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The enhancement of the concentration of a defect in the space-charge region near a grain boundary in an intrinsic ionic system was used to specify the change in the corresponding diffusion coefficient as a function of distance, x, normal to the boundary. This zone of enhanced and continuously varying diffusivity serves as a vehicle for enhanced transport along the boundary in addition to the expected contribution arising from the different structure at the boundary core. A two-dimensional diffusion equation was established for concentration c(x,y,t) in the space-charged region, solved numerically using the Crank–Nicolson finite-difference method, and integrated normal to the boundary to provide the average concentration gradient c(y,t) along the boundary—the solute distribution usually measured by experiment. The present gradients were analyzed in the same fashion as experimental data using a recent solution to the conventional model for the grain–boundary diffusion problem in which the interface is trea...

An equation developed by Le Claire is widely used to obtain a grain‐boundary diffusion product, aD′, from the measured solute concentration gradients produced under conditions of constant surface concentration in grain‐boundary diffusion experiments. However, a numerical assessment of the accuracy of Le Claire’s equation has revealed errors as large as 70% when applied outside of its range of validity to the shallow gradients (∼102 nm) that are provided by high‐resolution analytical methods. To provide a relation applicable to this region, a numerical analysis of the variation of computed values of ∂ ln C/∂η6/5 with β has been used to develop a new and greatly improved expression of the form aD′=D3/2t1/2[10A(−∂ ln C/∂η6/5)B ], where A and B are parameters whose value depends on the experimental value of ∂ ln C/∂η6/5. The relation is valid for the experimentally useful range of solute penetrations 6√Dt∼10√Dt and for 1≤β≤105, and has been shown to produce a grain‐boundary diffusion product accurate to wi...

Control of the interface structure of atomic scale multilayers is a fundamental issue to improve the performance of spin electronic devices. Deposition behavior of Co on Al surface at 300 K was investigated by molecular dynamics simulation. The deposited Co resulted in the formation of CoAl surface alloy regardless of the Al surface orientation. Structure of the surface alloy was dependent on the substrate orientation. Crystalline B2 structure was formed on Al (001) surface. On the contrary, amorphous mixed layers were evolved on Al (011) and (111) surfaces. In the case of Al (001) surface, 4 ML of the surface alloy were observed, which is consistent with the previous experimental observation. The present work shows that the formation of CoAl surface alloy should be considered even at the low substrate temperature and the low incident energy of deposited atoms.

Using ab-initio simulations, we investigated the effects of ferroelectric polarization on the magnetic anisotropy of FePt/BaTiO3 heterojunctions. The changed electronic structure at the interface of FePt and BaTiO3 reduced the magnetic anisotropy energy (MAE) under ferroelectric polarization. Through the electronic density of states analysis, it was found that the MAE change is mainly due to the changed hybridization state between d-orbitals of interface Ti and Fe atoms.

The potential for nitrogen doped graphene with dispersed Co metal atoms as a candidate material for future spintronic applications was studied using first-principles calculations. Among the three types of defective structures, the pyridinic and pyrrolic systems were demonstrated to be structurally stable since the binding energy between the Co adatom and these substrate layers exceeded the cohesive energy of Co metal alone. Also, Co adatom on these two structures showed high magnetic moment values, which surpassed that of the Co metal bulk. From these results, it is shown that the pyridinic and pyrrolic N defects on graphene have a positive effect on the geometric stability and magnetic property of decorated Co atoms.

A multi-scale approach connecting the atomistic process simulations to the device-level simulations has been applied to the Si(100)/SiO2 interface system. The oxidation of Si(100) surface was simulated by the atomic level molecular dynamics, the electronic structure of the resultant Si/suboxide/SiO2 interface was then obtained by the first-principles calculations, and finally, the leakage currents through the SiO2 gate dielectric were evaluated, with the obtained interface model, by the non-equilibrium Green's function method. We have found that the suboxide layers play a significant role for the electronic properties of the interface system and hence the leakage currents through the gate dielectric.

While almost all Si nanostructures, including Si nanowires (SiNWs), Si nanocrystals, and Si nanotrench-like structures on a supra- or sub-10 nm scale exhibit self-limiting oxidative behavior, herein we report full oxidation of SiNWs 5 nm in diameter. We investigated the oxidative behavior of SiNWs with diameters of 5 nm and compared our findings with those for SiNWs with diameters of 30 nm. Single-crystalline SiNWs 5 and 30 nm in diameter were grown by a chemical vapor deposition (CVD) process using Ti as a catalyst. The SiNWs were then oxidized at 600–1000 °C for 30 min to 240 min in O2. The thicknesses of the resulting oxide layers were determined by transmission electron microscopy (TEM). As expected, the SiNWs 30 nm in underwent self-limiting oxidation that was parabolic in nature. However, under the same conditions, the SiNWs 5 nm in diameter underwent full oxidation that was linear in nature. Atomic-scale molecular dynamic simulations revealed that the compressive stress in the oxide layer, which is...

We investigated the effects of interfacial bonding on the mechanical properties in the Si-carbon nanotube (CNT) nanocomposite by a molecular dynamics approach. To describe the system appropriately, we used a hybrid potential that includes Tersoff, AIREBO (adaptive intermolecular reactive empirical bond order), and Lennard–Jones potentials. With increasing bonding strength at the interface of Si matrix and CNT, toughness as well as Young’s modulus and maximum strength increased steadily. CNT pull-out and load transfer on the strong CNT were identified as the main mechanisms for the enhanced properties. At optimum bonding, crack tip was deflected around CNT and the fracture proceeded in plastic mode through Si matrix owing to the strong reinforcement of CNT, and resulted in a further enhancement of toughness. At maximum bonding, however, only load transfer is operative and the fracture returned to brittle mode. We concluded that a strong interface as long as the CNT maintains its structural integrity is des...

Using the ab initio calculations, the changes of the magnetic properties of Co atoms on graphene for biaxial strains on graphene were investigated. Our calculation results showed that the compression of graphene changed the magnetic moments of Co adatoms more significantly than the elongation of graphene. From the 3d-electron density of states of Co atoms, the changes of magnetic properties of Co atoms on graphene were found to result from the significant hybridization between Co 3dyz and C 2pz orbitals. It was demonstrated that this 3d-electron hybridization of Co/graphene system originated from the strain-induced change of the local atomic structure.

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.

Using a reactive molecular dynamics simulation, the oxidation of Si nanowires (Si-NWs) with diameters of 5, 10, and 20 nm was investigated. The compressive stress at the interface between the oxide and the Si core decreased with increasing curvature in the sub-10 nm regime of the diameter, in contrast to the theory of self-limiting oxidation where rigid mechanical constraint of the Si core was assumed. The Si core of the thinner Si-NW was deformed more with surface oxidation, resulting in a lower compressive stress at the interface. These results explain the experimental observation of full oxidation of very thin Si-NWs.

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.

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.

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.

Using the first-principles calculations, the change in magnetism and electronic structures during Cu nucleation in bcc Fe system was investigated. For modeling the Fe-rich bcc FeCu alloy, FexCu1−x (x≥0.75) were employed in the supercell system. Nonmagnetic Cu atoms were precipitated on the (100) plane of bcc Fe crystal, whereas the magnetized Cu atoms preferred to be precipitated on the (110) plane. Magnetization energies showed a linear decrease as the Cu concentration increased in bcc Fe. The magnetic moments of Fe atoms increased for larger concentration of Cu atoms. Electron density of states showed that the enhanced magnetic moment of Fe atoms resulted from the shift of minority spin states toward higher levels, which was associated with the bond formation between Cu and Fe atoms.

Using molecular dynamics simulation, the effect of Pb surfactant for the thin film growth of Co atoms on Cu(111) substrate was investigated. Specifically, the behavior of Co atoms being deposited on Cu(111) substrate with predeposited Pb layer was extensively investigated and compared with the case of without Pb layer to explain the effect of Pb surfactant. It was observed that Pb layer was floating during the Co deposition. It was, quantitatively, found that Pb surfactant played an important role in suppression of active diffusion of Co atoms, which was accomplished by the increase in the surface diffusion barrier energy. The energy change in the deposited Co adatom on the Cu(111) substrate with predeposited Pb layer showed that the approaching Co adatom penetrated into the Pb layer; then, the Co adatom settled down on the Cu(111) substrate. Consequently, Pb atoms around Co adatom suppressed the further diffusion of Co adatom.

Deposition and annealing behaviors of Al atoms on rough Cu (111) surface were investigated on the atomic scale by three-dimensional classical molecular dynamics simulation. The rough Cu surface was modeled by depositing 5 ML of Cu on Ta (011) substrate. Most Al atoms deposited on the rough Cu surface placed on the atomic steps, preserving the major features of the surface during Al deposition. This behavior was discussed in terms of the smaller barrier of the surface diffusion than Ehrlich–Schwoebel barrier of Al on Cu (111) surface. By annealing at 700 K, significant intermixing between Al and Cu rapidly occurs with decrease in the surface roughness. This behavior reveals that the exchange process of Al with substrate Cu dominates during the initial stage of high temperature annealing.

The interface and surface structure of transition metal (TM) (Fe, Co, and Ni) thin film deposited on Al(001) substrate at atomic level were investigated by molecular dynamics simulation. In spite of the low adatom incident energy of 0.1 eV, TM-Al intermixing occurred actively at the surface of Al(001) at room temperature. At the interface region of TM/Al(001), an intermetallic compound was formed and found to be of B2 structure. The Co/Al system showed different layer coverage and pair correlation function characteristics, in comparison with Fe/Al and Ni/Al systems. The different structural and intermixing characteristics at the interface were successfully explained in terms of lattice matching, cohesive energy, and local acceleration effect.

The energetics and the electronic structure of the Si∕Mo(110) surface were investigated using the density functional theory calculations in an attempt to clarify the initial stage of silicide layer formation. The energy barriers for the migration of Si adatom to an adjacent hollow site passing the bridge site were calculated to be 0.64eV [Mo (110)] and 0.74eV [Si-substituted Mo (110)]. In order to analyze the bonding nature between Si adatom and Mo (110) surface, layer density of states and total valence electron density were obtained. It can be reasonably inferred that Mo 4d states mainly contributed to the bonding between Mo and Si and, consequently, four Mo–Si covalent bonds were formed around the Si atom at the hollow site.

The structural and magnetic properties of Co(001)/MgO(001)/Co(001) magnetic tunnel junctions with two types of junction interface, on an O-site adsorption case (Case I) and on a Mg-O twofold hollow site adsorption case (Case II), were investigated using density functional theory. Interfacial spin polarization values of electrodes were calculated to be −81.3% for Case I and −90.5% for Case II, resulting from the adsorption site dependency of electrode atoms on the MgO(001) surface. Interestingly, Mg and O atoms also showed significantly induced spin polarization values due to the 2p−3d hybridization between the Mg/O and Co-interface atoms. The magnetic moments of the electrode atoms were calculated to be 1.67 μB and 1.72 μB on average without significant variation across the electrode, except for the surface layer.

Using the density functional theory calculations, the surface diffusion and incorporation processes for Fe–Cu multilayer system were investigated. The minimum energy path calculations revealed that the hollow site of Cu(001) and Fe(001) surfaces was the most stable adsorption site, and the energy barriers for the surface diffusion were 1.04eV for Fe∕Cu(001) and 0.50eV for Cu∕Fe(001) systems. During the process, small fluctuation of magnetic moment, as amount of 0.11μB for Fe adatom and 0.02μB for Cu adatom, was observed. The energy barrier for the incorporation of the Fe adatom into the surface layer of Cu substrate was 0.75eV, and the energy gain of the system was 0.92eV. At the substitutional site, the magnetic moment for Fe adatom was decreased by 2.64μB. The Cu adatom required a relatively small energy barrier, 0.50eV, for incorporation into the Fe substrate; however, it was found that, after the process, the total energy was rather increased by 0.37eV.

The deposition behavior for Ni thin film growth on Al substrates of various orientations according to the incident energy of adatoms was investigated by molecular dynamics simulation. In spite of the low adatom incident energy of 0.1eV, Ni–Al intermixing occurred actively at the surfaces of Al(001), Al(011), and Al(111) at 80K and Ni atoms apparently favored the island growth mode irrespective of the Al surface orientation. The highest surface roughness was shown for the case of Al(111) surface. The steering effect, which results in rougher surface, was significantly observed at low incident energy. The steering effect was quantitatively investigated through the extensive measurement of the trajectory and deposition flux of atoms with the variation of incident energy near the artificially structured Ni step positioned on Al surfaces.

The effects of the interface fluctuations on the magnetic properties of a Co∕α-Al2O3∕Co magnetic tunnel junction were intensively investigated using first-principles calculations. Hybridization of Co-3d and O-2p orbitals resulted in the covalent bonds between Co and three O atoms at the interface of Co(111)∕α-Al2O3(0001) and, subsequently, interfacial Co atoms became undulated as much as 34.6% of the interlayer distance measured in the z direction. The magnetic moment of Co atoms at the rippled interface was enhanced to 1.95μB, while the value of the other ferromagnetic Co atoms remained unchanged. Significantly induced minority-spin density of states at the Fermi energy of the Co-interface atoms led to a highly negative spin polarization, −52.0%, at the junction interface. Interestingly, the spin polarization value for the barrier layer was in the range of −6.3%–59.2%.