Dong Ryeol Lee

Quantifying interlayer exchange coupling via layer-resolved hysteresis loops in antiferromagnetically coupled manganite/nickelate superlattices

In superlattices made of a half metallic ferromagnet La2/3Ba1/3MnO3(LBMO) and a metallic paramagnet LaNiO3(LNO), the field dependence of the LBMO magnetization was studied using depth- and element-sensitive x-ray resonant magnetic scattering measurements. The superlattices have ten bilayers of LBMO and LNO, and the LBMO layers were antiferromagnetically coupled across LNO spacer layers. From the x-ray measurements, the magnetic hysteresis loop of each LBMO layer was obtained, and subsequently the obtained layer-resolved LBMO hysteresis loops were utilized to determine the interlayer exchange coupling.

Asymmetric magnetic proximity effect in a Pd/Co/Pd trilayer system

In spintronic devices consisting of ferromagnetic/nonmagnetic systems, the ferromagnet-induced magnetic moment in the adjacent nonmagnetic material significantly influences the spin transport properties. In this study, such magnetic proximity effect in a Pd/Co/Pd trilayer system is investigated by x-ray magnetic circular dichroism and x-ray resonant magnetic reflectivity, which enables magnetic characterizations with element and depth resolution. We observe that the total Pd magnetic moments induced at the top Co/Pd interface are significantly larger than the Pd moments at the bottom Pd/Co interface, whereas transmission electron microscopy and reflectivity analysis indicate the two interfaces are nearly identical structurally. Such asymmetry in magnetic proximity effects could be important for understanding spin transport characteristics in ferromagnetic/nonmagnetic systems and its potential application to spin devices.

Inducing vortex formation in multilayered circular dots using remanent curves

We report field manipulation of magnetic vortex states in Co(30u2009nm)/Cu(3u2009nm)/Ni80Fe20 (20u2009nm)-multilayer dot arrays via remanent curve. The element-resolved resonant x-ray magnetic measurements, combined with micromagnetic simulations, show vortex formation in the Co layer but not in the NiFe layer along the major hysteresis loop. Although the two magnetic layers are not directly coupled due to the presence of the Cu interlayer, the NiFe layer is strongly influenced by the dipolar field from uncompensated magnetic poles in the Co layer. Using remanent curves, we demonstrate that the single vortex state can be induced simultaneously in both layers.

Direct measurements of multi-photon induced nonlinear lattice dynamics in semiconductors via time-resolved x-ray scattering.

Nonlinear optical phenomena in semiconductors present several fundamental problems in modern optics that are of great importance for the development of optoelectronic devices. In particular, the details of photo-induced lattice dynamics at early time-scales prior to carrier recombination remain poorly understood. We demonstrate the first integrated measurements of both optical and structural, material-dependent quantities while also inferring the bulk impulsive strain profile by using high spatial-resolution time-resolved x-ray scattering (TRXS) on bulk crystalline gallium arsenide. Our findings reveal distinctive laser-fluence dependent crystal lattice responses, which are not described by previous TRXS experiments or models. The initial linear expansion of the crystal upon laser excitation stagnates at a laser fluence corresponding to the saturation of the free carrier density before resuming expansion in a third regime at higher fluences where two-photon absorption becomes dominant. Our interpretations of the lattice dynamics as nonlinear optical effects are confirmed by numerical simulations and by additional measurements in an n-type semiconductor that allows higher-order nonlinear optical processes to be directly observed as modulations of x-ray diffraction lineshapes.

Significance of the gate voltage-dependent mobility in the electrical characterization of organic field effect transistors

We studied the effect of the addition of free hole- and electron-rich organic molecules to organic semiconductors (OSCs) in organic field effect transistors (OFETs) on the gate voltage-dependent mobility. The drain current versus gate voltage characteristics were quantitatively analyzed using an OFET mobility model of power law behavior based on hopping transport in an OSC. This analysis distinguished the threshold voltage shifts, depending on the materials and structures of the OFET device, and properly estimated the hopping transport of the charge carriers induced by the gate bias within the OSC from the power law exponent parameter. The addition of pentacene or C60 molecules to a one-monolayer pentacene-based OFET shifted the threshold voltages negatively or positively, respectively, due to the structural changes that occurred in the OFET device. On the other hand, the power law parameters revealed that the addition of charge carriers of the same or opposite polarity enhanced or hindered hopping transport, respectively. This study revealed the need for a quantitative analysis of the gate voltage-dependent mobility while distinguishing this effect from the threshold voltage effect in order to understand OSC hopping transport in OFETs.We studied the effect of the addition of free hole- and electron-rich organic molecules to organic semiconductors (OSCs) in organic field effect transistors (OFETs) on the gate voltage-dependent mobility. The drain current versus gate voltage characteristics were quantitatively analyzed using an OFET mobility model of power law behavior based on hopping transport in an OSC. This analysis distinguished the threshold voltage shifts, depending on the materials and structures of the OFET device, and properly estimated the hopping transport of the charge carriers induced by the gate bias within the OSC from the power law exponent parameter. The addition of pentacene or C60 molecules to a one-monolayer pentacene-based OFET shifted the threshold voltages negatively or positively, respectively, due to the structural changes that occurred in the OFET device. On the other hand, the power law parameters revealed that the addition of charge carriers of the same or opposite polarity enhanced or hindered hopping transp...

A New Period-Doubled Modulation on the In/Si(111)4 ×1 Surface Induced by Defects

We report observations of defect-induced period-doubled (×2) modulations in scanning tunneling microscopy images of the In/Si(111)4 ×1 surface at room temperature. The perturbation is one-dimensional, ranging up to ten lattice constants in the row direction. The modulation is attributed to a perturbation in electronic charge density rather than to a lattice distortion. The defect-induced ×2 structure differs from the low-temperature (LT) 4 ×2 observed for the defect-free surface. First-principle calculations found that the defects stabilize a hypothetical structure with small lattice distortions, whose calculated energy is lower than that of the large lattice-distorted structure proposed for the 4 ×2-LT phase. This symmetry-broken, defect-induced modulation is extraordinary in that it does not mimic the LT phase observed in the defect-free system.