Sug-Bong Choe

Seoul National University, Sŏul, Seoul, South Korea

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Publications (102)256.71 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: The effects of annealing on the Gilbert damping constant and the magnetic properties are investigated in CoFeB films sandwiched by either Ta, Ru, or Pd layers. As the annealing temperature increases, the damping constant is found to be slightly decreased first and then, rapidly increased. The minimum damping constant is, thus, obtained with a 300 °C-annealed Ta/CoFeB/Ta film while the samples with Ru and Pd layers exhibit their minimum at 200 °C. The coercive field also exhibits a behavior similar to the damping constant; thus, an empirical correlation between them is proposed.
    Journal- Korean Physical Society 12/2014; 65(10):1611. · 0.43 Impact Factor
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    ABSTRACT: Unidirectional motion of magnetic domain walls is the key concept underlying next-generation domain-wall-mediated memory and logic devices. Such motion has been achieved either by injecting large electric currents into nanowires or by employing domain-wall tension induced by sophisticated structural modulation. Herein, we demonstrate a new scheme without any current injection or structural modulation. This scheme utilizes the recently discovered chiral domain walls, which exhibit asymmetry in their speed with respect to magnetic fields. Because of this asymmetry, an alternating magnetic field results in the coherent motion of the domain walls in one direction. Such coherent unidirectional motion is achieved even for an array of magnetic bubble domains, enabling the design of a new device prototype-magnetic bubblecade memory-with two-dimensional data-storage capability.
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    ABSTRACT: We report an experimental observation that indicates that a direct relation exists between the speed of the magnetic domain-wall (DW) motion and the magnitude of the perpendicular magnetic anisotropy (PMA) in Pt/Co/Pt films. It is found that by changing the thicknesses of the nonmagnetic Pt layers, the PMA magnitude can be varied significantly and the field-driven DW speed can also be modified by a factor of up to 50 under the same magnetic field. Interestingly, the DW speed exhibits a clear scaling behavior with respect to the PMA magnitude. A theory based on the DW creep criticality successfully explains the observed scaling exponent between the DW speed and the PMA magnitude. The presented results offer a method of maximizing the DW speed in DW-mediated nanodevices without altering the thickness of the magnetic Co layer.
    Applied Physics Letters 04/2014; 104(14):142410-142410-3. · 3.52 Impact Factor
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    ABSTRACT: We report here an analytic prediction of the domain-wall (DW) tilting caused by the Oersted field in the current-driven DW motion along ferromagnetic nanowires that have perpendicular magnetic anisotropy. By adopting the variational principle for energy minimization, the DW tilting angle is determined as a function of the current density with a finite threshold current density, above which the DW becomes elongated along the nanowire with two narrow domains at its edges. These results predict the minimum data bit size as well as the maximum current density needed for realizing stable DWs in DW-mediated nanodevices.
    Journal of Magnetism and Magnetic Materials 10/2013; 343:234–238. · 2.00 Impact Factor
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    ABSTRACT: We demonstrate here that ultrathin ferromagnetic Pt/Co/Pt films with perpendicular magnetic anisotropy exhibit a sizeable Dzyaloshinskii-Moriya interaction (DMI) effect. Such a DMI effect modifies the domain-wall (DW) energy density and consequently, results in an asymmetric DW expansion driven by an out-of-plane magnetic field under an in-plane magnetic field bias. From an analysis of the asymmetry, the DMI effect is estimated to be strong enough for the DW to remain in the N\'eel-type configuration in contrast to the general expectations of these materials. Our findings emphasize the critical role of the DMI effect on the DW dynamics as the underlying physics of the asymmetries that are often observed in spin-transfer-related phenomena.
    Physical Review B 07/2013; · 3.66 Impact Factor
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    ABSTRACT: We propose here a method for compensating the Joule-heating effects in the current-induced domain wall motion (CIDWM). In CIDWM experiments, the current induces not only the spin-transfer torque (STT) effects but also the Joule-heating effects, and both effects influence the domain wall (DW) motion. It is thus desired to develop a way to compensate the Joule-heating effects, in order to determine the pure STT effects on the DW motion. Up to now, in studies of DW creeping motions, such Joule-heating effects have been eliminated based on the Arrhenius law by assuming the temperature-independent creep scaling constants. However, here we find that such scaling constants are sensitive to the temperature, from the DW creeping experiment in Pt/Co/Pt wires with temperature control in a cryostat. By accounting the temperature dependence of the scaling constants, we demonstrate that all the DW speeds with various temperatures are exactly collapsed onto a single universal curve, which enables us to examine the pure STT effects on the DW motion.
    IEEE Transactions on Magnetics 07/2013; 49(7):3207-3210. · 1.21 Impact Factor
  • Physical Review Letters 03/2013; 110(13). · 7.73 Impact Factor
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    ABSTRACT: We demonstrate here that the current-driven domain wall (DW) in two dimensions forms a "facet" roughness, distinctive to the conventional self-affine roughness induced by a magnetic field. Despite the different universality classes of these roughnesses, both the current- and field-driven DW speed follow the same creep law only with opposite angular dependences. Such angular dependences result in a stable facet angle, from which a single DW image can unambiguously quantify the spin-transfer torque efficiency, an essential parameter in DW-mediated nanodevices.
    Physical Review Letters 03/2013; 110(10):107203. · 7.73 Impact Factor
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    ABSTRACT: Magnetic domain-wall (DW), interface between different magnetic domains, has received great attention due to its opportunities toward memory and logic devices as well as its abundant physical properties as a driven interface. Since recent advances of fabrication techniques allow us to scale down the devices, we are facing lower dimensional properties that should be elucidated undoubtedly. Here, we review recent progresses on DW dynamics in ferromagnetic nanowires and our recent experimental observation on the dimensionality transition of the DW dynamics driven by magnetic field and/or current. Our results show that the DW dynamics shows a transition from two to one dimensional behavior as the wire width decreases. In addition, we also demonstrate that the magnetic-field- and electric-current-driven DW dynamics in metallic ferromagnetic nanowires belong to the same universal class.
    Current Applied Physics 01/2013; 13(1):228–236. · 2.03 Impact Factor
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    ABSTRACT: We report on the basis of experiments that magnetic domain structures exhibit a transition between single and dendrite domains with respect to the width of ferromagnetic nanowires. This transition is directly observed in CoFe/Pt multilayered nanowires having a width in the range of 580 nm to 4.2 with a magnetic force microscope. Nanowires wider than 1.5 show typical dendrite domain patterns, whereas the nanowires narrower than 690 nm exhibit single domain patterns. The transition occurs gradually between these widths, which are similar to the typical widths of the dendrite domains. Such a transition affects the strength of the domain wall propagation field; this finding was made by using a time-resolved magneto-optical Kerr effect microscope, and shows that the domain wall dynamics also exhibit a transition in accordance with the domain configuration.
    Journal of Magnetics 12/2012; 17(4). · 0.32 Impact Factor
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    ABSTRACT: We observe a transition of domain-wall (DW) dynamics in ferromagnetic wires made of Co/Ni multilayers by use of transport measurement. As the wire width reduces, DW dynamics exhibits a transition from dendrite growth to pure DW motion. The threshold width is found to be about 300 nm and strongly depends on the relative dragging direction of the magnetic field and the current on DW: parallel (antiparallel) direction results in much smaller (larger) threshold width. It should be considered as a building block for DW-motion-based device applications.
    Applied Physics Letters 07/2012; 101(2). · 3.52 Impact Factor
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    ABSTRACT: We investigate two distinct pinning mechanisms--denoted as static and kinetic pinning of magnetic domain wall (DW) in Permalloy nanowires with different widths. Both pinning situations are realized at an artificial notch on U-shaped Permalloy nanowires, depending on the initial DW states, moving or pinned. We find experimentally that the kinetic and static depinning fields simultaneously increase as the width of the nanowire decreases, whereas a difference between static and kinetic depinning fields monotomically decreases. This is ascribed to the shape anisotropy field of the DWs depending on the geometry of nanowires based on one-dimensional collective model.
    Journal of Applied Physics 04/2012; 111(7). · 2.19 Impact Factor
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    ABSTRACT: Giant magnetoresistance (GMR) signals and the magneto-optic Kerr effect (MOKE) are combined to investigate the asymmetric domain wall (DW) motion in a GMR spin-valve stripe consisting of a wire and a circular ring. In the propagation of a tail-to-tail DW, the left-hand side, top-half ring, bottom-half ring, and the right-hand side are reversed in sequence. However, in the propagation of a head-to-head DW, the left-hand side, bottom half-ring, right-hand side, and top-half ring are switched in sequence. In addition, the critical current density for DW depinning shows asymmetric behavior. For tail-to-tail DW depinning, the critical current density of negative current pulses are lower than that of current pulses in the positive direction, and vice versa for head-to-head DW depinning.
    Journal of Applied Physics 03/2012; 111(7). · 2.19 Impact Factor
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    ABSTRACT: The energy barrier of a magnetic domain wall trapped at a defect is measured experimentally. When the domain wall is pushed by an electric current and/or a magnetic field, the depinning time from the barrier exhibits perfect exponential distribution, indicating that a single energy barrier governs the depinning. The electric current is found to generate linear and quadratic contributions to the energy barrier, which are attributed to the nonadiabatic and adiabatic spin-transfer torques, respectively. The adiabatic spin-transfer torque reduces the energy barrier and, consequently, causes depinning at lower current densities, promising a way toward low-power current-controlled magnetic applications.
    Physical Review Letters 11/2011; 107(21):217205. · 7.73 Impact Factor
  • Kab-Jin Kim, Sang-Jun Yun, Sug-Bong Choe
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    ABSTRACT: We propose a method to reduce the operation current of domain-wall (DW) motion in ferromagnetic nanowires with perpendicular magnetic anisotropy. It is based on the phenomena that a DW placed in wedged nanowire geometry moves spontaneously without any external magnetic field by consumption of the DW energy. Based on energy consideration, we found that there exists a geometry-induced effective field, which depends solely on the radius of circular arc intersecting normal to both the nanowire edges. Geometry for a constant effective field is analytically given with a curved wedge shape, which keeps the radius unchanged irrespective of the DW position. The validity of the analytic prediction is confirmed by a micromagnetic simulation.
    IEEE Transactions on Magnetics 11/2011; · 1.21 Impact Factor
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    ABSTRACT: We report local concentration of the Oersted field at the junction of electric contact geometry. The local Oersted field is numerically calculated with a simplified analytical model. This local field is then verified experimentally by observing magnetization reversal due to the current injection into Permalloy nanowires under magnetic field bias. The threshold current of magnetization reversal is found to be linearly dependent on the strength of the magnetic-field bias with a proportional coefficient (6.9 ± 0.2) × 10<sup>-11</sup> Oe·m<sup>2</sup>/A, which provides evidence of the local concentration of the Oersted field at the junction.
    IEEE Transactions on Magnetics 11/2011; · 1.21 Impact Factor
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    ABSTRACT: We examine magnetic domain wall motion in metallic nanowires Pt-Co-Pt. Regardless of whether the motion is driven by either magnetic fields or current, all experimental data fall onto a single universal curve in the creep regime, implying that both the motions belong to the same universality class. This result is in contrast to the report on magnetic semiconductor (Ga,Mn)As exhibiting two different universality classes. Our finding signals the possible existence of yet other universality classes which go beyond the present understanding of the statistical mechanics of driven interfaces.
    Physical Review Letters 08/2011; 107(6):067201. · 7.73 Impact Factor
  • Jisu Ryu, Sug-Bong Choe, Hyun-Woo Lee
    Physical review. B, Condensed matter 08/2011; 84. · 3.66 Impact Factor
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    ABSTRACT: It is experimentally reported herein that the injection field of domain walls (DWs) from the nucleation pad to the nanowire is controlled by the angle of the initializing magnetic field with the use of asymmetric sample structures. The injection field is abruptly varied twice between two distinct values at a certain angle. Micromagnetic simulation is used to model the injection of a DW into the nanowire with respect to the angle of the initializing magnetic field. This is ascribed to the different structure of the DW at the junction between the pad and the nanowire, resulting in the different pinning strength of the DW. These observations provide a way to control the injection field of DWs into nanostructures and give a possibility of the fast, reliable motion of the DW with field strengths less than the so-called Walker field on the nanowire by injecting the DW with a known magnetic structure.
    Journal of Nanoscience and Nanotechnology 07/2011; 11(7):6476-8. · 1.34 Impact Factor
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    ABSTRACT: Reported herein is a possible way of controlling the depinning field of magnetic domain walls (DWs) by using a magnetic field H(T) transverse to the nanowire. A typical notch structure-in the form of triangles on both edges of ferromagnetic Permalloy nanowires-is employed to pin the DWs. The depinning field of the DW initially pinned at the notch is then measured with respect to H(T). Interestingly, it is experimentally found that the depinning field is drastically decreased to almost 0 with increasing H(T), due to the internal shift of the DW position at the notch. Moreover, it is experimentally observed that an oscillatory behavior of the depinning field occurs with respect to H(T), Micromagnetic calculation is performed to model the depinning behavior of the DW pinned at the notch structure with respect to H(T). It is ascribed to the natural edge roughness of the nanowire, which means the edge roughness plays an important role in determination of the depinning field.
    Journal of Nanoscience and Nanotechnology 07/2011; 11(7):6472-5. · 1.34 Impact Factor

Publication Stats

629 Citations
256.71 Total Impact Points


  • 2005–2014
    • Seoul National University
      • Department of Physics and Astronomy
      Sŏul, Seoul, South Korea
  • 2010
    • Chungbuk National University
      • Department of Physics
      Chinsen, North Chungcheong, South Korea
  • 2004
    • Lawrence Berkeley National Laboratory
      • Center for X-Ray Optics
      Berkeley, CA, United States
  • 1996–2004
    • Korea Advanced Institute of Science and Technology
      • Department of Physics
      Seoul, Seoul, South Korea
  • 2003
    • University of Minnesota Twin Cities
      • School of Physics and Astronomy
      Minneapolis, MN, United States