Kyungsoo Ahn

Electrical and magnetic properties of NiCrxFe2−xO4 spinel (0 ≤ x ≤ 0.6)

The spinel ferrite NiCrxFe2−xO4 system was synthesized under atmospheric pressure, and showed single spinel phase in the XRD pattern. The lattice parameter decreased with increasing amount of Cr. For the study of cation distribution in this system, magnetic measurements and Mossbauer spectroscopy were carried out. It was found that ions at the B site moved to the A site, and that this system varied from an inverse to a normal spinel structure. It was observed that the magnetic moment of the systems and Curie Temperature decreased by substitution effect. Using a thermopower measurement, this system was found to exhibit n-type semiconductivity. In the electrical conductivity measurements, an abrupt change exists near the Tc, owing to the magnetic spin arrangement relaxation. The electrical conduction mechanism of spinel is an electron transfer between the +2 and +3 cations at the B site, so called polaron hopping mechanism. From the conductivity result, this system is closely related to the electron-hole-compensation between electrons and holes.

Physicochemical properties of electrochemically prepared polypyrrole perchlorate

Abstract Polypyrrole perchlorate (PPy-ClO 4 ) has been prepared by electrochemical polymerization in an acetonitrile solution. We present the results of extensive measurements on the physicochemical properties of the PPy-ClO 4 films by electrical, magnetic, electrochemical, and thermal analysis. Cyclic voltammetry demonstrates that both of the peak current and potential strongly depend on the scan rate, implying the electrode reaction process of PPy-ClO 4 is irreversible. Thermal analyses by thermogravimetric analysis (TGA) and by differential thermal analysis (DTA) show that a weight loss occurs between 250–320°C with a glass transition temperature T g ≈268°C, which is much higher than the previous works. The temperature dependence of the electrical conductivity, σ ( T ), is presented and analyzed using several models. The results demonstrate that the transport properties of PPy-ClO 4 can be described well by ln[ σ ( T ) T 1/2 ] vs. T −1/4 plot, corresponding to the phonon-assisted hopping process for the charge carriers. We discuss the implication of the data in relation with the microscopic and morphological structure of the samples.

Giant negative magnetoresistance in GdI2

Abstract GdI 2 is a layered d 1 compound which is isostructural with and nominally isoelectronic to the superconductors 2H–TaS 2 and 2H–NbSe 2 . GdI 2 orders ferromagnetically at 276(2) K and displays large negative magnetoresistance ∼70% at 7 T close to room temperature. At 10 K the saturation magnetization is 7.33(5) μ B in good agreement with the value predicted from spin polarized band structure calculations.

Magnetic ordering within the layered terbium carbide iodide, Tb2C2I2

Abstract The compound Tb 2 C 2 I 2 crystallizes with a layered structure (space group= C2/m ), which has a close-packed arrangement of I–Tb–Tb–I slabs. C 2 units are located within the Tb octahedral voids. Powder magnetic susceptibility measurements suggest two antiferromagnetic ordering transitions below 90 K. From the refinement of neutron powder diffraction patterns, the Tb moments order with a propagation vector k =(0,0,0). The refined Tb moments are 8.0(1) μ B at 10 K, and they are aligned antiferromagnetically in the ac plane.

Pressure dependence of the transition temperature of the layered superconductors Y2C2Br2−xIx

Abstract We report the effect of hydrostatic pressure up to 1.5 GPa on the superconducting transition temperatures Tc of four compounds in the series Y2C2Br2−xIx(0

Formation of high-Tc phase in Sn-incorporated (Bi,Pb)-Sr-Ca-Cu-O superconducting system

Superconductor Bi2Pb1 − xSnxSr2Ca2Cu3Oy (x = 0.0, 0.05, 0.08, 0.1, 0.2 and 0.3) samples were prepared by the conventional solid state reaction and analyzed by XRD, SEM, TGA and DSC and their resistivities were measured by standard ac four-probe method. When a small quantity of Sn (x < 0.1 ) was added, it was found from the SEM and XRD results that high-Tc phase occurred preferentially and the critical temperature was elevated. With the results of XRD, EDX, DSC and TG, it was concluded that a small amount of Sn promotes the formation of liquid phase, which makes the weak link of grain boundary better. When a large amount (x ≥ 0.1) of Sn was added to the (Bi,Pb) 2223 system, however, the solid state reactions became more important than liquid phase-assisted reactions. Thus, the reaction rate decreased and there is no longer a significant role of tin on the formation of 2223 phase in that region (0.1 ≤ x ≤ 0.3).

Electrical and magnetic properties of the Sn4+ substituted spinel ferrite, ZnSnxFe2−4x/3O4 (0.00 ≤ x ≤ 0.30)

Abstract ZnSnxFe2−4x/3O4 spinels (0.0 ≤ x ≤ 0.3) which have cubic symmetry were synthesized using a ceramic technique at 1150°C for 24 h in air and rapidly quenched to room temperature. From the XRD. TGA and DTA results, it was found that all samples had single phase and thermal stability in the range of room temperature up to 1000 K. As Sn4− concentration was increased from x = 0 to 0.3, the lattice parameter and electrical conductivity increased. The results of Mossbauer spectroscopy and magnetic moment measurements showed that the tetrahedral (A) site is occupied by a Zn+ ion and magnetic moments depend only upon Fe3+ ion in an octahedral (B) site. The magnetic moments decreased with an increasing amount of non-magnetic Sn (d10) ion. n-type semiconductivity was observed in all samples. However, the conductivities did not depend on Sn substitution. It was assumed that the conduction mechanism would be derived from the competitive relation between the formation of Fe2+ and elongation of distance between neighboring Fe cations.

Superconductivity in layered lanthanum carbide halides: La2C2(X,X′)2 (X,X′=Cl, Br, I)

Abstract The superconducting properties of the layered lanthanum carbide halides La 2 C 2 X 2 (X=Br, I) and mixed halides are investigated. These compounds show superconductivity at 7.03(5) for X=Br and at ≈1.7 K for X=I, respectively. Selected compositions were investigated using high-resolution neutron powder diffraction measurements from room temperature to 1.5 K. Rietveld refinements of the powder patterns yield the structural parameters, in particular of the C atoms. These allow us to follow in detail the variation of the interatomic distances in the C 2 group. The low-temperature structural parameters are taken as inputs to perform Extended Huckel band structure calculations which allow us to identify a band of low dispersion along the Γ–N direction in the Brillouin zone. We discuss correlations of structural and electronic properties of the La 2 C 2 (X,X′) 2 phase with the superconducting transition temperatures.

Spin fluctuations in the quasi-two-dimensional Heisenberg ferromagnet GdI 2 studied by electron spin resonance

The spin dynamics of

Magnetoresistance study of Ta4Te4Si fibers

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