arXiv subjects

Byong-Guk Park

Publications and source records attributed to Byong-Guk Park.

16 recordsLinked to original sources

arXiv2026

Observation of current-induced orbital quadrupole accumulation

Spintronics and orbitronics rely on current-induced accumulations of magnetic dipoles: spin and orbital angular momentum. However, electronic orbitals inherently carry multipoles beyond the dipole, with the rank-2 orbital quadrupole as the leading term. Here we use polarization-resolved Kerr microscopy to observe current-induced orbital-quadrupole accumulation at the surfaces of Ti and Pt, metals with markedly different spin--orbit-coupling strengths. By separating the symmetric and antisymmetric components of the off-diagonal optical conductivity, we isolate the time-reversal-even quadrupolar response from the conventional time-reversal-odd magnetic-dipolar one, and find that the quadrupolar optical response exceeds the dipolar one in both metals. First-principles analysis of the measured responses indicates that the quadrupole accumulations are of the same order of magnitude in the two metals despite their widely different spin--orbit-coupling strengths, consistent with a previously unidentified channel of charge-to-orbital conversion that does not require spin--orbit coupling. Our findings establish that current-induced orbital polarization is fundamentally multipolar, expanding current-induced phenomena from the dipolar to the multipolar regime and opening a route to electrical control of orbital-ordered phases.

cond-mat.mtrl-sci↗

arXiv2026

Bio-resorbable magnetic tunnel junctions

Magnetic tunnel junctions (MTJs) play a crucial role in spintronic applications, particularly data storage and sensors. Especially as a non-volatile memory, MTJs have received substantial attention due to its CMOS compatibility, low power consumption, fast switching speed, and high endurance. In parallel, bio-resorbable electronics have emerged as a promising solution for systems requiring temporary operation and secure data disposal, especially in military, intelligence, and biomedical systems where devices must safely disintegrate under physiological conditions. In this study, we investigate the bio-resorbability of MTJ by analyzing the dissolution behavior of its nanometer-thick constituent layers in phosphate-buffered saline (PBS) solution at pH 7.4, simulating physiological environments. The MTJ structures, composed of bio-resorbable materials, exhibit well-controlled degradation behaviors. Critically, as one of the ferromagnetic layers dissolves, binary information is irreversibly lost, within 10 hours of immersion. These findings highlight the potential of MTJs not only as high-performance memory elements but also as secure, transient data storage platforms. The ability to modify the dissolution lifetime by materials and thickness selection offers unique advantages for short-lived implantable devices, paving the way for integrating spintronic functionality into next-generation bioresorbable electronics.

cond-mat.mtrl-sci↗

arXiv2025

Effect of Magnetic Anisotropy on Magnetoelastic Waves in Ni/LiNbO3 Hybrid Device

We study the effects of magnetic anisotropy and crystalline axes in surface acoustic waves (SAWs) driven magnetic resonances of Ni/LiNbO3 hybrid devices. SAW absorption from the interaction with magnons in Ni displays a strong anisotropic dependence on the direction of the applied in-plane magnetic field. Magnetic anisotropy is further investigated by magneto-optical Kerr effect measurements to show both uniaxial and biaxial anisotropy components in Ni films on LiNbO3. By introducing a dipolar interaction term in addition to the anisotropies, we successfully explain the anisotropic SAW absorption in our devices. These findings show the importance of substrate-induced anisotropy and long-range dipolar effects in SAW-magnons hybrid devices and indicate future directions for optimizing these spin-acoustic devices through comprehensive anisotropy engineering.

cond-mat.mes-hall↗

arXiv2022

Long-Range Orbital Magnetoelectric Torque in Ferromagnets

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.

cond-mat.mes-hall↗

arXiv2022

Efficient conversion of orbital Hall current to spin current for spin-orbit torque switching

Spin Hall effect, an electric generation of spin current, allows for efficient control of magnetization. Recent theory revealed that orbital Hall effect creates orbital current, which can be much larger than spin Hall-induced spin current. However, orbital current cannot directly exert a torque on a ferromagnet, requiring a conversion process from orbital current to spin current. Here, we report two effective methods of the conversion through spin-orbit coupling engineering, which allows us to unambiguously demonstrate orbital-current-induced spin torque, or orbital Hall torque. We find that orbital Hall torque is greatly enhanced by introducing either a rare-earth ferromagnet Gd or a Pt interfacial layer with strong spin-orbit coupling in Cr/ferromagnet structures, indicating that the orbital current generated in Cr is efficiently converted into spin current in the Gd or Pt layer. Furthermore, we show that the orbital Hall torque can facilitate the reduction of switching current of perpendicular magnetization in spin-orbit-torque-based spintronic devices.

cond-mat.mtrl-sci↗

arXiv2021

Identifying intrinsic and extrinsic mechanisms of anisotropic magnetoresistance with terahertz probes

Identifying the intrinsic and extrinsic origins of magneto-transport in spin-orbit coupled systems has long been a central theme in condensed matter physics. However, it has been elusive owing to the lack of an appropriate experimental tool. In this work, using terahertz time-domain spectroscopy, we unambiguously disentangle the intrinsic and extrinsic contributions to the anisotropic magnetoresistance (AMR) of a permalloy film. We find that the scattering-independent intrinsic contribution to AMR is sizable and is as large as the scattering-dependent extrinsic contribution to AMR. Moreover, the portion of intrinsic contribution to total AMR increases with increasing temperature due to the reduction of extrinsic contribution. Further investigation reveals that the reduction of extrinsic contribution is caused by the phonon/magnon-induced negative AMR. Our result will stimulate further researches on other spin-orbit-interaction-induced phenomena for which identifying the intrinsic and extrinsic contributions is important.

cond-mat.mtrl-sci↗

arXiv2019

Perfect optical absorption-enhanced magneto-optic Kerr effect microscopy

Magnetic and spintronic media have offered fundamental scientific subjects and technological applications. Magneto-optic Kerr effect (MOKE) microscopy provides the most accessible platform to study the dynamics of spins, magnetic quasi-particles, and domain walls. However, in the research of nanoscale spin textures and state-of-the-art spintronic devices, optical techniques are generally restricted by the extremely weak magneto-optical activity and diffraction limit. Highly sophisticated, expensive electron microscopy and scanning probe methods thus have come to the forefront. Here, we show that perfect optical absorption (POA) dramatically improves the performance and functionality of MOKE microscopy. For 1-nm-thin Co film, we demonstrate a Kerr amplitude as large as 20 degree and magnetic domain imaging visibility of 0.47. Especially, POA-enhanced MOKE microscopy enables real-time detection and statistical analysis of sub-wavelength magnetic domain reversals. Furthermore, we exploit enhanced magneto-optic birefringence and demonstrate analyser-free MOKE microscopy. The POA technique is promising for optical investigations and applications of nanomagnetic systems.

physics.optics↗

arXiv2018

Enhanced spin-orbit torque via interface engineering in Pt/CoFeB/MgO heterostructures

Spin-orbit torque facilitates efficient magnetization switching via an in-plane current in perpendicularly magnetized heavy metal/ferromagnet heterostructures. The efficiency of spin-orbit-torque-induced switching is determined by the charge-to-spin conversion arising from either bulk or interfacial spin-orbit interactions, or both. Here, we demonstrate that the spin-orbit torque and the resultant switching efficiency in Pt/CoFeB systems are significantly enhanced by an interfacial modification involving Ti insertion between the Pt and CoFeB layers. Spin pumping and X-ray magnetic circular dichroism experiments reveal that this enhancement is due to an additional interface-generated spin current of the nonmagnetic interface and/or improved spin transparency achieved by suppressing the proximity-induced moment in the Pt layer. Our results demonstrate that interface engineering affords an effective approach to improve spin-orbit torque and thereby magnetization switching efficiency.

cond-mat.mtrl-sci↗

arXiv2017

Complementary logic operation based on electric-field controlled spin-orbit torques

Spintronic devices as alternatives to traditional semiconductor-based electronic devices attract considerable interest as they offer zero quiescent power, built-in memory, scalability, and reconfigurability. To realize spintronic logic gates for practical use, a complementary logic operation is essential but still missing despite a recent progress in spin-based logic devices. Here, we report the development of a complementary spin logic device using electric-field controlled spin-orbit torque (SOT) switching. In heavy metal/ferromagnet/oxide structures, the critical current for SOT-induced switching of perpendicular magnetization is efficiently modulated by an electric field via voltage-controlled magnetic anisotropy (VCMA) effect in a non-volatile manner. Moreover, the polarity of the VCMA is tuned by the modification of oxidation state at the ferromagnet/oxide interface. This allows us to fabricate both n-type and p-type spin logic devices and to enable a complementary logic operation, paving the way for the development of non-volatile and reconfigurable logic devices.

physics.app-ph↗

arXiv2017

Observation of transverse spin Nernst magnetoresistance induced by thermal spin current in ferromagnet/non-magnet bilayers

Electric generation of spin current via spin Hall effect is of great interest as it allows an efficient manipulation of magnetization in spintronic devices. Theoretically, spin current can be also created by a temperature gradient, which is known as spin Nernst effect. Here, we report spin Nernst effect-induced transverse magnetoresistance in ferromagnet (FM)/non-magnetic heavy metal (HM) bilayers. We observe that the magnitude of transverse magnetoresistance (i.e., planar Nernst signal) in FM/HM bilayers is significantly modified by HM and its thickness. This strong dependence of transverse magnetoresistance on HM evidences the spin Nernst effect in HM; the generation of thermally-induced spin current in HM and its subsequent reflection at the FM/HM interface. Our analysis of transverse magnetoresistance shows that the spin Nernst angles of W and Pt have the opposite sign to their spin Hall angles. Moreover, our estimate implies that the magnitude of the spin Nernst angle would be comparable to that of the spin Hall angle, suggesting an efficient generation of spin current by the spin Nernst effect.

cond-mat.mtrl-sci↗

arXiv2017

Spin-orbit torques induced by interface-generated spin currents

Magnetic torques generated through spin-orbit coupling promise energy-efficient spintronic devices. It is important for applications to control these torques so that they switch films with perpendicular magnetizations without an external magnetic field. One suggested approach uses magnetic trilayers in which the torque on the top magnetic layer can be manipulated by changing the magnetization of the bottom layer. Spin currents generated in the bottom magnetic layer or its interfaces transit the spacer layer and exert a torque on the top magnetization. Here we demonstrate field-free switching in such structures and attribute it to a novel spin current generated at the interface between the bottom layer and the spacer layer. The measured torque has a distinct dependence on the bottom layer magnetization which is consistent with this interface-generated spin current but not the anticipated bulk effects. This other interface-generated spin-orbit torque will enable energy-efficient control of spintronic devices.

cond-mat.mtrl-sci↗

arXiv2017

Contributions of Co and Fe orbitals to Perpendicular Magnetic Anisotropy of MgO/CoFeB Bilayers with Spin-Orbit-Torque-Related (Ta, W, IrMn, and Ti) Underlayers

We study the perpendicular magnetic anisotropy (PMA) of the CoFeB/MgO bilayers in contact with W, Ta, IrMn and Ti which has been suggested as the spin-orbit-torque-related underlayers. The saturation magnetization of the CoFeB depends on the underlayer materials due to formation of a dead-layer, affecting PMA strength of each film. The x-ray magnetic circular dichroism measurement reveals that the interfacial intermixing suppresses only the perpendicular orbital moment of Fe, while the intermixing simultaneously suppresses both the perpendicular and in-plane orbital moments of Co.

cond-mat.mtrl-sci↗

arXiv2016

Antiferromagnetic domain wall motion driven by spin-orbit torques

We theoretically investigate dynamics of antiferromagnetic domain walls driven by spin-orbit torques in antiferromagnet/heavy metal bilayers. We show that spin-orbit torques drive antiferromagnetic domain walls much faster than ferromagnetic domain walls. As the domain wall velocity approaches the maximum spin-wave group velocity, the domain wall undergoes Lorentz contraction and emits spin-waves in the terahertz frequency range. The interplay between spin orbit torques and the relativistic dynamics of antiferromagnetic domain walls leads to the efficient manipulation of antiferromagnetic spin textures and paves the way for the generation of high frequency signals in antiferromagnets.

cond-mat.mtrl-sci↗

arXiv2015

Large spin Hall magnetoresistance and its correlation to the spin-orbit torque in W/CoFeB/MgO structures

The spin-orbit interaction in heavy metal/ferromagnet/oxide structures has been extensively investigated because it can be employed in manipulation of the magnetization direction by in-plane current. This implies the existence of an inverse effect, in which the conductivity in such structures should depend on the magnetization orientation. In this work, we report a systematic study of the magnetoresistance (MR) of the W/CoFeB/MgO structures and its correlation to the current-induced torque to the magnetization. We observe that the MR is independent of the angle between magnetization and current direction, but is determined by the relative magnetization orientation with respect to the spin direction accumulated by spin Hall effect, which is the same symmetry of so-called spin Hall magnetoresistance. The MR of ~1% in W/CoFeB/MgO samples is considerably larger than those in other structures of Ta/CoFeB/MgO or Pt/Co/AlOx, which indicates a larger spin Hall angle of W. Moreover, the similar W thickness dependence of the MR and the current-induced magnetization switching efficiency demonstrates that they share the same underlying physics, which allows one to utilize the MR in non-magnet/ferromagnet structure in order to understand closely related other spin-orbit coupling effects such as inverse spin Hall effect, spin-orbit spin transfer torques, etc.

cond-mat.mtrl-sci↗

arXiv2015

Thermoelectric Signal Enhancement by Reconciling the Spin Seebeck and Anomalous Nernst Effects in Ferromagnet/Non-magnet Multilayers

The utilization of ferromagnetic (FM) materials in thermoelectric devices allows one to have a simpler structure and/or independent control of electric and thermal conductivities, which may further remove obstacles for this technology to be realized. The thermoelectricity in FM/non-magnet (NM) heterostructures using an optical heating source is studied as a function of NM materials and a number of multilayers. It is observed that the overall thermoelectric signal in those structures which is contributed by spin Seebeck effect and anomalous Nernst effect (ANE) is enhanced by a proper selection of NM materials with a spin Hall angle that matches to the sign of the ANE. Moreover, by an increase of the number of multilayer, the thermoelectric voltage is enlarged further and the device resistance is reduced, simultaneously. The experimental observation of the improvement of thermoelectric properties may pave the way for the realization of magnetic-(or spin-) based thermoelectric devices.

cond-mat.mtrl-sci↗

arXiv2014

Ferromagnetic resonance spin pumping in CoFeB with highly resistive non-magnetic electrodes

The relative contribution of spin pumping and spin rectification from the ferromagnetic resonance of CoFeB/non-magnetic bilayers was investigated as a function of non-magnetic electrode resistance. Samples with highly resistive electrodes of Ta or Ti exhibit a stronger spin rectification signal, which may result in over-(or under-)estimation of the spin Hall angle of the materials, while those with low resistive electrodes of Pt or Pd show the domination of the inverse spin Hall effect from spin pumping. By comparison with samples of single FM layer and an inverted structure, we provide a proper analysis method to extract spin pumping contribution.

cond-mat.mtrl-sci↗