Jeongyong Choi

Mn-doped V2VI3 semiconductors: Single crystal growth and magnetic properties

We have grown Mn-doped V2VI3 single crystals using the temperature gradient solidification method. We report on the structural and magnetic properties of Mn-doped Bi2Te3, Sb2Te3, Bi2Se3, and Sb2Se3 compound semiconductors. The lattice constants of several percent Mn-doped V2VI3 were slightly smaller than those of the undoped V2VI3 due to the smaller Mn atomic radius (1.40A) than those of Bi (1.60A) and Sb (1.45A). Mn-doped Bi2Te3 and Sb2Te3 had ferromagnetic ordering at TC=10 and 17K, respectively. However, Mn-doped Bi2Se3 and Sb2Se3 showed spin glass and paramagnetic properties, respectively.

Ferrimagnetism in strained Fe2As thin films on Si(001)

We present the structural and magnetic properties of Fe2As thin films grown on Si(100) by molecular beam epitaxy. From the reflection high-energy electron diffraction and x-ray diffraction patterns, the orientation of Fe2As film on Si(100) was found to be a c-axis in the tetragonal crystal structure. The tetragonal Fe2As thin film exhibited ferrimagnetic (FIM) ordering at room temperature. The magnetic moment of Fe2As was 0.1 μB/unit cell determined from the saturated magnetic moment. The observed FIM in Fe2As films was attributed to the strain in the film.

Perpendicular ferrimagnetism in strained Mn2As film

Ferrimagnetic Mn2As thin films with perpendicular magnetic anisotropy were successfully grown on Si(100) by molecular-beam epitaxy. From the reflection high-energy electron diffraction and X-ray diffraction patterns, the orientation of the Mn2As film on Si was along the c-axis in the tetragonal crystal structure. Mn2As film exhibited ferrimagnetic ordering at temperatures greater than 300 K, which differs from antiferromagnetic or paramagnetic behaviors in the bulk form. The magnetic moment of Mn2As determined by saturated magnetization was 0.51 µB per unit cell.

Perpendicular Exchange Bias and Thermal Stability of [Pd / Co]N / FeMn Films

Perpendicular magnetization curves and crystal textures of [Pd(0.8 ㎚)/Co(0.8 ㎚)]_5/FeMn multilayers having an exchange-biased perpendicular magnetic anisotropy as a function of FeMn thickness and annealing temperature were measured. As FeMn thickness increases from 0 to 21 ㎚, the perpendicular exchange biasing field (Hex) obtained the maximum value of 130 Oe at FeMn thickness 12 ㎚. As the annealing temperature increases to 240 ℃, the Hex increased from 115 Oe to 190 Oe and the exchange biased perpendicular magnetic anisotropy disappeared at 330 ℃.

Single Crystal Growth and Magnetic Properties of Mn-doped Bi 2 Se 3 and Sb 2 Se 3

We have grown Mn-doped Bi₂Se₃ and Sb₂Se₃ single crystals using the temperature gradient solidification method. We report on the structural and magnetic propertis of Mn-doped Bi₂Se₃ and Sb₂Se₃ compound semiconductors. The lattice constants of several percent Mn-doped Bi₂Se₃ and Sb₂Se₃ were slightly smaller than those of the un-doped samples due to the smaller Mn atomic radius (1.40 Å) than those of Bi (1.60 Å) and Sb (1.45 Å). Mn-doped Bi₂Se₃ and Sb₂Se₃ showed spin glass and paramagnetic properties, respectively.

Control of current-jump induced by voltage, temperature, light in p-type GaAs: Programmable critical temperature sensor

For two-terminal devices fabricated by Be (or Mn)-doped p-type epitaxial GaAs thin films, when the Mott metal-insulator transition (MIT) as current jump occurs, we observe that the energy gap of GaAs is not shifted, its peak intensity decreases in an applied voltage, and that the MIT temperature is between 410 and 440 K, and that the current jump is controlled by temperature, voltage and light intensity. The control of the jump voltage, a characteristic of the Mott MIT, reveals that these devices can be applied for programmable critical temperature sensors or optical sensors with high sensitivity.

Highly adhesive carbon nanotube field emitters with a carbide filler

We have developed highly adhesive carbon nanotube field emitters with a carbide filler. The adhesion was reinforced by a chemical reaction between the carbide filler and the substrate. We observed the field emission properties to be improved by the increment of the effective field emitters through repeated surface treatments.

Neutrophilic urticarial dermatosis and Sweet-like neutrophilic dermatosis: under-recognized neutrophilic dermatoses in lupus erythematosus:

Background/Objectives Neutrophilic dermatoses can be associated with autoimmune connective tissue diseases such as systemic lupus erythematosus (SLE). We analyzed clinical and histological features of neutrophilic urticarial dermatosis (NUD) and Sweet-like neutrophilic dermatosis (SLND)—the most recently delineated entities of the neutrophilic dermatoses. Methods We retrieved database medical records of patients with SLE whose skin biopsy demonstrated a neutrophilic-predominant infiltrate of the skin, and included those whose biopsies revealed findings of SLND or NUD. Results SLND skin lesions lasted longer than those of NUD and were localized to sun-exposed areas. All NUD cases resolved within one week either spontaneously or with treatment such as antihistamines, but SLND skin lesions lasted longer than one week; prednisone was used in four of these five patients. All NUD cases were found in existing SLE patients and were not associated with systemic signs of flare-up of SLE. However, 80% of SLND cases experienced flare-up of SLE; and in 60%, SLND developed concomitantly with SLE as a presenting sign. Conclusion Different clinical courses and relationships with SLE suggest that NUD and SLND have different pathogeneses for neutrophilic inflammatory reactions.

VEGF protein expression is associated with a poor prognosis in cutaneous extranodal NK/T-cell lymphoma patients

Vascular endothelial growth factor (VEGF) can stimulate angiogenesis and lymphangiogenesis, which are crucial processes in the growth and metastasis of tumour cells.1,2 The immunohistochemical status of VEGF protein expression in tumour tissue of extranodal NK/T-cell lymphoma (ENKTL) has not yet been evaluated. Herein, we investigated VEGF protein expression, as well as its prognostic value in cutaneous ENKTL patients. This article is protected by copyright. All rights reserved.

Phase analysis of VO2 thin film and the mechanism of the electrically triggered metal-insulator transition of VO2

Many researchers have produced various results for the mechanism of the metal-insulator transition (MIT) of VO2. This seems to be because of the influence of various VOx phases in the sample thin-film. VOx has many phases, and VO2 is in only a small window in the VOx phase diagram. In this work, VO2 thin films on Al2O3 substrates were prepared by a pulsed laser deposition method, and their I-V properties were measured over a temperature range from 50 K to 300 K. In the thin-films, not only the VO2 phase, but also at least two other VOx phases were present, including V2O3 and V5O9. For electrically triggered MIT, a space charge limit current (SCLC) appeared at high voltages while ohmic current appeared at low voltages, and MIT occurred when the space charge density became greater than the critical carrier density, nc. This is a current-driven device, basically, because MIT occurs after the charge carrier density, which is injected from the electrode becomes higher than nc. The switching time, which is the time for the whole sample to transition from insulator to metal because of bias application, depends on the charge carrier mobility in the insulator state. The switching speed of VO2 due to electrical triggering must be slower than that of optical triggering because the mobility of the charge carrier in the insulating state is only 0.5 cm2/V•s. For a simple two terminal model, the switching speed is proportional to the square of the length between the two electrodes and the mobility of the charge carrier, and inversely proportional to the voltage. Therefore, a proper switching speed can be designed for one’s use if the appropriate material and dimension of the device are used.