Chung-Seok Kim

Crystallization of amorphous alloy Co68Fe4Cr4Si13B11

Crystallization of amorphous alloy Co68Fe4Cr4Si13B11 was analyzed by X-ray diffraction, transmission electron microscopy and differential scanning calorimetry. The polymorphic crystallization of cobalt and eutectic crystallization of cobalt and Co3B were observed during the crystallization of the alloy. The Avrami constant was found to be 1.65, indicating that crystallization of the glass is dominated by three-dimensional diffusion controlled growth with a near zero nucleation rate. The onset of crystallization of Co68Fe4Cr4Si13B11 alloy was found to be above 773 K which was higher than those of Cr-free Co-rich amorphous alloy. The magnetic property of crystallized samples was also correlated to the microstructure during crystallization.

Microstructure and crystallization kinetics of amorphous metallic alloy: Fe54Co26Si6B14

Abstract Magnetic properties and microstructural evolution of the amorphous Co26Fe54B14Si6 alloy during thermal annealing were characterized using transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). TEM demonstrated that, similar to the Co-free iron glass system, the amorphous Co26Fe54B14Si6 alloy crystallizes through two stages: initial development of α-(Fe,Co) dendrites and eutectic reaction due to the solute enrichment. Through the DSC analysis, the activation energy for the initial dendritic crystallization was determined to be 208 kJ/mol, which is considerably lower than that of Co-free system, indicating that the addition of Co in the glass system destabilize the amorphous structure and promotes devitrification. The magnetic property of the material was also correlated to the microstructure developed during the crystallization process. The coercivity of the material was observed to increase steadily as the crystallization process proceeded due to increasing density of α-(Fe,Co) crystallites providing domain wall pinning sites.

Harmonic generation of an obliquely incident ultrasonic wave in solid-solid contact interfaces.

The conventional acoustic nonlinear technique to evaluate the contact acoustic nonlinearity (CAN) at solid-solid contact interfaces (e.g., closed cracks), which uses the through-transmission of normally incident bulk waves, is limited in that access to both the inner and outer surfaces of structures for attaching pulsing and receiving transducers is difficult. The angle beam incidence and reflection technique, where both the pulsing and receiving transducers are located on the same side of the target, may allow the above problem to be overcome. However, in the angle incidence technique, mode-conversion at the contact interfaces as well as the normal and tangential interface stiffness should be taken into account. Based on the linear and nonlinear contact stiffness, we propose a theoretical model for the reflection of an ultrasonic wave angularly incident on contact interfaces. In addition, the magnitude of the CAN-induced second harmonic wave in the reflected ultrasonic wave is predicted. Experimental results obtained for the contact interfaces of A16061-T6 alloy specimens at various loading pressures showed good agreement with theoretical predictions. Such agreement proves the validity of the suggested oblique incidence model.

Failure of exchange-biased low resistance magnetic tunneling junctions upon thermal treatment

Transmission electron microscopy (TEM) and Rutherford backscattering spectroscopy (RBS) were used to characterize low resistance (100–1000 Ω μm2) tunneling junctions consisting of Ta/NiFe/Cu/NiFe/IrMn/CoFe/Al (6.6 and 7.7 A)–oxide/CoFe/NiFe/Ta multilayers after annealing at temperatures ranging from 250 to 500 °C. The Al (7.7 A) junction showed continual improvement in the magnetoresistance (MR) ratio when annealed up to 300 °C while the MR ratio of the Al (6.6 A) junction dropped sharply above 250 °C in spite of the only 1 A difference in the deposited thickness of aluminum metal prior to plasma oxidation. TEM measurement provided evidence that the annealing process improves, in general, structural uniformity in the insulation layer, but thermal treatment can also degrade junction performance at a relatively low temperature due to current leakage through the electrodes. Current leakage can be problematic for a junction whose insulation barrier may be too thin (less than ∼10 A). Both RBS and TEM analyses ...

Thermal stability of the exchanged biased CoFe/IrMn electrode for the magnetic tunnel junction as a function of CoFe thickness

Pinned electrodes of the magnetic tunnel junction (MTJ) consisting of Ta/AlOx/CoFe(x)/IrMn/NiFe/Ta, where 35 A<x<87.5 A, were thermally annealed and analyzed using Auger electron spectroscopy and transmission electron microscopy in order to study the effects of the CoFe thickness on the interdiffusion of elements in the pinned electrode. Increasing CoFe thickness reduced the Mn migration out of the IrMn layer towards the tunnel barrier. An 87.5−A−thick CoFe layer completely blocked the Mn diffusion up to 350 °C with minimal reduction of the tunneling magnetoresistance (TMR) ratio when full junction was fabricated. Although other mechanisms could be responsible for the thermal degradation of the MTJ, the Mn diffusion appears to be related to the reduction of the TMR at 300 °C. Since the presence of Mn in the tunnel barrier as an impurity is detrimental to the junction performance, reduction of Mn migration towards the tunnel barrier by increasing the CoFe electrode should improve the postannealed performan...

Fatigue-Induced Micro-damage Characterization of Austenitic Stainless Steel 316 Using Innovative Nonlinear Acoustics

We present innovative nonlinear acoustics for characterizing fatigue-induced micro-damage of austenitic stainless steel 316 subjected to high-cycle fatigue. Various fatigue-driven deformations are accumulated at several positions near the middle of hourglass-shaped specimens. A bell-shaped curve of acoustic nonlinearity as a function of position is observed, and the variation in acoustic nonlinearity is attributed to the evolution of a lattice defect (dislocation) and stress-induced martensite based on transmission electron microscopy (TEM) observations. An oblique incidence technique using a longitudinal waveform is a potentially viable method for characterizing the high-cycle fatigue deformation of austenitic stainless steel 316 alloys.

Epitaxial integrated dE1 − dE2 silicon detectors

Abstract Epitaxial integrated d E 1 − d E 2 silicon detectors have been developed by using the multilayer epitaxial crystal growth technique combined with the chemical preferential etching technique. These detectors are useful for eliminating events affected by channeling and blocking effects in the identification of heavy ions using multiple detector telescope systems. Characteristics of d E detectors of the integrated d E 1 − d E 2 type are confirmed to as good as those of integrated E − d E detectors.

Microstructural Degradation Assessment in Pressure Vessel Steel by Harmonic Generation Technique

The harmonic generation technique has been used to characterize the isothermal degradation of pressure vessel steel. The isothermal degradation was conducted at 630°C with forged 2.25Cr-1Mo steel. The variation in the normalized ultrasonic nonlinearity parameter (β=β0) was interpreted as having resulted from microstructural evolution, which was supported by the results of electron microscopy and X-ray diffraction, in addition to Vickers hardness and ductile-brittle transition temperature. The normalized ultrasonic nonlinearity parameter increased progressively with the isothermal degradation time due to the increase in the volume fraction of equilibrium M6C carbide and the variation in the lattice parameter of M23C6 carbide. It was found that the nonlinearity parameter was very sensitive to the microstructure during the isothermal degradation of 2.25Cr-1Mo steel. The harmonic generation technique has the potential to assess microstructural changes due to isothermal degradation.

Effects of the dissolved oxygen in Ti films on Ti reactions in Cu/Ti/SiO2/Si system upon annealing

The reactions of Cu/Ti/SiO2 structures at temperatures ranging from 200 to 700 °C have been studied for various Ti thicknesses. X-ray and Rutherford backscattering spectroscopy (RBS) analyses were used to identify the reaction products resulting from Ti reactions in Cu/Ti/SiO2 systems and the oxygen composition in the unreacted Ti, and revealed a correlation between the oxygen concentration in Ti films and the sequences of the Ti reactions. The reaction products initially formed, at around 300 °C, were a series of Cu–Ti intermetallics (Cu3Ti/CuTi) at the Cu–Ti interface with the oxygen dissolved in the Ti moving from the compounds into the remaining unreacted Ti. At 500 °C, the Cu3Ti was converted into Cu-rich intermetallics, Cu4Ti, which grew at the expense of the CuTi due to the increased oxygen content in the Ti. In addition, the outdiffusion of Ti, to the Cu surface, and the Ti–SiO2 reactions caused an abrupt increase in the oxygen content in the Ti layer, which placed thermodynamic restraints on further Ti reactions. Furthermore, thinner Ti layers showed a higher increased rate of oxygen accumulation for the same consumption of Ti, which led to significantly reduced Ti consumption. The diffusion barrier properties of SiO2 for Cu metallization decreased with an increasing Ti thickness.

Effect of diffusion barrier in the thermally annealed exchange-biased IrMn-CoFe electrode in magnetic tunnel junctions

Exchange-biased electrodes IrMn/Ta(2 /spl Aring/)/CoFe/AlO/sub x/ and IrMn/CoFe/Ta(2 /spl Aring/)-AlO/sub x/ used in a magnetic tunnel junction were annealed at 300/spl deg/C to study the Mn diffusion characteristics. Auger electron spectroscopy and X-ray photoelectron spectroscopy analysis of the annealed electrodes show that the Ta diffusion barrier effectively blocked the Mn migration, regardless of the barrier location. Also concluded from the study was that the Mn migration is largely enhanced by the preferential oxidation of Mn.