Eung-Kwan Lee

Surface structures and magnetic anisotropies of a Fe/Pt (001) surface: An ab initio study

Using ab initio calculations, we obtained the surface phase diagram of a Fe/Pt(001) surface and the magnetic anisotropy energies of the equilibrium Fe/Pt(001) surface structures. From the obtained surface phase diagram, Fe-rich L12 B and perpendicular L10 B were found to be the most stable Fe–Pt surface phases. The calculated magnetic anisotropy energies of the Fe-rich L12 B and perpendicular L10 B Fe/Pt(001) structures revealed that the magnetic easy axes of the surface structures prefer to align in the [001] direction. Through systematic calculations, we showed that the magnetic anisotropy reduction in Fe/Pt(001) originates from the changed electron filling in the 3dz2 orbital of Fe atoms due to the surface formation.

Interface-Dependent Spin-Reorientation Energy Barrier in Fe/MgO(001) Thin Film

Using the density-functional-theory-based atomic modeling, the stable interface structure and the resultant magnetocrystalline anisotropy (MCA) of the Fe/MgO(001) for magnetic random access memory have been studied. The most stable surface structure of Fe/MgO(001) thin-film system was found to be either defect free or possessing oxygen vacancies in a c(2 ×1) periodicity. The formation of the oxygen vacancies in c(2 ×1) periodicity on MgO(001) surface reduced the MCA of Fe layer from 1.38 to 0.31 meV/atom. The reduced MCA is originated from the filling of the minority states of the Fe orbital below Fermi level.

Effects of uniaxial strains on the magnetic properties and the electronic structures of Fe/graphene system: An ab initio study

Using ab initio calculations, we investigated the changes of the magnetic moment and electronic structures of Fe adatoms on strained graphene sheets. By the uniaxial tensile strains in armchair and zig-zag directions on graphene sheets, the amounts of charge transfers from graphene 2pz orbital to Fe adatom 3d orbitals were linearly increased. The magnetic moments of Fe, however, show the tendency of linear decrements with the uniaxial tensile strains. The increased Fe magnetic moments by uniaxialy graphene compressions resulted from the shifting of spin-minority states of electrons while the decreased Fe magnetic moments were due to the reduction in the spin-majority states of 3dxy-orbitals of the Fe adatom.

Atomic behavior of carbon atoms on a Si removed 3C-SiC (111) surface during the early stage of epitaxial graphene growth

The understanding of the formation of graphene at the atomic scale on Si-terminated 3C-SiC for obtaining high-quality graphene sheets remains elusive, although epitaxial graphene growth has been shown to be a well-known method for economical mass production of graphene/SiC heterojunctions. In this paper, the atomic behavior of carbon atoms on a Si removed 3C-SiC (111) surface for the formation of graphene buffer layer during the early stage of epitaxial graphene growth was investigated using a molecular dynamics simulation. Observation of the behavior of the remaining carbon atoms on the Si-terminated 3C-SiC (111) surface after removal of the silicon atoms revealed that graphene clusters, which were formed by sp2-bonded carbon atoms, start to appear at annealing temperatures higher than 1300 K. Our simulations indicated that the structural stability of the whole system increased as the number of sp2-bonded carbon atoms on the Si-terminated 3C-SiC (111) surface increased. It was also found that the diffusi...

Interface-dependent magnetic anisotropy of Fe/BaTiO3: A first principles study

Using first principles calculations, we investigated interface structure effects on the magnetic properties of the Fe/BaTiO3 system. On the BaO-terminated surface, a Fe monolayer is formed as two Fe atoms are adsorbed on the top sites of Ba and O in the (1×1) surface unit and a Fe monolayer (ML) is formed on the TiO2-terminated surface as two Fe atoms are adsorbed on the two O top sites. The magnetic anisotropy energy of Fe was higher on the TiO2–terminated surface (1.5 eV) than on the BaO-terminated surface (0.5 eV). The decomposed electron density of the states showed that the stronger hybridization of Fe with the TiO2 layer than with the BaO layer is the most important reason for the higher magnetic anisotropy energy.

Energetics of Pb heterostructures formation on the Cu (111) in the early stage of the deposition process

The structural and self-assembling characteristic of Pb heterostructures on the Cu (111) substrate in the early stage of the deposition process were investigated using a molecular dynamics simulation and density functional theory. The Pb islands formed on the Cu (111) surface were observed to diffuse actively in lateral directions following the layer-by-layer growth mode. A heptameric hexagonal island was found to be most stable under highly nonequilibrium conditions. This result can be explained by the tendency of Pb heterostructures, which have minimum surface energy, to have the maximum number of Pb–Pb bondings. In addition, the atomic binding energy, the surface diffusion coefficient prefactor, and the surface diffusion energy barrier for Pb adatoms were quantitatively calculated according to various shapes of Pb islands to determine the stability of the corresponding island.

Configuration Dependency of Attached Epoxy Groups on Graphene Oxide Reduction: A Molecular Dynamics Simulation

The atomic behavior of epoxy groups on a graphene oxide sheet was observed during high thermal heat annealing using a reactive force-field based on molecular dynamics simulations. We found the oxygen-containing functional groups interplay with each other and desorbed from the graphene oxide sheet by a form of O2 gas if they were initially in close distance. Through comparing reduction results of graphene oxide with different densities of the nearest neighboring epoxy pairs, we confirmed that the amount of released O2 gas has a clear tendency to increase with a higher density of epoxy pairs in close distance on a graphene oxide sheet.

Atomic-Scale Simulations of Early Stage of Oxidation of Vicinal Si(001) Surfaces Using a Reactive Force-Field Potentials

The early stages of the oxidation process on vicinal Si(001) surfaces were studied at the atomic scale using reactive-force field-based molecular dynamics simulations. Oxygen molecules at step edges on the vicinal Si(001) surface showed higher reactivity than those on flat terraces. In macroscopic simulations of oxidation on vicinal Si(001) surfaces with different miscut angles (0°, 5.5°, 10.5°), we found that the initiation of oxidation with higher miscut angles was earlier than with lower angles. These results clearly show that a high density of step edges on the vicinal Si surface accelerates the initial oxidation.

Strain-induced Wurtzite to h-BN Phase Transformation in Zinc Oxide nanorods

Atomistic simulations are utilized to demonstrate a strain-induced phase transformation in Zinc Oxide (ZnO) nanowires. The strain-induced transformation occurs by the propagation and annihilation of (0001) planes, and the Wurtzite nanorod is transformed into a hexagonal boron nitride (h-BN) phase. Quantitative structural, mechanical and energetic analyses were performed to verify Wurtzite to h-BN transformation during uniaxial tensile test in [1010] direction.