Kyoung-il Lee

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.

Evidence for carrier-induced ferromagnetic ordering in Zn1−xMnxO thin films: Anomalous Hall effect

The intrinsic origin of the ferromagnetic ordering in Zn1−xMnxO thin films grown by pulsed-laser deposition was investigated. The ferromagnetic behaviors for a Zn1−xMnxO (x=0.26) film grown at 700 °C under oxygen pressures of 10−1 Torr were observed at 4 and 300 K. The anomalous Hall effect (AHE) was found at temperatures of up to 210 K for the Zn0.74Mn0.26O thin film. The anomalous Hall coefficients (RA) were determined to be approximately proportional to the square of resistivity in the low field region, indicating the side-jump process for the AHE. Our results provide direct experimental evidence that a carrier-mediated mechanism is responsible for the ferromagnetic ordering in Zn1−xMnxO thin films grown by pulsed-laser deposition.

Spin transfer switching characteristics in a [Pd/Co]m/Cu/[Co/Pd]n pseudo spin-valve nanopillar with perpendicular anisotropy

We successfully demonstrate spin transfer switching (STS) characteristics in a [Pd/Co]m/Cu/[Co/Pd]n pseudo spin-valve nanopillar with 100 nm diameter. We observed lower critical current density and high giant magnetoresistance (GMR) ratio in our devices compared to other fully perpendicularly magnetized pseudo spin-valve structures. The devices showed a current-perpendicular-to-plane GMR of 1.2% and a STS critical current density of JAP-P = −2.6 × 107 A/cm2 and JP-AP = 3.8 × 107 A/cm2. The observed low critical current density is thought to be due to higher spin-transfer efficiency arising from smaller spin orbital scattering, longer spin diffusion length of the thinner Pd, and thinner soft-layer-film thickness and coercivity in the nanopillar devices.

Effects of media stray field on electromigration characteristics in current-perpendicular-to-plane giant magnetoresistance spin-valve read sensors

Effects of magnetic stray field retrieved from both longitudinal and perpendicular magnetic recording media (denoted by “media stray field”) on electromigration (EM) characteristics of current-perpendicular-to-plane (CPP) giant magnetoresistance spin-valve (GMR SV) read sensors have been numerically studied to explore the electrical and magnetic stability of the read sensor under real operation. The mean-time-to-failure (MTTF) of the CPP GMR SV read sensors was found to have a strong dependence on the physical parameters of the recording media and recorded information status, such as the pulse width of media stray field, the bit length, and the head moving velocity. According to the numerical calculation results, it was confirmed that in the longitudinal media, the shorter the stray field pulse width (i.e., the sharper the media transition) allows for the longer MTTF of the CPP GMR SV read sensors; while in the perpendicular media, the sharper the media transition gives rise to a shorter MTTF. Interesting...

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 .

Interfacial spin polarization and resistance in lateral spin-valves incorporating Bi and BiPb thin films

Electrical spin injection and detection have been investigated at cryogenic temperatures using a lateral spin-valve structure. Either Bi or BiPb was used as a nonmagnetic spin medium, while CoFe and NiFe were employed as the spin injector and spin detector, respectively. A large magnetoresistance signal corresponding to ΔR=1.2 mΩ was detected from the BiPb-based spin-valves. From this result, a large spin diffusion length of 230 μm and a high interfacial spin polarization of 10% were derived. From an independent calculation, it was found that the interfacial spin polarization and the associated spin accumulation are strongly correlated with the interfacial resistance.

Real-time Detection of Nano-sized Magnetic Beads by Using Spin-valve Devices for Biological Applications

We report the real-time detection of nano-sized magnetic beads for biological applications using highly sensitive spin-valve devices with nano-oxide layer (NOL) exhibiting an 8.5% magnetoresistance (MR) ratio. A combination of photolithography and lift-off process was employed to place nano-sized magnetic beads on the active area of a spin-valve device. The spin-valve device was found to show a signal voltage change of 0.28 μV in the presence of a cluster of nano-sized magnetic beads by comparing the signal voltage difference between the sensing spin-valve device and a reference spin-valve device. The real-time detection of nano-sized magnetic beads was found successfully achieved by direct measurement of the magnetic dipole fields emanating from clusters of nano-sized magnetic beads on the spin-valve device.