KIST, United States
KIST, United States

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Joo S.,Korea University | Lee J.,Korea University | Kim T.,Korea University | Rhie K.,Korea University | And 3 more authors.
Current Applied Physics | Year: 2011

The spin polarization of a high g-factor bulk semiconductor is theoretically investigated in the presence of a magnetic field parallel to a driving electric field. Calculations have been carried out using the energy-dependent relaxation time approximation in association with spin-flip scattering. As the magnitude of the magnetic field increases, the spin-polarized current alternates between the spin-up and spin-down states for the low spin-scattering system. This implies that the current polarization can be tuned by controlling the magnetic field strength, suggesting possible applications to spintronic devices. An experimental method for investigating alternative current polarization is also considered. © 2010 Elsevier B.V. All rights reserved.

Lee J.-K.,Korea Institute of Science and Technology | Lee S.,Korea Institute of Science and Technology | Lee S.,Dongguk University | Kim J.-G.,Korea Basic Science Institute | And 5 more authors.
Small | Year: 2014

Evidence is presented in this paper that certain single-wall carbon nanotubes are not seamless tubes, but rather adopt a graphene helix resulting from the spiral growth of a nano-graphene ribbon. The residual traces of the helices are confirmed by high-resolution transmission electron microscopy and atomic force microscopy. The analysis also shows that the tubular graphene material may exhibit a unique armchair structure and the chirality is not a necessary condition for the growth of carbon nanotubes. The description of the structure of the helical carbon nanomaterials is generalized using the plane indices of hexagonal space groups instead of using chiral vectors. It is also proposed that the growth model, via a graphene helix, results in a ubiquitous structure of single-wall carbon nanotubes. In this work, single-wall carbon nanotubes (SWNTs) are shown to result from the spiral growth, as the minimum energy configuration, of zigzag graphene ribbons. The resulting graphene helices are not seamless tubes. The evidence and analysis addresses mounting inconsistencies in the measured mechanical and electrical properties of SWNTs. The growth and material properties, including chirality, need to be re-interpreted in terms of non-idealized structures of SWNTs. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Sohn J.-W.,KAIST | Sohn J.-W.,Daegu Gyeongbuk Institute of Science and Technology | Song H.-S.,KAIST | Song H.-S.,Daegu Gyeongbuk Institute of Science and Technology | And 6 more authors.
Current Applied Physics | Year: 2016

We investigate the ultrafast magnetization dynamics of perpendicularly magnetized MgO/CoFeB/Ta and Ta/CoFeB/MgO stack structures using an all-optical time-resolved magneto-optical Kerr measurement technique. We find that the effective Gilbert damping parameter αeff converges to a low value of ∼0.02 in the high external field regime, and it stays unchanged at ∼0.02 regardless of the pump fluence. Two asymmetric stack structures of CoFeB show identical dynamic behavior and Gilbert damping, implying that the influence of the under and top interfaces is not distinct. © 2015 Elsevier B.V. All rights reserved.

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