Won Young Jeung

On-Film Formation of Bi Nanowires with Extraordinary Electron Mobility

A novel stress-induced method to grow semimetallic Bi nanowires along with an analysis of their transport properties is presented. Single crystalline Bi nanowires were found to grow on as-sputtered films after thermal annealing at 260-270 degrees C. This was facilitated by relaxation of stress between the film and the thermally oxidized Si substrate that originated from a mismatch of the thermal expansion. The diameter-tunable Bi nanowires can be produced by controlling the mean grain size of the film, which is dependent upon the thickness of the film. Four-terminal devices based on individual Bi nanowires were found to exhibit very large transverse and longitudinal ordinary magnetoresistance, indicating high-quality, single crystalline Bi nanowires. Unusual transport properties, including a mobility value of 76900 cm(2)/(V s) and a mean free path of 1.35 mum in a 120 nm Bi nanowire, were observed at room temperature.

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.

Correlation between magnetic properties of electrodeposited Co(P) and NH4Cl concentrations in the electrolyte

The effect of ammonium chloride on the electrodeposition process and resulting magnetic properties of Co(P) alloy films was investigated. It was observed that the magnetic properties of the films varied extensively with the ammonium chloride contents in the solution. Cyclic voltammetry was employed as an electroanalytical technique to characterize the variation of interfacial polarization at different electrolyte compositions. In addition, microstructural characterization of electrodeposited Co(P) thin films by transmission electron microscopy revealed a significant increase in both grain size and preferred orientation of Co(P) grains with ammonium chloride contents. Results of characterizations indicated that the addition of ammonium chloride controlled the kinetics of electrocrystallization of Co(P) by modifying the charge transfer process and thus gave rise to increased grain size and preferred orientation in the Co(P) thin films. These microstructural features provide reasonable explanation on the variation of the magnetic properties of electrodeposited Co(P) films with the amount of ammonium chloride in the solution.

Angle Sensors Based on Oblique Giant Magneto Impedance Devices

The measurement of external magnetic field orientation using Giant Magneto Impedance (GMI) sensors has been performed. A soft magnetic alloy of Co30Fe34Ni36 was electroplated on a Si wafer with a CoFeNi seed layer. V-shaped microwire patterns were formed using a conventional photolithography process. An external magnetic field was generated by a rectangular AlNiCo permanent magnet. The reference direction was defined as the external magnetic field direction oriented in the middle of 2 GMI devices. As the orientation of the magnetic field deviated from the reference direction, variation in the field component along each device introduced voltage changes. It was found that, by taking the voltage difference between the left and right arms of the Vshaped device, the nonlinearity of each device could be significantly reduced. The fabricated angle sensor had a linear range of approximately 70 o and an overall sensitivity of approximately 10 mV.

Microstructure and magnetic properties of electrodeposited CoPtP alloys

CoPtP alloys have been grown by electrochemical process in an external magnetic field in order to enhance the perpendicular magnetic properties. The coercivity and squareness of the alloys were improved as the external magnetic field was increased. The alloys have a mosaic microstructure consisting of grains with size in the range of 10–15nm. The grains are separated by highly well defined 1–2nm grain boundaries. The alloys exhibit a columnar microstructure with a hcp [002] fiber texture. The films exhibit significant perpendicular magnetic anisotropy and permanent magnetic properties in the perpendicular direction, with coercivity of up to 6.1kOe and squareness of up to 0.9.

Tailoring the magnetic properties of CoFeNi alloys with variations in copper contents

CoFeNi alloys are some of the most studied soft magnetic materials because of their superior properties over FeNi alloys as write head core materials in hard disk drives. Pulsed electrodeposition was shown to be an interesting approach to vary the crystalline structure of the fcc-bcc mixed phase CoFeNi and CoFeNiCu films without changing the composition and the grain size by using a single bath. The permeability μ of CoFeNiCu films plated from bath composition exceeded 4.6×106. Coercivity Hc was 20–73A∕m. The high saturation magnetic flux Bs was 1.8T. The soft magnetic CoFeNiCu film prepared satisfies all the requirements needed for the preparation of magnetic recording heads, as all magnetic properties were improved in this direction.

Thickness Dependence of (001) Texture Evolution and Magnetic Properties of Sputter-Deposited FePt:MgO Nanocomposite Films

We investigate the effect of the number of bilayers (total film thickness) on the (001) texture development, L10 ordering reaction and magnetic properties in multilayered [FePt/MgO]6 films that were sputter deposited on thermally oxidized Si substrates. We obtained nearly perfectly L10 (001) oriented nanocomposite films when the initial multilayer structure was [FePt 2.8 nm/MgO 3.2 nm]6 and the total film thickness was 36 nm. When the film thickness was thicker than about 36 nm or the film was overaged, the grain orientation of the film developed in a random direction. The evolution of the (001) texture was favorable when the total film thickness was thin enough for the strain generated during the ordering transformation to relax in the plane normal direction, thus satisfying the plane stress condition. In addition, the disordered fcc FePt (200) texture grown on (200)-textured MgO in the as-deposited state also affected the L10 (001) texture evolution.

Operating Field Optimization of Giant Magneto Impedance (GMI) Devices in Micro Scale for Magnetic Bead Detection

Giant magneto impedance (GMI) effect devices for magnetic bead detections were examined. Since commercialized magnetic beads in biological applications are superparamagnetic in nature, not only for improvement of maximum sensitivity but also precise optimizations of operating fields are crucial for sensor applications. We have found that operating fields and maximum sensitivity can be enhanced by the tailoring aspect ratio in microscopic GMI sensors. With an aspect ratio of 3.3-3.8, the GMI sensors were found to exhibit an overall GMI ratio of more than 110% and a maximum sensitivity of 1.61 mV/Oe (15.4%/Oe) at the operating field of 43.1 Oe. Our results demonstrate that microscopic GMI sensors are highly sensitive to magnetic bead detections.

Study on Exchange-Biased Perpendicular Magnetic Tunnel Junction Based on Pd/Co Multilayers

We investigated top and bottom exchange biased perpendicularly magnetized magnetic tunnel junctions (pMTJs) with IrMn and the effects of the multilayer structure on the magnetoresistance and perpendicular exchange bias. The TMR ratio of the top exchange biased (TEB)-pMTJ was significantly deteriorated compared with that of the pseudo-pMTJ, due to the high junction resistance and the in-plane anisotropy induced in the Co/IrMn interface. The TMR ratio and perpendicular exchange bias were improved in the bottom exchange biased (BEB)-pMTJ as compared with those of the TEB-pMTJ. Perpendicularly magnetized buffer layers, such as (Pd/Co)n, should be used to induce perpendicular exchange coupling with antiferromagnets such as IrMn in BEB-pMTJs.

Shubnikov-de Haas Oscillations in an Individual Single-Crystalline Semimetal Bismuth Nanowire

The magneto-transport properties of an individual single crystalline Bi nanowire grown by a spontaneous growth method are reported. A four-terminal device based on an individual 400-nm-diameter nanowire was successfully fabricated using a plasma etching technique that removed an oxide layer that had formed on the surface of the nanowire. Large transverse ordinary magnetoresistance (1401%) and negative longitudinal ordinary magnetoresistance (−38%) were measured at 2 K. It was observed that the period of Shubnikov-de Haas oscillations in transverse geometry was 0.074 , 0.16 and 0.77 , which is in good agreement with those of bulk Bi. However, it was found that the period of SdH oscillation in longitudinal geometry is 0.24 , which is larger than the value of 0.16 reported for bulk Bi. The deviation is attributable to the spatial confinement arising from scattering at the nanowire surface boundary.