Chandan Mazumdar

Valence electron distribution in La2Li1/2Cu1/2O4, Nd2Li1/2Ni1/2O4, and La2Li1/2Co1/2O4

Abstract The distribution of valence electrons between transition metals (TM) and oxygen has been studied via X-ray absorption near-edge fine structure (XANES) at the TM-L 2,xa03 and O-K thresholds for La 2 Li 1/2 Cu 1/2 O 4 , Nd 2 Li 1/2 Ni 1/2 O 4 , and La 2 Li 1/2 Co 1/2 O 4 . Simulations of the TM-L 2,xa03 XANES by charge-transfer multiplet calculations result in 3d-hole concentrations that increase from ∼30% 3d 8 in the Cu(III) to ∼57% 3d 7 in the Ni(III) to ∼72% 3d 6 in the Co(III) compound. Concomitantly, the O-K XANES spectra reflect an increase of bond ionicity in the same sequence. The results show that the Li-doped holes in Nd 2 Li 1/2 Ni 1/2 O 4 are distributed between Ni-3d and O-2p orbitals.

Difference in spin state and covalence between La1−xSrxCoO3 and La2−xSrxLi0.5Co0.5O4

We present a comparative study of the spin states and electronic properties of La1-xSrxCoO3 and La2-xSrxLi0.5Co0.5O4 using X-ray absorption near-edge structure spectroscopy at both the O-K and Co-L-2.3 thresholds. In the La2-xSrxLi0.5Co0.5O4 system the CoO6 octahedra are isolated, the holes induced by Sr doping are trapped in the isolated Co(IV)O-6 octahedra, and a low-spin state is found for the Co ions, which does not change upon Sr doping. In the La1-xSrxCoO3 system, the interconnected CoO6 octahedra, with a 180degrees Co-O-Co bond angle, give rise to a transition from low-spin to intermediate-spin state with a ferromagnetic alignment of the Co spins. The double-exchange, ferromagnetic coupling between Co ions mediated by the 180degrees bond angle is responsible for suppressing the low spin-state. We find that the branching ratio of spectral intensities at the L-2 and L-3 thresholds in the Co-L-2.3 X-ray absorption spectra is sensitive to the spin state of the Co ions allowing its direct spectroscopic determination

Resonant magnetic X-ray scattering from ultrathin Ho–metal films down to a few atomic layers

Abstract The magnetic structure of ultrathin Ho–metal films grown on W(110) was studied by resonant magnetic X-ray scattering at the L3 and M5 resonances. For Ho films down to 14 monolayer (ML) thickness, a bulk-like helical antiferromagnetic structure is observed. For a 10-ML thick film, an altered magnetic structure and enhanced layer spacing is found.

Magnetic behavior and local electronic structure of Ce2Ni3Si5 and CeNi4B

Abstract We report on the results of magnetic susceptibility (2–300 K) and X-ray absorption near-edge structure (XANES) studies of the two polycrystalline compounds Ce 2 Ni 3 Si 5 and CeNi 4 B. XANES spectra at the Ce-M IV,V threshold exhibit the signature of mixed valence behavior of Ce in both compounds, with a larger 4f 1 occupancy in Ce 2 Ni 3 Si 5 in agreement with the magnetic results. Ni-L II,III XANES spectra of the two compounds reveal a 3d 9 configuration of Ni in the ground state of both compounds. The 3d band, however, is more strongly delocalized in Ce 2 Ni 3 Si 5 .

Magnetic effects in the band structure of ferromagnetic and antiferromagnetic lanthanide-metal films

Abstract Systematic magnetic effects in the band structure of the heavy lanthanide metals Gd, Tb, Dy, and Ho were studied by angle-resolved photoemission from epitaxial films. The temperature-dependent exchange splitting exhibits a different behavior for delocalized and localized band states. For the delocalized Δ 2 bulk band states, a Stoner-like temperature dependence of the splitting is observed in ferromagnetic and helical antiferromagnetic phases. A more localized oxygen-induced surface state shows a temperature-independent contribution to the splitting. All observed magnetic splittings scale with the 4f spin moment.