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This work investigates charge redistribution in a series of Ni-Cu alloys using x-ray photoemission spectroscopy (XPS) and Ni/Cu ${L}_{3,2}$- and $K$-edge x-ray-absorption near-edge structure (XANES). XPS results show that the constituent $d$ bands are well separated and shifted to a slightly higher binding energy upon dilution into the other host, indicating that the atomic sites in the alloy are not as well screened relative to the pure metal. However, no significant $d$-band narrowing is observed, suggesting that there is modest $d\ensuremath{-}d$ interaction in the alloys. In contrast to the XPS observation, XANES results show a reduction in white-line intensity at both edges relative to the pure metal suggesting that both Ni and Cu sites gain $d$ charge. The unoccupied Ni $d$ band is far from fully occupied even at infinite dilution. The discrepancy between the implications of the XPS and XANES results is dealt with using a charge redistribution model in which $s\ensuremath{-}p\ensuremath{-}d$ rehybridization takes place at both sites within the framework of electroneutrality and electronegativity considerations. It appears that both Ni and Cu gain a small but measurable amount of $d$ charge in alloy formation through rehybridization (loss of non-$d$ conduction charge). Possible connection between these results and the disappearance of ferromagnetism in ${\mathrm{Ni}}_{1\ensuremath{-}x}{\mathrm{Cu}}_{x}$ alloys at $xg0.6$ is discussed.

O K-edge and Cu ${\mathrm{L}}_{23}$ -edge x-ray-absorption near-edge-structure spectra for the series of ${\mathrm{Nd}}_{1+\mathrm{x}}$ ${\mathrm{Ba}}_{2\mathrm{\ensuremath{-}}\mathrm{x}}$ ${\mathrm{Cu}}_{3}$ ${\mathrm{O}}_{7+\mathrm{\ensuremath{\delta}}}$ compounds (x=0--0.6) were measured using a bulk-sensitive x-ray fluorescence yield technique. Near the O 1s edge, pre-edge peaks at \ensuremath{\sim}527.5 and \ensuremath{\sim}528.2 eV are ascribed to the excitations of O 1s electrons to O 2p holes located in the ${\mathrm{CuO}}_{3}$ ribbons and ${\mathrm{CuO}}_{2}$ planes, respectively. The peak at \ensuremath{\sim}528.2 eV decreases in intensity with increasing the Nd doping, indicating the filling of holes in the ${\mathrm{CuO}}_{2}$ planes by the substitution of ${\mathrm{Nd}}^{3+}$ for ${\mathrm{Ba}}^{2+}$ in the ${\mathrm{Nd}}_{1+\mathrm{x}}$ ${\mathrm{Ba}}_{2\mathrm{\ensuremath{-}}\mathrm{x}}$ ${\mathrm{Cu}}_{3}$ ${\mathrm{O}}_{7+\mathrm{\ensuremath{\delta}}}$ system. For low levels of Nd doping, the holes are located mainly in the ${\mathrm{CuO}}_{2}$ planes. At higher Nd doping, the holes are located predominantly in the ${\mathrm{CuO}}_{3}$ ribbons. The depression in ${\mathrm{T}}_{\mathrm{c}}$ for the ${\mathrm{Nd}}_{1+\mathrm{x}}$ ${\mathrm{Ba}}_{2\mathrm{\ensuremath{-}}\mathrm{x}}$ ${\mathrm{Cu}}_{3}$ ${\mathrm{O}}_{7+\mathrm{\ensuremath{\delta}}}$ system correlates closely with the hole concentration in the ${\mathrm{CuO}}_{2}$ planes and the number of disrupted fourfold-coordinated Cu on the chain sites. The high-energy shoulders in the Cu ${\mathrm{L}}_{23}$ -edge absorption spectra arise from ${\mathrm{Cu}3\mathrm{d}}^{9}$ L defected states to ${\mathrm{Cu}2\mathrm{p}}^{\mathrm{\ensuremath{-}}1}$ ${3\mathrm{d}}^{10}$ L excited states, where L denotes the ligand hole on the ${\mathrm{CuO}}_{3}$ ribbons and ${\mathrm{CuO}}_{2}$ planes. With increasing the Nd doping, the shoulders shift to higher energy \ensuremath{\sim}0.3 eV from x=0 to 0.6.

We present the electronic structure of ferrimagnetic ${\mathrm{Zn}}_{\mathit{x}}$${\mathrm{Ni}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Fe}}_{2}$${\mathrm{O}}_{4}$ compounds obtained by a combination of O K-edge x-ray-absorption measurements and first-principles spin-unrestricted calculations using the pseudofunction method. The two distinct preedge features are found to vary systematically as a function of the Zn content. From both experimental and theoretical analyses, we find that substitution of Ni with Zn enhances localization of the 3d states of Fe on the octahedral sites, so that the O 2p\char21{}Fe 3d hybridized states can be resolved into two distinct twofold and threefold features. \textcopyright{} 1996 The American Physical Society.

C. F. Chang,1 D. J. Huang,1,* A. Tanaka,2 G. Y. Guo,3,1 S. C. Chung,1 S.-T. Kao,4,5 S. G. Shyu,4,5 and C. T. Chen1 1National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan 2Department of Quantum Matters, ADSM, Hiroshima University, Higashi-Hiroshima 739-8530, Japan 3Department of Physics, National Taiwan University, Taipei 106, Taiwan 4Institute of Chemistry, Academia Sinica, Nankang, Taipei 115, Taiwan 5Department of Chemistry, National Central University, Tao-Yuan 320, Taiwan sReceived 17 September 2004; published 23 February 2005d

Measurements of magnetic circular dichroism in Fe $2p$ resonant photoemission and calculations based on a full-multiplet cluster model reveal the parameters of the electronic structure of ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}.$ We obtained on-site Coulomb energy ${U}_{\mathrm{dd}}$ of $3d$ electrons, charge-transfer energy $\ensuremath{\Delta}$ which is defined as the energy required to transfer an electron from O $2p$ to Fe $3d,$ and the hybridization strength between Fe $3d$ and O $2p$ at different Fe ion sites of ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ by analyzing the data of magnetic circular dichroism. The charge-transfer energy of ${\mathrm{Fe}}^{3+}$ ions was found to be significantly smaller than that of ${\mathrm{Fe}}^{2+}$ ions. We determined the energy positions of the low-lying multiplets of each Fe ion site. These energy positions for ${\mathrm{Fe}}^{3+}$ ion sites are 1.5\char21{}2.5 eV higher than those obtained from band-structure calculations. Our calculations also show that the photoemission spectral weight in binding energy bigger than 10 eV is substantial, in contrast to the results of band-structure calculations. Our findings conclude that ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ is a system with strong electron-electron interactions.

Spin-resolved O $1s$ x-ray absorption measurements on ${\mathrm{CrO}}_{2}$ reveal that the spin polarization of the states at the Fermi level approaches 100%, providing direct evidence of the half-metallic nature of ${\mathrm{CrO}}_{2}.$ The measurements also show the existence of an atomiclike Cr $3d$ state not far away from the Fermi level with a spin polarization of only 50%, establishing its Mott-Hubbard character. We conclude that ${\mathrm{CrO}}_{2}$ has a dualistic electronic nature, in which the states at the Fermi level are bandlike, while those at higher energies are strongly localized of completely different origin and symmetry. The finding of an exceptionally large number of O $2p$ holes suggests their important role in preventing the strongly correlated ${\mathrm{CrO}}_{2}$ from being an insulator.

With measurements of the magnetic circular dichroism and band-structure calculations in the local spin density approximation (LSDA+U) approach, we revealed that the orbital moment of O in ${\mathrm{CrO}}_{2}$ is ferromagnetically coupled to that of Cr, whereas the spin moment of O is antiferromagnetically coupled to that of Cr. Spin and orbital magnetic moments of Cr and O are enhanced as Coulomb interaction energy U increases. Comparing the magnetic circular diachromism data with the LSDA+U calculations, we conclude that it is essential to include the on-site Coulomb energy for adequately describing the orbital magnetic moments of ${\mathrm{CrO}}_{2}.$