Kwang-Seok Kim

Samsung Advanced Institute of Technology, Usan-ri, Gyeonggi-do, South Korea

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Publications (14)29.62 Total impact

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    ABSTRACT: We report first-principles calculations on the magnetocrystalline anisotropy energy (MAE) of an Fe monolayer sandwiched by MgO. We found that by increasing the interlayer distance between Fe and O by about 8% from its equilibrium value, the perpendicular interfacial magnetic anisotropy can be enhanced as high as 2.75 erg/cm2, which is three times larger than that at the equilibrium distance. The analysis of MAE based on the second-order interactions of the spin-orbit coupling shows that the energy position of the majority-spin dz2 orbital is of central importance in determining MAE. Our results suggest that increasing the Fe–O distance in the Fe/MgO system is an important material-design direction for high-performance magnetic memories.
    No preview · Article · Jan 2013 · Journal of Applied Physics
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    ABSTRACT: We studied the steady state precession of the spins in the magnetic tunnel junctions. In the field range below the spin-flop transition of synthetic antiferromagnetic pinned layer, the precession mode depends upon the current bias polarity. Above the spin-flop transition, however, the spin precession shows symmetric behavior on the current bias. The dominant mechanism of the spin precession changes from the spin transfer torque to thermal excitation at around the spin-flop transition. The enhancement of thermal fluctuation above the spin-flop transition is analyzed in relation to the static magnetic susceptibility.
    No preview · Article · Jul 2011 · Applied Physics Letters
  • Kisu Lee · Seungkyo Lee · B. K. Cho · Kwang-Seok Kim · Byungchul Kim
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    ABSTRACT: For the detection of specific bio-molecules, such as cells, proteins and DNA, it is necessary to verify the limit of detection (LOD) of an investigated giant magnetoresistive (GMR) sensor. We measured voltage drops across a GMR sensor before and after spreading magnetic particles on the surface of the GMR sensor and compared them to find the LOD of the GMR sensor. Magnetic particles (Dynabeads®) of 2.8 pm in diameter were used. The threshold for the detection was approximately 6 beads. This result indicates that the GMR sensor studied in this study can be used to detect bio-molecules, such as micron-sized cells.
    No preview · Article · Jul 2009 · Journal- Korean Physical Society
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    ABSTRACT: We investigated the top and bottom interfaces of a Co Fe B / Mg O / Co Fe B tunnel junction using transmission electron microscope (TEM) and x-ray photoemission spectroscopy (XPS) in order to understand the origin of the asymmetry of dI/dV in terms of bias polarity. It was found, from a TEM image, that there is no clear cut at the top interface, while the bottom interface has relatively clean boundary. Furthermore, XPS data show that more hydroxides were formed at the top interface than at the bottom interface. These indicate that the hydroxides would hinder the epitaxial crystallinity at the interface in Co Fe B / Mg O / Co Fe B tunnel junctions. Therefore, it is most likely that the asymmetry of dI/dV is caused by the disappearance of minority Bloch state, which is closely correlated with the existence of hydroxides at the top interface of a Co Fe B / Mg O / Co Fe B tunnel junction.
    No preview · Article · May 2009 · Journal of Applied Physics
  • Kisu Lee · Youngman Jang · Juyoung Kim · Seungkyo Lee · Kwang-Seok Kim · B.K. Cho
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    ABSTRACT: We investigated the out-of-plane anisotropy of Co<sub>50</sub>Fe<sub>50</sub>/Tb multilayer film with changing the thickness of Tb layer and applying an inplane filed of 200 Oe during film deposition. A strong out-of-plane anisotropy was found only for the multilayer with small thickness of Tb layer (t<sub>Tb</sub> = 1 nm) and was suppressed significantly by the application of an inplane magnetic field of 200 Oe during deposition, resulting in an inplane anisotropy. It was also found that the application of the magnetic field caused the enhanced intermixing between CoFe and Tb layers to form Co-Tb and Fe-Tb alloys at the interface of the layers. While the origin of the out-of-plane anisotropy and the change of the out-of-plane to inplane anisotropy with the application of inplane field of 200 Oe are not understood completely, it seems that there is a strong correlation between the out-of-plane anisotropy and the formation of the alloys and the stress at the interface.
    No preview · Article · Dec 2008 · IEEE Transactions on Magnetics
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    ABSTRACT: Variation in the quality of the interface in a CoFeB/MgO/CoFeB tunnel junction during thermal annealing was investigated using x-ray photoemission spectroscopy. The formation of B oxide and the reduction of Fe oxide at the bottom interface after thermal annealing near Ta = 300 °C were found to enhance the tunneling magnetoresistance ratio significantly. At the same time, an asymmetry of the conductance (dV/dI) in the bias polarity and a local minimum of conductance in a positive bias state were measured which were attributed to the presence of a minority state at the bottom interface. The authors believe that the existence of the Bloch state was also responsible for the failure of the application of the Brinkman-Dynes-Rowell or Simmons models to the CoFeB/MgO/CoFeB junction.
    No preview · Article · Sep 2007 · Applied Physics Letters
  • Kwang-Seok Kim · B. K. Cho · T. W. Kim
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    ABSTRACT: Magnetoresistive random access memory (MRAM) is one of the promising universal memories, which combines the high speed of static RAM, high-density of dynamic RAM, and non-volatility of FLASH memory. However, MRAM faces several challenges prior to the appearance at the commercial market on a large scale. The most important challenge among them will be an issue, related to the magnetic switching. The conventional MRAM structures and writing methods cause several issues to be overcome for high packing density without cross-talk. Furthermore, small ferromagnetic elements will require high magnetic field, which will increase the power consumption of the devices. This article will review new writing schemes, such as thermal-assisted switching and spin-transfer switching methods. In addition, reliability characteristics and thermal stability of magnetic tunnel junctions will be presented, because ensuring the reliability of magnetic tunnel junctions emerges as an another challenging problem for the successful application of the new writing schemes to the high-density MRAM devices in the next generation.
    No preview · Article · Aug 2006 · Current Applied Physics
  • Injun Hwang · Kwang-Seok Kim · YJ Cho · K. W. Kim · Taewan Kim
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    ABSTRACT: Thermally activated magnetization reversals of submicron sized magnetic tunnel junctions with NiFe single and NiFe / Ru / NiFe synthetic antiferromagnet (SAF) free layers are investigated by varying the pulse durations and current amplitudes in switching pulse measurements. The measured data show good agreement with the switching probability predicted by the Arrhenius-Neel theory and switching behaviors with a single activation energy barrier. Estimated activation barriers for magnetization reversals are higher for magnetic tunnel junctions (MTJs) with SAF free layers than MTJs with single free layers. It is believed that the high activation barrier is achieved by the larger magnetic volume of SAF layers.
    No preview · Article · May 2006 · Journal of Applied Physics
  • Kwang-Seok Kim · Y. M. Jang · C. H. Nam · Ki-Su Lee · B. K. Cho
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    ABSTRACT: Time-dependent dielectric breakdown (TDDB) measurements under constant voltage stress with positive and negative bias polarities are carried out for magnetic tunnel junctions (MTJs) with different oxidation status (under-, optimal, and overoxidation). We found that there is significant polarity dependence in the TDDB and speculated that the polarity dependence is due to both intrinsic and extrinsic origins. Optimally oxidized MTJs with positive bias on the top electrode show shorter times to breakdown (tBD’s) and lower barrier height than with negative bias, indicating that asymmetric band structure, in part, causes the polarity dependence. On the other hand, under- and overoxidized MTJs show much shorter tBD’s than optimally oxidized one and show a higher 1/f noise power density for positive bias than for negative bias, indicating that the polarity dependence is also, in part, due to the interface states, which acts like precursors for the dielectric breakdown.
    No preview · Article · Apr 2006 · Journal of Applied Physics
  • Kwang-Seok Kim · Ki-Su Lee · C.H. Nam · B.K. Cho
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    ABSTRACT: Magnetic tunnel junctions (MTJs) were fabricated using nitrogen-mixed oxygen plasma (O<sub>2</sub>:N<sub>2</sub>=10:1). From the measurements of time-dependent dielectric breakdown (TDDB) under a constant voltage stress, the reliability of MTJs with an oxy-nitride barrier was investigated and compared with that of MTJs fabricated using pure oxygen plasma. The reliability of MTJs with an oxy-nitride barrier is much improved with the incorporation of nitrogen to oxidation process. In addition, the reliability of the oxy-nitride barrier is gradually enhanced with increasing oxy-nitridation time even after the time exceeds the optimal value. It is believed that the enhancement is due to the bonding of nitrogen to electron traps both in the oxide barriers and at the bottom interface. The characteristics of the bonding were examined by XPS measurements, which reveal a nitrogen 1s peak in Al-N bond. The lifetime of the two barriers was also estimated for comparison.
    No preview · Article · Feb 2006 · IEEE Transactions on Magnetics
  • Kwang-Seok Kim · B. K. Cho
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    ABSTRACT: Time dependent dielectric breakdown (TDDB) measurements were carried out for magnetic tunneling junctions (MTJs) with different electron trap densities at the interface between a bottom electrode and an insulating barrier. The TDDB shows a strong bias-polarity dependence, which becomes bigger with increasing trap density. In addition, the current creep before total dielectric breakdown consistently shows bias-polarity dependence. The polarity dependence of the breakdown in MTJs with an ultrathin tunneling barrier (15–20 Å) is believed to be caused by precursor effect of the traps at the bottom interface, which enhances the trap generation rate in a tunneling barrier, resulting in acceleration of the breakdown process.
    No preview · Article · Mar 2005 · Applied Physics Letters
  • Kwang-Seok Kim · Heejea Shim · B. K. Cho
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    ABSTRACT: Time-dependent dielectric breakdown (TDDB) of magnetic tunnel junctions (MTJs) under a constant voltage stress, tunneling magnetoresistance (TMR), and barrier properties, e.g. effective barrier height and thickness, were investigated as a function of thermal annealing temperatures. A Weibull failure distribution function was plotted in terms of time to breakdown (TBD) of MTJs. The TBD when 63 fraction of as-fabricated MTJs cumulatively failed increased significantly after thermal annealing at 210 °C while the TMR also increased from 8.85% to 14.22% before and after thermal annealing at 210 °C, respectively. We believe that the enhanced reliability of the MTJs is due to healing effect of bulk defects in the barrier during the annealing process, likely leading to the reduction of defect trap density. The reduction of bulk defects in the barrier was also confirmed by the lowered /f noise power spectral density from voltage fluctuation measurements.
    No preview · Article · Jun 2004 · physica status solidi (a)
  • Kwang-Seok Kim · Y. M. Jang · J. M. Park · B. K. Cho
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    ABSTRACT: Time-dependent dielectric breakdown measurements under constant voltage stress were carried out for magnetic tunnel junctions (MTJs), prepared by different oxidation techniques. Insulating barriers in MTJs were fabricated by oxidation of a predeposited Al layer with different oxidation techniques, such as conventional O2 plasma, off-axis O2 plasma, Ar-mixed O2 plasma, and off-axis Ar-mixed O2 plasma oxidations. The time to breakdown (TBD) of the MTJs was estimated from Weibull failure distribution plot. The estimated TBD when 63% of the junctions formed by conventional O2 plasma oxidation failed was estimated to be about 55 s. The TBD increased up to about 250, 750, and 2,500 s for the junctions of off-axis, Ar-mixed, and off-axis Ar-mixed O2 plasma oxidation, respectively. We believe that the enhanced reliability of MTJs is due to the reduction of plasma damage during oxidation process, resulting in low electron trap density in the Al2O3 tunneling barrier. Low-frequency 1/f noise power density was measured to estimate the relative electron trap density as a function of the oxidation method, which supports our conjecture about the enhanced reliability of MTJs. © 2004 American Institute of Physics.
    No preview · Article · May 2004 · Journal of Applied Physics
  • Kwang-Seok Kim · B. K. Cho · T. W. Kim · W. J. Park
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    ABSTRACT: Breakdown characteristics of the magnetic tunnel junctions (MTJ) with different junction areas of S=200 μm2 and S=0.5 μm2 are investigated under constant voltage stress. The breakdown process is found to be quite different for the two junction areas. For the large junctions with S=200 μm2, magnetoresistance (MR) ratio decreases gradually with increasing time of constant voltage stress and lasts for 10–30 min. The low frequency 1/f noise power also increases with increasing stress time due to the Johnson and shot noises, caused by current flowing through ohmic shorts or pinholes across tunnel barrier. Meanwhile, the junctions with S=0.5 μm2 show abrupt junction breakdown after stress time of 1–7 h. There is no significant change in both MR ratio and its bias dependence during the voltage stress. In particular, soft-breakdown events are observed before total breakdown occurs. The junction breakdown of small junction area is of intrinsic nature of the junction while the one of large junction area is due to extrinsic factors. © 2003 American Institute of Physics.
    No preview · Article · May 2003 · Journal of Applied Physics

Publication Stats

69 Citations
29.62 Total Impact Points


  • 2007-2013
    • Samsung Advanced Institute of Technology
      Usan-ri, Gyeonggi-do, South Korea
  • 2009
    • SAIT Polytechnic
      Calgary, Alberta, Canada
  • 2003-2006
    • Gwangju Institute of Science and Technology
      • School of Materials Science and Engineering
      Gwangju, Gwangju, South Korea