Suk-Hee Han

Spin-orbit coupling in double-sided doped InAs quantum well structures

We have investigated Rashba spin-orbit interaction [Bychkov and Rashba, JETP Lett. 39, 78 (1984)] parameter (α) in double-sided doped InAs quantum well structures of different potential asymmetries created by introducing two separated carrier supply layers. The internal potential asymmetry is manipulated between negative and positive potential gradient by adjusting the relative doping concentrations of the two carrier supply layers. The larger potential asymmetry results in the more extensive variation in α with respect to gate electric field (Vg). The structures of the negative and positive potential gradients exhibit the opposite variation in α with respect to Vg which evidently supports the fact that the sign of α can be changed by the reversed potential asymmetry.

Magnetotransport properties of an individual single-crystalline Bi nanowire grown by a stress induced method

The magnetotransport properties of an individual crystalline Bi nanowire have been investigated in the range of 2–300 K using four-point measurements. I-V measurements show that the contacts were Ohmic at both 2 and 300 K, corresponding to resistivities of 1.0×10−4 and 8.2×10−5 Ω cm, respectively. The transverse magnetoresistance (MR) (2496% at 110 K) and longitudinal MR (−38% at 2 K) for the Bi nanowire were found to be larger than any values reported in the literature, demonstrating that the Bi nanowires grown by a stress induced method are high-quality single crystalline. The observed transverse and longitudinal MR behaviors in the Bi nanowire are consistent with variations in carrier concentrations as well as electronic structures, such as Fermi level and band overlap, based on simple two band model.

Strain-induced microstructural evolution in epitaxial Fe/MgO layers grown on InxGa1−xAs(001) substrates

Epitaxial Fe/MgO layers have been grown on InxGa1−xAs substrates to examine the epitaxial relationship and the morphological variation with respect to indium content, x and the growth temperature of MgO interlayer. The in-plane epitaxial relationship of Fe[010]//MgO[11¯0]//InxGa1−xAs[11¯0] is found in the structures of all x values for 4 nm thick MgO layers grown at room temperature. Epitaxial MgO interlayers grow in two-dimensional layer regardless of x while the morphology of subsequent Fe changes from two-dimensional layer to three-dimensional islands with the increase of x. Furthermore, the average size of Fe islands becomes smaller at higher x value due to enhanced underlying strain. The elevated growth temperature of MgO has led to partial strain relaxation, resulting in the suppression of three-dimensional Fe island formation.

Spin transport in polycrystalline Sb films

Spin transport in polycrystalline Sb films has been studied by using NiFe∕Sb∕NiFe lateral spin valve devices. A clear spin valve effect was detected in the magnetoresistance measurements, which were carried out using a conventional four-terminal geometry. The observation of a memory effect in these results indicates that the spin valve signal originates from spin injection and detection determined by the magnetization orientations of the two ferromagnetic electrodes in the NiFe∕Sb∕NiFe device. From our analysis of the spin valve signals, the authors estimate that the spin diffusion length in the Sb film is ∼2.3μm, with an injected spin polarization across the NiFe∕Sb interface of 0.8% at 20K.

Two magnetic phases in the co-sputtered Fe-Si/sub 3/N/sub 4/ thin film

The structure and magnetic properties of Fe-Si/sub 3/N/sub 4/ thin films fabricated by a co-sputtering technique were studied by X-ray diffractometry and magnetometry. With increasing area fraction of Si/sub 3/N/sub 4/ pieces on the Fe target, magnetization of the films decreased due to reduced Fe content and reduced grain size of the bcc crystalline phase. The temperature dependence and field dependence of magnetization of co-sputtered thin films can be explained by a mixture of two types of particles showing paramagnetism and ferromagnetism.

Ordinary Magnetoresistance of an Individual Single-crystalline Bi Nanowire

We report the magneto-transport properties of an individual single crystalline Bi nanowire grown by a spontaneous growth method. We have successfully fabricated a four-terminal device based on an individual 400-nm-diameter nanowire using plasma etching technique to remove an oxide layer forming on the outer surface of the nanowire. The transverse MR (2496% at 110 K) and longitudinal MR ratios (38% at 2 K) for the Bi nanowire were found to be the largest known values in Bi nanowires. This result demonstrates that the Bi nanowires grown by the spontaneous growth method are the highest-quality single crystalline in the literatures ever reported. We find that temperature dependence of Fermi energy () and band overlap () leads to the imbalance between electron concentration () and hole concentration () in the Bi nanowire, which is good agreement with the calculated from the respective density of states, N(E), for electrons and holes. We also find that the imbalance of plays a crucial role in determining magnetoresistance (MR) at T and at T, while mean-free path is responsible for MR at T>75 K for and T>205 K for .

Soft magnetic properties of Fe-(Si/sub 3/N/sub 4/, Al/sub 2/O/sub 3/) thin films

We have studied the structure and soft magnetic properties of Fe-ceramic (Si/sub 3/N/sub 4/, Al/sub 2/O/sub 3/) thin films fabricated by an rf magnetron sputtering apparatus using a composite target. The concentration of solute in the film increases linearly with the area fraction of ceramic pieces in the target. In the case of Si/sub 3/N/sub 4/, the concentration of O increases largely with the Ar pressure, especially at high Ar pressures of more than 20 mTorr. In Al/sub 2/O/sub 3/, the composition of Al and O exhibits a maximum at the Ar pressure of 20 mTorr. As the area fraction of the ceramic increases, the structure of the film changes from a crystalline phase to an amorphous phase. The electrical resistivity increases significantly with increasing area fraction of the ceramics. The saturation magnetization decreases inversely with the logarithm of the electrical resistivity. The gradient of the decrease in saturation magnetization against electrical resistivity is lower in the Al/sub 2/O/sub 3/ system than that in the Si/sub 3/N/sub 4/ system. The Ar pressure dependences of the electrical resistivity and the saturation magnetization are similar to that of the thin film composition. Fe-Si/sub 3/N/sub 4/ films exhibited a saturation magnetization of about 8 kG, a hard-axis coercivity of 0.2 Oe, an electrical resistivity of about 800 /spl mu/ /spl Omega/-cm and a permeability (100 MHz) of higher than 400.

Spin Interaction Effect on Potentiometric Measurements in a Quantum Well Channel

Spin-orbit interaction induced magnetic field, which can arise from an asymmetry of the potential well, causes imbalance of carrier densities between spin-up and spin-down electrons. In the potentiometric measurement, the detected signal follows the magnetization status of the detection ferromagnet (FM1). In case of adding the neighboring ferromagnet (FM2), the measured potential is decided by both of FMs. When the magnitude of external field is between the coercive field of the FM1 and FM2, the detector reads the intermediate potential. The detector interacts with the neighboring ferromagnet and shows four levels of potential states.

Magnetization correlations in lateral NiFe/Sb/NiFe spin valve devices

We have presented measurements of spin transport in antimony (Sb) which is an important electronic material for III-V semiconductors. Antimony films have been used for a conducting channel between two ferromagnetic electrodes in a lateral spin valve device. The Sb films show a metallic behavior in temperature dependence of resistivity. We demonstrate the first evidence of correlations of magnetization through the Sb channel in lateral NiFe/Sb/NiFe spin valve devices. A clear spin valve effect and the memory effect have been observed in a measurement using the conventional 4-probe geometry. The memory effect ensures that the observed spin valve signal originates from the spin polarized current in the Sb film.

Gate Controlled Hall Effect Devices

A novel active magnetoelectronic device consisting of a ferromagnetic film and gate electrodes on a GaAs/AlGaAs heterostructure has been fabricated and characterized. Magnetization of the ferromagnetic film is manipulated by external magnetic field which is applied in parallel to the ferromagnetic film axis. Magnetic fringe field at the edge of the ferromagnetic film gives rise to the Hall voltage, which shows hysteretic behavior upon magnetic field sweep. When negative voltage is applied to the gate electrodes, the Hall voltage has increased dramatically. The Hall voltage can be amplified by a factor more than 200.