Kwang-Su Ryu

Korea Advanced Institute of Science and Technology , Sŏul, Seoul, South Korea

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Publications (32)121.6 Total impact

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    ABSTRACT: Domain walls can be driven by current at very high speeds in nanowires formed from ultra-thin, perpendicularly magnetized cobalt layers and cobalt/nickel multilayers deposited on platinum underlayers due to a chiral spin torque. An important feature of this torque is a magnetic chiral exchange field that each domain wall senses and that can be measured by the applied magnetic field amplitude along the nanowire where the domain walls stop moving irrespective of the magnitude of the current. Here we show that this torque is manifested when the magnetic layer is interfaced with metals that display a large proximity-induced magnetization, including iridium, palladium and platinum but not gold. A correlation between the strength of the chiral spin torque and the proximity-induced magnetic moment is demonstrated by interface engineering using atomically thin dusting layers. High domain velocities are found where there are large proximity-induced magnetizations in the interfaced metal layers.
    Nature Communications 01/2014; 5:3910. · 10.02 Impact Factor
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    ABSTRACT: Spin-polarized currents provide a powerful means of manipulating the magnetization of nanodevices, and give rise to spin transfer torques that can drive magnetic domain walls along nanowires. In ultrathin magnetic wires, domain walls are found to move in the opposite direction to that expected from bulk spin transfer torques, and also at much higher speeds. Here we show that this is due to two intertwined phenomena, both derived from spin-orbit interactions. By measuring the influence of magnetic fields on current-driven domain-wall motion in perpendicularly magnetized Co/Ni/Co trilayers, we find an internal effective magnetic field acting on each domain wall, the direction of which alternates between successive domain walls. This chiral effective field arises from a Dzyaloshinskii-Moriya interaction at the Co/Pt interfaces and, in concert with spin Hall currents, drives the domain walls in lock-step along the nanowire. Elucidating the mechanism for the manipulation of domain walls in ultrathin magnetic films will enable the development of new families of spintronic devices.
    Nature Nanotechnology 06/2013; · 31.17 Impact Factor
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    ABSTRACT: Unconventional current-driven domain wall motion (CDDWM) faster than 300 m/s has been reported in perpendicularly magnetized atomically thin Co/Ni bilayers deposited on Pt underlayers [1], making these materials extremely promising for DW-based memory and logic devices. The driving force of the motion is believed to originate from Pt/Co interface. To investigate the interfacial effect we carried out AMR and AHE experiment on thin Pt/Co/Ni and Au/Co/Ni perpendicularly magnetized layers since CDDWM in Au/Co/Ni is induced by conventional spin transfer torque. We find that AMR ratios on Pt/Co/Ni as functions of magnitude and orientation of applied magnetic fields are significantly different from those on Au/Co/Ni. Also it is observed that the AHE values for Pt/Co/Ni are greater than those for Au/Co/Ni by more than one order of magnitude. Recently anisotropic interface magnetoresistance [2] was reported in Pt/Co/Pt sandwiches, but its underlying mechanism is not yet to be understood. Here we discuss the origin of the unconventional AHE and AMR behaviors from Pt/Co/Ni. [1] Kwang-su Ryu, Luc Thomas, See-Hun Yang, S.S.P. Parkin, Applied Physics Express 5, 093006 (2012). [2] A. Kobs et al, PRL 106, 217207 (2011).
    03/2013;
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    ABSTRACT: Spin-orbit-induced anisotropic transport in magnetic materials, studied for more than a century, has recently experienced a renewed interest thanks to the formulation of anisotropic spin scattering in terms of Berry's curvature. Anisotropic magnetoresistance (AMR) is related to the scattering of the transport electrons on the orbitals of localized electrons, depending on the magnetization direction. The contributions of the interfaces on AMR has been scarcely studied. We consider a trilayer composed of one ferromagnetic layer sandwiched between two normal metals. The normal metals display spin Hall effect (SHE), whereas the ferromagnetic layer polarize the flowing current. We propose that SHE present in the top and bottom layers might contribute to the AMR. The charge and spin currents are analyzed by drift-diffusion equations including the role of inverse SHE as well as anomalous Hall effect. Longitudinal and transverse spin accumulations at the interfaces are captured through spin dependent conductance and the mixing conductance. It is shown that the presence of a spin accumulation in the normal metal close to the interface is transformed into a charge current through inverse SHE hence altering the conductivity of the normal metal. The obtained total resistivity calculation indicates its own spin accumulation profile dependance.
    03/2013;
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    ABSTRACT: We report an asymmetry of magnetic disorder in exchange-biased IrMn(tIrMn=5–20 nm)/CoFe(50 nm) films observed by means of a Kerr microscope, capable of direct domain observation. From the correlation between the magnetization half-reversal time and applied magnetic field, we find that the magnetization switching in all the films occurs via a thermally activated reversal mechanism for both branches of hysteresis loops. Surprisingly, in the forward branch reversal where the applied magnetic field is antiparallel to the direction of exchange-bias field, degree of magnetic disorder decreases as exchange-bias field increases, which is definitely contrasted with the case of backward branch reversal. This result is likely ascribed to the fact that the local values of exchange-bias field and coercive field are oppositely fluctuating with each other in the film.
    Journal of Magnetism and Magnetic Materials 01/2013; 325:13–16. · 1.83 Impact Factor
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    ABSTRACT: Kerr microscopy is used to investigate domain wall motion in response to nanosecond-long current pulses in perpendicularly magnetized micron-sized Co/Ni/Co racetracks. Domain wall velocities greater than 300 m/s are observed. The velocity is independent of the pulse length for a wide range of current densities. However, the domain wall dynamics depends on the pulse length just above the threshold current for motion, where slow creep motion occurs, and at very high current densities, where domain nucleation takes place. We also observe a tilting of the domain wall that cannot be accounted for by the Oersted field from the driving current.
    Applied Physics Express 09/2012; 5(9):3006-. · 2.73 Impact Factor
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    ABSTRACT: In this study, we investigated the stochastic nature of domain reversal dynamics in exchange-biased IrMn/CoFe film using a time-resolved Kerr microscope. Interestingly, the statistical distributions of the magnetization half-reversal times for both forward and backward applied fields show that the magnetization reversal dynamics is much more stochastic for the backward branch, where an applied field is parallel to the exchange-bias field direction. The enhanced stochasticity is ascribed to the large degree of magnetic disorder during backward reversal, which induces discrete and random Barkhausen jumps, whereas the forward branch reversal is dominated by a thermally activated depinning process caused by a single potential barrier. This result can be explained by the asymmetry of the magnetic disorder between both branches of a hysteresis loop.
    Journal of Applied Physics 03/2012; 111(7). · 2.21 Impact Factor
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    ABSTRACT: One of the key problems for realization of domain wall motion devices is the reliable and energy efficient injection of domain walls (DWs) into magnetic nanowires. In this work, we explore the injection of domain walls in perpendicular magnetic anisotropy nanowires (Co/Ni multilayers) which are locally softened by ion irradiation. We observe a minimum in the domain wall injection critical current, which occurs where the anisotropy of the irradiated region transitions from out of plane to in plane anisotropy. Furthermore, we find that the irradiated site acts as a pinning site for the DWs. At the irradiation site, we are able to create localized nanosecond long pulsed magnetic fields used to inject the DWs. By performing DC resistance measurements after each injection event, we are able to probe for the existence of the domain wall, and also find the strength of the ion irradiated pinning site. Using the above technique, we have demonstrated a five fold reduction in the domain wall injection current.
    02/2012;
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    ABSTRACT: We report in this paper the decrease of the stochasticity of magnetization half-reversal time with increasing domain wall (DW) pinning in Fe films investigated by means of time-resolved magneto-optic Kerr microscopy. The domain images in the films reveal that the density of DW pinning sites increases with increasing Fe thickness. However, we found that the stochasticity of the magnetization half-reversal time significantly decreases with increasing DW pinning. The major reason for the reduced stochasticity is shown to be due to a thermally activated DW creep mechanism that becomes dominant during magnetization reversal due to increased DW pinning.
    New Journal of Physics 08/2011; 13(8):083038. · 4.06 Impact Factor
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    ABSTRACT: We have investigated the Barkhausen critical scaling behavior of NixFe1−x (x = 0−0.5) films using a magneto-optical microscope, capable of time-resolved domain observation. Real-time direct observations of the domain evolutions in these films revealed that magnetization reversal occurred with a sequence of random Barkhausen jumps. From more than 1000 repeated experiments with each sample, it was found that the distribution of the Barkhausen jump size followed a power-law distribution. The scaling exponent of the power-law distribution was found to have the same value of 1.1, independent of the film composition, revealing a universal critical scaling behavior in these alloy films.
    Journal of Applied Physics 03/2011; 109(7):07E101-07E101-3. · 2.21 Impact Factor
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    ABSTRACT: We report a breakdown of Barkhausen critical-scaling behavior in NiO/Fe films with increasing domain-wall pinning by means of a Kerr microscope capable of direct domain observation. The time-resolved domain images in the films revealed that the Barkhausen jump size is generally decreased with increasing NiO thickness, showing an enhanced domain-wall pinning effect. Interestingly enough, the power-law scaling behavior of the Barkhausen jump size distribution gradually disappears as pinning of domain walls in the Fe layer is increased. This is ascribed to the fact that the magnetization reversal mechanism is changed from a Barkhausen avalanche dominant mode to domain-wall creep dominant mode.
    Physical review. B, Condensed matter 01/2011; 83(6). · 3.77 Impact Factor
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    ABSTRACT: We have demonstrated the key components of the Racetrack Memory. In particular, we have shown that a series of DWs can be moved at high speed along magnetic nanowires by using nanosecond long current pulses for both in-plane and perpendicularly magnetized racetracks. The spacing between DWs has been reduced significantly by using a perpendicularly magnetized RT. Higher storage densities can be achieved by optimizing the materials of the RT (Fig.11) and by scaling the racetrack width.
    Electron Devices Meeting, 1988. IEDM '88. Technical Digest., International 01/2011;
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    ABSTRACT: We report a large converse magnetoelectric (CME) effect at room temperature in a multiferroic heterostructure formed from thin layers of perpendicularly magnetized CoxPd1-x alloys deposited on a piezoelectric single-crystal of lead magnesium niobate-lead titanate PMN-PT(001). The CME results from a strain-induced reorientation of the CoPd magnetization. By varying the composition and thickness of the CoxPd1-x film, a large converse magnetoelectric coupling constant, α = 8×10−7 s/m, at room temperature was found for 10 nm Co0.25Pd0.75. This large CME effect results from combining a highly magnetostrictive CoPd alloy with highly piezoelectric PMN-PT.
    Applied Physics Letters 12/2010; 97(25):252508-252508-3. · 3.79 Impact Factor
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    ABSTRACT: Spin precessions in the stripes of α-MnAs films prepared on GaAs(001) are investigated using an all-optical pump-probe method. We find that a large-angle spin precession appears while the stripe width decreases. In addition, the large-angle precession considerably changes the resonance frequency, resulting in a significant decrease in the relaxation time. These changes in the precessional motion are mainly ascribed to the dephasing of the nonuniform spin waves existing at the large-angle precession, as experimentally confirmed by varying the precession angle via tuning pump fluence. Micromagnetic simulations using a single Gilbert damping constant well predict the experimental observations, which verifies the interpretation of the change in the precessional motion.
    Physical review. B, Condensed matter 01/2010; 82(14). · 3.77 Impact Factor
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    ABSTRACT: We have investigated the magnetization reversal behavior in exchange-coupled NiO/Fe films with varying the NiO thicknesses using a magneto-optical microscope magnetometer capable of direct domain observation in real time. Interestingly enough, the magnetization reversal mechanism is gradually changed from a domain wall-motion process to a nucleation process as the NiO thickness increases. This result clearly demonstrates that the exchange coupling effect between the NiO and Fe layers increases the domain wall pinning effect of the Fe layer, resulting in the nucleation reversal mode.
    Journal of Applied Physics 01/2010; 107. · 2.21 Impact Factor
  • Journal of The Korean Magnetics Society. 01/2009; 19(5):176-179.
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    ABSTRACT: We investigate the origin of in-plane uniaxial magnetic anisotropy of epitaxial ferromagnetic MnAs film on GaAs(001). Interestingly, as temperature increases, the in-plane uniaxial magnetic anisotropy along the MnAs[110] direction changes and then disappears. Direct microscopic domain observations show that the type of domain structure changes from a simple domain to a closure one with increasing temperature. From these results, the temperature-dependent change of the in-plane magnetic anisotropy is ascribed to a decrease in the shape anisotropy induced by the decrease in the width of the ferromagnetic α-stripe.
    Applied Physics Letters 02/2008; 92(8):082503-082503-3. · 3.79 Impact Factor
  • Physica Status Solidi (c). 01/2008; 5(1):405-408.
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    ABSTRACT: We report the pinning role of the nonferromagnetic beta phase in the domain wall motion of ferromagnetic MnAs film on GaAs(001) in the temperature range of 20-35 °C. The dependence of the coercivity on the applied field direction at all temperatures was found to follow an inverse cosine law, as expected from a theoretical model considering the domain wall motion in ferromagnetic films. From this analysis, it was found that the domain wall pinning energy density increases as the temperature increases. This is ascribed to the increase in the volume fraction of the nonferromagnetic beta phase, resulting in an increase of the domain wall pinning effect during the domain wall motion.
    Journal of Applied Physics 01/2008; 103. · 2.21 Impact Factor
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    ABSTRACT: We have investigated critical scaling behavior of Barkhausen avalanches of two-dimensional Co and MnAs films using time-resolved magneto-optical microscopy, enabling to image Barkhausen avalanches at criticality. A stochastic analysis of the fluctuating size of Barkhausen jumps from numerous repetitive experiments shows a power-law scaling behavior in both systems. Strikingly, it is found that the variation of scaling exponent in the power-law distribution is closely correlated with the variation of the domain-evolution patterns. This result is understood by the fact that both the scaling exponent and the domain-evolution patterns strongly depend on the relative contribution between the dipolar interaction and domain wall energies. © 2008 American Institute of Physics.
    Journal of Applied Physics 01/2008; 103. · 2.21 Impact Factor