In-Sang Yang

Raman studies on single-phasic HgBa2CuO4+δ superconductors

Abstract Polycrystalline samples of HgBa 2 CuO 4+δ superconductors were prepared by solid state reaction of HgO and Ba 2 CuO 3+ x precursor in an evacuated silica tube. The HgBa 2 CuO 4+ δ has a T c of 92 K and has a tetragonal cell (P 4/mmm) with lattice parameters a and c with the values of 3.8868 (2) A and 9.4886 (1) A, respectively. Raman spectra of polycrystalline HgBa 2 CuO 4+δ show four peaks at 108 cm −1 , 158 cm −1 , 570 cm −1 and 592 cm −1 . Using a factor group analysis and Raman spectral data of the cuprate superconductors, these modes are assigned as E g and A 1g modes of Ba atoms and O(2) oxygens. The higher peaks at 570 cm −1 and 592 cm −1 are assigned to E g and A 1g modes of O(2) oxygen, respectively. The lower two modes at 108 cm −1 and 158 cm −1 are ascribed to vibrations of Ba atoms. Details of mode assignments are discussed based on symmetry and local structure.

Raman studies of spin-phonon coupling in hexagonal BaFe12O19

Phonon softening in hexagonal BaFe12O19 single crystal is observed in temperature dependent polarized Raman spectroscopy. The phonon softening is significantly enhanced below ∼80 K, at which the magnetization in a magnetic field along c-axis shows a clear change in the temperature dependence, indicating a magnetic phase transition near 80 K. Possible interpretations of the anomalous temperature dependence of the phonon frequencies–phonon softening behaviors are discussed. The phonon softening would be correlated to local structural and magnetic phase transition in hexagonal BaFe12O19, in which the spin-phonon coupling would have major contribution for the phonon softening. We also discuss that the spin-phonon coupling would be affected by both the spin ordering and the individual phonon vibration in magnetic material.

X-ray photoelectron spectroscopy study of HgBa2CuO4+δ

Abstract X-ray photoelectron spectroscopy was used to investigate the oxidation states of Hg and Cu ions in the HgBa2CuO4+δ compound. In contrast to the previous report on the scraped samples [C.S. Gopinath and S. Subramanian, Physica C 232 (1994) 222], our spectra from the fresh surfaces of fractured samples do not show the unusual 3 + oxidation state of Hg ions, while the satellite structure of the Cu 2 p 3 2 core-level spectrum which indicates the 2 + oxidation state of Cu ions is observed. These results confirm that, in the HgBa2CuO4+δ compound, the Hg atom does not act as an electron reservoir, and that the CuO layers are hole-doped due to the extra oxygens in the HgOδ planes.

Raman spectra of HgBa2Ca2Cu3O8+d superconductors

Abstract We have measured Raman spectra of HgBa2Ca2Cu3O8+d (Hg-1223) superconductors. The spectra show peaks at 92, 127, 138, 230, 479, 538, 574, and 585 cm-1. From a factor group analysis and the structural information about Hg-1223, we assign the modes at 92, 574, and 585 cm-1 to be A1g modes of Ba, O(2) and O(3) atoms, respectively. The modes at 479 and 538 cm-1 are proposed to be Eg modes of O(3) bending and O(2) stretching vibrations, respectively. Details of mode assignments are given in the discussion.

Resonant A1 phonon and four-magnon Raman scattering in hexagonal HoMnO3 thin film

We present the results of resonant Raman scattering of the A1 phonon at 680 cm−1 and of the four-magnon at 760 cm−1 in hexagonal HoMnO3 thin film. We find that the A1 phonon at 680 cm−1 shows a strong resonance effect near the on-site Mn d–d transition at ~1.7 eV. Our Raman results show that the four-magnon scattering can be selectively excited with red lasers of 647 nm (1.92 eV) and 671 nm (1.85 eV), but are not detectable with green lasers of 532 nm (2.33 eV), indicating that the four-magnon scattering in hexagonal HoMnO3 has an extremely strong resonance effect also near the on-site Mn d–d transition at ~1.7 eV. Furthermore, through the analyses of our study of the resonant four-magnon Raman scattering and earlier studies of the resonant two-magnon Raman scattering, we propose a simple general model for all resonant magnon scattering. Our simple general model predicts a simple method for the investigation of the spin-flipping/spin-wave in magnetic materials, which would have significant impacts on the applications of spintronic devices.

A Magnetization Study of Prussian-blue Analogue Na x Mn y [Fe(CN) 6 ]

In this report, we present the results of a study on the effects of the particle size on the properties of the Prussian blue (PB) analog Na x Mn y [Fe(CN)?]. A novel synthesis method of the Na x Mn y [Fe(CN)?] nano-particles using an organic solvent, formamide, is employed. The size of the PB particles is found to be 100-150 ㎚ for the samples prepared in the formamide solvent, which is much smaller than that of the samples prepared using water only. The broadening of the X-ray diffraction peaks of the nano-sized PB samples is attributed to the lattice disorder and a dramatic reduction in the particle size. The compositions of the samples are confirmed by an energy-dispersive X-ray analysis (EDAX), and the result proves that the samples are actually Na x Mn y [Fe(CN)?] Prussian blue. The UV-vis spectra show a broad intervalence charge-transfer (CT) band in the visible region between 400 and 700 ㎚, and the absorption decreases abruptly in the green region for the nano-sized PB sample. A divergence between the field cooled (FC) and zero field cooled (ZFC) magnetization curves is observed for the nano-sized PB sample at 11 K, indicating that nanoparticles in the sample are single domain superparamagnets with a blocking temperature of 11 K. Our results reveal that the nano-sized PB samples show significantly different optical and magnetic properties than those of the bulk PB samples.

Study of spin-ordering and spin-reorientation transitions in hexagonal manganites through Raman spectroscopy.

Spin-wave (magnon) scattering, when clearly observed by Raman spectroscopy, can be simple and powerful for studying magnetic phase transitions. In this paper, we present how to observe magnon scattering clearly by Raman spectroscopy, then apply the Raman method to study spin-ordering and spin-reorientation transitions of hexagonal manganite single crystal and thin films and compare directly with the results of magnetization measurements. Our results show that by choosing strong resonance condition and appropriate polarization configuration, magnon scattering can be clearly observed, and the temperature dependence of magnon scattering can be simple and powerful quantity for investigating spin-ordering as well as spin-reorientation transitions. Especially, the Raman method would be very helpful for investigating the weak spin-reorientation transitions by selectively probing the magnons in the Mn3+ sublattices, while leaving out the strong effects of paramagnetic moments of the rare earth ions.

Raman Studies of

Ti_1-xFe_xO₂ (x = 0.00-0.13) nanoparticle samples were prepared by hydrolysis method. We investigated the effects of Fe doping on the structural and magnetic properties of the Ti_1-xFe_xO₂ nanoparticle system. Scanning electron microscopy and X-ray diffraction measurements confirm that the particle size of the powder is in nanoscale, and that the magnetic Fe impurities substitute for the Ti sites in the anatase TiO₂ phase. All the samples with x > 0 were found to be super-paramagnetic at room temperature by magnetization measurements. Raman spectra also strongly support that the Fe atoms go into the Ti-site in theTiO₂ structure. For comparison, ceramic Ti_1-xFe_xO₂ samples were also prepared by usual ceramic method. Ferromagnetism was observed only in the ceramic Ti_1-xFe_xO₂ system. Additional Raman peak at around 610 cm^-1 is observed only in the ceramic samples. This may be related to the clusters created by mixture of various valence state of Fe, which probably would be the cause for ferromagnetism observed in the ceramic Ti_1-xFe_xO₂ system.

{\rm Ti}_{\bm{ 1-x}} {\rm Fe}_{\bm x} {\rm O}_{\bm 2}

We present the results of an optical method of quantitatively estimating the spin exchange interactions in hexagonal manganites RMnO3 (R = Tb, Dy, Ho, Er) epitaxial thin films. The two in-plane (a-b plane) spin exchange integrals J1 (intratrimer Mn-Mn interaction) and J2 (intertrimer Mn-Mn interaction) are deduced from the magnon scattering peak wavenumbers. We found that J2 decreases systematically when the R ionic radius increases, while J1 is nearly independent of R ionic radius, contrary to the expectation in single crystals. We show that the R dependence of J1 could be understood in terms of the stress in the thin films. Our result indicates that the stress has stronger effect on the atomic displacement of the intratrimer Mn-Mn distance than the intertrimer Mn-Mn distance.

Nanoparticles

We investigated the electronic structure and lattice dynamics of multiferroic