyl nodes. Our result suggests the possibility that the nonlinear properties in WSMs can be overestimated, and provides the validity condition for the conventional approximation. We also show the distinct Fermi level dependencies of the chiral magnetic effect and the negative longitudinal magnetoresistance, as a consequence of non-diverging chiral charge pumping.

cond-mat.mes-hall↗

arXiv2023

Robustness of the intrinsic anomalous Hall effect in Fe3GeTe2 to a uniaxial strain

Fe3GeTe2 (FGT), a ferromagnetic van der Waals topological nodal line semimetal, has recently been studied. Using first-principles calculations and symmetry analysis, we investigate the effect of a uniaxial tensile strain on the nodal line and the resultant intrinsic anomalous Hall effect (AHE). Our results reveal their robustness to the in-plane strain. Moreover, the intrinsic AHE remains robust even for artificial adjustment of the atomic positions introduced to break the crystalline symmetries of FGT. When the spin-orbit coupling is absent, the nodal line degeneracy remains intact as long as the inversion symmetry or the two-fold screw symmetry is maintained, which reveal that the nodal line may emerge much more easily than previously predicted. This strong robustness is surprising and disagrees with the previous experimental report [Y. Wang et al., Adv. Mater. 32, 2004533 (2020)], which reports that a uniaxial strain of less than 1 % of the in-plane lattice constant can double the anomalous Hall resistance. This discrepancy implies that the present understanding of the AHE in FGT is incomplete. The possible origins of this discrepancy are discussed.

cond-mat.mtrl-sci↗

arXiv2023

Intrinsic Magnon Orbital Hall Effect in Honeycomb Antiferromagnets

We theoretically investigate the transport of magnon orbitals in a honeycomb antiferromagnet. We find that the magnon orbital Berry curvature is finite even without spin-orbit coupling and thus the resultant magnon orbital Hall effect is an intrinsic property of the honeycomb antiferromagnet rooted only in the exchange interaction and the lattice structure. Due to the intrinsic nature of the magnon orbital Hall effect, the magnon orbital Nernst conductivity is estimated to be orders of magnitude larger than the predicted values of the magnon spin Nernst conductivity that requires finite spin-orbit coupling. For the experimental detection of the predicted magnon orbital Hall effect, we invoke the magnetoelectric effect that couples the magnon orbital and the electric polarization, which allows us to detect the magnon orbital accumulation through the local voltage measurement. Our results pave a way for a deeper understanding of the topological transport of the magnon orbitals and also its utilization for low-power magnon-based orbitronics, namely magnon orbitronics.

cond-mat.mes-hall↗

arXiv2023

Observation of long-range orbital transport and giant orbital torque

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.

cond-mat.mes-hall↗

arXiv2022

Microscopic study of orbital textures

Many interesting spin and orbital transport phenomena originate from orbital textures, referring to $\vec{k}$-dependent orbital states. Most of previous works are based on symmetry analysis to model the orbital texture and analyze its consequences. However the microscopic origins of orbital texture and its strength are largely unexplored. In this work, we derive the orbital texture Hamiltonians from microscopic tight-binding models for various situations. To form an orbital texture, $\vec{k}$-dependent hybridization of orbital states are necessary. We reveal two microscopic mechanisms for the hybridization: (i) lattice structure effect and (ii) mediation by other orbital states. By considering the orbital hybridization, we not only reproduce the orbital Hamiltonian obtained by the symmetry analysis but also reveal previously unreported orbital textures like orbital Dresselhaus texture and anisotropic orbital texture. The orbital Hamiltonians obtained here would be useful for analyzing the orbital physics and designing the materials suitable for spin-orbitronic applications. We show that our theory also provides useful microscopic insight into physical phenomena such as the orbital Rashba effect and the orbital Hall effect. Our formalism is so generalizable that one can apply it to obtain effective orbital Hamiltonians for arbitrary orbitals in the presence of periodic lattice structures.

cond-mat.mes-hall↗

arXiv2022

Spin current driven by ultrafast magnetization of FeRh

Laser-induced ultrafast demagnetization is an important phenomenon that probes arguably ultimate limits of the angular momentum dynamics in solid. Unfortunately, many aspects of the dynamics remain unclear except that the demagnetization transfers the angular momentum eventually to the lattice. In particular, roles of electron-carried spin current are debated. Here we experimentally probe the spin current in the opposite phenomenon, i.e., laser-induced ultrafast magnetization of FeRh, where the laser pump pulse initiates the angular momentum build-up rather than its dissipation. Using the time-resolved magneto-optical Kerr effect, we directly measure the ultrafast-magnetization-driven spin current in a FeRh/Cu heterostructure. Strong correlation between the spin current and the net magnetization change rate of FeRh is found even though the spin filter effect is negligible in this opposite process. This result implies that the angular momentum build-up is achieved by an angular momentum transfer from the electron bath (supplier) to the magnon bath (receiver) and followed by the spatial transport of angular momentum (spin current) and dissipation of angular momentum to the phonon bath (spin relaxation).

cond-mat.mtrl-sci↗