Namsoo Shin

Giant magneto-elastic coupling in multiferroic hexagonal manganites

The motion of atoms in a solid always responds to cooling or heating in a way that is consistent with the symmetry of the given space group of the solid to which they belong. When the atoms move, the electronic structure of the solid changes, leading to different physical properties. Therefore, the determination of where atoms are and what atoms do is a cornerstone of modern solid-state physics. However, experimental observations of atomic displacements measured as a function of temperature are very rare, because those displacements are, in almost all cases, exceedingly small. Here we show, using a combination of diffraction techniques, that the hexagonal manganites RMnO3 (where R is a rare-earth element) undergo an isostructural transition with exceptionally large atomic displacements: two orders of magnitude larger than those seen in any other magnetic material, resulting in an unusually strong magneto-elastic coupling. We follow the exact atomic displacements of all the atoms in the unit cell as a function of temperature and find consistency with theoretical predictions based on group theories. We argue that this gigantic magneto-elastic coupling in RMnO3 holds the key to the recently observed magneto-electric phenomenon in this intriguing class of materials.

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.

Mechanoluminescent SrAl2O4:Eu,Dy phosphor for use in visualization of quasidynamic crack propagation

The propagation of a macroscale crack was visualized in Mg partially stabilized zirconia ceramic using the mechanoluminescence (ML) of SrAl2O4:Eu,Dy. The transformation zone around the crack was also clearly detected by the ML. The ML technique permitted on a relatively fast time frame the quasidynamic R curve to be measured for crack speeds up to 15m∕s. The transformation zone width grew to a maximum in the initial transient part of the quasidynamic R curve. The applied stress intensity factor then commenced to increase to 16MPam.

Structural Characterization of the Surface-Modified Li x Ni0.9Co0.1O2 Cathode Materials by MPO4 Coating (M = Al , Ce, SrH, and Fe) for Li-Ion Cells

Structural characterization of surface-modified Li x Ni 0.9 Co 0.1 O 2 cathodes (x = 0.3 and 0.15) using an MPO 4 coaling (M = Al, Ce, SrH, and Fe) were investigated for their potential applications to Li-ion cells. MPO 4 nanoparticles that were precipitated from metal nitrate and (NH 4 ) 2 HPO 4 in water at pH = 10 were coated on the cathodes via mixing and heat treatment at 700°C. The CePO 4 and SrHPO 4 -coated Li 0.3 Ni 0.9 Co 0.1 O 2 cathodes heat treated at 300°C were mainly made up of the rock-salt phase (Fm3m), while AlPO 4 and FePO 4 -coated cathodes showed disordered [Li 1-x (Ni,Co) x ] 3b [(Ni,Co) x ] 3a O 2 -type hexagonal structure (R3m) with a cation mixing. However, when the x value decreased from 0.3 to 0.15, bare and coated cathodes which had a spinel (Fd3m) or hexagonal structure (R3m) at x = 0.3 were transformed into a NiO-type rock-salt structure. AlPO 4 -coated sample exhibited lowest degree of oxygen generation after 300°C annealing at x = 0.15, indicating the highest thermal stability among the bare and coated cathodes.

Luminescence of pulsed laser deposited Gd2O3:Eu3+ thin film phosphors on quartz glass substrates

Thin film phosphors of Gd2O3:Eu3+ were deposited on quartz glass plates by pulsed laser deposition and compared with typically used thin film phosphors composed of Y2O3:Eu3+ in terms of cathodoluminescence (CL) and photoluminescence (PL). Both the CL and PL of Gd2O3:Eu3+ thin film phosphor were superior to those of corresponding data on currently widely used Y2O3:Eu3+ red thin film phosphor. In particular, the Gd2O3:Eu3+ thin film phosphor showed a much better cathodoluminescence at low excitation energy (less than 1 keV) than a Y2O3:Eu3+ film of the same thickness, indicating that the Gd2O3:Eu3+ thin film phosphor is a much better candidate for use in field emission displays.

Effect of corrugated substrates on light extraction efficiency and the mechanism of growth in pulsed laser deposited Y2O3:Eu3+ thin-film phosphors

The luminance level of thin-film Y2O3:Eu3+ phosphors deposited by a pulsed laser deposition techniques are not candidates for field emission display because of their low extraction efficiency. A square lattice nanorod-type two-dimensional SiO2 photonic crystal layer (PCL) was built on a quartz glass substrate in an attempt to improve the efficiency of light extraction. The pitch of the PCL was varied, such that 500, 550, and 600nm pitches were produced while maintaining the depth of the PCL fixed at 200nm. The integrated photoluminescence efficiency of an Y2O2:Eu3+ thin-film phosphor deposited on the PCL was improved by 4.8 times and well matched with the calculated result based on the finite-difference time-domain method. The PCL played a significant role not only in enhancing the extraction efficiency but also aiding in the control of the film growth.

High-resolution synchrotron x-ray diffraction study of Zr-rich compositions of Pb(ZrxTi1- x)O3 (0.525<x<0.60) : Evidence for the absence of the rhombohedral phase

Results of Rietveld analysis of the synchrotron x-ray diffraction data on Pb(ZrxTi1−x)O3 (PZT) for 0.525⩽x⩽0.60 are presented to show the absence of rhombohedral phase on the Zr-rich side of the morphotropic phase boundary. Our results reveal that the structure of PZT is monoclinic in the Cm space group for 0.525⩽x⩽0.60. The nature of the monoclinic distortion changes from pseudotetragonal for 0.525⩽x⩽0.54 to pseudo-rhombohedral for x>0.54.

Origin of high piezoelectric response of Pb(ZrxTi1−x)O3 at the morphotropic phase boundary: Role of elastic instability

Temperature dependent structural changes in a nearly pure monoclinic phase composition (x=0.525) of Pb(ZrxTi1−x)O3 (PZT) have been investigated using Rietveld analysis of high-resolution synchrotron powder x-ray diffraction data and correlated with changes in the dielectric constant and planar electromechanical coupling coefficient. Our results show that the intrinsic piezoelectric response of the tetragonal phase of PZT is higher than that of the monoclinic phase. It is also shown that the high piezoelectric response of PZT may be linked with an anomalous softening of the elastic modulus (1∕S11E) of the tetragonal compositions closest to the morphotropic phase boundary.

Evidence for monoclinic crystal structure and negative thermal expansion below magnetic transition temperature in Pb(Fe1∕2Nb1∕2)O3

The existing controversy about the room temperature structure of multiferroic Pb(Fe1∕2Nb1∕2)O3 is settled using synchrotron powder x-ray diffraction data. Results of Rietveld refinements in the temperature range of 300–12K reveal that the structure remains monoclinic in the Cm space group down to 12K, but the lattice parameters show anomalies at the magnetic transition temperature (TN) due to spin-lattice coupling. The lattice volume exhibits negative thermal expansion behavior, with α=−4.64×10−6K−1, below TN.