H.C. Koo

Magnetization Behavior of Co Nanodot Array

We performed magnetic force microscopy (MFM) observation on array of Co dots in order to understand magnetic state and magnetization behavior of submicron sized Co dots patterned on GaMnAs bridge. MFM observations showed the magnetization reversal and processes of local magnetization of individual ferromagnetic Co nanodots. Magnetic state of Co dots either single domain or vortex is dependent on geometrical size and thickness. Transition from single domain to vortex state can be realized with MFM tip assisted local field. Magnetization reversal process takes place through sequential reversal of individual dots. Localized inhomogeneous magnetic field can be manipulated by controlling magnetic state of individual Co dot in the array structure.

Spin transport in an InAs based two-dimensional electron gas nanochannel

A spin device composed of two ferromagnetic electrodes and InAs two-dimensional electron gas was fabricated. Submicron spin channels were defined to enhance spin transport characteristics. Electrical transport measurement was performed to detect spin-polarized electrons. In potentiometric geometry a voltage change, ΔV=0.17mV, sensed by a ferromagnetic electrode was obtained at 5 and 77K. In the nonlocal method ΔV=0.057mV, which resulted from accumulated spin-polarized electrons, was obtained at 77K. The main reason for theses large signals is that the short and narrow spin channels increase the possibility for spin-polarized electrons to arrive at the spin detector.

Electrical Detection of Spin Hall Effect in a Two-Dimensional Electron Gas

In this study, the stray field induced by ferromagnetic patterns is used to align spins in flowing electrons. The spin polarized electrons will experience asymmetric scattering due to the strong spin-orbit coupling in InAs based semiconductors, and yield to a transverse voltage even in absence of external field.

Rashba-effect-induced spin polarization and anisotropic magnetoresistance in a quantum well layer

The Rashba spin splitting arises from the strong spin-orbit coupling in an asymmetric heterostructure [1, 2].

Electric field control of spin precession in a spin-injected Field Effect Transistor

Two decades ago, Datta and Das [1] predicted that the source-drain conductance of a spin-injected Field Effect Transistor (spin FET) would oscillate periodically with monotonically increasing gate voltage as a consequence of the Rashba spin-orbit interaction in the channel. The effect relies on ballistic transport and a relativistic transformation, and is unique because an electric field modulates the orientation of a magnetic moment. We have observed Datta-Das oscillations in a spin FET at cryogenic temperatures [2].

Silicon based spin valve device

The spin injection and transport properties of the Permalloy/Si/Permalloy (Py) hybrid device fabricated on 50 nm thick silicon film on insulator (SOI) wafer were investigated in this study. Spin valve effect was found in the field range of 100/spl sim/200 Oe over which magnetization of two Py is aligned antiparallel showing maximum resistance. The result indicates that the spin polarized electrons are injected and are detected after transporting through Si channel on insulator.

Spin accumulation and detection in an InAs based two-dimensional electron gas with novel measurement geometry

A novel spin measurement geometry of a nanosized spin device was observed in this study where the current does not pass the ferromagnetic electrode. Also, the spin signals resulted from the interaction between the two ferromagnetic metals (FMs) at the spin device was presented. It was found that the device can be a candidate for spin field effect transistor.

A novel type of spin injection barrier in a GaAs based two-dimensional electron gas system

A low transmission barrier is a crucial factor for the efficient spin injection, and an oxide barrier is commonly used for the insulator between the ferromagnet and the semiconductor. After heat treatment at the furnace, an AlAs layer was converted to an aluminum oxide layer, and arsenic gas was evaporated. This new method of forming spin injection barrier on the two-dimensional electron gas (2-DEG) system is very efficient to obtain tunneling behavior.

Electron Transport of Low Transmission Barrier between Ferromagnet and Two - Dimensional Electron Gas (2DEG)

The junction properties between the ferromagnet (FM) and two-dimensional electron gas (2DEG) system are crucial to develop spin electronic devices. Two types of 2DEG layer, InAs and GaAs channel heterostructures, are fabricated to compare the junction properties of the two systems. InAs-based 2DEG layer with low transmission barrier contacts FM and shows ohmic behavior. GaAs-based 2DEG layer with Al₂ O₃ tunneling layer is also prepared. During heat treatment at the furnace, arsenic gas was evaporated and top AlAs layer was converted to aluminum oxide layer. This new method of forming spin injection barrier on 2DEG system is very efficient to obtain tunneling behavior. In the potentiometric measurement, spin-orbit coupling of 2DEG layer is observed in the interface between FM and InAs channel 2DEG layers, which proves the efficient junction property of spin injection barrier.