Ung Hwan Pi

We devised a method to measure the virtual magnetic field induced by Rashba effect in ferromagnetic metal layer. Transverse Rashba magnetic field makes the magnetization direction tilted out of the easy axis, which could be detected by the change in anomalous hall resistances. Through a specified measurement of the second harmonics of the hall resistance, the Rashba field could be obtained with high sensitivity even at low current regime. The results are compared with the prior reports based on the measurement of the transverse field required for the nucleation of reversed domain.

The electrical responses of vanadium pentoxide nanowires to helium gas and environmental pressures are demonstrated. The devices feature well-aligned nanowires that are oriented by electrophoresis technique in the submicron scale. The electrical conductance is found to increase and decrease upon exposure to helium gas and air, respectively. This electrical response to helium is due to physical adsorption of the helium atoms into the interlayer of vanadium pentoxide nanowires. Furthermore, we observe flow-rate-dependent conductance variations such that the conductance is increased with stepwise behavior to the increase of flow rate of helium.

Applying magnetic exchange force microscopy with an Fe-coated tip, we experimentally resolve the atomic-scale antiferromagnetic structure of the Fe monolayer on W(001). On the basis of first-principles calculations, using an Fe nanocluster as a tip, we determine the distance dependence of the magnetic exchange forces. Significant relaxation of tip and sample atoms occurs, which depend sensitively on the local magnetic configuration. This shifts the onset of magnetic interactions toward larger separations and facilitates their observation. Implementing a multiatom tip in the calculations and accounting for relaxation effects are crucial to obtain the correct sign and distance dependence of the magnetic exchange interaction. By comparison with our calculations, we show that the experimentally observed contrast is due to a competition between chemical and magnetic forces.

Scalability of interface driven perpendicular magnetic anisotropy (i-PMA) magnetic tunnel junctions (MTJs) has been improved down to 1X node which verifies STT-MRAM for future standalone memory. With developing a novel damage-less MTJ patterning process, robust magnetic and electrical performances of i-PMA MTJ cell down to 15 nm node could be achieved.

We have studied the vortex structure in ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8}$ single crystal with low-density artificial columnar defects formed by the irradiation of 1.3 GeV uranium ions by using a low-temperature magnetic force microscope. We observed that some of the topographic steps are acting as strong line pinning centers for magnetic vortices in this material. We confirmed that these line steps correspond to the stacking fault dislocations. The stacking fault dislocation showed a direction dependent pinning behavior due to the line-shape geometry of the dislocation. The movement of the vortices across the dislocation line is impeded, while the movement along the dislocation line is quite free.