Dong-Seok Yang

Local structure and paramagnetic properties of Zn1-xMnxO

Having based on x-ray absorption spectroscopy, we point out structural phase separation in polycrystalline Zn1-xMnxO ceramics prepared by solid-state reaction. The samples are single phase in the wurtzite structure as x < 0.04. A Mn-concentration increase with x ≥ 0.04 results in the formation of a secondary phase of tetragonal ZnMn2O4 spinel. The feature of x-ray absorption spectra for the Mn K edge reveals an existence of Mn2+ and Mn3+ ions in Zn1-xMnxO, which are responsible for room-temperature paramagnetism. Particularly, when Mn content is increased, there is a chemical shift of the absorption edge toward higher energies, indicating an increase in concentration of Mn3+ ions. Fourier analyses for the spectra of extended x-ray absorption fine structure (EXAFS) shows up the substitution of Mn for Zn sites in the ZnO host lattice. The parameters related to the local structure have been determined and discussed in detail.

Origin of ferromagnetism in BaTiO3 nanoparticles prepared by mechanical milling

Recent studies have pointed out an existence of the room-temperature ferromagnetism in nanostructured BaTiO3 materials. It has been thought that the ferromagnetism is due to intrinsic defects. To elucidate this issue, we have prepared BaTiO3 nanoparticles by mechanical milling, starting from diamagnetic/nonmagnetic BaTiO3 powders, and then investigated their magnetic properties. Our idea is motivated by the fact that the mechanical milling introduces more lattice defects to a ground material. If so, the ferromagnetic (FM) order will increase with increasing the density of defects. Here, the defect density can be changed upon the milling time (tm). Our magnetic measurements at 300 K have indicated that the FM order increases with increasing tm as expected. To understand the nature of ferromagnetism, we studied X-ray absorption fine structure (XAFS) spectra of the samples for the Ti K-edge. The shift of the absorption edge towards lower energies of Ti3+ with increasing tm, proving an increase in Ti3+ concen...

Local structure and magnetic inhomogeneity of nano-sized La0.7Sr0.3MnO3 manganites

We present the study of the local structure and magnetic inhomogeneity of nano-sized La0.7Sr0.3MnO3 manganites performed by x-ray absorption fine structure (XAFS) spectroscopy and nuclear magnetic resonance (NMR). Nano-powders with particle size of 50–200 nm (NP1) and 20 nm (NP2), and bulk samples (BS) were studied. EXAFS data show the difference between the local structure of core and outer shell of the nano-particle. The data evidenced that the shell of particles presents the disordered crystalline array. According to the NMR, at low temperature the magnetic state of NP1 sample is homogenous. However, with increase of temperature the two ferromagnetic phases, with strong and weakened double exchange, coexist. The NP2 powder is magnetically homogeneous and characterized by weakened double exchange at all temperatures of the existence of the magneto-ordered state. The results specify on possible existence of characteristic spatial scale below which the phase stratification becomes energetically unfavorable.

Anomaly of local structure of La0.7ca0.3-xBaxMnO3 manganites at Curie temperature

The local structure of La0.7Ca0.3−xBaxMnO3 (x=0; 0.03; 0.06;…; and 0.3) lanthanum manganites were studied by extended x-ray absorption fine structure analysis. Dispersion, σMn–O2, and asymmetry, σMn–O3, of the pair distribution function for the Mn–O bond distances of MnO6 octahedron showed a maximum on x dependence. The maximum of σMn–O2 is caused by an increase of dynamic displacements of the Mn–O bond distances near the Curie temperature, TC. The observed x dependence of σMn–O3 is caused by a reduction in mobility of the charge carriers approaching TC from low as well as high temperatures.

EXAFS study for mechanically alloyed Co50C50

Co 50 C 50 alloys were fabricated by mechanical alloying process from the mixed powder of pure cobalt and carbon. The variations of the local structure and magnetic properties were examined by EXAFS analysis and the VSM magnetometer. It is shown that the alloying was progressed as the milling time increases. The significant changes of the magnetization and the structure has been occured in 12 h milling. EXAFS spectrum and its Fourier transform confirms that the final product is in the amorphous state. The EXAFS spectrum of the alloy milled for 24 h was fitted to the conventional EXAFS equation. The interatomic distances obtained from the fitting were 1.94 ±0.02 A for Co–C pair and 2.52 ±0.02 A for Co–Co pair, respectively. The magnetization was almost saturated in 24 h milling.

Exafs study on nanocrystalline Fe40Co10Cu50 alloy processed by mechanical alloying

Metastable Fe40Co10Cu50 alloys were fabricated by mechanical alloying process from the mixed powder. The structural changes during mechanical alloying process and post heat treatment were examined by Fe, Co, and Cu K-edge EXAFS measurements. The EXAFS analysis verified that Fe and Co atoms diffused to the Cu matrix so that the metastable FeCoCu with fcc structure was formed by the mechanical alloying. Also we found that the metastable FeCoCu alloy was decomposed into FeCo alloy and Cu by annealing process.

Effect of different thickness crystalline SiC buffer layers on the ordering of MgB2 films probed by extended x-ray absorption fine structure

Extended X-ray absorption fine structure (EXAFS) spectroscopy is a powerful method to investigate the local structure of thin films. Here, we have studied EXAFS of MgB2 films grown on SiC buffer layers. Crystalline SiC buffer layers with different thickness of 70, 100, and 130 nm were deposited on the Al2O3 (0001) substrates by using a pulsed laser deposition method, and then MgB2 films were grown on the SiC buffer layer by using a hybrid physical-chemical vapor deposition technique. Transition temperature of MgB2 film decreased with increasing thickness of SiC buffer layer. However, the Tc dropping went no farther than 100 nm-thick-SiC. This uncommon behavior of transition temperature is likely to be created from electron-phonon interaction in MgB2 films, which is believed to be related to the ordering of MgB2 atomic bonds, especially in the ordering of Mg–Mg bonds. Analysis from Mg K-edge EXAFS measurements showed interesting ordering behavior of MgB2 films. It is noticeable that the ordering of Mg–B bo...

Structural evolution of Fe80C20 alloy with alloying times

The structural evolution of mechanically alloyed Fe-C alloys was studied as a function of alloying times. The effect of alloying time on local structural changes of Fe-C has been investigated by means of Fe57 Mossbauer spectrometry, extended x-ray-absorption fine structure (EXAFS), and x-ray diffraction (XRD). XRD pattern from 24h alloyed Fe-C powder indicates at least the mixture of bcc-Fe and Fe3C phases. Mossbauer spectra analysis reveals that bcc-Fe decreases to the detriment of Fe3C phase with increasing alloying time, while both carbon-containing bcc-Fe and amorphouslike phase assigned to Fe located in grain boundaries (estimated at two atomic layers) remain alloying time independent. The variation of Fe3C phase content is in a good agreement with that observed by EXAFS analysis.

Phase Diagram and EXAFS Study of La0.7Ca0.3-xBaxMnO3 Manganites

The phase diagram and local structure of La 0.7 Ca 0.3- x Ba x MnO 3 ( x = 0; 0.03; 0.06; ...; 0.3) lanthanum manganites were studied. The Curie temperature, T c , of the compositions showed a sharp change near the concentrational structural orthorhombic–rhombohedral phase transition. Maximums of dispersion, σ Mn–O 2 , and asymmetry, σ Mn–O 3 , of pair distribution function for the Mn–O bond distances of MnO 6 octahedron on x -dependence were observed by extended X-ray absorption fine structure (EXAFS) analysis. The maximum of σ Mn–O 2 is caused by increase of dynamic rms displacements of Mn–O bond distances near the T c . The observed x dependence of σ Mn–O 3 reflects the reduction of charge carriers mobility at approaching to T c .

Divalent manganese in A-position of perovskite cell: X-ray absorption finite structure study of La0.6Sr0.4−xMnTixO3 manganites

Local structure and magnetic properties of Ti doped A-site deficient La0.6Sr0.4−xMnTixO3+δ manganites (0.15⩾x⩾0) have been studied. The compositions belong to rhombohedral R3¯c phase. Segregation of (La0.6Sr0.4−xMny)(Mn1−y−zTix)O3+δ phase and fallout of (z∕3)Mn3O4 oxide was observed with x increase. Some amount (y) of Mn, being in divalent valence state, occupies the A(=La,Sr)-position of perovskite cell. Samples with x=0 and 0.05 are ferromagnetic with Curie temperature TC=350 and 172K, respectively. Samples with x=0.1 and 0.15 are in spin(cluster)-glass states at low temperatures.