Eunsook Lee

Valence states and electronic structures of Co and Mn substituted spin gapless semiconductor PbPdO2

Electronic structures of Pb(Pd0.9T0.1)O2 (T = Mn, Co) spin gapless semiconductors have been investigated by employing soft X-ray absorption spectroscopy (XAS) and photoemission spectroscopy (PES). The valence states of Co and Mn ions are found to be mixed-valent (∼2.7) and tetravalent, respectively. The measured valence-band PES and O 1s XAS spectra show that both PbPdO2 and PbPd0.9Co0.1O2 are small-gap semiconductors. This finding is supported by the calculated band structures, obtained in the density functional theory with the modified Becke-Johnson potential (mBJ) scheme. This work also shows evidence for the existence of the phase separation in Mn-substituted PbPd0.9Mn0.1O2.

Size-dependent structural evolution of the biomineralized iron-core nanoparticles in ferritins

The structural identity of the biomineralized iron core nanoparticles in Helicobacter pylori ferritins (Hpfs) has been determined by employing soft x-ray absorption spectroscopy and soft x-ray magnetic circular dichroism. Valence states of Fe ions are nearly trivalent in all Hpfs, indicating that the amount of magnetite (Fe3O4) is negligible. With increasing filling of Fe ions, the local configurations of Fe3+ ions change from the mixture of the tetrahedral and octahedral symmetries to the octahedral symmetry. These results demonstrate that the biomineralization of the ferritin core changes from maghemite-like (γ-Fe2O3) formation to hematite-like (α-Fe2O3) formation with increasing Fe content.

Phase separation in thermoelectric delafossite CuFe1−xNixO2 observed by soft x-ray magnetic circular dichroism

Electronic structures of Ni-doped CuFe1−xNixO2 delafossite oxides (0 ≤ x ≤ 0.03) have been investigated by employing soft x-ray magnetic circular dichroism (XMCD). Finite XMCD signals are observed for Fe, Ni, and Cu 2p states, and valence states of Cu, Fe, and Ni ions are nearly monovalent (Cu+), trivalent (Fe3+), and divalent (Ni2+), respectively, for all x ≤ 0.03. Tiny magnetic impurities could be detected by employing XMCD. Fe and Ni 2p XMCD signals are identified due to ferrimagnetic spinel impurities of CuFe2O4 and NiFe2O4. XMCD signals for Cu 2p states arise from divalent Cu2+ ions. Thermoelectrical properties are found to be very sensitive to the very little impurity phase present in delafossite oxides.

Soft x-ray magnetic circular dichroism study of valence and spin states in FeT2O4 (T = V, Cr) spinel oxides

Electronic structures of spinel oxides FeT2O4 (T = V, Cr) have been investigated by employing soft x-ray magnetic circular dichroism (XMCD) and soft x-ray absorption spectroscopy (XAS). XAS reveals that Cr and V ions are trivalent, and that Fe ions are nearly divalent in FeT2O4 (T = V, Cr). Finite XMCD signals are observed in FeV2O4 at T = 80 K, while they are very weak in FeCr2O4. XMCD shows that Fe spins are antiparallel to V and Cr spins, with the V and Cr spins being canted from Fe spins, which suggests a Yafet-Kittel type triangular spin configuration in FeT2O4 (T = V, Cr).

The 7 × 1 Fermi Surface Reconstruction in a Two-dimensional f -electron Charge Density Wave System: PrTe3

The electronic structure of a charge density wave (CDW) system PrTe3 and its modulated structure in the CDW phase have been investigated by employing ARPES, XAS, Pr 4 f RPES, and first-principles band structure calculation. Pr ions are found to be nearly trivalent, supporting the CDW instability in the metallic Te sheets through partial filling. Finite Pr 4 f spectral weight is observed near the Fermi level, suggesting the non-negligible Pr 4 f contribution to the CDW formation through the Pr 4 f -Te 5p hybridization. The two-fold symmetric features in the measured Fermi surface (FS) of PrTe3 are explained by the calculated FS for the assumed 7 × 1 CDW supercell formation in Te sheets. The shadow bands and the corresponding very weak FSs are observed, which originate from both the band folding due to the 3D interaction of Te sheets with neighboring Pr-Te layers and that due to the CDW-induced FS reconstruction. The straight vertical FSs are observed along kz, demonstrating the nearly 2D character for the near-EF states. The observed linear dichroism reveals the in-plane orbital character of the near-EF Te 5p states.

Differences in the Electronic Structures of Bulk and Powder FeV 2 O 4 Spinel Oxide Investigated by Using Synchrotron Radiation

The electronic structure of ferrimagnetic spinel oxide of has been investigated by employing soft x-ray absorption spectroscopy (XAS) and soft x-ray magnetic circular dichroism (XMCD). The Fe 2p and V 2p XAS spectra show that the valence states of Fe and V ions are mixed-valent states and states, respectively. In Fe 2p XMCD spectra, finite XMCD signals are observed for divalent states only, but not for states. This finding indicates that the magnetic moments of ions are ordered ferromagnetically but that those of ions are cancelled, implying that ions play an important role in determining magnetic properties of .

Soft X-ray absorption spectroscopy study of multiferroic Bi-substituted Ba1−xBixTi0.9Fe0.1O3

The electronic structures of multiferroic oxides of Ba1−xBixTi0.9Fe0.1O3 (0 ≤ x ≤ 0.12) have been investigated by employing photoemission spectroscopy and soft X-ray absorption spectroscopy (XAS). The measured Fe and Ti 2p XAS spectra show that Ti ions are in the Ti4+ states for all x and that Fe ions are Fe2+-Fe3+ mixed-valent for x > 0. The valence states of Fe ions are found to be nearly trivalent for x=0 and decrease with increasing x from being nearly trivalent (v(Fe) ~ 3) for x = 0 to v(Fe) ~ 2.6 for x = 0.12. The valence states of both Ti and Ba ions do not change for all x ≤ 0.12. Based on the obtained valence states of Fe ions, the electronic and the magnetic properties of Ba1−xBixTi0.9Fe0.1O3 are explored.

Soft X-ray Synchrotron-Radiation Spectroscopy Study of [Co/Pd] Multilayers as a Function of the Pd Sublayer Thickness

We have investigated the electronic structures of intermetallic multilayer (ML) films of [Co(2A)/Pd(xA)] (x: the thickness of the Pd sublayer; x = 1 A, 3 A, 5 A, 7 A, 9 A) by employing soft X-ray absorption spectroscopy (XAS) and soft X-ray magnetic circular dichroism (XMCD). Both Co 2p XAS and XMCD spectra are found to be similar to one another, as well as to those of Co metal, providing evidence for the metallic bonding of Co ions in [Co/Pd] ML films. By analyzing the measured Co 2p XMCD spectra, we have determined the orbital magnetic moments and the spin magnetic moments of Co ions in [Co(2A)/Pd(xA)] ML films. Based on this analysis, we have found that the orbital magnetic moments are enhanced greatly when x increases from 1 A to 3 A, and then do not change much for x ≥ 3 A. This finding suggests that the interface spin-orbit coupling plays an important role in determining the perpendicular magnetic anisotropy in [Co/Pd] ML films.

Soft X-ray Synchrotron-Radiation Spectroscopy Study of Half-metallic Mn3Ga Heusler Alloy

By employing photoemission spectroscopy (PES) and soft X-ray absorption spectroscopy (XAS), the electronic structure of the candidate half-metallic antiferromagnet of Mn3Ga Heusler compound has been investigated. We have studied two ball-milled Mn3Ga powder samples, one after annealing and the other without annealing, respectively. Based on the Mn 2p XAS study, we have found that Mn ions are nearly divalent in Mn3Ga and that the Mn ions having the locally octahedral symmetry and those having the locally tetrahedral symmetry are both present in Mn3Ga. We have found relatively good agreement between the measured valence-band PES spectrum of Mn3Ga and the calculated density of states, which is in agreement with the half-metallic electronic structure of Mn3Ga.

Photoelectron Spectroscopy Study of the Semiconductor Electrode Nanomaterials for the Dye Synthesized Solar Cell

The electronic structures of the potential candidate semiconductor nanoparticles for dye-sensitized solar cell (DSSC), such as and , have been investigated by employing X-ray photoemission spectroscopy (XPS). The measured X-ray diffraction patterns show that and samples have the single-phase ilmenite-type structure and the inverse spinel structure, respectively. The measured Zn 2p and Sn 3d core-level XPS spectra reveal that the valence states of Zn and Sn ions are divalent (Zn 2+) and tetravalent (Sn 4+), respectively, in both and . On the other hand, the shallow core-level measurements show that the binding energies of Sn 4d and Zn 3d core levels in are lower than those in . This work provides the information on the valence states of Zn and Sn ions and their chemical bonding in and .