Y. M. Koo

Photoelectron bremsstrahlung spectrum in synchrotron radiation excited total reflection x-ray fluorescence analysis of silicon wafers

When synchrotron radiation is used as an excitation source, the total reflection x-ray fluorescence analysis of surface contamination on silicon wafer has an extremely low background intensity that determines the minimum detection limit. In this article, the background spectrum originating from the photoelectron bremsstrahlung is calculated using the Monte Carlo method. The doubly differential electron bremsstrahlung cross sections obtained from the Born approximation modified by the Elwert factor and with the use of the form factor approach for screening are used instead of empirical formulas. In addition to the bremsstrahlung spectrum produced from the silicon wafer, the bremsstrahlung intensity that photoelectrons, which escape from the silicon wafer, produce in the filter attached to the detector is also calculated in accordance with the usual synchrotron radiation excited total reflection x-ray fluorescence experimental conditions. The calculated photoelectron bremsstrahlung spectra are compared with...

An effect of high magnetic field on phase transformation in Fe-C system

Abstract The effect of a magnetic field on the Gibbs free energy of a material depends on its magnetization behaviors. To investigate the change in the Fe–Fe3C phase diagram caused by a high external magnetic field, the magnetic Gibbs free energies of the phases austenite, ferrite, and cementite are calculated on the basis of the molecular field theory. Using the calculated Gibbs free energy as a function of weight percentage carbon and temperature at a particular magnetic field, a phase diagram of the Fe–Fe3C system is drawn. The phase diagram is shifted upwards so that the Ac1 and Ac3 temperatures increase as the magnetic field is applied, but the Acm temperature change is almost independent of applied magnetic field value. The increase of eutectoid temperature and composition and its application to microstructural control are discussed.

The effect of residual strain on (001) texture evolution in FePt thin film during postannealing

Residual strain in a Fe55Pt45 thin film was investigated in order to provide experimental support for a previous theoretical study [J.-S. Kim et al., J. Appl. Phys. 99, 053906 (2006)] in which a strong evolution of (001) texture during the postannealing of deposited FePt thin films was attributed to amorphous substrates. Residual strain measurements using dual diffraction peaks to analyze the strain state were used in the study. The findings show that transformation and biaxial in-plane strains coexist in annealed Fe55Pt45 films and their effect could lower the strain energy of the (001) crystal (crystal with a [001] crystallographic orientation into the surface normal) when grain growth proceeds actively. Therefore, we propose that anisotropic strain due to ordering transformations under tensile in-plane strain is a key factor that affects texture evolution. The importance of the subsequent grain growth is also discussed.

Effects of cobalt-intercalation and polyaniline coating on electrochemical performance of layered manganese oxides

Cobalt-intercalated layered manganese oxide (CLMO, K0.08Co0.12MnO2), in which 75% of the potassium ions of the pristine K-birnessite (K0.32MnO2) in the interlayer region were exchanged with cobalt ions by an ion-exchanging reaction, and the polyaniline-coated CLMO prepared by chemical oxidative polymerization of aniline in an acidic medium were studied using synchrotron powder X-ray diffraction (XRD), thermogravimetric analysis (TGA), Fourier transform infrared (FT-IR) spectroscopy, extended X-ray absorption fine structure (EXAFS), X-ray absorption near edge spectroscopy (XANES) and scanning electron microscopy (SEM). The electrochemical performance of CLMO and polyaniline-coated CLMO was investigated for application as cathode electrode material for lithium ion batteries. Due to the effective pillaring effect of exchanged cobalt ions in the interlayer region, CLMO showed excellent cyclability over 40 cycles, although it exhibited a low discharge capacity (143 mAh/g at 40 mA g−1 current density). Polyaniline-coated CLMO, in addition to its excellent cyclability, presented a much higher discharge capacity (253 mAh/g) than that of CLMO at a current density of 40 mA g−1 in the range 1.5–4.0 V. These results demonstrate that intercalated cobalt ions in the interlayer region can supply sufficient structural stability during charge-discharge cycles and, furthermore, subsequent polyaniline coating on the surface of a cathode electrode can bring out surface stabilization that prevents an unwanted reaction between the electrode and electrolytes.

Dependence of the magnetic properties of Co/Pd multilayered films on the structural parameters estimated accurately by x‐ray diffraction

In this study, Co/Pd multilayered films with a few atomic layers of Co were prepared by alternating deposition in an ultrahigh‐vacuum physical‐vapor‐deposition system. The structural parameters were estimated accurately making use of only the angular positions of x‐ray diffraction peaks. The magnetic properties were found to vary greatly depending on Pd predeposition and Pd‐sublayer thicknesses as well as Co‐sublayer thickness. The Pd‐predeposited films were found to have a remarkably high coercivity of 4723 Oe and a greatly enhanced interfacial magnetic anisotropy of 0.72 mJ/m2, which indicates an excellent potential as a magneto‐optical recording medium.

The covalent bonding interaction in the ferroelectric LuMnO3

The electron density distributions of paraelectric and ferroelectric LuMnO3 are analyzed using high temperature synchrotron x-ray powder diffraction data with the Rietveld method, the maximum entropy method (MEM), and MEM-based pattern fitting. Bonding electrons due to orbital hybridization are clearly seen in the Lu1–O3 and Lu2–O4 bonds along the c-axis polarization direction. The Lu1–O3 bond was determined to be covalent due to hybridization below the ferroelectric transition temperature, and was identified as the driving force for ferroelectricity in LuMnO3. However, the Lu2–O4 bond shows covalent character in both paraelectric and ferroelectric states. Also, we suggest that the Lu2–O4 covalent bonding is responsible for large spontaneous polarization in LuMnO3, associated with the small radius of the Lu3+ ion.

Glide strains dependency on Schmid's factor of secondary slip systems in copper single crystals

The deformation behavior of Cu single crystal has been investigated by synchrotron radiation. The variation point of slope in the difference of glide strains corresponds to the exchange point of Schmids factor of the secondary slip systems. This phenomenon may be originated by the interaction between the primary and secondary slip systems.

Interface structure and magnetic anisotropy of a Co/Pt multilayer

A Co/Pt multilayer with strong perpendicular magnetic anisotropy was prepared by electron beam evaporation. Using polarized extended x-ray absorption fine structure (EXAFS) measurements at the Co K edge, the interface structure of the Co/Pt multilayer was investigated. In quantitative analysis, the EXAFS data measured with in-plane polarization showed considerable intermixing at the Co/Pt interfaces, while those measured with out-of-plane polarization indicated an abrupt interface structure. More detailed atomic configurations of the Co/Pt interface were simulated using the measured coordination numbers. The results of the computer simulation showed that the Pt clusters in Co layers and the Co clusters in Pt layers were interfacing. With the obtained structural information and a symmetry-based Neel model, the magnetic anisotropy of the multilayer was estimated. The estimated magnetic anisotropy was consistent with the measured one.

Origin of unknown x‐ray diffraction peaks from incommensurate superlattices

Unexpected x‐ray diffraction peaks have been observed in some thin film modulated structures in which each constituent element has a nonintegral number of atomic layers. The origin of these peaks has not been clearly identified. The positions and intensities of these peaks were analyzed by numerical calculation from a model superlattice. The results indicate that the positions of the anomalous peaks are caused by a new long range periodicity due to the nonintegral number of atomic layers of each constituent element and that the intensities of the anomalous peaks are determined by the interfacial structure between the two different kinds of atomic layers.

Synchrotron radiation excited total reflection x-ray fluorescence quantitative analysis of Si wafer by absolute fluorescence intensity calculation

One of the principal industry standard means of measuring surface and near surface wafer contamination is the total reflection X-ray fluorescence (TXRF). Quantification by theoretical calculation of the absolute fluorescence intensity is introduced instead of the use of standards in the TXRF experiment using synchrotron radiation. The surface densities of contaminants in and on Si wafers are determined by comparing calculated results with measured intensities.