We investigated electronic structure of 5d transition-metal oxide Sr 2 IrO 4 using angle-resolved photoemission, optical conductivity, and x-ray absorption measurements and first-principles band calculations. The system was found to be well described by novel effective total angular momentum J eff states, in which relativistic spin-orbit (SO) coupling is fully taken into account under a large crystal field. Despite of delocalized Ir 5d states, the J eff -states form so narrow bands that even a small correlation energy leads to the J eff = 1/2 Mott ground state with unique electronic and magnetic behaviors, suggesting a new class of the J eff quantum spin driven correlated-electron phenomena.

We investigated ferromagnetism of a newly discovered ferromagnetic semiconductor Co-doped anatase TiO2 thin film, using the magnetic circular dichroism (MCD) at the Co L(2,3) absorption edges. The magnetic moment was observed to be approximately 0.1 micro(B)/Co in the measurements, but the MCD spectral line shape is nearly identical to that of Co metal, showing that the ferromagnetism is induced by a small amount of clustered Co. With thermal treatments at approximately 400 degrees C, the MCD signal increases, and the moment reaches up to approximately 1.55 micro(B)/Co, which is approximately 90% of the moment in Co metal. In the latter case, the cluster size was observed to be 20-60 nm.

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The electronic and magnetic structures of Fe 1−x Cu x Cr 2 S 4 (0.1 x 0.5) spinel sulfides have been investigated systematically by performing photoemission spectroscopy (PES), soft x-ray absorption spectroscopy (XAS), and soft x-ray magnetic circular dichroism (XMCD) measurements using synchrotron radiation. Cr and Cu ions are found to be nearly trivalent (Cr 3+ ) and monovalent (Cu + ), respectively, and their valence states do not change with x. The Fe 2p XAS spectra of Fe 1−x Cu x Cr 2 S 4 are very similar to that of Fe metal, indicating that the Fe 3d electrons are strongly hybridized to other valence electrons. The Fe and Cr 2p XMCD spectra show that the magnetic moments of Cr ions and Fe ions are aligned antiparallel to each other and that both the Cr and Fe magnetic moments increase with increasing x. The valenceband PES study reveals that the Cr 3+ (t 3 2g ↓) 3d states are located at ∼1.5 eV below E F . The occupied Fe 3d states consist of the broad t 3 2g ↑ states, the e 2 g ↑ states at ∼4 eV below E F , and the e g ↓ states very close to E F . The filled Cu 3d 10 states lie at ∼2.5 eV below E F . This study suggests that the hybridized Fe e g ↓ and S 3p states near E F play an important role in determining the transport properties of Fe 1−x Cu x Cr 2 S 4 for x 0.5.

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