Dae-Eun Jeong

Resistive switching mechanism of TiO2 thin films grown by atomic-layer deposition

The resistive switching mechanism of 20- to 57-nm-thick TiO2 thin films grown by atomic-layer deposition was studied by current-voltage measurements and conductive atomic force microscopy. Electric pulse-induced resistance switching was repetitively (> a few hundred times) observed with a resistance ratio ⪢102. Both the low- and high-resistance states showed linear log current versus log voltage graphs with a slope of 1 in the low-voltage region where switching did not occur. The thermal stability of both conduction states was also studied. Atomic force microscopy studies under atmosphere and high-vacuum conditions showed that resistance switching is closely related to the formation and elimination of conducting spots. The conducting spots of the low-resistance state have a few tens times higher conductivity than those of the high-resistance state and their density is also a few tens times higher which results in a ∼103 times larger overall conductivity. An interesting finding was that the area where the ...

Electric-current-driven vortex-core reversal in soft magnetic nanodots

The authors report on electric-current-driven vortex-core (VC) reversal (switching) and the accompanying spin-wave emission, driven by spin-polarized ac currents of different amplitudes and frequencies, investigated by micromagnetic calculations of the dynamic evolution of a magnetic vortex in Permalloy nanodots. The magnetization orientation of the VC is effectively switchable between its upward and downward bistates and controllable by applying current above its threshold density, but with sufficiently small magnitude at frequencies close to the vortex eigenfrequency. This VC reversal phenomenon occurs through the creation of a vortex-antivortex pair and the subsequent annihilation of the initial vortex and the created antivortex, when the velocity of the initial VC reaches its critical value of approximately 340±20m∕s for the given material and geometry. In the course of these serial processes and immediately after VC switching, strong spin waves are emitted. These results provide physical insights int...

Soft x-ray resonant magneto-optical Kerr effect as a depth-sensitive probe of magnetic heterogeneity: A simulation approach

We report a noticeable depth sensitivity of soft x-ray resonant magneto-optical Kerr effect able to resolve depth-varying magnetic heterostructures in ultrathin multilayer films. For various models of depth-varying magnetization orientations in an ultrathin Co layer of realistic complex layered structures, we have calculated the Kerr rotation, ellipticity, intensity spectra versus grazing incidence angle ϕ, and their hysteresis loops at different values of ϕ for various photon energies hν’s near the Co resonance regions. It is found from the simulation results that the Kerr effect has a much improved depth sensitivity and that its sensitivity varies remarkably with ϕ and hν in the vicinity of the resonance regions. These properties originate from a rich variety of wave interference effects superimposed with noticeable features of the refractive and absorptive optical effects near the resonance regions. Consequently, these allow us to resolve depth-varying magnetizations and their reversals varying with de...

Oppositely rotating eigenmodes of spin-polarized current-driven vortex gyrotropic motions in elliptical nanodots

The authors found that there exist two different rotational eigenmodes of oppositely rotating sense in spin-polarized current-driven vortex gyrotropic motions in soft magnetic elliptical nanodots. Simple mathematical expressions were analytically calculated by adopting vortex-core (VC)-rotation-sense-dependent dynamic susceptibility tensors based on the linearized Thiele equation [Phys. Rev. Lett. 30, 230 (1973)]. The numerical calculations of those analytical expressions were confirmed by micromagnetic simulations, revealing that linear-regime steady-state VC motions driven by any polarized oscillating currents can be interpreted simply by the superposition of the clockwise and counterclockwise rotational eigenmodes. The shape of the orbital trajectories of the two eigenmodes is determined only by the lateral dimension of elliptical dots. Additionally, the orbital radii and phases of the two eigenmodes’ VC motions were found to markedly vary with the frequency of applied currents, particularly across the...

REFRACTIVE INDEX AND SNELL'S LAW FOR DIPOLE-EXCHANGE SPIN WAVES IN RESTRICTED GEOMETRY

We derived an analytic expression of refractive index for dipole-exchange spin waves (DESWs) in a laterally confined heterogeneous magnetic thin-film system and also derived Snells law for DESWs by employing a dispersion relation and a specific boundary condition at a heterogeneous magnetic interface. These analytical results mimic well the macroscopic behaviors of propagation, reflection and refraction of DESWs, as obtained from micromagnetic simulations, for example, on a specific yttrium-iron-garnet (Y3Fe5O12) and permalloy (Ni80Fe20) interface.

Soft x-ray polarizer for optical productions of any orthogonal state of the linear and circular polarization modes

An efficient soft x-ray polarizer that is able to optically convert a linear polarization state to any orthogonal state of not only linear but also circular polarization modes is found by means of numerical calculations of the intensities of individual orthogonal polarization components in reflected waves. Calculation results, using the known linear-polarization-mode based Kerr matrix as well as a newly derived circular-polarization-mode based Kerr matrix, indicate that a +45° or −45° linearly polarized incident wave can be readily converted to any orthogonal states of both circular and linear polarization modes, i.e., left- and right-handed circular and s- and p-linear polarizations through reflection, at certain grazing angles of incidence near the critical angle from a simple ferromagnetic thin film of Co(9.0nm)∕Si substrate. The intensities of almost pure circularly or linearly polarized reflected waves are about 10% or less in a certain spectral soft x-ray range just below the absorption edges of con...

Genetic inhibition of an ATP synthase subunit extends lifespan in C. elegans

Mild inhibition of mitochondrial respiration leads to longevity. Disruption of mitochondrial respiratory components extends lifespan in Caenorhabditis elegans, but the effects appear to be complex and the underlying mechanism for lifespan regulation by mitochondrial respiratory genes is still not fully understood. Here, we investigated the role of Y82E9BR.3, a worm homolog of the ATP synthase subunit C, in modulating longevity in C. elegans. We found that the Y82E9BR.3 protein is localized in mitochondria and expressed in various tissues throughout development. RNAi knockdown of Y82E9BR.3 extends lifespan, decreases the accumulation of lipofuscin, and affects various physiological processes, including development delay, reproduction impairment and slow behavior. Further tissue-specific RNAi analysis showed that the intestine is a crucial organ for the longevity effects conferred by Y82E9BR.3 RNAi. Moreover, we demonstrated that lifespan extension by Y82E9BR.3 RNAi is associated with reduced mitochondrial function, as well as the suppression of complex I activity in mitochondria. Unexpectedly, Y82E9BR.3 RNAi knock down did not influence the whole-worm ATP level. Our findings first reveal the crucial role of Y82E9BR.3 in mitochondrial function and the underlying mechanism of how Y82E9BR.3 regulates lifespan in C. elegans.

Differential destructive interference of the circular polarization eigenmodes of scattered soft x rays at the grazing incidence in magnetic thin films

Recently, the authors found that an additional magneto-optical effect that linearly polarized soft x rays incident on a single magnetic layer on a nonmagnetic substrate can be converted to any states among the linearly s- and p- and circularly left- and right-handed polarizations by changing the grazing angle of incidence in specular reflection geometry. In this article, the authors report that the physical origin of such an effect is the differential interference of the circular polarization eigenmodes of scattered soft x rays at the grazing incidence. Totally destructive interference takes place selectively for one helicity but not for the other one at a certain grazing angle and in a specific energy region just below the absorption edges, thus leading to differential circular reflectivity. Numerical calculations using an iterative method of transmission, reflection, and propagation matrices allow us not only to verify the underlying mechanism but also to find the necessary specific conditions of photon...

Giant asymmetry of soft x-ray magnetic scattering between opposite circular polarizations near the Brewster angle

We first report a novel phenomenon that manifests itself in a colossal difference in soft x-ray reflectivity from ferromagnetic transition-metal films between the left- and right-handed circular polarization (LCP and RCP) modes at a resonance near normal Brewsters angle. Theoretical and numerical studies of sft x-ray resonant magnetic scattering using the circular-polarization-mode basis reveal that this effect arises from a totally destructive interference of photons scattered individually from chargem orbital, and spin degrees of freedom in magnetized thin films that selectively occurs only for one helicity of the opposite circular modes when the required criteria are fulfilled. Across the normal Brewsters angle. the polarization state of scattered soft x-ray is continuously variable from the RCP to the LCP mode (or vice versa) through the linear s polarization mode by changing the incidence angle of linear p-polarized x rays at the resonance.